Patent Application
20120198867
This invention relates generally to refrigerant vapor compression systems, such as refrigeration systems and air conditioning systems. More particularly, this invention relates to controlling the dehumidification of air or an air-gas mixture being conditioned for supply to a climate controlled space.
Refrigerant vapor compression systems are well known in the art and are commonly used for conditioning air to be supplied to a climate controlled space. For example, refrigerant vapor compression systems are used for conditioning air to be supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression systems are also used in refrigerating air supplied to display cases, merchandisers, freezer cabinets, cold rooms or other perishable/frozen product storage area in commercial establishments.
Refrigerant vapor compression systems are also commonly used in transport refrigeration systems for refrigerating air or an air-gas mixture supplied to a cargo storage space of a truck, trailer, container or the like for transporting perishable/frozen items by truck, rail, ship or intermodally. Refrigerant vapor compression systems used in connection with transport refrigeration systems are generally subject to more stringent operating conditions due to the wide range of operating load conditions and the wide range of outdoor ambient conditions over which the refrigerant vapor compression system must operate to maintain product within the cargo space at a desired temperature. The desired temperature and relative humidity within the cargo space at which the cargo needs to be controlled can also vary over a wide range depending on the nature of cargo to be preserved. Consequently, in transport refrigeration systems, it is desirable that the refrigerant vapor compression system be capable of controlling relative humidity to a desired level for the perishable product being shipped, typically in the range of from about 50% relative humidity to about 75% RH, over a wide range of product transport temperatures, for example from −5° C. (23° F.) to 25° C. (77° C.). At the high end of temperature range, the control of relative humidity to the desired low levels becomes more challenging.
A method is provided for controlling dehumidification of an airflow to be conditioned for supply to a climate controlled space. The method includes the steps of: passing the airflow to be conditioned over a plurality of refrigerant conveying conduits of an evaporator of a refrigerant vapor compression thereby cooling the airflow; operating the refrigeration vapor compression system to maintain the airflow at a set point air temperature indicative of a desired temperature within the climate controlled space; and adjusting the evaporator expansion device so as to reduce the temperature of the refrigerant within the refrigerant conveying conduits of the evaporator whenever further dehumidification of the air flow to be conditioned is desired. The step of operating the refrigerant vapor compression system to maintain said airflow at a set point air temperature indicative of a desired temperature within the climate controlled space may include the step of reheating said airflow having passed over the plurality of refrigerant conveying conduits of the evaporator prior to supplying said airflow to the climate controlled space. The step of adjusting the evaporator expansion device so as to reduce the temperature of the refrigerant within the refrigerant conveying conduits of the evaporator may be implemented if a sensed relative humidity reflective of the relative humidity within the climate controlled space exceeds a desired set point relative humidity. The set point relative humidity may lie in the range from a preselected relative humidity minus a preselected tolerance to the preselected relative humidity plus the preselected tolerance, inclusive of the end point relative humidities.
The step of adjusting the evaporator expansion device so as to reduce the temperature of the refrigerant within the refrigerant conveying conduits of the evaporator may include reducing a refrigerant flow area through the expansion device.
The step of adjusting the expansion device so as to reduce the temperature of the refrigerant within the refrigerant conveying conduits of the evaporator may include increasing a pressure drop in the refrigerant passing through the expansion device.
The step of adjusting the expansion device so as to reduce the temperature of the refrigerant within the refrigerant conveying conduits of the evaporator may include decreasing a flow rate of refrigerant passing through the expansion.
The step of adjusting the expansion device so as to reduce the temperature of the refrigerant within the refrigerant conveying conduits of the evaporator may include setting a superheat setting of the expansion device to a maximum superheat setting.
An apparatus for supplying conditioned air to a climate controlled space includes a refrigerant vapor compression system including a compression device, a refrigerant heat rejection heat exchanger, an electronic expansion valve, and an evaporator heat exchanger arranged in series refrigerant flow relationship in accordance with a refrigerant vapor compression cycle; an evaporator fan for passing a flow of air to be conditioned over a plurality of refrigerant conveying conduits of the evaporator heat exchanger in heat exchange relationship with the refrigerant thereby cooling the airflow flow of air to be conditioned; and a controller operatively associated with the refrigerant vapor compression system. The controller is configured to operate the refrigeration vapor compression system to maintain the airflow at a set point air temperature indicative of a desired temperature within the climate controlled space, and to adjust the electronic expansion valve so as to reduce the temperature of the refrigerant within the refrigerant conveying conduits of the evaporator whenever further dehumidification of the air flow to be conditioned is desired. In an embodiment, the controller adjusts a superheat setting of the electronic expansion valve to a maximum superheat setting so as to reduce the temperature of the refrigerant within the refrigerant conveying conduits of the evaporator.
For a further understanding of the disclosure, reference will be made to the following detailed description which is to be read in connection with the accompanying drawing, wherein:
Referring initially to
Additionally, as used throughout this application, the terms “air” and “airflow” are intended to include and refer to not only air, but also air-gas mixtures. For example, without limiting the scope of air-gas mixture, the terms “air” and “airflow” includes air-gas mixtures containing air and one or more of carbon dioxide gas, nitrogen gas, and ethylene gas. Such air-gas mixtures may be used in connection with establishing a desired controlled atmosphere within the cargo storage space of a container or the like for transporting certain perishable/frozen items by truck, rail, ship or intermodally container.
The refrigerant vapor compression system 100 includes a compressor 20, a condenser 30 including a condenser heat exchanger 32 and associated condenser fan(s) 34, an evaporator 40 including an evaporator heat exchanger 42 and associated evaporator fan(s) 44, and an expansion device 50 associated with the evaporator 40, connected in a refrigeration cycle in a conventional manner by refrigerant lines 2, 4 and 6, which collectively define a refrigerant flow circuit. As in conventional practice, refrigerant line 2 connects the discharge outlet of the compressor 20 in refrigerant flow communication with the inlet to the condenser heat exchanger 32, refrigerant line 4 connects the outlet of the condenser heat exchanger 32 in refrigerant flow communication with the inlet to the evaporator heat exchanger 42, and refrigerant line 6 connects the outlet of the evaporator heat exchanger 42 in refrigerant flow communication with the suction inlet of the compressor 20, thereby completing the primary refrigerant flow circuit. The expansion device 50 is disposed in refrigerant line 4 in operative association with and upstream with respect to refrigerant flow of the inlet to the evaporator heat exchanger 42. The expansion device 50 comprises an expansion device having a selectively variable flow area therethrough, such as an electronic expansion valve (EXV), whereby the amount of flow of refrigerant through the evaporator heat exchanger 42 may be controlled by selectively varying the flow area through the expansion device 50.
The compressor 20 is depicted in the exemplary embodiment as a scroll compressor. However, the compressor 20 may be a reciprocating compressor or other type of compressor, the particular type of compressor used not being germane to or limiting of the invention. In the depicted embodiment, each of the condenser heat exchanger 32 and the evaporator heat exchanger 42 comprises one or more refrigerant-conveying heat exchange tube coils formed of conventional round tubes. However, it is to be understood that either or both of the condenser heat exchanger 32 and the evaporator heat exchanger 42 may comprise other forms of heat exchangers such as for example a tube bank heat exchanger formed of one or more banks of flat, multiple flow channel, refrigerant-conveying tubes extending between an inlet header and an outlet header. The particular type of heat exchanger used for either of the condenser heat exchanger 32 or the evaporator heat exchanger 42 is not germane to or limiting of the invention.
As in conventional practice, the refrigerant vapor compression system 100 may also include a compressor unloader valve 25 disposed in a compressor unload refrigerant line 22 that connects the discharge chamber of the compressor 20 in refrigerant flow communication with the refrigerant line 6 between the outlet of the evaporator heat exchanger 42 and the suction inlet to the compressor 20. In addition, the refrigerant vapor compression system 100 may include other components such as, but not limited to, a filter/drier 16 and a receiver 18 disposed in refrigerant line 4 upstream with respect to refrigerant flow of the economizer 60.
The refrigerant vapor compression system 100 may, if desired, as in the embodiment depicted in
When the economizer circuit flow control valve 63 is open, refrigerant having traversed the condenser heat exchanger 32 passes from refrigerant line 4 into refrigerant line 8, thence through the economizer expansion device 65 wherein the refrigerant is expand to a lower pressure, lower temperature before passing through the second pass 64 of the refrigerant-to-refrigerant economizer heat exchanger 60. In traversing the second pass 64, the lower temperature refrigerant passes in heat exchange relationship with the higher temperature refrigerant passing through the first pass 62, whereby the refrigerant passing through the first pass 62 is cooled before passing on through refrigerant line 4 to the evaporator expansion device 50 and the refrigerant passing through the second pass 64 is heated and typically at least partially, if not fully, evaporated, before continuing through the economizer circuit refrigerant line 8 to be injected into an intermediate stage of the compression process. The first refrigerant pass 62 and the second refrigerant pass 64 of the refrigerant-to-refrigerant heat exchanger economizer 60 may be arranged in a counter flow heat exchange relationship, as depicted in
The refrigeration unit includes a controller 150 that is operatively associated with and forms part of the refrigerant vapor compression system 100. As shown in the depicted embodiment, the controller 150 may include a microprocessor 154 and its associated memory 156, as well as an input/output interface 152 with an associated analog-to-digital converter 158. The memory 156 of controller 150 may contain operator or owner preselected, desired values for various operating parameters within the system, including, but not limited to temperature set points for various parameters within the system 100 or the climate controlled space 110, pressure limits, current limits, and any variety of other desired operating parameter set points or limits associated with the refrigeration unit or its operation. The input/output interface 152 includes various drive circuits or field effect transistors (“FETs”) and relays which, under the command of the controller 150, communicate with and/or manipulate various devices in the refrigerant vapor compression system 100, including without limitation: drives motors (not shown) operatively associated with the compressor 20, the condenser fan(s) 34 associated with the condenser heat exchanger coil 32, and the evaporator fan(s) 44 associated with the evaporator heat exchanger coil 42; various valves, such as the compressor unloader valve 25, the economizer expansion device 50, and the economizer solenoid valve 63; and various pressure sensors, for example pressure transducers, and temperature sensors such as a compressor discharge pressure transducer 101, a compressor suction pressure transducer 103, an evaporator pressure transducer 105, a compressor discharge temperature sensor 107, an evaporator outlet refrigerant temperature sensor 109, a box air temperature sensor 113, a humidity sensor 115, an ambient air sensor 117, a return air sensor 119 and such other sensors as desired.
The controller 150 is configured to operate the refrigerant vapor compression system 100 to maintain a temperature and humidity controlled environment within the climate controlled space 110, which in the embodiment described herein constitutes the cargo box of the truck, trailer or container wherein the product is stored. As in conventional practice, the controller 150 maintains a predetermined box temperature, TSPACE, by selectively controlling the operation of the compressor 20, the condenser fan(s) 34 associated with the condenser heat exchanger coil 32, and the evaporator fan(s) 44 associated with the evaporator heat exchanger coil 42, as well as selectively opening or closing the compressor unloader valve 25 and the economizer solenoid valve 63.
In operation, when the temperature of the product within the space 110 is above a predetermined temperature, for example when the product has been loaded into the box in a “hot” condition, such as produce loaded directly from the field, the controller 150 will operate the refrigerant vapor compression system in a pull-down mode so as to provide maximum or near maximum cooling capacity to rapidly pull-down the temperature, TSPACE, of climate-controlled space 110 to a temperature within a preset tolerance, ΔT, typically about 0.25° C. (0.45° C.), of a desired set point temperature, TSP, that represents the desired temperature at which the product loaded into the box is to be transported. Typically, in the pull-down mode, the controller 150 will energize the economizer solenoid valve 63 to position the solenoid valve 63 in its open position, thereby permitting refrigerant to flow from refrigerant line 4 through refrigerant line 8 pass through the second pass 64 of the economizer 60 so as to increase the cooling capacity of the system. The controller 150 monitors the temperature, TSPACE, of climate-controlled space 110 by means of one or more temperature sensors 113 disposed within the climate controlled space 110 at one or more locations. The temperature sensors 113 may be thermostat type sensors, thermister type sensors or other types of temperature sensing devices.
When the sensed temperature indicates that the temperature, TSPACE, within the enclosed space 110 has been pulled down to the set point temperature, TSP, the controller 150 will switch operation of the refrigerant vapor compression system 100 from the “pull-down” mode” into an “equilibrium” mode to maintain the temperature, TSPACE, within the enclosed space 110 stable at the set point temperature, TSP, that is at a temperature within a temperature range from TSP−ΔT to TSP+ΔT, inclusive of the end points. In this mode, the controller 150 maintains the predetermined box temperature, TSPACE, by selectively controlling the operation of the compressor 20, the condenser fan(s) 34 associated with the condenser heat exchanger coil 32, and the evaporator fan(s) 44 associated with the evaporator heat exchanger coil 42, as well as selectively opening or closing the compressor unloader valve 25 and the economizer solenoid valve 63, in various schemes.
When the refrigerant vapor compression system is operating in the “equilibrium” mode with the temperature, TSPACE, lying within the equilibrium range of equal to or greater than TSP−ΔT and equal to or less than TSP+ΔT, the controller 150 will enter a humidity control mode in which the controller 150 monitors the relative humidity, RHSPACE, by means of a humidity sensor 115, such as for example a humidistat or other type of humidity sensing device, disposed to sense a relative humidity reflective of the relative humidity with the climate controlled space 110. The humidistat 115 may be disposed in the flow of supply air being delivered to the climate controlled space 110 downstream of the airside outlet of the evaporator heat exchanger 42 as depicted in
Referring now to
However, if, at step 205, the sensed relative humidity within the climate controlled space 110 is indeed greater than the desired relative humidity set point, the controller 150 will enter a dehumidification mode. Upon entering the dehumidification mode, at step 210, the controller 150 will generate and transmit a command signal to the expansion device 50, disposed in refrigerant line 4 downstream with respect to refrigerant flow of the economizer 60 and upstream with respect to the refrigerant inlet to the evaporator heat exchanger 42 of the evaporator 40, to raise the superheat level of the refrigerant vapor leaving the evaporator heat exchanger 42 by causing the refrigerant flow area through the electric expansion valve 50 to further close. For example, the controller 150 will, at step 210, transit a command signal to the expansion device 50 setting the superheat level to its maximum value, thereby closing the flow area through the expansion device 50 to a minimum open area. Reducing the flow area opening within the expansion device 50 causes a further restricting of the flow of refrigerant through the expansion device 50 which results in a lower refrigerant flow and a lower refrigerant pressure within the evaporator heat exchanger 42.
As a result of the drop in refrigerant pressure, the temperature of the refrigerant passing through the evaporator heat exchanger 42 also drops to a lower temperature. As a result of the lower temperature of the refrigerant passing through the tubes of the evaporator heat exchanger 42, the surface temperature of the outer surface of the tubes of the evaporator heat exchanger 42 also decreases. Because the outer surface temperature of the refrigerant conveying tubes of the evaporator heat exchanger 42 is now colder and the temperature difference between the airflow circulating from the climate controlled space 110, which remains at the set point temperature, TSP, is now greater, more moisture contained in that airflow will condense out of the airflow onto the outer surface of the tubes as the airflow passes over the outer surface of the tubes of the evaporator heat exchanger 42, thereby removing moisture from the airflow. As a result, the supply airflow, that is the airflow being supplied back to the climate controlled space 110 after having traversed the evaporator 40, will have a lower moisture content than the return airflow, that is the airflow drawn from the climate controlled space to be passed through the evaporator 40.
Additionally, the supply airflow leaving the airside outlet of the evaporator 40 will have a lower temperature due to the increased heat transfer from the air flow to the colder refrigerant as the air flow traverses the evaporator heat exchanger 42 in heat exchange relationship with the colder refrigerant. Therefore, at step 210, the controller 150 will also initiate reheat of the supply air flow leaving the air side outlet of the evaporator 40 by activating an air reheater 70 operatively associated with the evaporator 40. The air reheater 70 heats the supply airflow having traversed the evaporator heat exchanger 42 prior to delivery back to the climate controlled space 110. The air reheater 70 may comprise an electric heater, a hot refrigerant air heater coil, or other type of means for adding sensible heat to the air flow. The air reheater 70 may be disposed at the air side outlet of the evaporator 40 downstream of the evaporator heat exchanger 42 whereby the supply air flow having traversed the evaporator heat exchanger 42 is heated prior to entering the climate controlled space 110.
Without reheating the airflow during the dehumidification mode, the amount of dehumidification attainable is limited in that temperature of the airflow traversing the evaporator heat exchanger 42 could not be reduced lower than the set point temperature. However, with reheating available, the controller 150 can allow the temperature of the airflow having traversed the evaporator heat exchanger 42 to drop below the set point temperature so as to achieve increased condensation of moisture from the airflow since the reheater 70 may be operated to raise the temperature of the airflow up to the set point temperature prior to the airflow passing back into the climate controlled space. Thus, the controller 150 may maintain the superheat setting of the electronic expansion valve 50 at the maximum superheat setting without worrying that the temperature of the airflow has dropped below the set point temperature. Rather, the controller 150 may maintain the superheat setting of the electronic expansion valve 50 at the maximum superheat setting so long as the temperature of the refrigerant passing through the evaporator heat exchanger 42 drops so low that the temperature of the surface of the evaporator heat exchanger 42 exposed to condensation of moisture from the airflow is so low as to result in the rapid build-up of frost on the exposed surface of the evaporator heat exchanger 42. Therefore, lower levels of relative humidity may be attained when desired, such as when the climate controlled space is the cargo box of a truck, trailer or container wherein a cargo such as flower bulbs is being transported.
Over a period of time of operation in this enhanced dehumidification mode, the relative humidity will necessarily drop to the set point relative humidity, RHSP. Therefore, at selected time intervals during operation in the enhanced dehumidification mode, the controller 150 will, at step 215, repeatedly compare the sensed relative humidity, RHSPACE, to the relative humidity set point, RHSP, minus the preselected relative humidity tolerance, ΔRH, to determine whether the sensed relative humidity, RHSPACE, has dropped to a level that is equal to or less than the desired relative humidity set point minus the relative humidity tolerance, that is, RHSP−ΔRH. If not, the controller 150 continues operation in the dehumidification mode with the reheater 70 activated and the superheat setting of the expansion device 50 set at maximum superheat.
If the sensed relative humidity, RHSPACE, has indeed dropped to a level that is equal to or less than the desired relative humidity set point minus the relative humidity tolerance, that is, RHSP−ΔRH, the controller 150, at step 220, terminates reheat by deactivating the reheater 70 and resetting the superheat setting of the electronic expansion valve 50 to a preprogrammed default value, and thence proceeds to step 225. At this point, the controller 150 periodically checks, so long as the system remains operating in the humidity control mode, whether the sensed relative humidity, RHSPACE, is greater than or equal to the desired relative humidity set point plus a relative humidity tolerance, that is RHSP+ΔRH. If no, and the controller 150 checks, at step 235, whether the system remains operating in a humidity control mode. If no, the controller 150 will simply exit the dehumidification mode at step 240. If yes, the controller 150 will continue operation in the dehumidification mode by repeating steps 220 and 235, in succession, and return to the dehumidification cycle at step 210 if and when the sensed relative humidity, RHSPACE, again exceeds the desired relative humidity set point plus a relative humidity tolerance, that is RHSP+ΔRH.
As noted previously, if the temperature of the refrigerant passing through the evaporator heat exchanger 42 drops too low, the temperature of the surface of the evaporator heat exchanger coil exposed to condensation of moisture from the airflow, the rapid build-up of frost could occur. Therefore, during operation in the dehumidification mode, the controller 150 will, at regular intervals, check the evaporator outlet pressure sensed by the evaporator outlet pressure sensor 105 against a low limit pressure value to ensure that the temperature of the evaporator heat exchanger 42 does not drop so low as to cause rapid build-up of frost on the surface of the evaporator heat exchanger 42 exposed to the airflow. In the event that the sensed evaporator outlet temperature does drop below the lower limit pressure value, the controller 150 will gradually reduce the superheat setting of the evaporator expansion valve 50 from its maximum superheat setting value until the controller 150 determines that the sensed evaporator outlet pressure again rises above the lower limit pressure value. The controller 150 will maintain the superheat setting of the electronic expansion valve 50 at this reduced superheat setting valve for the remainder of the dehumidification mode so long as the sensed evaporator outlet pressure does not again drop below the lower limit pressure value.
In an embodiment, the evaporator fans 44 may be variable speed fans having a variable speed motor/drive that is capable of operating at a lower speed setting and at a higher speed setting. The controller 150 controls the supply of electrical power to the fan motor/drive (not shown) associated with each evaporator fan 44 in a conventional manner, for example by controlling the frequency or the current output of the electrical power supply, to selectively operate the evaporator fan or fans 44 at either the lower speed setting or the higher speed setting. At the lower speed setting, the evaporator fan or fans 44 draw airflow from the climate controlled space 110 and pass the airflow over the refrigerant conveying conduits 45 of the evaporator heat exchanger 44 at a lower volume flow rate lower than the volume flow rate produced when the evaporator fan or fans 44 are operating at the higher speed setting. To further enhance the dehumidification process, the controller 150 may selectively operate the variable speed evaporator fan(s) 44 at the lower speed setting to reduce the volume flow rate of the airflow passed through the evaporator 40 in heat exchange relationship with the refrigerant passing through the refrigerant conveying conduits 45 of the evaporator heat exchanger 42. By operating at a lower volume flow rate of airflow through the evaporator 40, the range of dehumidification will potentially be extended as the airflow will take more time to traverse the evaporator heat exchanger 42, allowing more moisture to condense. Similarly, the capability to reheat the airflow by means of the reheater 70 is also potentially enhanced as the airflow will take more time to traverse the reheater 70.
Referring now to
Referring now to
The terminology used herein is for the purpose of description, not limitation. Specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as basis for teaching one skilled in the art to employ the present invention. While the present invention has been particularly shown and described with reference to the exemplary embodiments as illustrated in the drawing, it will be recognized by those skilled in the art that various modifications may be made without departing from the spirit and scope of the invention. Those skilled in the art will also recognize the equivalents that may be substituted for elements described with reference to the exemplary embodiments disclosed herein without departing from the scope of the present invention.
Therefore, it is intended that the present disclosure not be limited to the particular embodiment(s) disclosed as, but that the disclosure will include all embodiments falling within the scope of the appended claims.
Reference is made to and this application claims priority from and the benefit of U.S. Provisional Application Ser. No. 61/251,462, filed Oct. 14, 2009, and entitled DEHUMIDIFICATION CONTROL IN REFRIGERANT VAPOR COMPRESSION SYSTEMS, which application is incorporated herein in its entirety by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US10/51802 | 10/7/2010 | WO | 00 | 4/16/2012 |
| Number | Date | Country | |
|---|---|---|---|
| 61251462 | Oct 2009 | US |
