U.S. patent application number 10/488147 was filed with the patent office on 2004-12-09 for temperature control system.
Invention is credited to Ashe, Robert, Morris, David Charles.
Application Number | 20040247500 10/488147 |
Document ID | / |
Family ID | 9921481 |
Filed Date | 2004-12-09 |
United States Patent
Application |
20040247500 |
Kind Code |
A1 |
Ashe, Robert ; et
al. |
December 9, 2004 |
Temperature control system
Abstract
Temperature controllers are provided which employ a control
element of variable area containing flowing heat transfer fluid.
The area of the element available for temperature control is
changed by opening and closing a bank of conduits in a cascade and
the conduits are opened and closed according to a temperature
measuring device in the medium whose temperature is to be
controlled.
Inventors: |
Ashe, Robert; (Radlett,
GB) ; Morris, David Charles; (Warrington,
GB) |
Correspondence
Address: |
OHLANDT, GREELEY, RUGGIERO & PERLE, LLP
ONE LANDMARK SQUARE, 10TH FLOOR
STAMFORD
CT
06901
US
|
Family ID: |
9921481 |
Appl. No.: |
10/488147 |
Filed: |
August 3, 2004 |
PCT Filed: |
September 4, 2002 |
PCT NO: |
PCT/EP02/09956 |
Current U.S.
Class: |
422/200 ;
422/198; 422/201 |
Current CPC
Class: |
G05D 23/1919 20130101;
F28F 27/02 20130101 |
Class at
Publication: |
422/200 ;
422/198; 422/201 |
International
Class: |
F28D 001/00; F28D
007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 4, 2001 |
GB |
0121375.0 |
Claims
1. A temperature control system comprising: a control element of
variable area containing flowing heat transfer fluid wherein the
area available is changed by opening and closing a bank of conduits
in a cascade in which the conduits are opened and closed according
to a temperature measuring device in the medium whose temperature
is to be controlled.
2. The temperature control system according to claim 1 in which the
heat transfer fluid flows with a Reynolds number greater than
2000.
3. The temperature control system according to claim 1 wherein the
area of a heat transfer element that is available and the flow are
controlled to provide a substantially constant thermal gradient
between the process fluid and the heat transfer fluid.
4. The temperature control system according to claim 1, wherein a
Log mean thermal difference (LMTD) is greater than or equal to
1.degree. C.
5. The temperature control system according to claim 4, wherein the
LMTD is greater than or equal to 10.degree. C.
6. The temperature control system according to claim 5, wherein the
LMTD is greater than or equal to 20.degree. C.
7. The temperature control system according to claim 6, wherein the
LMTD is greater than or equal to 100.degree. C.
8. The temperature control system according to claim 1, wherein the
bank of conduits comprises a series of coils.
9. A temperature control system according to claim 8 in which the
individual coils are sized according the following formula:
A=m.Cp.(tsi-tso)/(U.LMTD) Where Q=designated nominal design load
per coil (W) U=overall heat transfer coefficient
(W.m.sup.-1.K.sup.-1) A=heat transfer area of the coil (m.sup.2)
(tsi-tso)=designated nominal temperature change in the heat
transfer fluid between inlet and outlet. (.degree. C.) LMTD=Log
mean thermal difference between the process and heat transfer fluid
(C)
10. The temperature control system according to claim 9, wherein
the heat transfer fluid temperature (tsi-tso) rises by 0.01.degree.
C. or more.
11. The temperature control system according to claim 10, wherein
the heat transfer fluid temperature (tsi-tso) rises by 0.1.degree.
C. or more.
12. The temperature control system according to claim 11, wherein
the heat transfer fluid temperature (tsi-tso) rises by 1.degree. C.
or more.
13. The temperature control system according to claim 12, wherein
the heat transfer fluid temperature (tsi-tso) rises by 10.degree.
C. or more.
14. The temperature control system according to claim 1, wherein
the linear velocity of the heat transfer fluid is greater than 0.1
ms.sup.-1
15. A temperature control system according to claim 1, wherein a
linear velocity of the heat transfer fluid is greater than 1
ms.sup.-1.
16. The temperature control system according to claim 15, wherein
the linear velocity of the heat transfer fluid is greater than 3
ms.sup.-1.
17. The temperature control system according to claim 16, wherein
the linear velocity of the heat transfer fluid is greater than 5
ms.sup.-1.
18. A temperature control system which comprises a control element
of variable area containing flowing heat transfer fluid wherein the
area available is changed by opening and closing a bank of conduits
in a cascade in which the conduits are opened and closed according
to a temperature measuring device in the medium whose temperature
is to be controlled, said system has sufficiently high resolution
to enable a disturbance or deviation in the temperature of the
process media to detected at a rate that will allow a real time
correction to be made in the flow rate and heat transfer area such
that the said disturbance or deviation results in a relatively
minor change in the process temperature.
19. The temperature control system according to claim 1, wherein
said system can respond to a change in load of a process fluid
within less than 2 seconds.
20. The temperature control system according to claim 19, wherein
said system can respond to a change in load of the process fluid
within less than 10 seconds.
21. The temperature control system according to claim 3, wherein
the nominal capacity of said heat transfer element is not more than
0.1 Watt.
22. The temperature control system according to claim 21, wherein
the nominal capacity of said heat transfer element is not more than
1 Watt.
23. The temperature control system according to claim 22, wherein
the nominal capacity of said heat transfer element is not more than
10 Watts.
24. The temperature control system according to claim 23, wherein
the nominal capacity of heat transfer element is not more than 100
Watts.
25. The temperature control system according to claim 24, wherein
the nominal capacity of said heat transfer element is not more than
1000 Watts.
26. The temperature control system according to claim 25, wherein
the nominal capacity of said heat transfer element is not more than
10000 Watts.
27. The temperature control system according to claim 26, wherein
the nominal capacity of said heat transfer element is not more than
100000 Watts.
Description
[0001] The present invention relates to temperature control
systems. In particular the invention relates to systems which
provide improved temperature control leading to faster and more
accurate temperature control which in turn can lead to more precise
operations in many industries and to energy savings.
[0002] Temperature control is widely used in operations ranging
from industrial reactions to air conditioning to heating and
refrigeration systems. Some operations require more precise control
than others, although all such operations involve high-energy
consumption and energy savings would be advantageous. The majority
of systems rely on activation of the heat transfer system by a
signal from the medium whose temperature is to be controlled and
the response is to bring heat transfer elements into or out of
operation depending upon the requirements of the medium. In many
instances, the heat transfer element comprises a conduit through
which a heat transfer fluid flows.
[0003] The disadvantage of the existing systems is that, outside of
a limited operating range, the response of the system to
temperature changes is coarse, it is often sluggish and has low
resolution. This tends to result in bumpy irregular temperature
profiles without the optimum degree of temperature control.
[0004] In an article in Collection Czechoslovak Chem. Comm. (Vol
47) (1982) pages 446 to 453 it is proposed to use a variable area
cooling surface in which a retractable cooler was immersed into a
reaction mixture. Introduction of the cooler into the reaction
mixture could be used to increase the cooling effect and retraction
from the mixture WOULD result in a decrease in the cooling effect.
The article shows that this technique, to some extent, reduces the
temperature surges and irregularities in the reaction mixture as it
is cooled or heated. Achieving turbulence in the heat transfer
fluid with this design is difficult, furthermore it requires
complex apparatus to allow for movement of the cooling element
whilst ensuring thermal insulation of the reaction vessel and
preventing leaking of the reaction mixture.
[0005] U.S. Pat. No. 5,762,879 relates to a reaction heat control
mechanism in which the heat exchange area is regulated. In U.S.
Pat. No. 5,762,879 the regulation is achieved by varying the height
of the heat transfer fluid in an external temperature control
jacket which surrounds the reactor. Controlling the heat transfer
area by this method requires a stable surface which is incompatible
with the need to maintain high flows and high turbulence of heat
transfer fluids. This leads to sluggish control response and
reduced heat transfer capacity.
[0006] The quality of temperature control is dependant on how fast
the temperature of the heat transfer surface can be raised and
lowered. This in turn is dependant on the resistance to heat flow,
the thermal gradient and how fast the heat transfer fluid can be
delivered to the heat transfer surface.
[0007] We have now developed temperature control systems which will
improve these factors and overcome the problems of the earlier
systems.
[0008] In our PCT Patent Applications PCT/EP02/04651,
PCT/EP02/04646, PCT/EP02/04650 and PCT/EP02104648 we describe
improved systems for the monitoring and control of physical and
chemical reactions. These systems are concerned with the improved
generation of calorimetric data to monitor a reaction and the use
of the caloRimetric data to control the reaction. We have now found
that certain of the techniques described in United Kingdom Patent
Applications can be modified and used to provide improved
temperature control in a wide range of activities.
[0009] The invention therefore provides a temperature control
system which employs a control element containing a heat transfer
fluid of variable area wherein the effective area of the control
element is changed by opening and closing a bank of conduits which
pass through the medium whose temperature is to be controlled in a
cascade fashion, to allow or prevent flow of the heat transfer
fluid into the conduits the conduits being opened and/or closed
according to a signal from a temperature measuring device in the
medium whose temperature is to be controlled.
[0010] The system of the present invention allows rapid reaction to
temperature change and enables the temperature of the heat transfer
surface to be raised and lowered rapidly. The factors which
influence the rate of heat transfer are:
[0011] i) turbulence of the heat transfer fluid. High turbulence
reduces the thickness of the stagnant layer of heat transfer liquid
at the wall of the heat transfer surface (service side boundary
layer). This results in a thin boundary layer which has a lower
resistance to heat flow. Accordingly our systems use a high heat
transfer fluid velocity for the purposes of achieving a low
boundary layer resistance
[0012] ii) delivery of the heat transfer fluid to the heat transfer
surface. For good temperature control, the heat transfer fluid in
the conduit should be changed as quickly as possible. The best
response is achieved by replacing the fluid in the heat exchanger
with a plug of new fluid. The faster this plug travels, the faster
the response.
[0013] iii) maintaining a high thermal gradient between the process
and service fluids over varying heat loads. A high thermal gradient
gives a high rate of temperature change at the heat transfer
surface. The invention enables a high thermal gradient to be
maintained with a falling heat load by reducing the heat transfer
area. By way of example, consider the two heat exchangers in the
table below:
1 Property Heat Exchanger A. Heat Exchanger B. Surface area 1
m.sup.2 0.1 m.sup.2 Heat transfer coefficient 1 kW .multidot.
m.sup.-2 .multidot. K.sup.-1 1 kW .multidot. m.sup.-2 .multidot.
K.sup.-1 Thermal gradient 10.degree. C. 100.degree. C. Heatload 10
kW 10 kW
[0014] Say the heat load changes to 11 kW and the temperature of
Heat Exchanger A overshoots by 1.degree. C. as the system adjusts
to the new set point. This represents an excess load of 1 kW. Heat
Exchanger B will have a 10.degree. C. overshoot to give the same
excess heat load of 1 kW. The rate of heat up in Exchanger B
however will be much faster since the rate of change of surface
temperature is greater (due to the higher thermal gradient between
the heat transfer surface and the heat transfer fluid). Thus the
smaller heat exchanger with higher thermal gradients will give
faster control response.
[0015] Whilst any form of conduit may be used for the heat
exchanger, pipes or coils are preferred and the invention will
hereafter be described in relation to a coil or coils.
[0016] In order for effective operation, the temperature control
system should have the following characteristics:
[0017] a. a high temperature difference is preferably maintained
between heat transfer fluid and the medium whose temperature is to
be controlled.
[0018] b. the heat transfer fluid must always flow at a reasonable
velocity. The velocity will vary with coil size and conditions but
it is preferred that it is greater than 0.1 m/s more preferably
greater than 1 m/s. Lower velocities will give slower temperature
control response.
[0019] c. when used for batch processes or multi-purpose duties,
the heat transfer equipment should be capable of stable operation
over a wide range of energy release/absorbtion rates. The range
will vary according to the nature of the reaction. In the case of
batch reactions a very wide operating range will be required.
[0020] The heat exchanger is made up of multiple elements. Each
element typically consists of a pipe or coil. Below is a simplified
method for calculating the size of an individual heat transfer coil
element:
[0021] The first step is to decide on a nominal size for a given
heat transfer coil in terms of heat carrying capacity Q (which is
expressed in Wafts). For example a nominal capacity of 100 Watts
might be selected for an individual coil.
[0022] The next step is to select a nominal temperature drop of the
heat transfer fluid through the pipe. A high temperature drop of
the heat transfer fluid is associated with a low thermal mass of
fluid in the pipe, which is desirable (since this will lose it's
heat rapidly when a coil in a cascade system is shut off). For
example, a nominal temperature drop of the heat transfer fluid
might be taken as 1.degree. C.
[0023] From this, the mass flowrate of the heat transfer fluid can
be calculated from the following formula:
Q=m.Cp.(tsi-tso)
[0024] Where Q=The nominal heat load (e.g. 100 Watts in this
example)
[0025] m=Mass flowrate of the heat transfer fluid (kg.s.sup.-1)
[0026] Cp=specific heat of the heat transfer fluid
(kJ.kg.sup.-1.K.sup.-1)
[0027] (tsi-tso)=temperature change of the heat transfer fluid in
(e.g. 1.degree. C. in this example)
[0028] The next step is to determine the diameter of the pipe. For
this, a diameter is selected such that a tolerable pressure drop in
the pipe is obtained. High pressure drops are preferred since these
are associated with turbulence and rapid control response. The heat
transfer fluid flowrate m (kg.s.sup.-1) can be estimated from a
graph of flowrate versus pressure drop. In practice this step needs
will be iterative since the true length (and hence total pressure
drop) will not be known at this stage and a guessed value must be
used.
[0029] The next step is to determine the area of the coil using the
formula below:
Q=U.A.LMTD
[0030] Where Q=nominal process load (100 Watts for this
example)
[0031] U=overall heat transfer coefficient
(W.m.sup.-2.K.sup.-1)
[0032] A=heat transfer area (m.sup.2)
[0033] LMTD=Log mean thermal difference between the process and
heat transfer fluid (K)
[0034] The overall heat transfer coefficient (U) can be calculated
or obtained from measured data.
[0035] The LMTD is calculated from the following:
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)]
[0036] Where Tp=process temperature
[0037] Tsi=temperature of heat transfer fluid in
[0038] Tso=temperature of heat transfer fluid out
[0039] By knowing the area and diameter of the coil, the length can
be calculated using simple geometry. The pressure drop should now
be checked against the true length. If pressure drop is too high or
too low, a new pipe diameter should be selected and the calculation
repeated.
[0040] This information may then be used to determine the optimum
diameter to length relationship of an individual coil whereby high
turbulence is achieved without incurring excessive pressure drop of
heat transfer fluid through the heat exchanger (as shown by a high
Reynolds number). In the systems of the present invention it is
preferred that:
[0041] a. the linear velocity of heat transfer fluid must be
reasonably high (preferably >0.1 m.s.sup.-1) in order to
maintain satisfactory control response and a good overall heat
transfer coefficient.
[0042] b. the pressure drop of heat transfer fluid flowing through
the coil is from 0.1 to 20 bar.
[0043] In practice, optimum coil lengths will vary according to the
temperature differences employed and the thermodynamic and physical
characteristics of the system. Calculating optimal coil length is
an iterative process. A general-purpose device will be sized using
conservative data based on fluids with low thermal conductivity and
a low temperature difference between the reaction fluid and the
heat transfer fluid. Each coil will have a limited operating
range.
[0044] In a preferred system in which the heat transfer equipment
is capable of stable operation over a wide range of energy
releases, the system is such that the area of heat transfer may be
varied according to the needs of the particular load. This may be
conveniently accomplished by providing multiple heat transfer pipes
each of which has a diameter and length relationship designed to
provide a certain degree of heat transfer. In the preferred
multiple pipe system, the pipes may be brought into and out of
operation, as the needs of the system dictates.
[0045] The system of the present invention is described with
reference to a chemical reactor as shown in the accompanying
drawings in which
[0046] FIG. 1 is a schematic illustration of a reaction vessel
served with a single heat transfer coil (of specified
diameter).
[0047] FIG. 2 is a schematic illustration of a comparable reactor
served with three heat transfer coils to provide variable heat
transfer.
[0048] FIG. 1 is a schematic illustration of a reactor (1)
containing a process fluid (2) and a cooling coil (3) which is
three metres long. This system is capable of controlling
temperature at energy liberation rates of between 72 and 260 Watts
by varying the flow rate of the heat transfer fluid.
[0049] The reactor in FIG. 2 has an improved range of up to 780
watts. The versatility has been increased by adding two more coils
(4) and (5). When one coil is operating the system can be
controlled with heat generation in the range of 72 to 260 watts (as
in the reactor of FIG. 1). With all three coils operating (at a
nominal maximum flow) the system can be controlled with a high
degree of accuracy with heat generation up to 780 watts.
[0050] In normal operation, the flow of heat transfer fluid to a
coil (or set of coils) will be increased using a flow control
valve. When a new coil switches in to accommodate a rising load,
the control valve will regulate the flow to ensure smooth
transition to the higher flow. This will require a rapid flow
control response to the step change in the system pressure drop. To
provide a smooth transition between operating conditions and wide
operating range a large number of coils is desirable. A multi port
flow control valve as described in our PCT Patent Application
PCT/EP02/09806 will be particularly useful.
[0051] Instrumentation is a key aspect of successful operation of
the systems of the present invention. Accurate and sensitive
instrumentation must be used for measuring temperatures and the
rate of flow of the heat transfer fluid. Instruments must operate
over a wide range of flows and this may be achieved by breaking up
the coil system into separate modules operated by manifolds. This
enables different coils to be brought into or out of operation,
according to the needs of the system.
[0052] Fast and accurate temperature measurements is a key
performance requirement. To achieve this, the temperature element
is conveniently mounted where liquid turbulence is high.
[0053] In a preferred process, in addition to the normal process
temperature transmitters, which constantly measure the process
across its entire range and provide the necessary safety
interlocks, a second pair of temperature elements can be provided
to monitor the specific process set point. The arrangement uses two
different types of measuring elements. The main device is
preferably an RTD, a 4 wire Pt100 RTD to {fraction (1/10)}.sup.th
DIN standard being especially suitable. The transmitter used to
provide the 4-20 mA output signal is spanned to the minimum
allowable for the transmitter (similarly any output signal type or
temperature span could be used). The temperature transmitter will
be calibrated specifically at the process set point. Larger ranges
will still give acceptable results, but reducing the span to the
minimum possible offers improved accuracy and resolution. Thus this
arrangement will provide an extremely accurate means of process
temperature measurement.
[0054] The element of the temperature measurement system is the
part of the device which is in contact with the liquid. In the case
of an RTD, its resistance will change in response to changing
temperature. The response of an RTD is not linear. The transmitter
is the calibrated part of a measuring device and is used to
linearise the output to the control system and convert the signal
to an industry standard, usually 4-20 mA, but it could also be 1-5
V or 0-10V. A thermocouple's response to a change in temperature is
a varying voltage. Usually milli volts per .degree. C. A
thermocouple transmitter will again convert this signal to an
industry standard, again more often than not, 4-20 mA. Accordingly
the term `element` when describing a physical mechanical presence
in the process, e.g., a temperature element is located in the
reactor and measures the temperature of the reactor contents. And
the term `transmitter` when describing aspects of temperature
measurement relating to the control system, e.g., a temperature
transmitter is calibrated 0-100.degree. C. and displays the
contents temperature of the reactor.
[0055] The limitation of any RTD is its speed of response to a step
change in temperature. Typically it can take up to four or five
seconds for an RTD to measure a change in temperature.
Thermocouples, on the other hand, can respond much more rapidly to
temperature fluctuations. For this reason a thermocouple is also
used to monitor the process set point, a T type thermocouple being
especially suited. Its transmitter will be similarly ranged to the
RTD. However, as a T type thermocouple has an accuracy of only + or
-1.degree. C., it will not be used to monitor the process
temperature. Its function is to monitor the rate of change of the
process temperature.
[0056] The combined use of these two different types of sensing
elements provides a temperature control system, which is both
extremely accurate and responsive. It should be noted that not all
process operations require this level of temperature measurement
accuracy and control. In such cases, more basic temperature control
and measurement systems will prove tolerable.
[0057] In order to fully utilize this two-element approach, custom
software is used to determine which process variable (temperature,
or rate of change of temperature) is the most significant at any
one instance in time.
[0058] Conventional reactors for example have fixed area heat
transfer surfaces (or occasionally several elements such as
separate sections on the bottom dish and walls). They perform most
effectively with a high and constant flow rate of heat transfer
fluid to the jacket (or coils). Process temperature is controlled
by varying the heat transfer fluid temperature. In the preferred
system of the present invention used to control chemical and
physical reactions, the area of the heat transfer surface may be
varied according to the needs of the reaction (although some
variation in heat transfer fluid temperature can also be used).
[0059] A typical control arrangement for control of the heat
transfer fluid using a variable area heat transfer surface is shown
in FIG. 3. In FIG. 3, valves (16) and (17) are control valves that
regulate flow of heat transfer fluid to the heat transfer coils.
The extent to which they are open is determined by a temperature
output measure from the reactor (or vessel). With the process at
idle, valve (18) is open and sufficient flow permitted to
compensate for heat gain from the agitator. As load is applied to
the process, valve (16) opens to permit the flow of more heat
transfer fluid. When valve (16) is open beyond a pre-set point (or
when flow rate dictates) valve (19) will open and valve (16) will
close up slightly to compensate. As valve (16) approaches the top
of its control range, valve (17) takes over. As valve (17)
progressively opens the valves (18) to (24) are opened in a cascade
fashion.
[0060] The heat transfer fluid is applied to the control equipment
at constant pressure and temperature. In some cases temperature can
also be varied where it is necessary to increase the operating
range.
[0061] The rate at which heat can be transferred between the
process fluid and the heat transfer fluid is dictated (in part) by
the overall heat transfer coefficient (U). The larger the value of
U, the smaller the heat transfer area required. The U value may be
calculated from three components.
[0062] The heat transfer resistance through the process fluid
boundary layer
[0063] The heat transfer resistance through the coil wall
[0064] The heat transfer resistance through the heat transfer fluid
boundary layer
[0065] The boundary layers are the stagnant layers of liquid either
side of the coil wall. The faster the agitation (or liquid flow),
the thinner the boundary layer. Thus high liquid velocities give
better heat transfer. Also liquids with good thermal conductivity
give better heat transfer through the boundary layers.
[0066] Heat transfer mechanism across the coil wall is similar,
except (unlike the boundary layers) the distance through which the
heat has to conduct is fixed. Higher heat transfer rates are
achieved where the coil material has high thermal conductivity.
Higher heat transfer rates are also achieved where the coil
material is thin.
[0067] Thus a high U value requires both a thin coil material (with
high thermal conductivity) and turbulent conditions in both liquids
(the more turbulent, the better). The higher the U value, the
smaller the area required for heat transfer. This means a shorter
heat transfer coil.
[0068] It is therefore preferred to use the thinnest walled coils
possible without compromising mechanical strength and corrosion
tolerance. A typical wall thickness would be 1/2 to 4 mm.
[0069] The material from which the coil is fabricated is not
critical but they should be inert to the medium whose surface is to
be controlled and have high thermal conductivity.
[0070] For purposes of illustration only the following examples
show the sizing of the heat transfer coils.
[0071] Example 1 illustrates the sizing an individual heat transfer
coil such as that used in FIG. 1. Examples 2 and 3 illustrate the
sizing and use of multiple heat transfer coil systems. These two
examples also employ the additional feature of also providing
caloimetric data.
[0072] In these examples some of the numbers used are arbitrary and
are chosen for purposes of illustration only. The examples
illustrate the sizing of coils for a batch reactor where an
exothermic reaction takes place. In this, a theoretical reaction
reagent A is reacted with product B to produce a new compound C as
follows:
A+B.fwdarw.C
[0073] where A=kg of A
[0074] B=kg of B
[0075] C=kg of C
[0076] The heat liberated .DELTA.Hr is as follows:
.DELTA.Hr.sub.c=1,000 (kJ/kg.sub.c) (1)
[0077] The batch reactor is prefilled with component B. Component A
is added slowly (alternatively the two components could be pumped
continuously through the reactor in the desired ratios). For the
purposes of this example it is assumed that it is a fast reaction
and component B reacts immediately on contact with A. The heat
liberated is therefore proportional to the rate of addition (of A).
If it is assumed that the addition rate is such that 0.001
kg/second of C is produced
[0078] The heat load of the reactor (q)=0.001.times.1000=1 kW.
[0079] The reaction is also assumed to take place at constant
temperature so that the heat load on the cooling fluid is also 1
kW.
[0080] FIG. 4 is a schematic illustration of a section through a
typical heating/cooling coil such as coil (3) of FIG. 1 in the
process fluid (2) through which flows the heat transfer fluid (25).
The boundary layer is shown at (26) and it is this boundary layer
which is kept as thin as possible by ensuring turbulent flow in the
heat transfer unit.
EXAMPLE 1
[0081] The heat transfer coil (3) controls the process temperature.
The temperature difference between the heat transfer fluid and the
process fluid must be as high as possible, but not so great that
boiling, burning or freezing occur on the pipe surface. Assume that
the process media temperature is 30.degree. C. (t.sub.p). Also
assume that the lowest temperature at which service fluid can be
delivered to the system is 5.degree. C. (to avoid freezing on the
outer surface). Thus the service fluid inlet temperature (t.sub.si)
is 5.degree. C. and the outlet temperature (t.sub.so) is 15.degree.
C. [since (t.sub.si-t.sub.so) is 10.degree. C.]
[0082] Once the choice for (t.sub.si-t.sub.so) is made, the mass of
the heat transfer fluid can be determined as follows:
m=q/Cp(t.sub.si-t.sub.so) (1)
[0083] where m=mass flow of heat transfer fluid (kg/s)
[0084] q=heat gain by the heat transfer fluid=1 (kW) (in this 1 kW
is the heat of reaction)
[0085] Cp=specific heat of heat transfer fluid=1.6
kJ.kg.sup.-1.K.sup.-1 (based on the choice of the synthetic heat
transfer fluid)
[0086] t.sub.si-t.sub.so=temperature change of heat transfer fluid
(selected to be 10.degree. C.)
[0087] Thus from equation (1), the mass flow
(m)=1/1.6.times.10=0.0625 kg/s Assume the density of the heat
transfer fluid=840 kg/m.sup.3
[0088] Thus the volume flowrate of the fluid
(W)=0.0625/840=0.000074 m.sup.3/s
[0089] Optimising coil geometry and the velocity of the heat
transfer fluid is an iterative process. Low velocity of the heat
transfer fluid through the heat exchange coil gives rise to poor
control and measurement response. Low velocity also results in a
large ratio of thermal mass of heat transfer fluid to heat load.
This tends to magnify any errors of temperature measurement. High
liquid velocity is desirable as it gives faster control response
and a better ratio of thermal mass to heat load. As the velocity is
increased however, the pressure drop through the coil gets
higher.
[0090] Accordingly the optimum coil will be long enough to give
adequate heat transfer area without incurring an excessive pressure
drop. If the diameter is too small, the pressure drop will be too
high (due to high liquid velocity and long pipe length). If the
diameter is too large, the liquid velocity will be too low.
[0091] In this example an initial calculation based on a 4 mm
diameter pipe is made for the first iteration as follows:
[0092] At a flowrate of 0.000074 m.sup.3/s through a 4 mm bore
pipe, the pressure drop of the heat transfer fluid is calculated as
being 1.24 bar/m (based on synthetic heat transfer fluid).
[0093] The pipe length is calculated from the relationship
L=A/.pi.D(m)
[0094] where L=pipe length=(m)
[0095] A=surface area of pipe (m.sup.2)
[0096] D=pipe diameter=0.004 (m)
[0097] .pi.=3.1416
[0098] The surface area (A) required for control of the reaction is
determined from the heat transfer capabilities of the pipe as
follows:
A=q/U.LMTD(m.sup.2)
[0099] where A=surface area of pipe (m.sup.2)
[0100] U=overall heat transfer coefficient=0.730
(kW.m.sup.-2.K.sup.-1) (estimate for organic process fluid and
synthetic oil heat transfer fluid)
[0101]
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)] (.degree. C.) (log mean thermal difference
between process and service fluids)
[0102] Also T.sub.p=30
[0103] T.sub.si=5
[0104] T.sub.so=15
[0105] Thus LMTD=19.6 (.degree. C.)
[0106] Therefore A=1/(0.730.times.19.6)=0.07 m.sup.2 (m.sup.2)
[0107] Therefore L=0.07/(3.1416.times.0.004)=5.6 (m)
[0108] The pressure drop through the line=5.6.times.1.24=6.9
bar
[0109] The linear velocity can also be calculated using the
continuity equation as follows:
V=W/A
[0110] where V=linear velocity (m/s)
[0111] W=volume flowrate (m.sup.3/s)
[0112] A=cross sectional area of the pipe (m.sup.2)
[0113] Thus V=0.000074/(.pi..times.0.004.sup.2/4)=5.9 (m/s)
[0114] A summary of the results of this calculation is shown in
Table 1 below.
2 TABLE 1 Coil duty 1 kW Pipe diameter 4 mm Liquid flowrate 0.074
l/s Liquid velocity 5.9 m/s Pipe length 5.6 m Pressure drop 6.9
bar
[0115] The table shows that although the 4 mm diameter coil is
capable of operating in a reaction that generates 1 kW of heat, it
does so at the expense of very high pressure drop (of the heat
transfer fluid). A small increase in process load beyond 1 kW would
require even higher flowrates and a longer coil which would result
in an unacceptably high pressure drop. Thus under the conditions
which have been chosen purely for the purposes of illustration, at
a load of 1 kW the 4 mm diameter coil is at the top end of its
operating range.
[0116] A larger pipe diameter of 5 mm internal bore is therefore
selected for the second iteration.
[0117] At a flowrate of 0.000074 m.sup.3/s through a 5 mm bore
pipe, the pressure drop of the heat transfer fluid is 0.42 bar/m
(based on a standard pressure drop calculation synthetic heat
transfer fluid).
[0118] The pipe length is again calculated from the
relationship
L=A/.pi.D
[0119] where L=pipe length=(m)
[0120] A=surface area of pipe (m.sup.2)
[0121] D=pipe diameter=0.005 (m)
[0122] .pi.=3.1416
[0123] The required area (A) is determined from the heat transfer
capabilities of the pipe using the same formula
A=q/U.LMTD(m.sup.2)
[0124] as was used in the first iteration.
[0125] With the 5 mm coil however, (note the value of U is lower in
this case (0.66 kW.m.sup.-2.K.sup.-1) this is due to the reduced
service fluid velocity (which gives a higher service side boundary
layer resistance).
[0126] A=1/(0.66.times.19.6)=0.077 m.sup.2
[0127] L=0.077/(3.1416.times.0.005)=4.9 m
[0128] The pressure drop through the line=4.9.times.0.42=2.1
bar.
[0129] Also the new velocity is calculated as follows: Thus
V=0.000074/(.pi..times.0.005.sup.2/4)=3.8 (m/s)
[0130] The results of this second calculation are shown in Table
2.
3 TABLE 2 Coil duty 1 kW Pipe diameter 5 mm Liquid flowrate 0.074
l/s Liquid velocity 3.8 m/s Pipe length 4.9 m Pressure drop 2.1
bar
[0131] The 5 mm diameter coil therefore offers good linear
velocities and a moderate pressure drop. Such a coil would
therefore be useful for controlling the temperature of the reaction
used for the purposes of this example. The velocity is also well
above the minimum preferred value (1 m/s).
[0132] To be of practical service, a heat transfer coil needs to
operate over a range of conditions as opposed to being limited to
one specific heat transfer rate. Table 3 shows the performance of
the 5 mm diameter coil under a variety of conditions (for organic
process fluid and synthetic heat transfer oil). The one constant in
the table is that the temperature change of the heat transfer fluid
flowing through the coil (t.sub.si-t.sub.so) is always 10.degree.
C.
4TABLE 3 CALCULATED COIL LENGTHS FOR A 5 mm .o slashed. COIL
Pressure Heat LMTD LMTD LMTD LMTD LMTD Drop capacity Velocity
5.degree. C. 10.degree. C. 15.degree. C. 20.degree. C. 25.degree.
C. (bar/m) (W) Flow (l/s) m/s (m) (m) (m) (m) (m) 0.1 457 0.033 1.7
8.9 4.4 2.9 2.2 1.8 0.25 761 0.055 2.8 12.4 6.2 4.2 3.0 2.5 0.50
1121 0.081 4.1 17.2 8.6 5.7 4.3 3.5 0.75 1439 0.104 5.3 20.8 10.4
6.9 5.2 4.2 1.00 1660 0.120 6.1 23.6 11.8 7.9 5.9 4.8
[0133] The first column in Table 3 shows pressure drop (per metre
of coil) through the coil for a given flow rate. The second column
gives the heating or cooling capacity of the coil based on the
10.degree. C. temperature change. The third and fourth columns give
the volume flow rate and velocity of the liquid. The last five
columns give minimum coil lengths required for the quoted LMTD
values. The LMTD temperature values quoted at the top of these
columns represent the log mean temperature difference between the
heat transfer fluid and the process fluid.
[0134] It can be seen from Table 3 that different coil lengths are
used depending on process heat load and log mean temperature
difference between the process and service fluids. Table 3 shows
that a large temperature difference is beneficial as it requires
shorter coil lengths.
[0135] From Table 3, a good general-purpose coil would be 5.9
metres in length. This would be capable of serving any of the
duties contemplated in Table 3 where the required coil length was
5.9 metres or less. It would be suitable for a process load of 1.66
kW providing the difference in temperature between process and heat
transfer fluid was at least 20.degree. C. Under these conditions
the pressure drop through the coil would be 5.9 bar.
[0136] The coil also offers adequate heat transfer area and
reasonable control response at heat loads down to 0.46 kW. Although
low velocities are tolerable the control system becomes
increasingly sluggish with low flows. Also low velocities result in
a large ratio of thermal mass (of heat transfer fluid) to heat
load. This tends to magnify any errors of temperature measurement.
High liquid velocity is therefore desirable as it gives faster
control response and a better (lower) ratio of thermal mass (of the
heat transfer fluid) to heat load.
[0137] For the reasons given above, high heat transfer fluid
velocities are generally desirable. Very high pressure drops
however also introduce greater energy from turbulence and friction.
There are also practical equipment constraints on how fast a liquid
can be pumped through a pipe. The single coil system of 1 is
useful, but has its limitations.
[0138] As Table 1 illustrates, a single coil has an optimum
operating range. Although it is capable of controlling a range of
heat transfer rates, it has its limitations. As Table 3 shows, at
heat transfer rates above 1121 W, the pressure drop across the coil
increases rapidly due to the need for increasingly longer pipes and
higher pressure drops per meter of pipe length.
[0139] The limitations of the single coil may be illustrated as
follows:
[0140] A coil 6.2 m long operating with an LMTD (log mean
temperature difference between the process fluid and service fluid)
of 10.degree. C. has a nominal operating range of 457-1121 W. At
maximum load, the pressure drop across the coil would be 1.55 bar.
If this coil was to be used with a heat load of 1660 W under the
same conditions, it would have to be 11.8 meters long and the
corresponding pressure drop would be 11.8 bar. If, under the same
conditions, the LMTD was reduced to 5.degree. C., the pipe would
need to be 23.6 meters long and the resulting pressure drop would
be 23.6 bar.
[0141] Although the range of a coil can be increased by varying the
inlet temperature (t.sub.si), there are limitations. If the
temperature difference (t.sub.si-t.sub.so) is reduced, the system
becomes progressively less accurate due to limitations of the
temperature measuring devices. If the temperature difference
(t.sub.si-t.sub.so) is expanded too far, there is a risk of
freezing the process fluid (or surface boiling or heat damage where
heat is being absorbed by the process fluid).
[0142] Although service fluid flow and supply temperatures are both
parameters that can be varied to alter the operating range,
reliable control methods favour using one control parameter at a
time (and step changing the other where necessary).
[0143] The 5 mm diameter coil illustrated in Table 1 gives a turn
down ratio of approximately 2.5 (1121/457). If the temperature
difference across the coil (t.sub.si-t.sub.so) was increased from
10.degree. C. to 20.degree. C., the turn down ratio could be
increased to 5. An alternative method of increasing the operating
range of the system is to use multiple coils in a cascade fashion
which provide a variable area heat transfer surface. Such a system
is illustrated by the following
EXAMPLE 2
[0144] Example 2 illustrates, the design of variable area heat
transfer systems employing multiple coil systems such as that
illustrated in FIG. 2.
[0145] Example 2 addresses the fact that a reactor might be
required to handle exothermic reactions which generate heat in the
range of 500 to 15,000 W. A range of this size exceeds the
operating capabilities of the single heat transfer coil system
illustrated in 1. Such a reactor can however be effectively
operated using multiple coils as illustrated in this example (in
this example identical coils each 11.8 m long are used) in a
cascade fashion. With one coil operating with the heat transfer
fluid at 1.7 m/s, a heat load of 457 W will give a temperature rise
in service fluid (t.sub.si-t.sub.so) of 10.degree. C. If, under the
same conditions the velocity of the heat transfer fluid is
increased to 6.1 m/second the capacity rises to 1,660 W (see 3). If
two coils are used at maximum flow the capacity is 3,320 W. By
adding coils in this manner ever greater heat loads can be
measured. If, for example, ten coils are used at the maximum flow,
the capacity is 16,600 W. This system therefore offers a turndown
ratio of approximately 36 (16,600/457). Accordingly, by varying the
velocity of the fluid and the number of coils, the heat capacity
can be measured with a high degree of accuracy over a wide
range.
[0146] The devices described so far have turndown capacities of up
36. In practice, a turndown of 1000 or more may be desirable. This
could be important with a batch reaction where the end point needs
to be identified with precision. Alternatively, high turndown would
be useful for process operations that switch from batch to
continuous operation. In other cases, the same piece of equipment
might be used on multiple applications of widely varying energy
release (or absorption) rates. Whilst the device previously
described has considerable use it has its limitation for this type
of application, because an impractical number of coils would be
needed. Therefore an alternative embodiment of the invention uses a
plurality of coils for varying surface area as illustrated in Table
3.
EXAMPLE 3
[0147] Table 4 sets out the heat transfer capacities of a series of
coils of varying diameter and length.
5TABLE 4 Coil diameter Coil length range Operating range (mm) (m)
(W) 1 0.13-0.6 4-22 2 0.9-2.3 40-141 3 1.9-4.7 118-429 4 3.0-7.8
249-913 5 4.4-11.8 457-1660
[0148] In order to arrive at the operating range, as with Examples
1 and 2, the LMTD is taken as 10.degree. C. and (t.sub.si-t.sub.so)
as 10.degree. C. The extremes of the ranges set out in columns two
and three of Table 4 represent the calculated values for minimum
and maximum flow of the heat transfer fluid. Minimum flow is that
which results in a pressure drop (of service fluid) of 0.1
bar.m.sup.-1 and maximum flow that which results in a pressure drop
(of service fluid) 1 bar.m.sup.-1.
[0149] This combination of coil diameters and lengths provides a
system capable of very high turndown rations. For example a six
coil reactor can be designed to operate at less than 4 W and up to
5000 W. Table 5 shows the cumulative capacity of 6 coils of varying
diameter.
6 TABLE 5 Cumulative Coil diameter Coil range range Coil number
(mm) (W) (W) 1 1 mm 4-22 4-22 2 1 mm 4-22 4-44 3 2 mm 40-141 4-185
4 3 mm 118-429 4-614 5 5 mm 457-1660 4-2274 6 5 mm 457-1660
4-3934
[0150] Each coil is sized for the maximum length shown in Table 4.
The nominal turndown ratio of the six coils is 984.
[0151] If (t.sub.si-t.sub.so) is stepped down to 5.degree. C. when
a single 1 mm diameter coil is operating, the nominal turndown
ratio is increased to 1967 (2-3934 W).
[0152] If (t.sub.si-t.sub.so) is stepped up to 20.degree. C. when
all the coils are operating the nominal turndown ratio is increased
to 3934 (2-7868 W).
[0153] Whilst coil sizing and systems operation have been
illustrated in relation to the control of reaction temperatures
they are equally applicable to any temperature control system.
[0154] The examples cited above are relatively crude approximations
of variable area. Although they utilise variable area, they still
exploit varying degrees of conventional flow/temperature control
philosophies. In practice, the smaller the incremental steps of the
heat transfer coils, the nearer the system becomes to a true
variable area device. The benefits of the true variable area system
are smooth simple control, small thermal inertia of individual
coils and good thermal gradient throughout the operating range.
Conventional control valves become very complex as the number of
coil increments is increased. For this reason, the valve shown in
FIG. 5 offers an effective means of controlling a large number of
coils with a single actuator. Conduits of the temperature control
system of the present invention may be opened and closed by the
valve system described in our Patent Application reference
PAAMBA096 filed 31 Aug. 2001, which provides a valve for the
control of the delivery of fluids to two or more conduits in a
cascade fashion wherein the valve has multiple outlet ports
operating in a cascade wherein the outlet ports are opened and/or
closed according to a signal expressing the requirement for fluid
in the conduits.
[0155] These valves can be designed to provide the same control
characteristics as a conventional control valve for each of its
multiple outlet ports and as such can be used to replace multiple
conventional valves with a single multi-port flow valve having a
single means of activation requiring one control signal only as
opposed to multiple valves and actuators and at least one control
signal per control valve.
[0156] The multi-port flow control valve can therefore be
constructed to operate either with a linear or rotary action. The
number of outlet ports will depend on the number of individual
flows, which need to be independently controlled. In the example
illustrated in FIG. 3, 6 connections for heat transfer coils are
shown, but the valve of the present invention can be designed for
use with any number of coils. By modulating the multi-port flow
control valve the effective heat transfer area in the reactor can
be varied. The maximum number of outlet ports on this type of valve
is limited only by the physical constraints of the
construction.
[0157] FIG. 5 shows a multi-port flow control valve which can be
used with the present invention in which (27) is the inlet port for
heat transfer fluid, (28 to 33) are the outlet ports, (34) is the
plunger. The Figure shows the plunger position with outlet port
(28) open, outlet port (29) partially open and outlet ports (30 to
33) closed. (35) is the seal between the heat transfer fluid and
hydraulic fluid employed in the actuator shaft (36) and (37) is the
actuator piston whose position is determined by a bi-direction
variable speed hydraulic pump (38) which drives the shaft up and
down the valve body to open and close the outlet ports. The arrows
in FIG. 5 show the flow of the heat transfer fluid.
[0158] FIG. 6 shows various options for the valve orifices (39 to
42) of the valve ports (28 to 33) of FIG. 5. (39 to 42A) is the
plan view of the orifices and (39 to 42B) shows the same orifices
in section view.
[0159] In some designs the port openings on the valve may overlap,
whilst in others the port openings may open in separate discreet
steps.
[0160] The invention can be used in any system requiring
temperature control. It is however particularly useful for
improving the operation of commercial chemical and physical
reaction systems. It can however also be used to provide
considerably smaller reaction systems with comparable commercial
throughput. For example the invention enables reduction of reactor
size by a factor of 10 and, in some instances, a factor of 100 or
greater. In particular it can be applied to current commercial
[0161] batch organic synthesis reactions currently carried out in
reactors of 10 to 20,000 litres.
[0162] bulk pharmaceutical synthesis reactions currently carried
out in reactions of 10 to 20,000 litres.
[0163] batch polymerisation reactions currently carried out in
reactors of 10 to 20,000 litres.
[0164] batch synthesis reactions of 10 to 20,000 litres currently
used for unstable materials (compounds susceptible to
self-accelerating runaways)
[0165] batch inorganic synthesis reactions currently carried out in
reactions of 10 to 20,000 litres.
[0166] The techniques may also be useful in larger scale chemical
and petrochemical operations.
[0167] The combination of ability to vary the area of the heat
transfer surface and the use of turbulent plug flow in the heat
transfer fluid enables the creation of a high thermal gradient
between the heat transfer fluid and the medium whose temperature is
to be controlled. This, in turn enables a fast and accurate
response to fluctuations in the temperature of the medium allowing
rapid response and the ability to maintain highly accurate
temperature control. These systems enable the LMTD to be maintained
stable and constant at high levels. The LMTD should be as high as
possible and we prefer that it be greater than 1.degree. C.,
preferably greater than 10.degree. C., more preferably greater than
20.degree. C. even more preferably greater than 100.degree. C. The
LMTD that can be attained depends upon the temperature of the
medium that is to be controlled. The optimum flow of the heat
transfer fluid will depend upon the system including the nature of
the fluid however, it is preferred that the fluid flows with a
Reynolds number greater than 2000. Alternatively high linear
velocities in the pipe should be maintained of greater than 0.1
m/s, more preferably greater than 1 m/s or even more preferably
greater than 3 m/s. This is important with small-bore pipes as a
high Reynolds number may be difficult to achieve.
[0168] As stated, the techniques of the present invention may be
used in any systems employing temperature control. Any industrial
process in which heat is absorbed or released during physical or
chemical change may be controlled by these techniques. For example,
the techniques may be used to control the temperature of reactors,
crystallisers, evaporators, driers, fermenters, stills, vapourisers
and gas evaporators. The techniques may also be used in industrial
and domestic processes requiring a controlled temperature, such as
in liquid heating and cooling systems and storage and
transportation of solids, liquids and gasses. The techniques of the
present invention may also be used in utlities such as heating and
ventillation systems, air conditioning and chilling and
refrigeration.
* * * * *