U.S. patent application number 10/522974 was filed with the patent office on 2005-07-28 for reduced volume heat exchangers.
This patent application is currently assigned to ASHE MORRIS LTD.. Invention is credited to Ashe, Robert, Morris, David.
Application Number | 20050161205 10/522974 |
Document ID | / |
Family ID | 31891814 |
Filed Date | 2005-07-28 |
United States Patent
Application |
20050161205 |
Kind Code |
A1 |
Ashe, Robert ; et
al. |
July 28, 2005 |
Reduced volume heat exchangers
Abstract
A heat exchanger for delivery of heat transfer fluid to a
process heat transfer surface is provided, the heat exchanger is in
contact with a process fluid and the heat transfer surface
comprises at least five heat transfer conduits each having a cross
sectional area for the flow path of less than 2000 square
millimetres wherein the linear velocity of the heat transfer fluid
through the heat transfer conduits is from 0.5 to 20
m.multidot.s.sup.-1 and adapted so that the temperature of the heat
transfer fluid changes by at least 1.degree. C. when the system is
operating at design load, the exchanger enables more accurate
temperature measurement and control in physical and chemical
reactions. Also provided is a heat transfer system in which the
heat transfer conduit for passage of the heat transfer fluid is
attached to an expansion plate which is in contact with the heat
transfer surface and enables independent movement of the heat
transfer conduit and the heat transfer surface as their temperature
change due to changes in temperature and/or pressure.
Inventors: |
Ashe, Robert; (Shenley,
GB) ; Morris, David; (Appleton, GB) |
Correspondence
Address: |
OHLANDT, GREELEY, RUGGIERO & PERLE, LLP
ONE LANDMARK SQUARE, 10TH FLOOR
STAMFORD
CT
06901
US
|
Assignee: |
ASHE MORRIS LTD.
Theobald Street
GB
|
Family ID: |
31891814 |
Appl. No.: |
10/522974 |
Filed: |
February 1, 2005 |
PCT Filed: |
August 8, 2003 |
PCT NO: |
PCT/EP03/08942 |
Current U.S.
Class: |
165/168 |
Current CPC
Class: |
F28F 2265/26 20130101;
F28D 7/0041 20130101; B01J 2219/00083 20130101; B01J 2219/00094
20130101; F28D 1/06 20130101; F28D 7/024 20130101; B01J 2219/0009
20130101 |
Class at
Publication: |
165/168 |
International
Class: |
F28F 003/12 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 9, 2002 |
GB |
0218442.2 |
Aug 20, 2002 |
GB |
0219372.0 |
Aug 30, 2002 |
GB |
0220159.8 |
Claims
1. A heat exchanger for delivery of heat transfer fluid to a
process heat transfer surface which is in contact with a process
fluid wherein the heat transfer surface fluid is delivered in at
least five heat transfer conduits each having a cross sectional
area for the flow path of less than 2000 square millimetres wherein
the linear velocity of the heat transfer fluid through the heat
transfer conduits is from 0.5 to 20 m.multidot.s.sup.-1 and adapted
so that the temperature of the heat transfer fluid changes by at
least 1.degree. C. when they system is operating at design heat
load.
2. A heat exchange according to claim 1 in which the time taken for
the heat transfer fluid to pass through the heat exchanger as
measured in seconds is not greater than twice length of the heat
transfer surface when said length is measured in metres.
3. A heat exchanger according to claim 1 in which the conduits have
a cross sectional area for the flow path of less than 180 square
millimetres.
4. (canceled)
5. A heat exchanger according to claim 1 where the heat transfer
fluid is delivered in 5 or more separate heat transfer fluid
conduits where the total inventory of gas, liquid or solid to be
heated or cooled within the device is less than 1000 litres.
6. A heat exchanger according to claim 1 where the heat transfer
fluid is delivered in 3 or more separate heat transfer fluid
conduits per 1000 litres of gas, liquid or solid to be heated where
the total inventory of said gas, liquid or solid within the heat
transfer device is greater than 1000 litres.
7. (canceled)
8. (canceled)
9. A heat exchanger according to claim 1 wherein the linear
velocity of the heat transfer fluid through the heat transfer
conduit is between 0.5 and 5 m.multidot.s.sup.-1 for liquid cooled
systems when the heat exchanger is operating at full design load
and between 2 and 20 m.multidot.s.sup.-1 for gas cooled systems
when the heat exchanger is operating at full design load.
10. (canceled)
11. (cancelled)
12. (canceled)
13. (canceled)
14. A heat exchanger according to claim 1, whereby the heat
transfer fluid flows within independent conduits which are not in
direct contact with the gas, liquid or solid which is being heated
or cooled and that the heat transfer fluid conduit is bonded,
fused, glued, brazed, welded or soldered to the surface which
serves as the containment barrier for the gas, liquid or solid
which is being heated or cooled.
15-58. (canceled)
59. A heat exchanger according to claim 1 wherein the heat transfer
fluid conduit or conduits is held to the surface which serves as
the containment barrier for the gas, liquid or solid which is being
heated or cooled by means of clamps, springs, wires, natural shape
of the conduit or some other form mechanical fixing and a layer of
a soft, thermally conductive material such as conductive grease,
fluid, conductive wool, fibrous conductive mat or a mixture thereof
is provided between the transfer fluid conduit and the surface
which serves as the containment barrier for the gas, liquid or
solid which is being heated or cooled.
60. A heat exchanger according to claim 1 wherein the conduit for
the heat transfer fluid is mounted on an expansion plate to permit
independent movement of the heat transfer conduit in relation to
the containment barrier for the gas, liquid or solid which is being
heated or cooled.
61. A heat exchanger according to claim 1 which uses a variable
area heat transfer surface.
62. A heat exchanger according to claim 1 in which the residence
time of the heat transfer fluid is less than 6 seconds.
63. A heat transfer system for the transfer of heat between a
process fluid and a heat transfer fluid across a heat transfer
surface comprising a heat transfer conduit for passage of the heat
transfer fluid attached to an expansion plate said expansion plate
being in contact with the heat transfer surface said expansion
plate enabling independent movement of the heat transfer conduit
and the heat transfer surface.
64. A heat transfer system according to claim 63 wherein the heat
transfer fluid is delivered in at least five heat transfer conduits
each having a cross sectional area for the flow path of less than
2000 square millimetres wherein the linear velocity of the heat
transfer fluid through the heat transfer conduits is from 0.5 to 20
m.multidot.s.sup.-1 and adapted so that the temperature of the heat
transfer fluid changes by at least 1.degree. C. when they system is
operating at full design load.
65. A heat transfer system according to claim 63 whereby the heat
transfer fluid flows within independent conduits which are not in
direct contact with the gas, liquid or solid which is being heated
or cooled and that the heat transfer fluid conduit is bonded,
fused, glued, brazed, welded or soldered to the surface which
serves as the containment barrier for the gas, liquid or solid
which is being heated or cooled.
66. A heat transfer system according to claim 63 where the heat
transfer fluid flows within independent conduits which are not in
direct contact with the gas, liquid or solid which is being heated
or cooled and the heat transfer fluid conduit is held to the
surface which serves as the containment barrier for the gas, liquid
or solid which is being heated or cooled by means of clamps,
springs, wires, natural shape of the conduit or some other form
mechanical fixing and the gap between the heat transfer fluid
conduit and the surface which serves as the containment barrier for
the gas, liquid or solid which is being heated or cooled is filled
by means of a soft, thermally conductive material such as
conductive grease, fluid, conductive wool, fibrous conductive mat
or a composite of several of these materials.
Description
[0001] The present invention relates to improvements in or relating
to heat exchangers.
[0002] Heat exchangers are used to add or remove heat from gases
liquids and solids. Different designs and applications exist but
the present invention is concerned with types where heating or
cooling fluid flows within a conduit which is within or in close
proximity to the material being heated or cooled. Of particular
interest however are those where the heat transfer fluid flows
through some form of jacket, coil or pipe. Examples of this include
tanks or pipes with internal or external jackets, coils or plates.
The present invention is also applicable to many types of equipment
designed for specific process functions such as reactors, reaction
calorimeters, fermenters, cell growth vessels, extruders, dryers,
mixers, mills, filters etc.
[0003] The heat exchangers with which the present invention is
concerned are designed to have reduced inventories of heat transfer
fluid within the jacket, coil or plate. They are referred to in
this document as reduced volume heat exchangers.
[0004] The gas or liquid which is used to deliver and remove heat
is referred to as the heat transfer fluid. The heat transfer fluid
may be a proprietary product such as Syltherm XLT, Dowtherm J or a
non proprietary fluid such as water, ethylene glycol or any other
fluid which is suitable as a heat transfer fluid. The heat transfer
fluid might also be a gas or vapour which can condense or boil.
[0005] The material which is to be heated or cooled is referred to
as the process material. The process material may be liquid, gas,
powder, solids or mixtures thereof. The surface which separates the
heat transfer fluid or heat transfer fluid conduit from the process
material and, that part of the surface which is in direct contact
with the process material is referred to as the process heat
transfer surface. The volume of heat transfer fluid contained
within the conduit may be referred to as hold up volume. Where
there is a containment conduit for heat transfer fluid which is not
part of the heat transfer surface, this is referred to as a
conductor pipe.
[0006] Conventional jackets heat exchangers employ (as illustrated
in FIG. 1), external half coils (such as those illustrated in FIGS.
2 & 4) and internal coils (as shown in FIG. 3). These heat
exchanger systems are designed on the principle that heat transfer
fluid turbulence should be maximised for good distribution and
transmission of heat. In order to achieve this, excess volumes of
heat transfer fluid are passed through the heat exchanger.
[0007] The traditional design approach for heat exchangers has been
to expose the process material to a surface behind which is a large
bath or large plugs of turbulent heat transfer fluid. The present
invention on the other hand is based on using multiple thin films
or thin tubes of heat transfer fluid. According to the invention
the flow paths of the heat transfer fluid are relatively short and
the residence time is also short.
[0008] The concept of the present invention described herein has
superfinite similarities to bolt on or clamp on jackets, sometimes
referred to as "limpet jackets". The key difference of the present
invention is that whilst the limpet jackets deliver compromised
heat transfer capabilities, the design of the present invention
delivers substantially enhanced heat transfer capabilities.
[0009] The present invention which embodies the reduced volume
concept enables different design principles to be employed. A
greater number of smaller heat transfer conduits are used for
delivering heat transfer fluid to the process heat transfer surface
(or directly to the process material in the case of internal pipes,
coils or plates). We have found that considerable benefits can be
realised using these techniques.
[0010] We have found that the reduced volume design usually (but
not always) contains conduits for the heat transfer fluid rather
than half coils or jackets. In addition the use of external jackets
or conduits provides for the designer, the freedom to pick a
material for containment of the heat transfer fluid. This has the
benefit that a fully contained pipe or conduit does not have to be
welded to the surface of the heat exchanger to maintain good
containment integrity (of the heat transfer fluid) and a variety of
attachment techniques such as glue, solder, clamps or guides can be
used. Thus a material like copper having higher thermal
conductivity can be used to deliver heat to the heat transfer
surface irrespective of the process material. Because the conduits
(carrying the heat transfer fluid) are small and made of a
conductive material, heat can be usefully transmitted from the heat
transfer fluid over the full wetted perimeter of the conduit. Good
heat transfer at the process heat transfer surface can therefore be
achieved by even distribution of the conductive heat transfer
conduits. The present invention therefore provides exchangers which
are easier to build and which offer better performance in a variety
of ways.
[0011] The design techniques described here can be applied to
systems which use heat transfer fluids which are either liquid,
gaseous or a mixture of liquids and gases. It is particularly
relevant however to systems using liquid or gaseous heat transfer
fluids.
[0012] The present invention therefore provides a method for
designing heat exchangers with reduced hold up volumes of heat
transfer fluid; the use of this smaller amount provides the
following benefits:
[0013] a more reliable measure of heat balance is possible since
the temperature change of the heat transfer fluid is large and
therefore easier to measure. This provides valuable and accurate
information about the rate and progress of any operation which
liberates or absorbs heat. The effects of thermal change within the
heat transfer fluid are also reduced;
[0014] a small inventory of heat transfer fluid with a
comparatively short flow path can be replaced more quickly leading
to improved temperature control;
[0015] better transmission of heat from the heat transfer fluid to
the heat transfer surface is possible with the reduced volume
design;
[0016] better distribution of heat transfer fluid is possible due
to the use of multiple small pipes.
[0017] In a first embodiment the present invention therefore
provides a heat exchanger for delivery of heat transfer fluid to a
process heat transfer surface which is in contact with a process
fluid wherein the heat transfer surface comprises at least five
heat transfer conduits each having a cross sectional area for the
flow path of less than 2000 square millimetres wherein the linear
velocity of the heat transfer fluid through the heat transfer
conduits is from 0.5 to 20 m/s and adapted so that the temperature
of the heat transfer fluid changes by at least 1.degree. C. when
the system is operating at full load.
[0018] In a second embodiment the present invention therefore
provides a process for the transfer of heat between a process fluid
and a heat transfer fluid across a heat transfer surface in which
the heat transfer surface comprises at least five heat transfer
conduits each having a cross sectional area for the flow path of
less than 80 square millimetres wherein the linear velocity of the
heat transfer fluid through the heat transfer conduits is from 0.5
to 20 m/s and the temperature of the heat transfer fluid changes by
at least 1.degree. C. when the system is operating at full
load.
[0019] It is preferred that the heat transfer fluid have a
relatively short residence time in the conduits. We prefer that
this residence time in seconds is not greater than twice the length
of the heat transfer surface as measured in metres.
[0020] The key to this design can be summarised in the following
statements:
[0021] the heat transfer fluid is delivered to the heat transfer
surface in multiple small elements. Typically a single element
would not carry more than 20% (and in many cases much less) of the
total heat transfer fluid.
[0022] there is no need to maximise turbulence in the heat transfer
conduit in order to achieve good transmission of heat between the
heat transfer fluid and the process material. Most reduced volume
heat exchangers of the present invention operate with the heat
transfer fluid flowing under laminar conditions (although it can be
turbulent in some cases).
[0023] maximising turbulence in the heat transfer conduit is not a
design requirement for achieving good distribution of heat transfer
fluid.
[0024] the heat transfer fluid may be carried within a fully
contained conduit or pipe and, this pipe or conduit may be of an
entirely different material to the containment wall for the process
material particularly where the heat transfer element is an
external heat transfer element. This can avoid the time consuming
and costly welding required when half coils are used as external
heat transfer elements.
[0025] where external heat transfer elements are used, the heat
transfer fluid conduit wall may be used to enhance the efficiency
of heat transmission and maintain even distribution of heating or
cooling.
[0026] in the case of external heat transfer elements, the full
hydraulic perimeter of the containment conduit for the heat
transfer fluid is used for transferring heat to and from the heat
transfer fluid.
[0027] the shape of the containment conduit for the heat transfer
fluid can be modified. to increase the heat transfer area between
the conduit and the heat transfer fluid.
[0028] each heat transfer element may be sized such that, at
maximum heating or cooling load, the heat transfer fluid undergoes
a significant temperature change (typically greater than 1.degree.
C.) whilst flowing at a comparatively high linear velocity
(typically 0.5 to 5 m.multidot.s.sup.-1). In many cases this
temperature change will be >3.degree. C.
[0029] In a further embodiment of the present invention which
employs external heat transfer elements which are of different
material to the process containment vessel, the design can be
developed to allow for differential expansion due to changes in
temperature and/or pressure of the heat transfer surface and the
conduit carrying the heat transfer fluid. The invention therefore
provides techniques whereby stress problems created by differential
expansion can be overcome by the use of thermally conductive
expansion plates as described herein below.
[0030] PCT/EP02/09956 and GB 2374 948 describe a design for
variable area heat exchangers. The heat transfer surfaces described
in those patents are broken up into multiple separate elements. In
sizing individual heat transfer elements, the linear velocity and
the temperature change of the heat transfer fluid passing through
the heat transfer element are important design criteria. By using a
multi element variable area heat exchanger, these earlier patents
are concerned with more effective use of heat transfer fluid. It
was also shown that such heat exchangers offer (amongst other
things) more accurate heat measuring capabilities and better
temperature control.
[0031] The present invention exploits the concept of multi-element
heat exchangers to achieve reduced hold up volumes of heat transfer
fluid. As with the variable area design, specified values of
temperature change and velocity of the heat transfer fluid are
design requirements. Unlike the earlier variable area design
however, the present invention regulates the log mean thermal
difference (between the process material and the heat transfer
fluid) to control temperature. In the design of the present
invention the full heat transfer area or large sections of heat
transfer area carry a continuous flow of heat transfer fluid. In
the design of the present invention, the prime purpose of using
multiple elements is to provide good heat transmission and even
distribution of heating or cooling fluid whilst maintaining small
hold up volumes of heat transfer fluid.
[0032] On very small heat exchangers, some of the design criteria
of the present invention are less easy to differentiate from those
of the traditional systems. For example a very small heat exchanger
will usually have laminar flow by default. However even on these
very small heat exchanger systems some of the key design criteria
of the present invention enhance the performance of the heat
exchanger. The most important being the division of the delivery of
the reduced volumes of the heat transfer surface into multiple
elements.
[0033] On large heat exchangers, the differences between the
present invention and conventional designs are very clear cut.
Although some reactors use multiple heat transfer elements they are
used for different reasons. In some existing systems, separate heat
transfer elements are used to allow different sections of the
equipment to be heated or cooled separately. In other cases, the
heat transfer elements are broken up to reduce the temperature
change of the heat transfer fluid. In the reduced volume design,
one of the design objectives is the very converse of this. The heat
transfer surface is broken up so as to give a comparatively large
temperature change in the temperature of the heat transfer
fluid.
[0034] The amount of heat which can be transmitted by a heat
exchanger is determined by the equation:
q=U.multidot.A.multidot.LMTD
[0035] Where
[0036] q=heat transmitted (W)
[0037] U=heat transfer coefficient
(W.multidot.m.sup.-2.multidot.K.sup.-1)
[0038] A=the heat transfer area (m.sup.2)
[0039] LMTD=temperature difference between heat transfer fluid and
process material (K)
[0040] The components of this equation and their implication for
heat exchanger design are discussed below.
[0041] The heat transfer coefficient (U) defines the ease with
which heat can be transmitted between the heat transfer fluid and
the process material.
[0042] FIG. 5 shows a heat transfer surface between two fluids. For
good heat transmission, the heat transfer surface (X.sub.b) in FIG.
5) should be as thin as possible and have high thermal
conductivity. In practice however wall thickness and choice of
material are usually governed by the need to maintain adequate
mechanical strength and resistance to chemical attack.
[0043] Boundary layers lie at the interface between the heat
transfer fluid and the conduit and between the process material and
the heat transfer surface. In these boundary layers, virtually no
mixing occurs and heat has to cross by conduction. The thickness of
the boundary layers (X.sub.HTF and X.sub.p of FIG. 5) reduces as
the turbulence in the respective bulk fluids increase. Thin
boundary layers have reduced resistance to heat transmission.
Traditionally, large heat exchangers promote high turbulence to
reduce the thickness of the boundary layer.
[0044] The amount of heat that can be transferred is directly
proportional to the area of the heat transfer surface available
(A). Folded surfaces offer greater heat transfer area but the scope
for this is limited for many applications. Ease of cleaning and the
need to avoid stagnant pockets are key design considerations. In
some cases however limited profiling is used (such as dimples or
ribs) to increase heat transfer area (this can also help to promote
turbulence).
[0045] The amount of heat that can be transferred is directly
proportional to the difference in temperature between the process
material and the heat transfer fluid. The average of the
temperature difference between the respective materials is referred
to as the Log Mean Thermal Difference (LMTD). Variation of the LMTD
is used as the temperature control parameter in fixed area heat
exchangers whereas variable area heat exchangers such as those
described in PCT/EP02/09956 and GB 2374948, the LMTD can be kept
substantially constant.
[0046] In conventional heat exchangers the size and shape of
jacketed vessels are largely determined by functional requirements
such as product capacity needs, the need for uniform agitation
within the process material, velocity control within the process
material and ease of cleaning. For these reasons many heat
exchangers have relatively simple internal geometry. The heat
transfer surface is often formed around the outer surface of the
vessel. In some cases, one or more internal coils or plates are
fitted.
[0047] The designer seeks to maximise the heat transfer capacity by
improving the heat transfer coefficient. In conventional large heat
exchangers, this is achieved by maximising turbulence. A measure of
how turbulent a system is can be related to the Reynolds number as
follows:
Re=.rho..multidot.v.multidot.dl/.mu.
[0048] Where
[0049] Re=Reynolds number [dimensionless]
[0050] .rho.=Fluid density [kG.multidot.m.sup.-3]
[0051] v=fluid velocity [m.multidot.s.sup.-1]
[0052] .mu.=fluid viscosity[N.multidot.s.multidot.m.sup.-2]
[0053] As a general rule, the flow conditions of fluids turn from
laminar to turbulent at a Reynolds number of around 2000. Above
2000 turbulence tends to increase with rising Reynolds number and
with this increase the boundary layer gets thinner thus reducing
resistance to heat transmission. Conventional large heat exchangers
maximise turbulence by increasing the liquid velocity. In the case
of jacketed systems, turbulence is promoted by injecting heat
transfer fluid in to the jacket at high velocity or with the use of
baffles. In the case of coils or tubes, heat transfer fluid is
pumped at high velocity.
[0054] The present invention on the other hand uses reduced volumes
of heat transfer fluid in a manner that secures a reasonable
temperature drop of the heat transfer fluid whilst keeping the
fluid velocity at an acceptable high level. The heat transfer fluid
flow on large systems (with a nominal heat transfer capacity of
greater than 3 kW per heat transfer conduit based on a temperature
rise of 5.degree. C. in the heat transfer fluid) usually has
reduced turbulence with a Reynolds number of less than 10,000. On
mid sized systems (with a nominal heat transfer capacity of greater
than 0.1 to 3 kW per heat transfer conduit based on a temperature
rise of 5.degree. C. in the heat transfer fluid) the Reynolds
number is usually below 2000. On small systems (with a nominal heat
transfer capacity of less than 0.1 kW per heat transfer conduit
based on a temperature rise of 5.degree. C. in the heat transfer
fluid) the Reynolds number is usually less than 500. It must be
recognized however, that systems can deviate from the typical
values shown above especially where very high pressure drops
through the heat transfer conduit are used. It is generally
preferable however that the pressure drop of the heat transfer
fluid is not excessive (>2 bar). There are however different
design considerations depending on whether the heat transfer
elements pass through the process material or are wrapped around
the external surface. The two cases are considered separately
below.
[0055] Reduced Volume Systems with External Heat Transfer
Elements
[0056] The amount of heat removed by the heat transfer fluid can be
calculated from the following equation.
q=mCp(t.sub.si-t.sub.so)
[0057] Where
[0058] q=heat transfer capacity of heat exchanger (W)
[0059] m=mass flow of heat transfer fluid
(kg.multidot.s.sup.-1)
[0060] Cp=specific heat of heat transfer fluid
(J.multidot.kg.sup.-1.multi- dot.K.sup.-1)
[0061] t.sub.si-t.sub.so=temperature change of heat transfer fluid
(K)
[0062] Where t.sub.si is the temperature of the heat transfer fluid
on entry unto the heat exchanger and t.sub.so is the temperature of
the fluid on exist from the heat exchanger.
[0063] In the reduced volume system of the present invention, the
linear velocity and the temperature change of the heat transfer
fluid are key design parameters. The velocity and temperature
change of the heat transfer fluid (t.sub.si-t.sub.so) are
preferably as large as is reasonably practical. In addition to
this, it is preferable to use the lowest acceptable design heat
load. Some of the design considerations which arise from these
objectives are considered below.
[0064] Consideration 1: The Design Heat Load (q.sub.des)
[0065] The design heat load will be based on the nominal maximum
heat load of the process whose temperature is to be controlled. If
the equipment is used for multiple purposes, this will be based on
the process operation which has the highest heat load. For example
biological processes tend to have a lower maximum heat load than
chemical processes. In some cases the design heat load may be based
on emergency conditions. It should be noted that some systems can
be designed on the basis that short term peak heat loads may be
higher than the maximum steady state cooling capacity of the
system. In this type of case, a design heat capacity which is
smaller than the peak process heat capacity can be beneficial since
it will require a smaller inventory of heat transfer fluid. The
design heat load is referred to as q.sub.des.
[0066] It is good practice to choose a q.sub.des which is larger
than the absolute value of the maximum heat load. This additional
safety margin allows for errors in calculation or unforeseen
operating conditions.
[0067] Consideration 2: The Heat Transfer Fluid Temperature Rise
(t.sub.si-t.sub.so),
[0068] The maximum acceptable value of heat transfer fluid
temperature rise (t.sub.si-t.sub.so), should be established bearing
in mind the operating conditions and capabilities of the
system.
[0069] The heat transfer coefficient (U) needs to be established.
The U value defines how easily heat can be transmitted between the
process material and the heat transfer fluid. This can be
calculated using standard heat transfer theory but is most easily
taken from test data or historical data.
[0070] The nominal area of the heat transfer surface (A) of the
heat exchanger needs to be established. For the purposes of this
description, this can be based on the heat transfer area in contact
with the process material. It should be noted that this is not the
true heat transfer area since the heat transfer area in contact
with the heat transfer fluid is likely to be different to the heat
transfer area in contact with the process material.
[0071] Thus using q.sub.des, U and A, the design value for the log
mean thermal difference can be determined as follows:
q.sub.des=U.multidot.A.multidot.LMTD.sub.des
[0072] Where
[0073] q.sub.des=the maximum process heat load (W)
[0074] U=the heat transfer coefficient
(W.multidot.m.sup.-2.multidot.K.sup- .-1)
[0075] A=the nominal heat transfer area (m.sup.2)
[0076] LMTD.sub.des=Design log mean thermal difference (K)
[0077] The LMTD.sub.des can now be used to establish a design value
for the heat transfer fluid temperature change (t.sub.si-t.sub.so).
The LMTD is the true average difference between the heat transfer
fluid temperature and the process temperature and is calculated
from the following formula:
LMTD=[(T.sub.p-t.sub.si)-(T.sub.p-t.sub.so)]/ln[(T.sub.p-t.sub.si)/(T.sub.-
p-t.sub.so)]
[0078] Where
[0079] T.sub.p=process temperature and is generally fixed
[0080] t.sub.si=inlet temperature of the heat transfer fluid
[0081] t.sub.so=exit temperature of the heat transfer fluid as
illustrated in FIG. 6.
[0082] By testing different values of T.sub.p-t.sub.si, alternative
values of t.sub.si-t.sub.so can be found which when calculated out,
give the design LMTD (LMTD.sub.des). In this evaluation the
following factors need to be considered:
[0083] the value of t.sub.si must not be so high that it causes
heat damage or unwanted boiling of the process material. The
temperature (t.sub.si) must also fall within the design
capabilities of the system.
[0084] the value of t.sub.si must not be so low that it causes cold
damage or unwanted freezing of the process material. The
temperature (t.sub.si) must also fall within the design
capabilities of the system.
[0085] a high value of t.sub.si-t.sub.so reduces the maximum heat
transfer capacity but improves the ability to measure heat balance
accurately.
[0086] a low value of t.sub.si-t.sub.so requires more pumping
energy to get the heat transfer fluid through the heat
exchanger.
[0087] We prefer that t.sub.si-t.sub.so is in the range of
0.1.degree. C. to 1000.degree. C. A more normal design range of
t.sub.si-t.sub.so however would be between 1.degree. C. and
20.degree. C., preferably between 3.degree. C. and 20.degree. C.
more preferably between 3.degree. C. and 15.degree. C.
[0088] The design value of t.sub.si-t.sub.so could be determined by
other criteria. For example, the designer may need to measure the
heat balance of the working system. If he has a temperature
measuring device which is capable of measuring to .+-.0.1.degree.
C., he might select a t.sub.si-t.sub.so of 5.degree. C. on the
basis that this gives a temperature measuring accuracy of
.+-.2%.
[0089] As a further embodiment therefore the present invention
provides the use of a predetermined t.sub.si-t.sub.so of a heat
transfer process fluid to be employed in a heat exchanger for the
design of the heat exchanger so as to reduce the hold up volume of
heat transfer fluid within the heat exchanger.
[0090] In a preferred system the hold up volume is reduced to the
minimum acceptable volume as determined by the nature of the heat
exchanger.
[0091] Consideration 3: The Heat Transfer Fluid Flow Rate (m)
[0092] Having determined the design heat load (q.sub.des) and the
design temperature change of the heat transfer fluid
(t.sub.si-t.sub.so) the required rate of flow of heat transfer
fluid at maximum load can be calculated from the following
equation.
m=q.sub.des/[Cp(t.sub.si-t.sub.so)]
[0093] Where
[0094] q.sub.des=design heat load
[0095] m=mass flow of heat transfer fluid
(kg.multidot.s.sup.-1)
[0096] Cp=specific heat of heat transfer fluid
(J.multidot.kg.sup.-1.multi- dot.K.sup.-1)
[0097] t.sub.si-t.sub.so=temperature change of heat transfer fluid
(K)
[0098] The mass flow of heat transfer fluid (m) may be used as one
of the factors for sizing the heat transfer elements. In practice
however this is not a fixed value since it can be varied by
increasing or decreasing the velocity of heat transfer fluid
through the heat transfer elements. The ability to vary the
velocity (and temperature) of the heat transfer fluid is useful and
gives the equipment operator the freedom to vary the heat exchanger
performance around a core design value.
[0099] As a further embodiment the present invention therefore
provides the use of a predetermined linear velocity of the heat
transfer fluid to be employed in a heat exchanger for the design of
the heat exchanger so as to reduce the hold up volume of heat
transfer fluid within the heat exchanger to the minimum acceptance
volume.
[0100] In a preferred system the hold up volume is reduced to the
minimum acceptable volume as determined by the nature of the heat
exchanger.
[0101] Consideration 4: The Number of Heat Transfer Elements
(n)
[0102] The underlying purpose of this invention is to minimise the
volume of heat transfer fluid held within the heat exchanger. It is
also desirable to utilise the maximum available heat transfer area.
The thickness of the heat transfer fluid layer surrounding the heat
exchanger can be calculated as follows:
[0103] At any time, the volume of heat transfer fluid in service is
as follows:
V=m.multidot.r/.rho.
[0104] Where
[0105] V=total volume of heat transfer fluid (V)
[0106] m=mass flow of heat transfer fluid
(kg.multidot.s.sup.-1)
[0107] .rho.=density of the heat transfer fluid
(kg.multidot.m.sup.-3)
[0108] r=residence time of the heat transfer fluid (s)
[0109] The residence time (r) is calculated from the following
relationship:
r=Z/v
[0110] Where
[0111] r=residence time of heat transfer fluid (s)
[0112] Z=total length of each heat transfer fluid conduit (m)
[0113] v=Velocity of heat transfer fluid (m.multidot.s.sup.-1)
[0114] Although there are aspects of Z and v which may have to be
tested by iterative methods, simple criteria may often be applied.
For example, on a cylindrical vessel Z will often be a simple
multiple of the vessel perimeter (half, once or twice the distance
of the perimeter) to make fabrication simple. The velocity may also
be set in say the range of 1 to 3 m.multidot.s.sup.-1, to deliver
fast temperature control response without being so high as to incur
excessive pressure drop.
[0115] Thus the thickness of the heat transfer fluid layer can be
calculated as follows:
W=V/A
[0116] Where
[0117] W=thickness of the heat transfer fluid layer (m)
[0118] V=total volume of heat transfer fluid (m.sup.3)
[0119] A=heat transfer area (m.sup.2)
[0120] The heat transfer area referred to is that which is in
contact with the process material. The reduced volume concept seeks
to reduce the heat transfer fluid to a thinnest possible layer
(W).
[0121] One consequence of designing a minimum volume system is that
the thickness of the heat transfer fluid layer surrounding the heat
transfer surface is reduced. If a single thin sheet of fluid is
used however, the fluid will tend to channel and not give uniform
distribution. As this layer gets thinner, problems of fluid
distribution arise and the heat transfer fluid starts to channel as
shown in the transition from FIG. 7(a) to FIG. 7(b).
[0122] The solution to the channelling problem is resolved by
breaking up the heat transfer surface into separate channels as
shown in FIG. (8). It is preferable that the ratio of height to
width of these channels is limited such that one dimension is not
more than five times the other, i.e. L is no more than five times W
in the system illustrated in FIG. 8.
[0123] In the full pipe version of the minimum flow design, the
side walls of the conduits not in contact with the heat transfer
surface also serve as conductors thus giving a second reason for a
low L:W ratios. In systems where there are very small channels
(<1 mm.sup.2 in cross sectional area) or multiple small
channels, the internal dimensions may use ratios of up to 10:1 (L
to W or W to L) where multiple conduits are arranged in parallel as
illustrated in FIG. 9, L can be 10 times W or less. In this case, W
is calculated as follows:
W=W1+W2+W3 . . .
[0124] depending on how many elements are used.
[0125] This relationship applies irrespective of the shape of the
conduit and three different shapes of conduit are illustrated in
FIG. 10.
[0126] There will be different ways of determining the number of
heat transfer conduits according to the geometry and layout of the
heat transfer surface. In the case of a cylindrical vessel (with
heat transfer conduits only on the cylindrical side), with conduits
around the cylinder of one full turn, the theoretical minimum
number of conduits would be:
n.sub.min=H/(L-Y)
[0127] Where
[0128] H=height of vessel cylinder (m)
[0129] W=thickness of the heat transfer fluid layer (m)
[0130] Y=wall thickness of the heat transfer fluid conduit (m)
[0131] This design process is indicative and not absolute and there
is no absolute rule for the ideal number of elements since the
fabrication method chosen will have a significant impact on how the
elements are assembled and how they perform. There are, however,
some general considerations to take into account:
[0132] larger systems require a greater number of elements.
[0133] the elements need to cover adequate heat transfer surface
area in contact with the process material.
[0134] as a general rule, a larger number of elements will permit
better distribution of heat transfer fluid and better transmission
of heat.
[0135] if the number of elements is too large, their individual
flow capacities will reduce to a point where blockage can be a
problem.
[0136] The number of elements used on a system can vary from 5 to
many thousands. A typical number however will vary from 5 to 200
depending on the size of the system, prefered systems contain for
10 to 200, most preferably 10 to 100, most preferably 10 to 50
elements.
[0137] The design process may be iterative and the number of
elements chosen may need to be reconsidered several times during
the design process.
[0138] Consideration 5: The Heat Transfer Element Cross-Section
(d)
[0139] High linear velocities (of heat transfer fluid) are
desirable as they give small hold up volumes of heat transfer
fluid. They also improve the response time for temperature
control.
[0140] Having specified a design value for the linear velocity, the
cross sectional area of flow can be calculated from the following
equation
a.sub.e=m/[v.multidot..rho..multidot.n]
[0141] Where a.sub.e=cross sectional area of flow path
(m.sup.2)
[0142] m=mass flow of heat transfer fluid
(kg.multidot.s.sup.-1)
[0143] v=linear velocity of heat transfer fluid through the conduit
(m.multidot.s.sup.-1)
[0144] .rho.=density of heat transfer fluid
(kg.multidot.m.sup.-3)
[0145] n=number of elements used
[0146] If the heat transfer element were circular, this would give
an internal pipe diameter of:
d=[4a.sub.e/II).sup.1/2
[0147] Where
[0148] d=Internal diameter of heat transfer element (m)
[0149] a.sub.e=cross sectional area of flow path (m.sup.2)
[0150] In practice, the conduits carrying the heat transfer fluid
may have a variety of different cross sectional shapes.
[0151] The cross sectional area is dependent on the size of the
heat transfer surface and the heat load it is required to carry it
should however be less than 2000, preferably less than 1300, more
preferably less than 500, more preferably less than 80 and in some
instances less than 20 perhaps less than 1 square millimetre.
[0152] Linear velocities of between 0.01 m.multidot.s.sup.-1 and 10
m.multidot.s.sup.-1 can be used for design purposes in liquid
cooled systems. In practice however very low velocities are
undesirable as they yield large hold up volumes in the heat
transfer conduit. The use of large volumes also increases the
response time for temperature control which is undesirable. Very
high velocities can also present problems as they create high
pressure drops through the heat transfer element which can put too
high a load on the pumps that deliver the fluid. A typical system
will be designed for linear velocities of between 0.5
m.multidot.s.sup.-1 and 5 m.multidot.s.sup.-1. In the case of
systems heated or cooled by gas, the maximum range can vary from
0.1 m.multidot.s.sup.-1 and 100 m.multidot.s.sup.-1 but a typical
range will be 2 m.multidot.s.sup.-1 to 20 m.multidot.s.sup.-1. It
is preferred that the pressure drop across the heat transfer
element be no greater than 10 bar and is preferably in the range
0.5 to 5 bar.
[0153] Consideration 6: The Heat Transfer Element Length (Z)
[0154] Once the cross sectional areas for the heat transfer
elements have been established, the optimum lengths can be
determined. Each heat transfer element has an optimum heat carrying
capacity. If the element is too short, the heating/cooling capacity
of the heat transfer fluid will be under utilised. If the heat
transfer element is too long, the heat carrying capacity of the
element will be insufficient for the heat transfer area
covered.
[0155] The nominal heat transfer capacity of each heat transfer
element may be calculated as follows:
q.sub.e=q.sub.des/n
[0156] Where
[0157] q.sub.e=nominal heat transfer capacity of each element
(V)
[0158] q.sub.des=design heat load for the system (W)
[0159] n=number of heat transfer elements used.
[0160] Each heat transfer element has to cover a specific area of
heat transfer surface. This area is calculated as follows:
A.sub.e=q.sub.e/U.multidot.LMTD.sub.des
[0161] Where
[0162] A.sub.e=area covered by the heat transfer element
(m.sup.2)
[0163] q.sub.e=nominal heat transfer capacity of each element
(W)
[0164] U=the heat transfer coefficient
(W.multidot..sup.-2.multidot.K.sup.- -1)
[0165] LMTD.sub.des=design log mean thermal difference (K)
[0166] As an approximation, the nominal width (w) of the heat
transfer element can be taken as the distance between the centre
line of one heat transfer element and the centre line of the next
heat transfer element. From this, the nominal length of the element
can be calculated as follows:
z.sub.e=A.sub.e/w.sub.e
[0167] Where
[0168] zl.sub.e=length of each heat transfer element (m)
[0169] A.sub.e=nominal heat transfer area of each element
(m.sup.2)
[0170] w.sub.e=nominal width of each heat transfer element (m)
[0171] Once the length of the heat transfer element has been
established, the pressure drop should be checked by conventional
calculation methods. If the pressure drop is too high, the
calculation may need to be repeated with a larger cross sectional
flow areas and/or different flow path lengths. This may in turn
lead to choosing a different number of heat transfer elements. The
evaluation process may require a number of iterations. In many
cases it will be preferable to start with a nominated length of
heat transfer element as one of the design criteria.
[0172] For external heat transfer conduits, the maximum effective
path length of a heat transfer conduit is preferably less than
twice the length of the heat transfer surface and more preferably
is approximately equal to the heat transfer surface of the heat
exchanger.
[0173] In the case of a cylindrical vessel, the length of the heat
transfer surface is:
P=.pi..multidot.D
[0174] Where
[0175] D=diameter of the cylinder
[0176] p=3.1416
[0177] P=perimeter of heat exchanger
[0178] This gives a maximum residence time of:
r=D/v
[0179] Where
[0180] r=residence time (s)
[0181] D=diameter of the cylindrical vessel (m)
[0182] V=linear velocity of the heat transfer fluid
(m.multidot.s.sup.-1)
[0183] On the basis that the preferred minimum velocity is 0.5
m.multidot.s.sup.-1, this limits the residence time of heat
transfer fluids in reduced volume systems. Thus, the maximum
residence time for a 3 metre diameter cylindrical vessel with a
single loop would be approximately 19 seconds with a fluid velocity
of 0.5 m.multidot.s.sup.-1. With a preferred linear velocity of say
2 m.multidot.s.sup.-1, the residence time would be under 5 seconds.
Typically the residence time of heat transfer fluid in a reduced
volume heat exchanger will be from 0.1 to 5 seconds according to
the size of the system. Very small (under 1 litre) and very large
systems (such as cylindrical vessels of greater than 5 metre in
diameter) may have residence times above and below the typical
value described above. The heat transfer conduits area generally
arranged so as to lie parallel with the hydraulic plane selected.
In other shapes of vessel, a different plane may be chosen to
define the length of the heat transfer surface. The plane which is
used to define the length of the heat transfer surface however must
have sufficient length to ensure efficient heat transfer without an
unmanageably large number of heat transfer conduits. In some cases
such as narrow pipes the conduit may be wrapped in such a was as to
be more than twice the length of the heat transfer surface.
[0184] The length of the heat transfer surface varies according to
the geometry of the vessel with which it is used. In the case of a
cylindrical vessel the length may be the perimeter or the height of
the cylinder, in the case of a conical vessel the length may be the
perimeter of an upper portion of the cone. In the case of a square
of rectangular vessel the length is the surface with which the
conduit is in contact.
[0185] The residence time of the heat transfer fluid depends upon
the size of the system but it should be from 0.01 to 100 seconds.
In small systems where the transfer surface is not greater than 10
metres in length, preferably the residence time of heat transfer
fluid is less than 6 seconds, preferably less than 5 seconds and
more preferably less than 4 seconds, most preferably less than 3
seconds and is in the range 0.01 to 6 seconds.
[0186] Under these conditions different cross sectional areas and
internal profiles of the heat transfer element need to be tested to
find the optimum hydraulic design. Developing the design on the
basis of defined lengths of heat transfer element can simplify the
mechanical. If for example, each element covers half, one or two
full loop of a cylinder; the manifolding will be simpler and
neater.
[0187] Consideration 7: Other Calculation Considerations
[0188] The methodology described above has been simplified for the
purpose of setting out the key design objectives. In practice more
rigorous methods can be used to good effect. For example, in the
method shown, the U value was assumed and the area was based on the
surface area in contact with the process material. An alternative
analysis would employ an incremental approach using the following
equation:
1/UA=1(h.sub.htf.multidot.A.sub.htf)+L.sub.c/(k.sub.c.multidot.A.sub.c)+L.-
sub.p(k.sub.p.multidot.A.sub.p)+1/(h.sub.p.multidot.A.sub.p)
[0189] Where
[0190] UA=nominal heat transfer capacity per Kelvin
(W.multidot.K.sup.-1)
[0191] h.sub.htf=heat transfer fluid coefficient
(W.multidot.m.sup.-2.mult- idot.K.sup.-1)
[0192] A.sub.htf=heat transfer area of the internal conduit wall
(m.sup.2)
[0193] L.sub.c=thickness of the conduit wall (m)
[0194] K.sub.c=thermal conductivity of the conduit wall
(W.multidot.m.sup.-1.multidot.K.sup.-1)
[0195] A.sub.c=Contact area between the conduit and the process
wall (m.sup.2)
[0196] L.sub.p=thickness of the process wall (m)
[0197] K.sub.p=thermal conductivity of the process wall
(W.multidot.m.sup.-1.multidot.K.sup.-1)
[0198] A.sub.p=Area of the process wall (m.sup.2)
[0199] H.sub.p=Process material heat coefficient
(W.multidot.m.sup.-2.mult- idot.K.sup.-1)
[0200] In the previous analysis it was also assumed that the wall
thicknesses were consistent. In practice the thickness of the
conduit (L.sub.c) wall may be varied or extended to ensure that
there is good continuity for transmission of heat between the heat
transfer fluid and the process material. An illustration of this is
shown in FIGS. 11 and 12 where good continuity is provided between
the heat transfer fluid and the process material. FIG. 20 shows how
this can be done even when expansion plates, as described
hereafter, are employed.
[0201] Design of reduced volume systems can be an iterative process
and some compromise may have to be made (e.g. t.sub.si-t.sub.so and
fluid velocity may not be ideal). The method described above is not
therefore intended to represent a definitive or rigorous design
approach but provides sufficient information to enable a design to
be established. The designer may use a variety of techniques for
reaching a solution; however the underlying objective is to deliver
a heat exchanger which relies on much smaller inventories of heat
transfer fluid than is typical for conventional heat
exchangers.
[0202] It should be recognised that some systems will not require
heat measurement and comparatively high values of t.sub.si-t.sub.so
can be tolerated. Even in these cases however, a value which is
slightly higher value than might be used in a comparable
conventional heat exchanger is beneficial since it serves to reduce
the hold up volume within the heat exchanger with negligible loss
of heat transfer capacity. By applying the reduced volume design
principles, fast temperature response is achieved and a simpler
fabrication method is possible.
[0203] For a fixed area heat exchanger, the ideal performance will
only hold good for one set of operating conditions. Even with this
compromise however, the design of the present invention will
deliver a system which is simple to build and generally closer to
an optimum design than most conventional heat exchangers.
[0204] Consideration 8: Conduit Design
[0205] In most conventional heat exchangers, excess heat transfer
fluid is used to achieve uniform fluid displacement and turbulence
throughout the heat transfer conduit.
[0206] In the reduced volume design of the present invention, flow
conditions are often laminar, particularly with small systems, and
several different techniques are used to enhance the heat transfer
coefficient and ensure even distribution of heating or cooling
fluid.
[0207] It is desirable to use the maximum available area of heat
transfer surface in contact with the process material. The heat
transfer conduits need to be laid out in manner which ensures that
the available area of heat transfer surface is properly covered.
For this reason, shapes and materials of conduits which can be
adapted to the profile of the process heat transfer wall are
preferably used.
[0208] In the case of external jackets or coils, the conduits used
to carry the heat transfer fluid do not need to be compatible with
the process material and are not subjected to the same thermal or
mechanical conditions as the process. In these circumstances, a
material of high thermal conductivity like copper makes an ideal
conduit material and has a thermal conductivity which can be more
than 20 times greater than that of stainless steel. Because the
conduits are comparatively small, the entire internal wetted
surface of the heat transfer fluid conduit can be used to transmit
heat between the heat transfer fluid and the process heat transfer
wall. Because the conduits carrying the heat transfer fluid have to
conduct heat across their walls, they are referred to as conductor
pipes. Whilst it is desirable to fabricate conductor pipes in
conductive material, less conductive materials can be made to work
well by virtue of their small size and adaptable shape. FIG. 11
shows external conductor pipes. In some cases (such as for accurate
calorimetry in variable area systems), it may be preferable to have
an air gap or insulation between the individual heat transfer
elements.
[0209] For external conductor pipes, a further increase in the heat
transfer area (for the heat transfer fluid) can be achieved by
using conductor pipes with an oblong profile as shown in FIG.
12.
[0210] FIGS. 11 and 12 show single conductor pipes with circular or
rectangular cross sections. In practice however a variety of shapes
and groups of conductor pipes can be used to optimise the contact
area between the heat transfer fluid and the conductor pipe whilst
maintaining good mechanical strength. Some examples are shown in
FIG. 13.
[0211] The internal profile of the conductor pipe can also be
modified in other ways to give an enhanced heat transfer area as
shown in FIG. 14.
[0212] Additionally the heat transfer performance can also be
improved by the addition of dimples, knurling or other surface
enhancement to the inner conduit wall.
[0213] Inserts can be used to good effect in reduced volume systems
(as shown in FIG. 15). Smooth inserts can be used to reduce the
internal hydraulic volume of the conductor pipe. This allows a
larger conduit to hold a reduced volume of heat transfer fluid. By
using different diameters of inserts, common pipe sizes can be used
for multiple flow duties. Larger conduits with removable inserts
can also be used to make cleaning easier. Profiled inserts (such as
flow disrupters) can also be used to promote mixing and improve
heat transfer conditions at the boundary layer.
[0214] Consideration 9 Assembly of the Heat Transfer Conduits
[0215] External heat transfer elements can be fixed to the process
heat transfer surface by a variety of methods. Reduced volume
jackets can be fabricated in the same manner as traditional half
coil designs as shown in FIG. 16.
[0216] Whilst conventional fabrication techniques can be used, they
can be labour intensive for fabrication and may not deliver the
optimum thermal performance. A preferred solution is to use fully
contained conductor pipes as shown below. These pipes can be fixed
to the heat transfer surface using adhesive, solder, brazing or
welding. It is preferable that the bonding agent has good thermal
conductive properties. The contact area between the heat transfer
conduit and the heat transfer surface can be as small or large as
the designer deems necessary. The example in FIG. 17 uses bonding
material on one side of the conductor pipe.
[0217] Alternatively conductor pipes can also be sprung or clamped
onto the heat transfer surface as shown in the FIG. 18 (clamping
arrangement not shown). The heat transfer capacity can be enhanced
by filling the air space between the conductor pipe and the heat
transfer surface with a soft conductive layer such as conductive
paste, fluid, conductive wool, or conductive matting. Composite
layers could also be used such as copper wool impregnated with
conductive grease.
[0218] Conductor pipes can be fabricated in several sections as
shown in the illustration below. The conductor pipes can be held in
place by springs or clamps (not shown). As with the system
illustrated in FIG. 18 a soft conductive layer can be used to fill
the air gap, FIG. 19 shows a two part mounted conductor pipe
configuration.
[0219] Another aspect of the present invention provides a method to
reduce or avoid problems of differential expansion and/or
contraction due to changes in temperature and/or pressure which can
cause stress if the conductor pipe is of a different material to
the heat transfer surface. According to the invention this problem
can be overcome by using expansion plates to link the conductor
pipe to the heat transfer surface. The use of an expansion plate
allows the conductor pipe to expand at a different rate and/or to a
different extent to the heat transfer surface. It is preferable
(but not always essential) that expansion plates are fabricated in
materials that have good thermal conductivity properties. An
example of an expansion plate is shown in FIG. 20.
[0220] The present invention therefore further provides a heat
transfer system for the transfer of heat between a process fluid
and a heat transfer fluid across a heat transfer surface comprising
a heat transfer conduit for passage of the heat transfer fluid
attached to an expansion plate said expansion plate being in
contact with the heat transfer surface said expansion plate
enabling independent movement of the heat transfer conduit and the
heat transfer surface.
[0221] With the system shown in FIG. 20 the conductor pipe is free
to move up and down in relation to the heat transfer surface.
[0222] The expansion plates for the conductor pipes can be made
into a variety of shapes to accommodate compact construction
methods. For example they can be wedge shape, notched or chamfered
as shown in FIG. 21. An example of a slightly different expansion
plate is shown in FIG. 22.
[0223] The expansion plate can be made up in several sections with
the conductor pipe clamped, welded, braised, bonded or soldered to
it.
[0224] In a further embodiment of the present invention the heat
transfer element is mounted on an expansion plate and the expansion
plate itself is provided with a channel. The channel in the
expansion plate being adapted to receive a band which can hold the
expansion plate against the surface of the process vessel. In this
way the expansion plate may be placed against the surface of the
process vessel, the band inserted and tensioned so as to force the
expansion plate against the surface of the vessel and hold the heat
transfer element in place against the surface of the vessel. In
this way, good transmission of heat between the heat transfer fluid
and the process material may be accomplished whilst the expansion
plate is free to alter its configuration independently of the
process vessel as the temperatures and/or pressures increase or
decrease and there is differential expansion due to the different
coefficients of expansion of the process vessel and the heat
transfer element. Such a system is illustrated in FIGS. 23 to 25 in
which FIG. 23 shows a heat transfer element material on an
expansion plate provided with a slot. FIG. 24 shows the heat
transfer element of FIG. 23 mounted on the process vessel with the
metal band fixing strip in place to hold the expansion element
against the surface of the process vessel. FIG. 25 shows how the
heat transfer element of FIG. 23 can move away from its position in
FIG. 24 due to the independent expansion of the expansion
plate.
[0225] The use of such a system also makes reactor construction and
assembly much easier in that welding, soldering or the use of
adhesives is not required to fix the heat transfer elements to the
process vessel. In addition the heat transfer elements can be
easily replaced if necessary perhaps because of damage or the need
to change the size or nature of the heat transfer element.
[0226] Consideration 10 Maintaining Uniform Flow in the Heat
Transfer Conduits
[0227] In the system of the present invention even distribution of
the heat transfer fluid to the multiple heat transfer elements is
generally desirable. For example, if a system has 10 heat transfer
elements of similar size and length, it would be desirable for each
element to receive about 10% of the flow of heat transfer fluid.
The flow characteristics of similar elements can be different
however due to small variations arising during fabrication.
[0228] However, it is preferred to prevent major flow variations.
This can be accomplished by providing a regulating valve to each
heat transfer element. In this way the flow characteristics can be
adjusted for each conduit by adjusting the valve setting.
Alternatively a restriction orifice can be fitted to each conduit
such that the restriction through the orifice is large in
comparison to the flow resistance of the conduit Flow restrictors
with similar pressure drop characteristics are simple to fabricate
and are easy to change. This makes the pressure drop variations
between the different conduits small in relation to the orifice
resistance, thus giving substantially constant pressure drop
characteristics on all elements of similar size and type.
[0229] Some systems may have different lengths and sizes of heat
transfer element and in this instance the same flow balancing
techniques can be applied although the valves or restrictors may be
set differently for different heat transfer elements.
[0230] Reduced Volume Designs with Internal Heat Transfer
Elements
[0231] The description above illustrates design and fabrication
methods where the heat transfer elements are fixed to the external
surface of the equipment. The present invention may also be applied
to systems where the heat transfer fluid conduit passes directly
into the process fluid (such as internal coils or plates).
[0232] For internal systems, a reduced hold up of heat transfer
fluid is also desirable and this is achieved by using multiple
internal heat transfer elements rather than one or a few large
coils.
[0233] For internal heat transfer elements, the same design
criteria apply as described above in relation to external heat
transfer elements. The analysis is made simpler however by the fact
that the conductor pipe is in direct contact with the process
material. Fabrication is also different as the conductor pipes are
free to expand and contract and do not have to be fixed in place
other than for mechanical support; expansion plates may not
therefore be required.
[0234] The following benefits are realised through use of the
systems of the present invention employing external and/or internal
heat transfer elements.
[0235] 1. Heat balance may easily be measured because with a
process held at constant temperature, the heat gained or lost by
the process material is the same as the heat gained or lost by the
heat transfer fluid. When using the techniques of the present
invention the heat gain or loss of the heat transfer fluid can be
determined by measuring the inlet and outlet temperature and the
mass flow of the heat transfer fluid. Thus:
q=m.multidot.Cp(t.sub.si-t.sub.so)
[0236] Where
[0237] q=heat liberated or absorbed by the process (W)
[0238] m=mass flow of heat transfer fluid
(kg.multidot.s.sup.-1)
[0239] Cp=specific heat of heat transfer fluid
(J.multidot.kg.sup.-1.multi- dot.K.sup.-1)
[0240] t.sub.si-t.sub.so=temperature change of heat transfer fluid
(K)
[0241] The use of reduced volume heat exchangers of the present
invention give better heat balance data by virtue of having
generally larger temperature changes in the heat transfer fluid
(which makes measuring the temperature change easier) and having
lower thermal inertia within the heat transfer fluid (changes in
thermal conditions of the jacket tend to mask the true heat change
within the process).
[0242] Even on systems where the process temperature can vary or
there is a phase change (e.g. crystallisation or boiling) in the
process material, useful heat balance data can still be
extracted.
[0243] Heat balance measurement is a valuable monitoring tool for
many chemical and physical processes. It can for example reveal the
rate and progress of chemical reactions. This, in turn, permits the
user to optimise addition rates and reaction times. In some cases
it serves a valuable safety function as heat monitoring can detect
the onset of runaways or accumulation of un-reacted material. Heat
monitoring can also be used to monitor and control a variety of
other processes such as crystallisation, cell growth systems and
drying.
[0244] 2. Temperature control can be improved. Good temperature
control requires fast response and a key factor in fast response is
the speed with which the heat transfer fluid temperature within the
heat exchanger can be modified. The heat transfer elements in a
reduced volume heat exchanger have comparatively short flow paths
and the fluid travels as a plug through the heat transfer element.
High linear velocities of the heat transfer fluid are also used.
These conditions all serve to give fast temperature control
response.
[0245] 3. On the external conductor pipe design, the heat transfer
capacity can be increased since the are of the heat transfer
surface between the heat transfer fluid and the conductor pipe can
be increased to a value which is greater than the process surface
area in contact with the process fluid. Because the heat transfer
elements are small and adaptable, they can be used to cover areas
that a normal Jacket or half coil could not reach (such as
peripheral components attached to the vessel). The higher heat
transfer capacity and better surface coverage will be of benefit
where heat transfer capacity is important.
[0246] Heating and cooling capacity constraints are a common
problem in many applications. In batch reactions for example, the
addition rate of reactant usually has to be reduced to the point
where the heat of reaction does not exceed the cooling capacity of
the system. In large fermenters, external jackets are often
inadequate for cooling purposes and internal coils have to be
added.
[0247] 4. The reduced volume design as described herein offers a
very simple construction method. Reduced volume heat exchangers
will be simpler and cheaper to fabricate than most comparable
conventional heat transfer devices, especially those with external
jackets or coils. Individual heat transfer elements are also simple
to repair or replace.
[0248] 5. The present invention allows for energy savings for the
same degree of heat transfer through reduced volumetric flow and
reduced pressure drop of heat transfer fluid through the heat
transfer conduit.
[0249] 5. The expansion plate concept used with external heat
transfer elements according to the present invention provides the
additional benefit that the heat transfer elements may be made of a
different material from the process vessel without causing problems
due to differential expansion of the materials. This then allows
materials with high thermal conductivity, such as copper, to be
used for the heat transfer elements.
[0250] 6. The reduced volume concept requires less volumetric flow
through the jacket than conventional heat exchangers and in many
cases will also be used with reduced pressure drops (of the heat
transfer fluid through the conduit). This will have the benefit of
reducing the size of pipework around the heat exchanger and will
contribute towards reduced energy requirements.
[0251] The reduced volume design and fabrication technique of the
present invention can be used for any heat transfer application
where better temperature control, better distribution of heat
transfer fluid, better heat transfer coefficients or better
measurement of heat balance is required. It can also be used to
simplify construction of heat exchangers where external jackets or
coils are used. It can also be used to save energy. This technique
will deliver a better design for industrial process equipment such
as reactors, dryers, mixers, fermenters, mills, cell growth
vessels, filters or extruders. It can also used for direct fired
equipment.
[0252] The technique can be employed on any size of system where
the inventory of process material is from 1 millilitre to 100,000
litres or even larger.
[0253] This design concept can also be applied to a variety of
other types of equipment which are not used within the process
industries. Examples included refrigeration systems, combustion
engines, hydraulic systems, heat exchangers in nuclear reactors,
air craft heating and cooling systems, heating and cooling systems
for ships, heating and cooling systems for road vehicles, HVAC
systems etc.
[0254] The invention can be used to improve the operation of
laboratory scale and commercial chemical and physical reaction
systems. It can however also be used to provide considerably
smaller reaction systems with comparable commercial throughput
where calorimetric data permits the process to operate in
continuous or semi-continuous manner. For example the invention
enables reduction of reactor size by a factor of 10 and, in some
instances, a factor of 100 or greater.
[0255] The invention is particular useful in the follow
reactions
[0256] batch organic synthesis reactions currently carried out in
reactors of 10 to 20,000 litres.
[0257] bulk pharmaceutical synthesis reactions currently carried
out with process material quantities of 10 to 20,000 litres.
[0258] batch polymerisation reactions currently carried out in
reactors of 10 to 20,000 litres.
[0259] batch synthesis reactions of 10 to 20,000 litres currently
used for unstable materials (compounds susceptible to
self-accelerating runaways)
[0260] batch inorganic synthesis reactions currently carried out in
reactions of 10 to 20,000 litres.
[0261] evaporates, batch dryers, holding tanks, crystallisers,
fermenters, cell growth vessels, mills, mixers and filters
typically carried out in systems of 10 to 20,000 litres
[0262] compressors, internal combustion engines, air conditioning
systems
[0263] The techniques may also be useful in larger scale chemical
and petrochemical operations.
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