U.S. patent application number 10/890585 was filed with the patent office on 2006-01-19 for chiller system with low capacity controller and method of operating same.
Invention is credited to Daniel Dominguez.
Application Number | 20060010893 10/890585 |
Document ID | / |
Family ID | 35597979 |
Filed Date | 2006-01-19 |
United States Patent
Application |
20060010893 |
Kind Code |
A1 |
Dominguez; Daniel |
January 19, 2006 |
Chiller system with low capacity controller and method of operating
same
Abstract
A method (126) and a low capacity controller (28) enable a
chiller system (20) to operate below a minimum allowable capacity.
The chiller system (20) includes a chiller (40) and a chiller fluid
loop (30), having a supply line (36) for conveying a chiller fluid
(34) from the chiller (40) and a return line (38) for returning the
chiller fluid (34) to the chiller (40). A capacity demand for the
chiller (40) is determined. When the capacity demand is less than a
minimum allowable capacity of the chiller (40), the chiller fluid
(34) is routed from the return line (38) to a heat exchanger (96)
of the low capacity controller (28) where the chiller fluid (34) is
warmed and returned to the return line (38). The warmed chiller
fluid (34) establishes a false capacity demand, detectable at the
chiller (40), that is at least the minimum allowable capacity.
Inventors: |
Dominguez; Daniel; (El Paso,
TX) |
Correspondence
Address: |
Jordan M. Meschkow;Meschkow & Gresham, PLC
Suite 409
5727 North Seventh Street
Phoenix
AZ
85014
US
|
Family ID: |
35597979 |
Appl. No.: |
10/890585 |
Filed: |
July 13, 2004 |
Current U.S.
Class: |
62/201 |
Current CPC
Class: |
F25D 17/02 20130101;
F24F 2221/54 20130101; F24F 11/30 20180101; F24F 11/62 20180101;
F24F 3/06 20130101 |
Class at
Publication: |
062/201 |
International
Class: |
F25D 17/02 20060101
F25D017/02 |
Claims
1. A method of operating a chiller system, said chiller system
including a chiller and a fluid loop, said fluid loop including a
supply line for conveying a fluid from said chiller and a return
line for returning said fluid to said chiller, said method
comprising: determining a capacity demand for said chiller; and
when said capacity demand is less than a minimum allowable capacity
of said chiller, warming said fluid in said return line to
establish a false capacity demand, detectable at said chiller, that
is at least said minimum allowable capacity.
2. A method as claimed in claim 1 wherein said determining
operation comprises: measuring a supply temperature of said fluid
in said supply line; measuring a return temperature of said fluid
in said return line; and ascertaining a difference between said
return temperature and said supply temperature to determine said
capacity demand.
3. A method as claimed in claim 2 wherein said determining
operation further comprises calculating said capacity demand as a
function of a flow rate of said fluid and said difference between
said return temperature and said supply temperature.
4. A method as claimed in claim 1 wherein said chiller system
further includes a heat exchanger in communication with said return
line, said fluid is a first fluid, and said warming activity
includes transferring heat from a second fluid in said heat
exchanger to said first fluid.
5. A method as claimed in claim 4 wherein said second fluid is
condenser fluid, and said method further comprises transporting
said condenser fluid to said heat exchanger via a condenser fluid
loop interposed between said chiller and a cooling tower of said
chiller system.
6. A method as claimed in claim 1 wherein said fluid is a first
fluid, and said method further comprises: routing a portion of said
first fluid in said return line through a secondary loop, said
secondary loop being in heat exchanging relation with a second
fluid; and returning said portion of said first fluid back to said
return line via said secondary loop.
7. A method as claimed in claim 6 wherein said returning activity
comprises modulating a return of said portion of said first fluid
to said return line.
8. A method as claimed in claim 6 wherein said secondary loop is
coupled with said supply line, and said returning operation
comprises: conveying said portion of said first fluid into said
supply line; and routing said portion of said first fluid to said
return line via a bypass valve of said chiller system interposed
between said supply line and said return line.
9. A method as claimed in claim 6 wherein said secondary loop is
coupled with said return line, and said returning operation
comprises routing said portion of said first fluid from said
secondary loop directly into said return line.
10. A method as claimed in claim 9 further comprising regulating a
flow of said portion of said first fluid into said return line.
11. A method as claimed in claim 6 further comprising: selectively
operating said chiller system in a chiller bypass mode; when in
said chiller bypass mode, deactivating said chiller; when in said
chiller bypass mode, transporting said portion of said first fluid
through said secondary loop to cool said portion of said first
fluid; and when in said chiller bypass mode, returning said portion
of said first fluid from said secondary loop to said supply
line.
12. A method as claimed in claim 1 wherein said warming activity
comprises adjusting a volume of said fluid warmed in response to
said capacity demand.
13. A method as claimed in claim 1 further comprising when said
capacity demand is greater than said minimum allowable capacity,
discontinuing said warming activity.
14. A chiller system for providing a fluid to a heat exchange unit
comprising: a chiller; a pump for forcing said fluid through said
chiller; a fluid loop having a supply line for conveying said fluid
from said chiller to said heat exchange unit, and a return line for
returning said fluid from said heat exchange unit to said chiller;
means for determining a capacity demand for said chiller; and when
said capacity demand is less than a minimum allowable capacity of
said chiller, means for warming said fluid in said return line to
establish a false capacity demand, detectable at said chiller, that
is at least said minimum allowable capacity.
15. A chiller system as claimed in claim 14 wherein said
determining means comprises: a first temperature sensor for
measuring a supply temperature of said fluid in said supply line; a
second temperature sensor for measuring a return temperature of
said fluid in said return line; means for determining a flow rate
of said fluid in said fluid loop; and means for calculating said
capacity demand as a function of said flow rate and a difference
between said return temperature and said supply temperature.
16. A chiller system as claimed in claim 14 wherein said warming
means comprises: a heat exchanger; and a secondary loop in
communication with said return line and in heat exchanging relation
with said heat exchanger, a portion of said fluid in said return
line being routed through said secondary loop to warm said fluid in
said heat exchanger when said capacity demand is less than said
minimum allowable capacity, and said portion of said fluid being
returned to said return loop via said secondary loop when said
fluid is warmed.
17. A chiller system as claimed in claim 16 wherein: said chiller
system further comprises a bypass valve interposed between said
supply line and said return line; and said secondary loop
comprises: a supply conduit for transporting said portion of said
fluid from said return line toward said heat exchanger; a return
conduit coupled with said supply line for conveying said portion of
said fluid from said heat exchanger; and a modulating valve in
fluid communication with said return conduit for modulating a
return of said portion of said fluid to said return line, said
portion of said fluid being returned to said return line via said
bypass valve.
18. A chiller system as claimed in claim 16 wherein: said secondary
loop comprises: a supply conduit for transporting said portion of
said fluid from said return line toward said heat exchanger; a
return conduit coupled with said return line for returning said
portion of said fluid from said heat exchanger to said return line;
a modulating valve in fluid communication with said return conduit
for modulating a return of said portion of said fluid to said
return line; and said chiller system further comprises means for
regulating a flow of said portion of said first fluid into said
return line.
19. A chiller system as claimed in claim 18 wherein said regulating
means is a pump interposed along said return conduit.
20. A chiller system as claimed in claim 18 wherein said regulating
means is a three-way valve positioned at a junction of said return
conduit with said return line.
21. A chiller system as claimed in claim 16 further comprising: a
cooling tower; and a condenser fluid loop interposed between said
chiller and said cooling tower, said second fluid loop being routed
through said heat exchanger, and said condenser fluid loop
conveying a second fluid through said heat exchanger when said
capacity demand is less than said minimum allowable capacity such
that heat from said second fluid is transferred to said portion of
said fluid.
22. A chiller system as claimed in claim 21 further comprising:
means for selectively operating said system in a chiller bypass
mode; means for, in said chiller bypass mode, deactivating said
chiller; means for, in said chiller bypass mode, transporting said
portion of said fluid through said secondary loop for enabling heat
transfer in said heat exchanger between said condenser fluid loop
and said secondary loop to cool said portion of said fluid; and
means for, in said chiller bypass mode mode, returning said portion
of said fluid from said secondary loop to said supply line.
23. A low capacity controller for a chiller system, said chiller
system including a chiller and a first fluid loop, said first fluid
loop including a supply line for conveying a chiller fluid from
said chiller and a return line for returning said chiller fluid to
said chiller, said chiller being operable above a minimum allowable
capacity for said chiller, and said low capacity controller
comprising: a heater; a secondary loop interposed between said
first fluid loop and said heater; means for enabling a transfer of
said chiller fluid through said heater via said secondary loop when
a capacity demand is less than said minimum allowable capacity,
said chiller fluid being warmed at said heater; and means for
returning said chiller fluid from said heater to said return line
via said secondary loop.
24. A low capacity controller as claimed in claim 23 wherein said
chiller system further includes a cooling tower and a condenser
fluid loop interposed between said chiller and said cooling tower,
said condenser fluid loop carrying a condenser fluid, and said low
capacity controller further comprises means for enabling a transfer
of said condenser fluid through said heater via said condenser
fluid loop when said capacity demand is less than said minimum
allowable capacity for said chiller.
25. A low capacity controller as claimed in claim 23 wherein said
returning means comprises a modulating valve for modulating a
return of said chiller fluid to said return line.
26. A low capacity controller as claimed in claim 23 wherein said
chiller system includes a bypass valve interposed between said
supply line and said return line, and said secondary loop
comprises: a supply conduit for transporting said portion of said
fluid from said return line toward said heat exchanger; a return
conduit coupled with said supply line for conveying said portion of
said fluid from said heat exchanger, said portion of said fluid
being returned to said return line via said bypass valve.
27. A low capacity controller as claimed in claim 23 wherein said
returning means adjusts a volume of said chiller fluid returned to
said return line in response to said capacity demand.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to the field of chilled water
systems. More specifically, the present invention relates to a
method of operating a chilled water system.
BACKGROUND OF THE INVENTION
[0002] Chiller systems provide a temperature conditioned fluid, for
use in conditioning the air within large buildings and other
facilities. The chilled fluid is typically pumped to a number of
remote heat-exchangers or system coils for cooling various rooms or
areas within a building. A chiller system enables the
centralization of the air conditioning requirements for a large
building or complex of buildings by using water or a similar fluid
as a safe and inexpensive temperature transport medium.
[0003] In general, a chiller of the chiller system provides chilled
water of a particular temperature, via a first fluid loop, for
cooling air in a building. Heat is extracted from the building air,
transferred to the fluid in the first fluid loop, and is returned
via the first fluid loop to the chiller. The returned fluid is
again cooled to the desired temperature by transferring the heat of
the fluid to the chiller's refrigerant. After the refrigerant is
compressed by a compressor, the heat in the refrigerant is
transported to the condenser and heat is transferred to a second
fluid conveyed in a second fluid loop. The second fluid loop
transports waste heat from the condenser of the chiller to a
cooling tower which then transfers the waste heat from the second
water loop to ambient air by direct contact between the ambient air
and the second fluid of the second loop.
[0004] A multiple chiller system has two or more chillers connected
by parallel or series piping to a common distribution system.
Multiple chillers offer operational flexibility, standby capacity,
and less disruptive maintenance. Through the use of multiple
chillers, when the cooling demand is low, only one chiller may need
to operate, and the operating chiller's capacity may be controlled
to match the demand. If the cooling demand is beyond a single
chiller's maximum capacity, one or more additional chillers may
need to be activated. As such, the operating chillers are
controlled so the system's total capacity (sum of the chillers'
individual capacities) meets the cooling demand.
[0005] Chiller capacities normally are based on the maximum load
anticipated. However, much of the time that a chiller is in use, it
is operating at less than full load. Indeed, a chiller system in a
building may operate over a wide range of demand conditions, with
significant dominance of low capacity demand. Low capacity demand
can result from seasonal fluctuations, when cooling a small office
or zone during "off-hours", when there is low or no load available
for the start up of the chiller system, and so forth.
[0006] Low capacity demand typically causes a chiller to operate
less efficiently. That is, lower capacity demand results in a
higher kilowatt use per ton. More critically however, low capacity
demand can force a chiller system to operate in unstable
conditions. Under such conditions, compressor and evaporator
capacities balance at ever lower suction pressures and
temperatures. Typically, chillers are outfitted with protection
mechanisms that cause them to shut down when the capacity demand
becomes very low, for example, less than approximately eighteen
percent of total chiller capacity. Left unchecked, the eventual
result is coil frosting and compressor flooding.
[0007] Accordingly, some chillers are unable to operate when the
capacity demand is very low. This is problematic for an operator
who wishes to cool a small office or zone during "off-hours", who
needs to keep one office or room open past normal closing time, who
has low or no load available for the start up of the chiller
system, and so forth.
[0008] Attempts have been made to circumvent this problem by the
inclusion of a hot gas bypass. A hot gas bypass can stabilize a
chiller system by diverting hot, high-pressure refrigerant vapor
from the discharge line directly to the low-pressure side of the
chiller. This technique keeps the chiller compressor more fully
loaded while the evaporator satisfies the part-load condition. In
addition, the diverted vapor raises the suction temperature, which
prevents frost from forming.
[0009] Although hot gas bypass can provide frost control and match
system capacity to load to allow the system to operate at safe
balance points during unsafe loads, hot gas bypass sometimes fails
to safely stabilize the chiller system. Moreover, hot gas bypass
can undermine the reliable operation of a chiller by introducing
problems stemming from insufficient oil return and refrigerant
logging in the hot gas bypass line. In addition, hot gas bypass
requires an additional refrigerant line thus increasing the initial
cost of a chiller system, and also increasing the likelihood of
refrigerant leaks. Hot gas bypass further reduces operating
efficiency because the bypassed vapor does no useful cooling.
[0010] Accordingly, what is needed is a method and apparatus for
operating a chiller system below a minimum allowable capacity for
the chiller system when a capacity demand is very low.
SUMMARY OF THE INVENTION
[0011] Accordingly, it is an advantage of the present invention
that a method and low capacity controller are provided for
operating a chiller system.
[0012] It is another advantage of the present invention that a
method and low capacity controller are provided that enable a
chiller system to be operated below a minimum allowable capacity
for the chiller system.
[0013] Yet another advantage of the present invention is that the
low capacity controller can be readily and cost effectively
incorporated into a chiller system.
[0014] The above and other advantages of the present invention are
carried out in one form by a method of operating a chiller system,
the chiller system including a chiller and a fluid loop, and the
fluid loop including a supply line for conveying a fluid from the
chiller and a return line for returning the fluid to the chiller.
The method calls for determining a capacity demand for the chiller.
When the capacity demand is less than a minimum allowable capacity
of the chiller, the fluid is warmed in the return line to establish
a false capacity demand, detectable at the chiller, that is at
least the minimum allowable capacity.
[0015] The above and other advantages of the present invention are
carried out in another form by a low capacity controller in a
chiller system. The chiller system includes a chiller and a first
fluid loop, the first fluid loop including a supply line for
conveying a chiller fluid from the chiller and a return line for
returning the chiller fluid to the chiller, the chiller being
operable above a minimum allowable capacity for the chiller. The
low capacity controller includes a heater, and a secondary loop
interposed between the return line and the heater. The low capacity
controller further includes means for enabling a transfer of the
chiller fluid through the heater via the secondary loop when a
capacity demand is less than the minimum allowable capacity, the
chiller fluid being warmed at the heater, and means for returning
the chiller fluid from the heater to the return line via the
secondary loop.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] A more complete understanding of the present invention may
be derived by referring to the detailed description and claims when
considered in connection with the Figures, wherein like reference
numbers refer to similar items throughout the Figures, and:
[0017] FIG. 1 shows a block diagram of a chiller system in
accordance with a preferred embodiment of the present
invention;
[0018] FIG. 2 shows a flowchart illustrating an operational process
of the present invention in accordance with the preferred
embodiment;
[0019] FIG. 3 shows a block diagram of the chiller system operating
in a low capacity mode in accordance with the preferred
embodiment;
[0020] FIG. 4 shows a block diagram of the chiller system operating
in a chiller bypass/economizer mode;
[0021] FIG. 5 shows a block diagram of a low capacity controller
for a chiller system in accordance with an alternative embodiment
of the present invention; and
[0022] FIG. 6 shows a block diagram of a low capacity controller
for a chiller system in accordance with another alternative
embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] FIG. 1 shows a block diagram of a chiller system 20 in
accordance with a preferred embodiment of the present invention. In
general, chiller system 20 includes a chiller fluid section 22, a
refrigeration section 24, a condenser fluid section 26, a low
capacity controller 28, and a system controller 29.
[0024] Chiller fluid section 22 includes a chiller fluid loop 30
and pumps 32. Pumps 32 are in fluid communication with chiller
fluid loop 30 for forcing a chiller fluid, represented by arrow
heads 34, to circulate within chiller fluid loop 30. Chiller fluid
loop 30 includes a supply line 36 and a return line 38. Supply line
36 conveys chiller fluid 34 from chillers 40 of refrigeration
section 24 to an air handler 42 for conditioning the air within a
space served by air handler 42. Air handler 42 uses chiller fluid
34 to transfer heat energy from the air being circulated from the
space by means of a fan 44 and ductwork (not illustrated) to a heat
exchange coil 46 of chiller fluid loop 30. Return line 38 returns
chiller fluid 34 to chillers 40, so that chiller fluid 34 can be
subsequently re-cooled and re-circulated through chiller fluid loop
30.
[0025] Chiller fluid section 22 further includes one or more pump
controllers or variable flow devices 48 for controlling a flow rate
of chiller fluid 34 through chiller fluid loop 30 and a
differential pressure sensor 50 for controlling a bypass valve 52
to maintain a minimum flow of chiller fluid 34 in chiller fluid
loop 30. Chiller fluid section 22 further includes a flow meter 54,
a supply temperature sensor 56, a return temperature sensor 58, and
a return mixed temperature sensor 60 for staging chillers 40 on and
off in response to a capacity demand (discussed below) and/or to
control bypass valve 52 as known to those skilled in the art.
[0026] In a preferred embodiment, pumps 32 form a primary variable
flow distribution system. Such a primary variable flow distribution
system lowers initial costs, due to the elimination of the
secondary pumps and associated fittings, vibration isolation,
starters, and so forth, and uses less energy than conventional
primary-secondary variable flow systems. However, the present
invention may alternatively be adapted for use in a
primary-secondary variable flow distribution system. In addition,
only two pumps 32 are shown for simplicity of illustration. However
it should be understood that chiller system 20 may include any
number of pumps 32.
[0027] Although only one air handler 42 is illustrated herein, it
should be readily apparent that chiller fluid loop 30 may
distribute chiller fluid 34 to various system coils or heat
exchangers for cooling rooms or other areas within a building.
[0028] Refrigeration section 24 includes one or more chillers 40.
In this exemplary embodiment, chiller system 20 includes two
chillers 40. However, the present invention may be adapted for use
in chiller systems that include any number of chillers 40. In
addition, chillers 40 need not be of the same capacity. As such,
one of chillers 40 may have a higher maximum cooling capacity than
the other.
[0029] In an exemplary embodiment, chillers 40 are screw
compressors utilizing R134A refrigerant. Screw compressors are
positive-displacement machines with nearly constant flow
performance, and are usually available in units from about 30 to
1250 tons. A slide valve of a screw compressor is used to adjust
the refrigerant's flow rate for varying the chiller's capacity or
cooling effect. Although screw compressors are described herein, it
should be apparent to those skilled in the art that other types of
chillers may be employed as an alternative to screw
compressors.
[0030] Generally, each of chillers 40 includes a compressor 62 that
forces a refrigerant in series through a condenser 64, an expansion
device 66 (for example, a flow restrictor, orifice, capillary,
expansion valve, and so forth), and an evaporator 68 via a
refrigeration conduit 70. Evaporator 68 conditions chiller fluid 34
to a predetermined temperature, for example, 45.degree. F., so that
chiller fluid 34 can be reused and conveyed in chiller fluid loop
30 to air handler 42. The energy extracted from chiller fluid 34 by
evaporator 68 is transported by refrigeration conduit 70 to
compressor 62 which lowers the condensation point of the
refrigerant so that the refrigerant can be condensed by condenser
64.
[0031] Condenser fluid section 26 includes a cooling tower 72,
pumps 74, and a condenser fluid loop 76 conveying a condenser
fluid, generally represented by arrow heads 78. Second fluid loop
76 is interposed between cooling tower 72 and condenser 64 of each
of chillers 40. When the refrigerant is condensed by condenser 64,
the energy, in the form of heat is transferred to condenser fluid
78 in condenser fluid loop 76. Pumps 74 force the circulation of
condenser fluid 78 through cooling tower 72 where the heat of
condenser fluid 78 is transferred to ambient air.
[0032] Condenser fluid section 26 may further include one or more
flow valves 80 and one or more check valves 82 associated with
pumps 74 that regulate a flow rate and flow direction of condenser
fluid 78 in condenser fluid loop 76. Condenser fluid section 26 may
further include a differential pressure sensor 84 for controlling a
bypass valve 86 to maintain a minimum flow of condenser fluid 78 in
condenser fluid loop 76. In addition, a condenser fluid temperature
sensor 88 and a condenser fluid flow meter 90 may be provided for
monitoring the temperature and flow rate of condenser fluid 78 in
condenser fluid loop 76.
[0033] As known to those skilled in the art, instead of operating
energy intensive chillers during cool, dry outdoor conditions, the
cooling effect of outside air may be utilized to provide direct
cooling to a facility or process using cooling tower 72. This
situation is sometimes known as "free cooling," and the chillers
are deactivated, i.e., the operation of chillers 44 is bypassed.
Thus, a three-way valve 92 may be included in condenser fluid
section 26 for selectively routing condenser fluid 78 through
cooling tower 72, or for bypassing cooling tower 72. In addition,
when chillers 44 are deactivated, condenser inlet valves 94 may be
provided that close partially or entirely to limit or prevent a
flow of condenser fluid 78 into condensers 64 when utilizing "free
cooling."
[0034] In accordance with a preferred embodiment of the present
invention, chiller system 20 includes low capacity controller 28.
Low capacity controller 28 advantageously enables chillers 44 to
operate below a minimum allowable capacity for chillers 44. In
general, the minimum allowable capacity may be approximately
eighteen percent of total chiller capacity. Conventional chiller
systems are outfitted with protection mechanisms that cause them to
shut down when the capacity demand becomes very low, for example,
less than approximately eighteen percent of total chiller capacity.
Left unchecked, the eventual result is coil frosting and compressor
flooding. Accordingly, conventional chiller systems cannot operate
at very low capacity demands.
[0035] Low capacity controller 28 of the present invention enables
chillers 44 to operate at a capacity demand much lower than
eighteen percent of total capacity, for example, as low as
approximately three percent of total capacity or lower. It will
become readily apparent below, that low capacity controller 28
provides precise capacity control for these minimal capacity
requirements. Minimum capacity requirements may occur when cooling
a small office or zone during "off-hours" of "after-hours", when
there is low or no load available for the start up of the chiller
system, at seasonal fluctuations, and so forth. Moreover, it will
become readily apparent in the ensuing discussion that low capacity
controller 28 may be further employed to take advantage of "free
cooling," by utilizing the cooling effect of outside air to provide
direct cooling to a building.
[0036] Low capacity controller 28 includes a heater, in the form of
a heat exchanger 96, and a secondary loop 98 interposed between
chiller fluid section 22 and heat exchanger 96. In a preferred
embodiment, secondary loop 98 includes an inlet conduit 100 in
fluid communication with return line 38 for conveying a portion of
chiller fluid 34 from return line 38 to heat exchanger 96.
[0037] Secondary loop 98 further includes an outlet conduit 102
having a first branch 104 selectively in fluid communication with
supply line 36 and a second branch 106 selectively in fluid
communication with supply line 36 of chiller fluid section 22.
First means, in the form of a modulating valve 108, is positioned
along first branch 104 of outlet conduit 102 for selectively
returning the portion of chiller fluid 34 to return line 38.
Modulating valve 108 enables a rate controlled flow of chiller
fluid 34 to return line 38 (discussed below). Second means, in the
form of a chiller side flow valve 110, is positioned along second
branch 106 of outlet conduit 102 for selectively returning chiller
fluid 34 to supply line 36 (discussed below).
[0038] Condenser fluid loop 76 includes a secondary loop section
112 interposed between condenser fluid section 26 and heat
exchanger 96. More specifically, secondary loop section 112
includes an inlet section 114 in fluid communication with a
condenser fluid supply line 116 of condenser fluid loop 76 and heat
exchanger 96. Secondary loop section 112 further includes an outlet
section 118 in fluid communication with heat exchanger 96 and a
condenser fluid return line 120 of condenser fluid loop 76. Means,
in the form of a condenser side flow valve 122, is positioned along
outlet section 118 for selectively enabling a transfer of condenser
fluid 78 from condenser fluid loop 76 through heat exchanger 96 and
back to condenser fluid loop 76 (discussed below).
[0039] In an exemplary embodiment, heat exchanger 96 is a plate and
frame heat exchanger that provides a safe, sanitary, and efficient
method of transferring heat from one fluid medium to another as the
two streams pass on opposing sides of intervening plates. In
particular, heat is transferred between chiller fluid 34 and
condenser fluid 78 in response to a particular mode of operation in
which chiller system 20 is being operated. This heat transfer
between chiller fluid 34 and condenser fluid 78 will be discussed
in detail in connection with the flowchart of FIG. 2.
[0040] System controller 29 generally oversees, manages, and
controls the various components of chiller system 20. System
controller 29 may encompass a wide variety of electrical devices
(programmable or not programmable) having the ability to provide
various output signals in response to various input signals. This
communication is schematically represented by a bi-directional
arrow 124. Examples of system controller 29 include, but are not
limited to, microcomputers, personal computers, dedicated
electrical circuits having analog and/or digital components,
programmable logic controllers, and various combinations
thereof.
[0041] In addition, system controller 29 may be utilized to oversee
and manage individual controllers of various equipment groups. In
such a situation, cooling tower 72 may be associated with an
individual controller, each of chillers 40 may be associated with
an individual controller, and signals may be forwarded between the
individual controllers and system controller 29. Such a controller
is typically supplied from the manufacturer as a complete
direct-digital-control (DDC) and monitoring package.
[0042] FIG. 2 shows a flowchart generally illustrating an
operational process 126 of chiller system 20 in accordance with the
preferred embodiment. In particular, system controller 29 (FIG. 1)
may control chiller system 20 to provide the necessary capacity to
satisfy a capacity demand, or load. Process 126 begins with a task
128.
[0043] At task 128, controller 29 determines the capacity demand.
The capacity demand is a measure of the amount of cooling required,
i.e., the total load, imposed upon chiller system 20 at a give
instant. In a preferred embodiment, capacity demand is calculated
as a function of the flow rate of chiller fluid 34 and the
difference between the supply and return temperatures of chiller
fluid 34, as follows: DEMAND (tons)=FLOW
RATE*(CHWST-CHWMRT)500/12,000 where flow rate is detected at flow
sensor 54 (FIG. 1), CHWST is the chilled water supply temperature
measured at supply temperature sensor 56, and CHWMRT is the chilled
water mixed return temperature measured at return mixed temperature
sensor 60. Although the above function may be utilized to determine
the capacity demand, those skilled in the art will recognize that
alternative functions may be employed that provide a measure of the
capacity demand.
[0044] Next, a task 130 is performed. At task 130, controller 29
determines the ambient conditions. Controller 26 can determine the
ambient conditions by receiving signals that include, for example,
outside air temperature, outside air temperature dew point, and the
like, from the appropriate sensors, as known to those skilled in
the art.
[0045] In response to data collection tasks 128 and 130, a query
task 132 is performed. At query task 132, system controller 29
determines whether load requirements and ambient conditions are
such that chiller system 20 may enter a chiller bypass mode, also
known as an economizer mode. As discussed above, under cool, dry
outdoor conditions, the cooling effect of outside air may be
utilized to provide direct cooling using cooling tower 72 (FIG. 1).
When ambient conditions indicate such is the case, process 126
proceeds to a task 134. At task 134, chiller system 20 (FIG. 1) is
operated in a chiller bypass/economizer mode. Thereafter, process
126 loops back to tasks 128 and 130 to continue monitoring capacity
demand and ambient conditions. The chiller bypass/economizer mode
will be discussed in connection with a chiller system fluid flow
schematic presented in FIG. 4.
[0046] However, when system controller 29 determines that load
requirements and ambient conditions are such that chiller system 20
should not enter a chiller bypass mode, process flow proceeds to a
query task 136. At query task 136, system controller 29 determines
whether the capacity demand calculated at task 130, is less than a
minimum allowable capacity. In a preferred embodiment, the capacity
demand is compared with an internal table of system controller 29
which is the cooling capacity of the installed chillers 40. The
internal table preferably contains a minimum allowable capacity
figure for chiller system 20. Since the individual capacities for
each of chillers 40 need not be the same, the minimum allowable
capacity figure may be approximately eighteen percent of the total
capacity of the lowest capacity one of chillers 40 in chiller
system 20.
[0047] When the capacity demand is not less than the minimum
allowable capacity, process flow proceeds to a task 138. At task
138, system controller 29 provides the signaling necessary to
operate chiller system 20 in a nominal mode. For example, system
controller 29 will signal a discrete output to energize the
required number of chillers 40. To enable chillers 40, system
controller 29 may generate the discrete outputs to energize
variable flow devices 48, as needed. In addition, system controller
29 may generate discrete outputs to close condenser side flow valve
122, chiller side flow valve 110, and modulating valve 108 so that
low capacity controller 28 is bypassed. Once one or more chillers
40 have been enabled, each chiller 40 may operate based on their
stand alone product integrated controls. Thereafter, process 126
loops back to tasks 128 and 130 to continue monitoring capacity
demand and ambient conditions.
[0048] When the capacity demand is less than the minimum allowable
capacity, signifying a very low desired capacity, tasks 140, 142,
and 144 are performed.
[0049] Referring to FIG. 3 in connection with tasks 140, 142, and
144, FIG. 3 shows a block diagram of chiller system 20 operating in
a low capacity mode 146 in accordance with the preferred
embodiment. FIG. 3 particularly illustrates fluid flow through low
capacity controller 28 of chiller system 20. In order to clearly
demonstrate the fluid flow, all of the reference numerals set forth
in FIG. 1 are not repeated in FIG. 3. Rather, only those reference
numerals that further the understanding of fluid flow in low
capacity mode 146 are shown in FIG. 3. It should be understood,
however, that the components of chiller system 20 in FIG. 1 are
equivalent to and present in chiller system 20 illustrated in FIG.
3.
[0050] At task 140, system controller 29 provides the necessary
signaling to route condenser fluid 78 to heat exchanger 96 via
inlet section 114 of secondary loop section 112. This is
accomplished by opening condenser side flow valve 122 which enables
a flow of condenser fluid 78 through secondary loop section
112.
[0051] At task 142, system controller 29 provides the appropriate
signaling to route a portion of chiller fluid 34, i.e., portion
34', to heat exchanger 96 via inlet conduit 100 of secondary loop
98. Chiller side flow valve 110 of secondary loop 98 is maintained
in a closed position. Accordingly, portion 34' of chiller fluid 34
is routed through heat exchanger 96 and returned to supply line 36
via first branch 104 of outlet conduit 102. Portion 34' of chiller
fluid 34 mixes with chiller fluid 34 supplied from chillers 40, as
denoted in FIG. 3 by the numerals 34+34'. Since chiller system 20
is operating at low capacity, much of this mixed chiller fluid
34+34' is returned to return line 38 via bypass valve 52 to
maintain a minimum flow of mixed chiller fluid 34+34' in chiller
fluid loop 30.
[0052] At heat exchanger 96, heat is transferred from the warmer
condenser fluid 78 to the cooler portion 34' of chiller fluid 34.
This causes the temperature of portion 34' of chiller fluid 34 to
increase. Consequently, portion 34' causes the chiller fluid in
return line 38, i.e., mixed chiller fluid 34+34', of chiller fluid
loop 30 to be warmer than what was originally measured at return
mixed temperature sensor 60. This warmed chiller fluid 34+34'
establishes a "false capacity demand" from the perspective of
chillers 40, so that chillers 40 will not enter into an automatic
chiller shutdown due to a freeze protection condition.
[0053] Low capacity mode 146 may be most clearly explained by
example. Under normal conditions, let it be assumed that the supply
and return differential temperature of chiller fluid 34 is
16.degree. F. when operating at minimum flow and full capacity.
Under this condition with a design supply temperature of 45.degree.
F., the temperature of returning chiller fluid 34 should be
approximately 61.degree. F. If the minimum allowable capacity of
chillers 40 is eighteen percent, then the minimum allowed return
temperature is approximately 48.degree. F. Anything below this
temperature is too cold for chiller 40. Because chiller 40 cannot
download any further, chiller fluid 34 leaving chillers 40 drops
below the set point of 48.degree. F. Without the inclusion of the
present invention, this "too cold" condition can cause the
initiation of a freeze alarm, and cause chillers 40 to shut down.
By warming chiller fluid 34 in return line 38, the temperature of
chiller fluid 34 can be kept above this alarm condition, for
example, above 52.degree. F.
[0054] Modulating valve 108 is adjusted, i.e., modulated, to allow
enough flow of portion 34' of chiller fluid 34 into return line 38.
This flow is tied to the capacity demand formula presented above.
That is, the temperature of chiller fluid 34 is warmed so that the
false capacity demand detectable at chillers 40 is above the
minimum allowable capacity.
[0055] Following tasks 140, 142, and 144 operational process 126
loops back to tasks 128 and 130 to continue monitoring capacity
demand and ambient conditions. Chiller system 20 remains in low
capacity mode 146 until the capacity demand, as determined at task
128, becomes greater than the minimum allowable capacity, or unless
chiller system 20 is able to enter the chiller bypass/economizer
mode as determined through ambient conditions at task 130.
[0056] FIG. 4 shows a block diagram of chiller system 20 operating
in a chiller bypass/economizer mode 148 in response to task 134
(FIG. 2) of process 126 (FIG. 2). FIG. 4 particularly illustrates
fluid flow through low capacity controller 28 of chiller system 20
when in chiller bypass/economizer mode 148. In order to clearly
demonstrate the fluid flow, all of the reference numerals set forth
in FIG. 1 are not repeated in FIG. 4. Rather, only those reference
numerals that further the understanding of fluid flow in chiller
bypass/economizer mode 148 are shown in FIG. 4. It should be
understood, however, that the components of chiller system 20 in
FIG. 1 are equivalent to and present in chiller system 20
illustrated in FIG. 4.
[0057] In chiller bypass/economizer mode 148, system controller 29
provides the necessary signaling to route condenser fluid 78 to
heat exchanger 96 via inlet section 114 of secondary loop section
112. This is accomplished by opening condenser side flow valve 122
which enables a flow of condenser fluid 78 through secondary loop
section 112. An "X" positioned over each portion of condenser fluid
loop 76 entering condensers 64 represents no- or minimal-flow
conditions of condenser fluid 78 into condensers 64 since chillers
40 may be deactivated in chiller bypass/economizer mode 148. This
may be accomplished by partially or entirely closing condenser
inlet valves 94.
[0058] In addition, system controller 29 provides the appropriate
signaling to route chiller fluid 34 to heat exchanger 96 from
return line 38 via inlet conduit 100 of secondary loop 98 and back
to supply line 36 of chiller fluid loop 30. This is accomplished by
opening chiller side flow valve 110 and maintaining modulating
valve 108 in a closed position. Accordingly, chiller fluid 34 is
routed through heat exchanger 96 and returned to supply line 36 via
second branch 106 of outlet conduit 102. An "X" positioned over
supply line 36 directed away from chillers 40 and return line 38
directed toward chillers 40 represents no- or minimal-flow
conditions of chiller fluid 34 into evaporators 68 since chillers
40 may be deactivated in chiller bypass/economizer mode 148.
[0059] At heat exchanger 96, heat is transferred from the warmer
chiller fluid 34 to the cooler condenser fluid 78. This causes the
temperature of chiller fluid 34 to decrease. This cooled chiller
fluid 34 is subsequently routed to air handler 42 to cool the space
served by air handler 42. Chiller bypass mode 148 is sometimes
referred to as an economizer, "waterside", or "free cooling" mode
because cooling is achieved utilizing the cooling effect of outside
air to provide direct cooling instead of operating the energy
intensive chillers 40. Accordingly, low capacity controller 28
achieves the secondary benefit of providing direct cooling, as well
as enabling chillers 40 to operate under very low load conditions
by establishing a false capacity demand, as discussed above.
[0060] FIG. 5 shows a block diagram of a low capacity controller
150 for chiller system 20 in accordance with an alternative
embodiment of the present invention. Low capacity controller 150
simply replaces low capacity controller 28 described in detail
above. Accordingly, all of the reference numerals and components
set forth in FIG. 1 are not repeated in FIG. 5. Rather, only those
reference numerals and components that further the understanding of
low capacity controller 150 are shown in FIG. 5.
[0061] Low capacity controller 150 includes heat exchanger 96, and
a secondary loop 152 interposed between chiller fluid section 22
(FIG. 1) and heat exchanger 96. In this alternative embodiment,
secondary loop 152 includes inlet conduit 100 in fluid
communication with return line 38 for conveying portion 34' of
chiller fluid 34 from return line 38 to heat exchanger 96.
[0062] Secondary loop 152 further includes an outlet conduit 154
having a first branch 156 selectively in fluid communication with
return line 38 and second branch 106 selectively in fluid
communication with supply line 36 of chiller fluid section 22. Like
low capacity controller 28 (FIG. 1), low capacity controller 150
also includes chiller side flow valve 110 positioned along second
branch 106 of outlet conduit 102 for selectively returning chiller
fluid 34 to supply line 36 when chiller system 20 (FIG. 1) operates
in the economizer mode (as discussed above).
[0063] A pump 158 is placed in series with modulating valve 108
along first branch 156 of outlet conduit 154 for selectively
returning portion 34' of chiller fluid 34 directly to return line
38. Due to the higher flow rate and volume of chiller fluid 34 in
return line 38, pump 158 is utilized to force the flow of portion
34' of chiller toward return line 38, while modulating valve 108
enables the rate controlled flow of chiller fluid 34 to return line
38. Accordingly, portion 34' of chiller fluid 34 and returning
chiller fluid 34 are mixed in return line 38, as denoted by
reference numerals 34+34', when chiller system 20 (FIG. 1) operates
in low capacity mode 146 (FIG. 3).
[0064] As discussed above, heat is transferred from the warmer
condenser fluid 78 to the cooler portion 34' of chiller fluid 34.
This causes the temperature of portion 34' of chiller fluid 34 to
increase. Consequently, portion 34' causes the chiller fluid in
return line 38, i.e., mixed chiller fluid 34+34', of chiller fluid
loop 30 to establish the "false capacity demand" from the
perspective of chillers 40, so that chillers 40 will not enter into
an automatic chiller shutdown due to a freeze protection
condition.
[0065] Like low capacity controller 28 (FIG. 1), low capacity
controller 150 also includes chiller side flow valve 110 positioned
along second branch 106 of outlet conduit 154 for selectively
returning chiller fluid 34 to supply line 36 when chiller system 20
(FIG. 1) operates in the economizer mode (as discussed above in
connection with FIG. 4).
[0066] FIG. 6 shows a block diagram of a low capacity controller
160 for chiller system 20 (FIG. 1) in accordance with another
alternative embodiment of the present invention. Low capacity
controller 160 simply replaces low capacity controller 28 described
in detail above. Accordingly, all of the reference numerals and
components set forth in FIG. 1 are not repeated in FIG. 6. Rather,
only those reference numerals and components that further the
understanding of low capacity controller 160 are shown in FIG.
6.
[0067] Low capacity controller 160 includes heat exchanger 96, and
a secondary loop 162 interposed between chiller fluid section 22
(FIG. 1) and heat exchanger 96. In this alternative embodiment,
secondary loop 162 includes inlet conduit 100 in fluid
communication with return line 38 for conveying portion 34' of
chiller fluid 34 from return line 38 to heat exchanger 96.
[0068] Secondary loop 162 further includes an outlet conduit 164
having a first branch 166 selectively in fluid communication with
return line 38 and second branch 106 selectively in fluid
communication with supply line 36 of chiller fluid section 22 (FIG.
1). A three-way valve 168 controls the flow of warmed portion 34'
of chiller fluid 34 into return line 38, thus, its subsequent
mixing with returning chiller fluid 34 in return line 38, while
modulating valve 108 enables the rate controlled flow of chiller
fluid 34 to return line 38 (as discussed above. Accordingly,
portion 34' of chiller fluid 34 and returning chiller fluid 34 are
mixed in return line 38, as denoted by reference numerals
34+34'.
[0069] Like low capacity controllers 28 (FIG. 1) and 150 (FIG. 5),
low capacity controller 160 also includes chiller side flow valve
110 positioned along second branch 106 of outlet conduit 164 for
selectively returning chiller fluid 34 to supply line 36 when
chiller system 20 (FIG. 1) operates in the economizer mode (as
discussed above in connection with FIG. 4).
[0070] In summary, the present invention teaches of a method and
low capacity controller for operating a chiller system. The low
capacity controller enables a chiller system to be operated below a
minimum allowable capacity for the chiller system by warming
chiller fluid prior to its return to the chillers. This establishes
a false capacity demand at the chillers to prevent the chillers
from going into an alarm and shut-down condition. In addition, the
low capacity controller achieves a secondary benefit through its
use in a chiller bypass/economizer mode to provide direct cooling
through the utilization of the cooling effect of outside air. The
low capacity controller is readily and cost effectively
incorporated into a chiller system through the use of a plate and
frame heat exchanger, and an extension of the fluid loops to route
the warmed chiller fluid back to the return line when in the low
capacity mode and to route the cooled chiller fluid back to the
supply line when in the chiller bypass/economizer mode.
[0071] Although the preferred embodiments of the invention have
been illustrated and described in detail, it will be readily
apparent to those skilled in the art that various modifications may
be made therein without departing from the spirit of the invention
or from the scope of the appended claims. For example, the
utilization of the heat exchanger in the chiller bypass/economizer
mode may be optional. As such, the present invention may be adapted
to include a heat exchanger that utilizes a fluid other than the
condenser fluid.
* * * * *