SYSTEM AND METHOD OF HOT GAS DEFROST CONTROL FOR MULTISTAGE CASCADE REFRIGERATION SYSTEM

Abstract

The present invention provides a system and method for an improved multistage, cascade refrigeration system using hot gas defrost to rid the evaporator of ice build-up which accumulates over time, while the air in the evaporator enclosure remains below the freezing point of water. The present invention thus provides greater defrost flexibility with increased ease of design and implementation than current refrigeration systems, which allows for more robust hot gas defrost function for multistage refrigeration systems, such that it is unaffected by temperature changes of the condensing fluid (ambient air temperature for air cooled condensers, water temperature for water cooled condensers), and can be readily adapted to any refrigerant suitable for a selected temperature range.

Claims

1-20. (canceled)

21. A multistage, cascade refrigeration system comprising: a first stage and a second stage thermally connected via a central heat exchanger, the first stage including a first stage liquid expansion valve and a first stage compressor, and the second stage including a second stage liquid expansion valve, a second stage hot gas valve, and a second stage compressor; and a controller operably coupled to the first and second stage compressors, the first and second stage expansion liquid valves, and the second stage hot gas valve, the controller configured to: operate the multistage, cascade refrigeration system in a hot gas defrost cycle, shut off the first stage compressor during at least a portion of the hot gas defrost cycle, fully close the first and second stage liquid expansion valves during at least a portion of the hot gas defrost cycle, and open the second stage hot gas valve during the hot gas defrost cycle.

22. The multistage, cascade refrigeration system of claim 21, further comprising a pressure sensing device, the pressure sensing device configured to monitor a suction pressure of a refrigerant associated with the first stage compressor; and wherein the controller is further operably coupled to the pressure sensing device, and the controller is further configured to: receive an indication of the suction pressure of the refrigerant associated with the first stage compressor from the pressure sensing device, and control the first stage liquid expansion valve based on the suction pressure of the refrigerant associated with the first stage compressor during at least a portion of the hot gas defrost cycle.

23. The multistage, cascade refrigeration system of claim 22, wherein the control circuitry configured to control the first stage liquid expansion valve based on the suction pressure is further configured to control the first stage liquid expansion valve when the hot gas defrost cycle is initiated to allow the suction pressure of the refrigerant associated with the first stage compressor to equalize and close the first stage liquid expansion valve when a target pressure is reached.

24. The multistage, cascade refrigeration system of claim 21, wherein the control circuitry configured to shut off the first stage compressor is further configured to shut off the first stage compressor when the hot gas defrost cycle is initiated.

25. The multistage, cascade refrigeration system of claim 21, further comprising a temperature sensing device, the temperature sensing device configured to monitor a temperature associated with the central heat exchanger; and wherein the controller is further operably coupled to the temperature sensing device, and the controller is further configured to: receive an indication of the temperature associated with the central heat exchanger, and control the first stage compressor based on the temperature associated with the central heat exchanger during at least a portion of the hot gas defrost cycle.

26. The multistage, cascade refrigeration system of claim 25, wherein the temperature associated with the central heat exchanger is a refrigerant temperature associated with a refrigerant in the first stage at the central heat exchanger, and wherein the controller configured to control the first stage compressor is further configured to operate the first stage compressor to maintain the refrigerant temperature associated with the refrigerant in the first stage at the central heat exchanger to be below a certain temperature.

27. The multistage, cascade refrigeration system of claim 21, wherein the controller includes a first stage superheat board, a second stage superheat board, and a system controller.

28. The multistage, cascade refrigeration system of claim 21, wherein the second stage hot gas valve controls the flow of a superheated vapor from the second stage compressor to a second stage evaporator coil enclosure during the hot gas defrost cycle.

29. The multistage, cascade refrigeration system of claim 21, wherein the controller is further configured to initiate the operation of the hot gas defrost cycle based on a timer.

30. The multistage, cascade refrigeration system of claim 21, wherein the controller is further configured to initiate the operation of the hot gas defrost cycle based on one or more of the following: suction pressure data, suction temperature data, PWM duty cycle data, data regarding quantity of door openings recorded from switches, and compressor cycle rate data.

31. A multistage, cascade refrigeration system comprising: a first stage and a second stage thermally connected via a central heat exchanger, the first stage including a first stage liquid expansion valve and a first stage compressor, and the second stage including a second stage liquid expansion valve, a second stage hot gas valve, and a second stage compressor; a temperature sensing device configured to monitor a temperature associated with the central heat exchanger; a pressure sensing device configured to monitor a suction pressure of a refrigerant associated with the first stage compressor; and a controller operably coupled to the first and second stage compressors, the first and second stage expansion liquid valves, the temperature sensing device, the pressure sensing device, and the second stage hot gas valve, the controller configured to: operate the multistage, cascade refrigeration system in a hot gas defrost cycle, receive an indication of the temperature associated with the central heat exchanger from the temperature sensing device, receive an indication of the suction pressure of the refrigerant associated with the first stage compressor from the pressure sensing device, control the first stage liquid expansion valve when the hot gas defrost cycle is initiated to allow the suction pressure of the refrigerant associated with the first stage compressor to equalize and close the first stage liquid expansion valve when a target pressure is reached, and open the second stage hot gas valve during the hot gas defrost cycle.

32. The multistage, cascade refrigeration system of claim 31, wherein the control circuitry is further configured to shut off the first stage compressor when the hot gas defrost cycle is initiated.

33. The multistage, cascade refrigeration system of claim 31, wherein the controller is further configured to control the first stage compressor based on the temperature associated with the central heat exchanger during at least a portion of the hot gas defrost cycle.

34. The multistage, cascade refrigeration system of claim 33, wherein the temperature associated with the central heat exchanger is a refrigerant temperature associated with a refrigerant in the first stage at the central heat exchanger, and wherein the controller is further configured to operate the first stage compressor to maintain the refrigerant temperature associated with the refrigerant in the first stage at the central heat exchanger to be below a certain temperature.

35. The multistage, cascade refrigeration system of claim 31, wherein the controller is further configured to shut off the first stage compressor during at least a portion of the hot gas defrost function, and fully close the first and second stage liquid expansion valves during at least a portion of the hot gas defrost function.

36. The multistage, cascade refrigeration system of claim 31, further comprising a second stage pressure sensing device, the second stage pressure sensing device configured to monitor a pressure associated a refrigerant in the second stage; and wherein the controller is further operably coupled to the second stage pressure sensing device, and the controller is further configured to: receive an indication of the pressure associated with the refrigerant in the second stage, and control the hot gas valve during the hot gas defrost cycle based on the pressure associated with the refrigerant associated with the second stage.

37. An improved method for a hot gas defrost cycle for a multistage, cascade refrigeration system, the multistage, cascade refrigeration system comprising a first stage and a second stage thermally connected via a central heat exchanger, and the improved method comprising: operating the multistage, cascade system in a hot gas defrost cycle, the hot gas defrost cycle configured to remove ice build-up on a second stage evaporator coil while maintaining air configured to flow across the second stage evaporator coil below the freezing point of water; shutting off a first stage compressor during at least a portion of the hot gas defrost cycle, the first stage compressor configured to circulate refrigerant within the first stage; fully closing a first stage liquid expansion valve and a second stage liquid expansion valve during at least a portion of the hot gas defrost cycle, the first stage liquid expansion valve coupled to the first stage and configured to control the flow of refrigerant within the first stage, and the second stage liquid expansion valve coupled to the second stage and configured to control the flow of refrigerant within the second stage; and opening a second stage hot gas valve during the hot gas defrost cycle, the second stage hot gas valve configured to control the flow of a superheated vapor from the second stage compressor to the second stage evaporator coil enclosure during the hot gas defrost cycle.

38. The improved method of claim 37, further comprising: receiving an indication of a suction pressure of a refrigerant associated with a first stage compressor from a pressure sensing device, and controlling the first stage liquid expansion valve based on the suction pressure of the refrigerant associated with the first stage compressor during at least a portion of the hot gas defrost cycle.

39. The improved method of claim 38, wherein controlling the first stage liquid expansion valve based on the suction pressure further includes controlling the first stage liquid expansion valve when the hot gas defrost cycle is initiated to allow the suction pressure of the refrigerant associated with the first stage compressor to equalize and closing the first stage liquid expansion valve when a target pressure is reached.

40. The improved method of claim 39, wherein shutting off a first stage compressor further includes shutting off the first stage compressor when the hot gas defrost cycle is initiated.

41. the improved method of claim 37, further comprising: receiving an indication of a pressure of a refrigerant associated with the second stage; and controlling the hot gas valve during the hot gas defrost cycle based on the temperature associated with the pressure associated with the second stage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] For the present disclosure to be easily understood and readily practiced, the present disclosure will now be described for purposes of illustration and not limitation in connection with the following figure, wherein:

[0015] FIG. 1 depicts one potential embodiment of a multistage, cascade refrigeration system of the present invention, containing two refrigeration systems linked through a central heat exchanger, a system controller, and superheat control boards.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention is not limited in application by the use of specific components outlined in the following descriptions or drawing as there may be various methods of control and refrigerant valving in many different embodiments. The terminology utilized throughout this description should also be understood to not be limiting, as it is only intended to be used in the efforts of describing the operation of the system and methodology.

[0017] The present invention relates generally to the function and ability of a refrigeration system using hot gas defrost to rid the evaporator coil enclosure of ice build-up which accumulates over time, while the air in the evaporator coil enclosure remains below the freezing point of water. This is especially important for systems whose operating temperatures require multiple refrigerants and refrigeration systems to achieve the desired temperatures, usually below −40° C. At these extreme temperatures, defrosting the evaporator coil can be quite difficult.

[0018] Referring now to FIG. 1, which illustrates an overview of an embodiment of the multistage, cascade refrigeration system (100) of the present invention, in the depicted embodiment, the first stage system (101) suction pressure transducer (107) and second stage (102) suction pressure transducer (114), as well as the suction line temperatures of the first stage (108) and second stage (115) are monitored and act as inputs to the controlling functions provided by the superheat boards (116, 117). The superheat boards SB-1 and SB-2 respectively (116, 117), in turn, control the electronic liquid refrigerant expansion control valves, such as, for example PWM valves, in their respective stages. SB-1 (116) controls the refrigerant flow of the first stage liquid expansion PWM valve (105). SB-2 (117) controls the refrigerant flow of the second stage liquid expansion PWM valve (112) and the second stage hot gas PWM valve (111). Beyond the refrigerant flow control capabilities, the first stage system is controlled by an overall system controller (118) which can be any number of possible control configurations using relays, contactors, PLC's, and other technologies commonly known by those skilled in the art. The first stage system has a compressor (103) which provides the mass flow and pressure differential of the system. The compressed superheated vapor produced by the compressor is cooled and condensed to liquid in the condenser (104) before that liquid is expanded through the PWM valve (105). The expanded refrigerant then absorbs energy from the central heat exchanger (106) and returns to the compressor as superheated vapor. The second stage operates similarly, using the system controller (118) to start and stop its functions and components, including the compressor (110). The compressed superheated gas from the compressor is cooled and condensed in the central heat exchanger (106), whose overall temperature is monitored by a temperature sensing device (109), which could include but is not limited to thermocouples (TC) and resistance temperature detector (RTD). The cooled liquid is expanded through the liquid expansion PWM valve (112) and moves to the evaporator housing (113) where energy is absorbed. The hot gas PWM valve (111) controls the flow of superheated vapor from the compressor to the evaporator coil enclosure (113) during a hot gas defrost cycle.

[0019] The initialization of the hot gas defrost cycle on an evaporator coil (113) can be done through several methods including, but not limited to, a timer where a period is chosen, or other more robust and active methodologies utilizing the information and data sent to the system controller (118). This information can include, but is not limited to, data such as suction pressure, suction temperature, PWM duty cycle, quantity of door openings recorded from switches, along with other performance factors such as compressor cycle rate. The data is then analyzed through various conditions and algorithms in the system controller (118), which can determine when a defrost cycle is required.

[0020] The design phase of the hot gas system of the present invention includes an in-depth analysis of the refrigeration properties of the refrigerants chosen for the application. The principles of saturated temperature and pressure are at the core of the present invention, utilizing the principles of the Ideal Gas Law and the thermodynamic properties of pressure and enthalpy, such as is described in refrigerant P-H diagrams and tables common to those skilled in the art. Using the refrigerant properties data, the refrigerants of the first and second stage are analyzed, and a temperature stability point is targeted, such that the same temperature results in desirable pressures for each refrigerant.

EXAMPLE

[0021] The present invention is more particularly described in the following non-limiting example, which is intended to be illustrative only, as numerous modifications and variations therein will be apparent to those skilled in the art.

Example

Hot Gas Defrost Cycle Operation

[0022] In this example, refrigerant R-404a is used in the first stage, and refrigerant R-508B is used in the second stage, given a design criterion where a refrigeration system must operate at −80° C. in the evaporator enclosure. As during normal operating conditions, the compressor operating conditions during defrost are critical to performance and longevity. As such, operational limits exist to which we must adhere. The upper limit of compressor operation can vary but is generally set by compressor manufacturers and industry experts to be 300 PSIA of discharge pressure and a compression ratio of around 10:1 between suction pressure and discharge pressure. If operating at 300 PSIA, any liquid R-508B that exists in the central heat exchanger, would be at a saturation temperature of almost −11° C. The lower limit of compressor operation exists only in the form of how quickly and efficiently we would like the hot gas defrost to operate. Product testing has indicated this defrost cycle discharge pressure to be about 200 PSIA of R-508B, yielding a saturation temperature of about −25° C.

[0023] This target temperature range for the R-508B, as defined by its refrigerant properties, becomes congruent with the desired temperature of the entire central heat exchanger, and it must be maintained within the given temperature window of −11° to −25° C. for a proper defrost to occur on the second stage. Targeting a temperature in the middle of the range, such as −16° C., and using the same principles of saturated temperature and pressure, the first stage refrigerant is analyzed. At −16° C., R-404 has a saturation pressure of about 50 PSIA. When the defrost cycle is initiated, the system controller turns off the first stage compressor and allows the pressure to equalize through the PWM valve. The superheat board monitors the first stage suction pressure transducer and closes the first stage PWM valve when the defined 50 PSIA suction pressure is reached. By maintaining the pressure of R-404A in the central heat exchanger, the pool of liquid refrigerant becomes a maintained mass, stabilizing its temperature around the targeted −16° C. The temperature of the central heat exchanger is measured by a temperature sensing device, and monitored by the system controller.

[0024] With the temperature of the central heat exchanger defined by first stage suction pressure of 50 PSIA and −16° C., that same temperature correlates to the discharge pressure for the second stage system. When the temperature increases, the second stage discharge pressure rises and when the temperature decreases, the discharge pressure also decreases, following the principles of the Ideal Gas Law. When the second stage refrigerant R-508B is saturated at −16° C., the corresponding pressure is about 260 PSIA, which is less than the defined 300 PSIA maximum. When a hot gas defrost is called for by the refrigeration system controller, the second stage compressor continues to run, the second stage superheat board closes the liquid expansion PWM valve and begins controlling the hot gas PWM valve. During the hot gas cycle, the second superheat board monitors the second stage suction pressure transducer, and uses that input to modulate the second stage hot gas PWM valve. Keeping in mind the desired compression ratio limit of 10:1 and 300 PSIA maximum discharge pressure, the targeted suction pressure is set at 30 PSIA and is controlled by the modulation of the hot gas PWM valve. This modulation continues throughout the cycle, continually targeting 30 PSIA suction pressure, allowing for safe operation of the second stage compressor, and eliminating the need for a separate crankcase pressure regulator (CPR) valve and other hardware common to those skilled in the art.

[0025] The first stage suction pressure will continue to be maintained and the second stage compressor will continue to run throughout the defrost process. Typical temperature measurement methods such as RTD's and TC's will act as inputs to the controller, signaling when the evaporator coil has become adequately defrosted, triggering the end of the defrost cycle. Should the temperature of the central heat exchanger rise to a point outside the defined operating limit of −11° C., the controller will re-engage the first stage compressor the super heat board will begin controlling the first PWM for liquid again, and the system will pull the central heat exchanger temperature back down to the defined set point of −16° C. Once reached, the compressor will once again shut off and the PWM valve will again allow pressure to equalize until it reaches the defined 50 PSIA set point. The first stage PWM will again be closed by the first superheat board and the pressure maintained to stabilize the heat exchanger. In such an event, as the temperature approaches −11° C., the discharge pressure of the second stage would continue to climb towards our upper limit of 300 PSIA. When the temperature is brought back down to −16° C. or colder, the pressure in the second stage will also drop back into the middle of the operating range.

[0026] The operating conditions used in this example would change with different selections of refrigerants in either the first or second stages of the system, due to the change of the refrigerant properties and points of saturated temperature and pressure. With correct operating conditions however, this methodology would be valid for any suitable hydrofluorocarbon (HFC) or hydrocarbon (HC) refrigerants chosen in a multistage, cascade refrigeration system.

[0027] While the invention has been particularly shown and described with reference to embodiments described above, it will be understood by those skilled in the art that various alterations in form and detail may be made therein without departing from the spirit and scope of the invention, as defined by the appended claims.