Variable evaporator water flow compensation for leaving water temperature control

A method of controlling a refrigerant chiller system is particularly suited for chillers where the water being chilled (or some other liquid) flows through the chiller's evaporator at a flow rate that is variable and is not directly known. To effectively control the chiller and maintain the temperature of the water leaving the evaporator at a desired target temperature, the cooling capacity of the chiller's evaporator is estimated based the degree of valve opening of an expansion valve, a pressure differential across the expansion valve, and a change in enthalpy per unit mass of the refrigerant flowing through the evaporator. In some embodiments, the chiller system includes multiple refrigerant circuits that are hermetically isolated from each other.

FIELD OF THE INVENTION

The subject invention generally pertains to the control of an HVAC chiller that includes an evaporator and more specifically to a method of controlling the evaporator's cooling capacity to achieve a desired temperature of the chilled water leaving the evaporator, wherein the water flow rate through the evaporator varies.

BACKGROUND OF RELATED ART

Typical refrigerant chillers basically comprise a compressor, condenser, expansion device and an evaporator. Within the evaporator, vaporizing refrigerant cools a supply of water that is then circulated through a network of heat exchangers to meet the cooling demand of rooms or other areas of a building.

As the cooling demand varies, the flow rate of the water might be adjusted according. Doing so, however, can make it difficult to control the chiller's response in providing the evaporator with appropriate cooling capacity because the chiller's controller might not be aware of the water's rate of flow. The goal is to maintain the temperature of the water as it leaves the evaporator at a desired target temperature (e.g., 35° F.). Without knowing the flow rate of the water, the chiller might overcorrect at low water flow rates or respond too sluggishly at higher flow rates.

To address this problem, a flow meter could be added to the water circuit; however, such meters can be rather expensive. Alternatively, water pressure sensors upstream and downstream of the evaporator could be used to help determine the approximate flow rate through the evaporator, but the accuracy of such a method can vary depending on the total water head and whether the physical condition of the evaporator remains constant over years of use. The design of the evaporator and the actual flow rate of the water can also affect the accuracy of measuring flow rate based on the pressure drop across the evaporator.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for controlling a refrigerant chiller, wherein the water flows through the chiller's evaporator at a rate that is variable and is not directly known, i.e., the flow rate is not determined by sensing the water's flow rate or pressure drop.

Another object of some embodiments is to estimate the water flow rate through an evaporator based on the rate of refrigerant flowing through an expansion valve.

Another object of some embodiments is to maintain the temperature of water leaving an evaporator at a desired target outlet temperature while the water's flow rate is variable and generally unknown.

Another object of some embodiments is to estimate the estimate the cooling capacity of an evaporator based on the degree of valve opening of an expansion valve that regulates the refrigerant flow rate, a pressure differential across the expansion valve, and a change in enthalpy per unit mass of the refrigerant flowing through the evaporator.

Another object of some embodiments is to estimate cooling capacity of an evaporator without having to measure the rate at which water flows through the evaporator.

One or more of these and/or other objects of the invention are provided by a method of controlling a chiller system having variable aqueous liquid flow through an evaporator wherein flow rate is not directly known and the cooling capacity of the chiller's evaporator is estimated based the degree of valve opening of an expansion valve that regulates the refrigerant flow rate to the evaporator, a pressure differential across the expansion valve, and a change in enthalpy per unit mass of the refrigerant flowing through the evaporator.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A chiller system10, shown inFIG. 1, includes an evaporator system12that is part of at least one refrigerant circuit, such as a circuit14and/or16. Chiller system10circulates a refrigerant18through circuit14and/or16to cool an aqueous liquid20flowing through evaporator system12. Refrigerant18and liquid20are hermetically isolated from each other. A pump22forces liquid20through evaporator system12and also pumps the cooled liquid20to wherever cooling may be needed. The term, “aqueous” refers to any liquid containing at least a trace of water. Aqueous liquid20, for example, can be pure water or a mixture of water and glycol. Other examples of liquid20are certainly possible and well within the scope of the invention.

To meet a varying cooling demand, liquid20is pumped through evaporator12at various flow rates, and a controller24responsive to various sensors controls system10such that the evaporator's cooling capacity (e.g., tons) is appropriate for any given liquid flow rate. Specifically, controller24adjusts chiller system10such that the cooling capacity of evaporator system12is at a level where liquid20leaving evaporator12is kept at a predetermined target outlet temperature (e.g., 35° F.), regardless of the liquid's flow rate.

The relationship between the evaporator's cooling capacity and the resulting temperature of liquid20leaving evaporator12can be determined based on the capacity being substantially equal to the mass flow rate of liquid20through evaporator12times the liquid's specific heat times the liquid's decrease in temperature as liquid20passes through evaporator12. Although the temperature of liquid20entering and leaving evaporator12is easy to determine using temperature sensors26and28, the mass flow rate of liquid20can be difficult or expensive to measure directly. Thus, the present invention provides an alternate, novel method of estimating the evaporator's cooling capacity without actually having to measure the liquid's flow rate.

Instead of determining the evaporator's capacity as a function of the liquid's flow rate through evaporator12, the capacity is determined based on the mass flow rate of the refrigerant flowing through one or more expansion valves and the refrigerant's change in enthalpy as refrigerant18passes through evaporator12. The refrigerant's flow rate through an expansion valve can be determined based on the valve's degree of opening, the pressure drop across the valve, and known flow characteristics of the valve. This method will be described in more detail with reference to the dual-circuit chiller system shown inFIG. 1; however, the same basic method can also be readily applied to single-circuit refrigerant circuits and numerous other system configurations as well.

For the illustrated example, circuit14(also referred to as a first circuit or circuit-A) comprises a refrigerant compressor30that discharges relatively high pressure, high temperature vaporous refrigerant18into a first condenser circuit31within a condenser system32. Compressor30can be any type of compressor including, but not limited to, a centrifugal compressor, screw compressor, scroll compressor, reciprocating compressor, etc. Condenser system32can be a single or duplex shell and tube heat exchanger with a cooling fluid34being conveyed through the tubes and refrigerant18passing through the shell across the tubes. As refrigerant18passes across the tubes, the refrigerant being in heat transfer relationship with fluid34condenses within the shell of condenser system32.

Downstream of condenser32, first circuit14has an expansion valve36(also referred to as a first expansion valve or a valve-A). The portion of circuit14that is downstream of compressor30and upstream of expansion valve36is referred to as a high-pressure side14aof circuit14. Expansion valve36provides an adjustable flow restriction that conveys refrigerant18from condenser circuit31to evaporator system12. Upon passing through valve36at a regulated mass flow rate, refrigerant18cools by expansion and then enters a first evaporator circuit38of evaporator system12. Evaporator system12can be a single or duplex shell and tube heat exchanger with liquid20being conveyed through the tubes and cooler refrigerant18passing through the shell across the tubes. As the relatively cool refrigerant18passes across the tubes, the refrigerant vaporizes upon cooling liquid18. After vaporizing, refrigerant18returns to a suction inlet40of compressor30to perpetuate the cycle of first circuit14. The portion of circuit14that is downstream of expansion valve36and upstream of compressor30is referred to as a low-pressure side14bof circuit14.

Likewise, second circuit16(also referred to as a second circuit or circuit-A) comprises a refrigerant compressor42(e.g., one similar to compressor30) that discharges relatively high pressure, high temperature vaporous refrigerant18into a second condenser circuit44within condenser system32. For this particular embodiment of the invention, circuits14and16each have their own separate charge of refrigerant, and the two charges do not mix with each other. With condenser system32being a shell and tube heat exchanger, as refrigerant18passes across the tubes and through the shell, the refrigerant is cooled by fluid34and condenses within the shell of condenser system32.

Downstream of condenser circuit44, second circuit16has an expansion valve46(also referred to as a second expansion valve or a valve-B). The portion of circuit16that is downstream of compressor42and upstream of expansion valve46is referred to as a high-pressure side16aof circuit16. Expansion valve46provides an adjustable flow restriction that conveys refrigerant18from second condenser circuit44to evaporator system12. Upon passing through valve46at a regulated mass flow rate, refrigerant18cools by expansion and then enters a second evaporator circuit48of evaporator system12. With evaporator system12being a shell and tube heat exchanger, the relatively cool refrigerant18passing across the tubes vaporizes upon cooling liquid20. After vaporizing, refrigerant18returns to a suction inlet50of compressor42to perpetuate the cycle of second circuit16. The portion of circuit16that is downstream of expansion valve46and upstream of compressor42is referred to as a low-pressure side16bof circuit16.

Although liquid20chilled within evaporator system12can be used for various purposes, system10is particularly suited for conveying chilled liquid20through a liquid circuit52that includes a network of heat exchangers54. It should be appreciated by those of ordinary skill in the art, however, that liquid circuit52is for sake of example and that countless other liquid circuit configurations are certainly possible and well within the scope of the invention. Nonetheless, in this example, heat exchangers54can each be associated with a fan56for supplying cool supply air to various comfort zones, such as rooms or other designated areas of a building. Control valves58upstream or downstream of heat exchangers54regulate the amount of cool liquid flowing to each heat exchanger54, thus valves58control the amount of cooling that each heat exchanger54provides.

As the total cooling demand applied to heat exchanger's54varies, the liquid mass flow rate through evaporator12is adjusted accordingly. This can be done by driving pump22with a variable speed motor, adding a variable bypass valve60in parallel with pump22, using a variable volume pump, or using various other adjustable flow means well known to those of ordinary skill in the art.

As liquid circuit52applies a varying load to refrigerant system10, controller24adjusts the operation of chiller system10such that evaporator system12has a cooling capacity that maintains the liquid leaving evaporator12at a predetermined target outlet temperature. Depending on the specific chiller system, the chiller's operation might be adjusted by various means including, but not limited to, adjusting the speed of one or more compressors, selectively operating and de-energizing multiple compressors, adjusting a centrifugal compressor's inlet guide vanes, adjusting a screw compressor's slide valve, adjusting the temperature or flow rate of a fluid cooling the refrigerant in a condenser, adjusting the degree of opening of one or more expansion valves, and/or various combinations thereof.

For the illustrated example, controller24operates according to an algorithm62ofFIG. 2. In control block64, controller24energizes compressors30and/or42to activate circuits14and/or16respectively.

In block66, controller24calculates a first capacity value (e.g., tons) representative of an estimate of the first capacity at which circuit14provides cooling in evaporator system12. The first capacity value is calculated as a function of a degree of valve opening of first expansion valve36, a pressure differential of the refrigerant between high pressure side14aand low pressure side14b,and a change in enthalpy per unit mass of refrigerant18flowing through evaporator circuit38of evaporator system12.

For accuracy, the pressure differential between high side14aand low side14bpreferably is sensed right at expansion valve36; however, the pressure differential can be sensed at other locations. Sensing the pressure differential is depicted by pressure sensors84and86providing controller24with pressure feedback signals78and80. The sensing of the pressure differential is schematically illustrated, and the actual sensing of these pressures could be achieved by a single differential pressure sensor that conveys a single differential pressure signal to controller24.

For sake of example, expansion valve36can be a Sporlan Y1187-1-SEH1-175 valve that is stepper-motor driven. Thus, the degree of opening of expansion valve36is known or can at least be determined by controller24because controller24is what provided an output signal76that commanded expansion valve36to open a certain degree in the first place. Alternatively, an encoder or some other suitable position feedback device could be added to expansion valve36, and such a device could provide controller24with a feedback signal that indicates the valve's degree of opening.

The refrigerant's change in enthalpy per unit mass as refrigerant18passes through evaporator12can be approximated and considered generally constant. For greater accuracy, however, the approximate change in enthalpy can be calculated based on various thermodynamic values such as, for example, the saturated vapor pressure in evaporator circuit38, the saturated liquid temperature of condenser circuit31, the temperature of fluid32entering condenser circuit31, and various combinations thereof. Converting pressure and/or temperature values to enthalpy can be done with reference to commonly known thermodynamic equations or lookup tables stored in controller24.

Controller24calculates the refrigerant's mass flow rate based on the known degree of opening of expansion valve36(output signal76), the sensed pressure differential across valve36(feedback signals80and84), the approximate known density of liquid refrigerant18, and the known flow characteristics of valve36(i.e., the valve's rated or empirically derived flow coefficient Cv).

In block90, controller24calculates a second capacity value (e.g., tons) representative of an estimate of the second capacity at which circuit16provides cooling in evaporator system12. The second capacity value is calculated as a function of a degree of valve opening of second expansion valve46, a pressure differential of the refrigerant between high pressure side16aand low pressure side16b,and a change in enthalpy per unit mass of refrigerant18flowing through evaporator circuit48of evaporator system12.

Again, for accuracy, the pressure differential between high side16aand low side16bpreferably is sensed right at expansion valve46; however, the pressure differential can be sensed at other locations. Sensing the pressure differential is depicted by pressure sensors108and106providing controller24with pressure feedback signals102and106. The sensing of the pressure differential is schematically illustrated, and the actual sensing of these pressures could be achieved by a single differential pressure sensor that conveys a single differential pressure signal to controller24.

Although expansion valves36and46do not necessarily have to be the same, expansion valve46can be another Sporlan Y1187-1-SEH1-175 valve. Thus, the degree of opening of expansion valve46is also known or can at least be determined by controller24because controller24is what provided an output signal100that commanded expansion valve46to open a certain degree in the first place. Alternatively, an encoder or some other suitable position feedback device could be added to expansion valve46, and such a device could provide controller24with a feedback signal that indicates the valve's degree of opening.

The refrigerant's change in enthalpy per unit mass as refrigerant18passes through evaporator12can be approximated and considered generally constant. For greater accuracy, however, the approximate change in enthalpy can be calculated based on various thermodynamic values such as, for example, the saturated vapor pressure in evaporator circuit48, the saturated liquid temperature of condenser circuit44, the temperature of fluid32entering condenser circuit44, and various combinations thereof. Converting pressure and/or temperature values to enthalpy can be done with reference to commonly known thermodynamic equations or lookup tables stored in controller24.

Controller24calculates the refrigerant's mass flow rate based on the known degree of opening of expansion valve46(output signal100), the sensed pressure differential across valve46(feedback signals106and102), the approximate known density of liquid refrigerant18, and the known flow characteristics of valve46(i.e., the valve's rated or empirically derived flow coefficient Cv).

In block114, controller24calculates a total capacity value, which in this example is the sum of the two capacity values determined in blocks66and90. If a refrigerant system were to have more than two refrigerant circuits, then the total capacity value would be the sum of all the individual capacity values of those circuits. If a refrigerant system had only one active refrigerant circuit, then the total capacity value would equal the capacity value of that one circuit. For some chiller systems, the calculated capacity values can be adjusted by an empirically derived adjustment factor so that the calculated capacity value more closely reflects the actual cooling capacity of the chiller's evaporator.

In block116, controller24receives temperature feedback signals118and120from temperature sensors28and26that sense the liquid's temperature as liquid20enters and leaves evaporator12. Based on signals118and120, controller24determines the liquid's drop in temperature as liquid20passes through evaporator12.

In block122, controller24notes the relationship between the liquid's temperature differential (block116) and the computed cooling capacity value of evaporator12(block114).

In block124, a predetermined target outlet temperature of liquid20leaving evaporator12is established.

Upon knowing the relationship of the liquid's change in temperature and the cooling capacity of evaporator12, in block126controller24adjusts the operation of chiller system10to achieve an evaporator cooling capacity that drives the liquid's leaving temperature (signal118) to the predetermined target temperature. Depending on the design of the chiller, adjusting the chiller's operation can involve adjusting the operation of compressor30and/or42, adjusting the position of inlet guide vanes, adjusting a screw compressor's slide valve, and/or adjusting the opening of expansion valve36and/or46.

Although the invention is described with respect to a preferred embodiment, modifications thereto will be apparent to those of ordinary skill in the art.