Abstract:
A test chamber that is capable of operating in a mode where the temperature of the chamber is efficiently cooled without removing a substantial amount of moisture from the air. In one aspect, the test chamber includes a structure defining a work space having air, and a temperature control system (e.g., a refrigeration system having a compressor, a condenser, and an evaporator valve). The temperature control system includes a heat exchanger (e.g., an evaporator) positioned to communicate with the air in the work space, a source of cold fluid (e.g., a compressed, condensed, and throttled refrigerant) coupled to the heat exchanger, a source of hot fluid (e.g., compressed refrigerant gas) coupled to the heat exchanger, and a controller for controlling a mixture of cold fluid and hot fluid entering the heat exchanger (e.g., by adjusting a cold fluid valve and/or a hot fluid valve). In order to limit the loss of humidity caused by condensation on the heat exchanger, it is preferred that the controller is programmed such that the temperature of the mixture entering the heat exchanger is controlled to limit a temperature differential between the heat exchanger and the air in the work space.

Description:
BACKGROUND 
     The present invention relates to a temperature- and humidity-controlled test chamber and a method of controlling the temperature and humidity thereof. 
     General purpose environmental test chambers typically are designed for several tasks requiring distinct modes of operation. One such task may be high and low temperature transitions and stabilizations with the temperature ranging from 180° C. to −70° C. Typically, to reach lower temperatures with mechanical refrigeration, a cascade refrigeration system is used. This requires two separate refrigeration circuits (stages) with a high pressure refrigerant in the low stage and a relatively lower pressure refrigerant in the high stage to “cascade” the heat out of the chamber, lowering the air temperature in the enclosed space. 
     Another task may be the precise control of temperature and humidity within the cabinet workspace. When operating in the temperature/humidity mode, it is important to keep the cooling coil above the freezing point of water to prevent excessive moisture migration (i.e., ice formation on the coil) and blockage of air flow through the cooling coil. To account for this, some designs incorporate a separate cooling coil within the chamber workspace and utilize the high stage refrigerant to maintain a cooling coil temperature above the freezing point of water. The refrigerant is expanded from a liquid to a vapor at a controlled pressure. The evaporating pressure is set based on the lowest temperature required for the temperature/humidity mode of operation, but above the freezing point of water. When cooling is required at the highest temperature/humidity combination in the operational range, a portion of the cooling coil temperature is significantly below the dew point of the air stream within the chamber, resulting in condensation and a considerable cooling requirement due to the latent heat of condensation. Moisture condensed from the air must be replaced to maintain the controlled humidity condition. Steam may be added by a boiler (not shown) that is open to the chamber atmosphere, or by pressurized steam rails (not shown). Moisture may also be added to the chamber by way of an atomizing spraying system. The re-introduction of moisture is often accompanied by sensible heat (steam), further increasing the cooling load. Additional cooling causes additional condensation, which increases the amount of steam required to replace the condensed moisture. As a result, temperature and humidity must be continuously monitored and corrected to ensure they stay within the desired ranges. 
     There is also a need in the market to operate at high temperature/humidity conditions while a product(s) within the chamber generates heat. A product, or thermal load, within the chamber may fall into one of two categories: a thermal load that generates heat is called a “live load,” and a thermal load that does not generate heat is called a “dead load.” Maintaining high temperature/humidity conditions in a system containing a live load is a challenge. The current systems either limit the temperature/humidity range, limit the allowable amount of heat dissipation by the live load, or are specialized such that the overall utility of the equipment is compromised. 
     SUMMARY 
     The present invention provides a test chamber that is capable of operating in a mode where the temperature of the chamber is efficiently cooled without removing a substantial amount of moisture from the air. This is particularly desirable when both temperature and humidity control are important. In one aspect, the test chamber includes a structure defining a work space having air, and a temperature control system (e.g., a refrigeration system having a compressor, a condenser, and an evaporator valve). The temperature control system includes a heat exchanger (e.g., an evaporator) positioned to communicate with the air in the work space, a source of cold fluid (e.g., a compressed, condensed, and throttled refrigerant) coupled to the heat exchanger, a source of hot fluid (e.g., compressed refrigerant gas) coupled to the heat exchanger, and a controller for controlling a mixture of cold fluid and hot fluid entering the heat exchanger (e.g., by adjusting a cold fluid valve and/or a hot fluid valve). In order to limit the loss of humidity caused by condensation on the heat exchanger, it is preferred that the controller is programmed such that the temperature of the mixture entering the heat exchanger is controlled to limit a temperature differential between the heat exchanger and the air in the work space. 
     The present invention is also embodied in a method of controlling the temperature of a test chamber having a temperature control system including a source of cold fluid, a control valve that limits the flow of cold fluid, a source of hot fluid, and a heat exchanger. The method comprises positioning a heat exchanger in the chamber, flowing a cold fluid (e.g., a compressed, condensed, and throttled refrigerant) toward the heat exchanger, flowing a hot fluid (e.g., compressed refrigerant gas) toward the heat exchanger, mixing the cold fluid with the hot fluid to produce a mixture, and controlling the ratio of hot fluid and cold fluid in the mixture (e.g., adjusting a cold fluid valve and/or a hot fluid valve to control the amount of cold fluid mixing with the hot fluid to control the temperature of the mixture in the heat exchanger). In order to limit the loss of humidity caused by condensation on the heat exchanger, it is preferred that controlling includes adjusting the temperature of the mixture in the heat exchanger to control the temperature differential between the heat exchanger and the air in the work space. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a first construction of the refrigeration apparatus in accordance with the present invention. 
         FIG. 2  is a schematic diagram of a second construction of the refrigeration apparatus in accordance with the present invention. 
         FIG. 3  is a flowchart illustrating one way of controlling the apparatus of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
     This is an apparatus and method for controlling the temperature in a temperature/humidity test chamber  10  using a vapor refrigerant flowing through a closed loop system. The vapor refrigerant is circulated through a temperature-controlled coil  12  within an environmental test chamber load space  14 . When cooling is required without a reduction in humidity, the vapor refrigerant is preconditioned to control (i.e., reduce substantially, while still achieving the desired cooling result) the temperature differential between the coil  12  and a moisture-laden air stream passing across the coil  12 , thereby reducing or eliminating the amount of moisture from the air stream that condenses on the coil  12 . Since less moisture is lost in the cooling process, the need to replace moisture by adding steam to the test chamber load space  14  is reduced. Because less sensible heat from steam is added and there is less latent heat transferred from condensation, the efficiency of the system is improved and the system is capable of accommodating test loads that dissipate more heat. When dehumidification is desired, the temperature-controlled coil  12  can act as an evaporator in a manner well understood by those of ordinary skill in the art. That is, a portion of the evaporator may be controlled to fall below the dew-point of the chamber air such that chamber air passing over the evaporator condenses on the coil. If necessary, a heater(s) (not shown) in the test chamber reheats the dehumidified air. 
     In accordance with the present invention, the refrigerant entering the temperature-controlled coil  12  is a mixture of cold liquid or liquid/vapor refrigerant and hot vapor refrigerant having, in total, a greater mass flow rate than conventional evaporator coils. The increased flow rate allows heat transfer to occur between the coil  12  and the load space  14  at a lower temperature differential. Thus, the temperature-controlled coil  12  can provide efficient cooling to the load space  14  without removing moisture from the load space air. The present invention may be applied to any refrigeration circuit. Two possible constructions are described below. 
     In one construction, shown in  FIG. 1 , a single stage closed-loop refrigeration system  16  includes a single stage compressor  18 , a condenser  20 , an expansion valve  22 , and a coil  12 . The compressor  18  compresses a refrigerant gas, which is then condensed into a liquid refrigerant by the condenser  20 , which could be an air-cooled, liquid-cooled or other suitable type of condenser. The liquid refrigerant travels to the expansion valve  22  by way of a liquid line  24 . The refrigerant then travels to the coil  12 , which is located in the environmental test chamber load space  14 . The evaporating refrigerant removes heat from the load space  14  in a manner well understood by those of ordinary skill in the art. 
     In accordance with the present invention, a superheated vapor line  26  fluidly connects the compressor  18  to the coil  12 , allowing superheated vapor to bypass the condenser  20  and mix with liquid or two-phase refrigerant from the liquid line  24  before entering the coil  12 . A manually-operated valve  28  and a first control valve  30  are located on the superheated vapor line  26 , and a second control valve  32  is located on the liquid line  24 . The first and second control valves  30 ,  32  are controlled by a chamber controller  34  to regulate the mixture of superheated vapor and liquid or two-phase refrigerant that enters the coil  12 . More appropriately, the coil  12  should be called a “temperature-controlled coil” in accordance with the present invention because the temperature of the refrigerant mixture entering the coil is controlled. It should be understood that the first and second control valves  30 ,  32  can be combined into a single three-way valve with an inlet from the superheated vapor line  26 , an inlet from the liquid line  24 , and an outlet to the coil  12 . 
     The chamber controller  34  operates in two modes: temperature control and temperature/humidity control. In each mode, the flow of refrigerant through the first and second control valves  30 ,  32  is regulated to achieve a mixture of superheated vapor and liquid or two-phase refrigerant that is appropriate to maintain the load space  14  at a temperature and humidity set-point inputted by a user. 
     In temperature control mode, the refrigerant mixture is controlled to bring the temperature in the test chamber  10  to the set point without concern for humidity levels. In this mode, cooling is accomplished by cooling the coil  12  to a low temperature in order to achieve the desired temperature in the chamber quickly. In this mode, a portion of the coil  12  could be below the dew-point of the air in the test chamber  10 , and thus could result in condensation and a reduction in the humidity of the air in the test chamber  10 . 
     In temperature/humidity control mode, a temperature-controlled refrigerant mixture is introduced to the temperature-controlled coil  12 . When high relative humidity and cooling are requested, it is undesirable and inefficient (for reasons explained above) to dehumidify the load space air. Accordingly, liquid refrigerant from the liquid line  24  is metered and mixed with a stream of vapor refrigerant from the superheated vapor line  26 . This causes the temperature of the refrigerant entering the coil  12  to be higher than normal, and thus the ΔT between the coil  12  and the air in the chamber  10  is relatively small. The result is little, if any, condensation on the coil  12 , and thus little, if any, loss of moisture in the air in the test chamber  10 . 
       FIG. 3  shows a flowchart illustrating the temperature-control portion of the temperature/humidity control mode. During this control process, the flow of superheated vapor through the superheated vapor line  26  is maintained constant, and thus all control of the refrigerant entering the coil  12  is accomplished by varying the amount of liquid refrigerant entering from the liquid line  24  by adjusting the second control valve  32 . First, the temperature inside the chamber load space T C  is measured and compared with a desired temperature range T D , which can be input by the user. Typically, the user enters a specific desired temperature, and the controller provides a reasonable temperature range to maintain. 
     If T C  is above T D , then the chamber is in need of cooling, and the controller  34  opens the second control valve  32  slightly to increase the amount of liquid refrigerant that is mixed with vapor refrigerant from the superheated vapor line  26 . This amount is initially set low to minimize the temperature difference between the load space air and the coil  12 . If no decrease is seen in the load space air temperature, then the controller  34  further increases the mass flow rate of liquid refrigerant by further opening the second control valve  32 . The valves may be pulse-width modulated to control the mass flow rate by pulsing the valve open and closed for calculated periods of time, as is known in the art. This process is continued until a decrease in T C  is detected. As soon as a decrease in T C  is detected, the process is held steady and monitored until T C is within T D , or until T C  is no longer moving toward T D . When T C  falls within T D , monitoring of temperature continues as the live load in the test chamber  10  will continue to dissipate heat. 
     If T C  is below T D , then the chamber is in need of less cooling, and the controller  34  closes the second control valve  32  slightly to decrease the amount of liquid refrigerant that is mixed with vapor refrigerant from the superheated vapor line  26 . If no increase is seen in the load space air temperature, then the controller  34  further decreases the mass flow rate of liquid refrigerant by further closing the second control valve  32 . The valves may be pulse-width modulated to control the mass flow rate by pulsing the valve open and closed for calculated periods of time, as is known in the art. This process is continued until an increase in T C  is detected. As soon as an increase in T C  is detected, the process is held steady and monitored until T C  is within T D , or until T C  is no longer moving toward T D . If T C  is no longer moving toward T D  and the second valve is fully closed, then it may be necessary to add heat (e.g., by an auxiliary heat source) in order to increase T C  to fall within T D . When T C  falls within T D , monitoring of temperature continues. 
     When dehumidification is requested, the refrigerant mixture is controlled to be below the dew-point of the load space air. Typically, the amount of superheated vapor refrigerant is reduced via the first control valve  30  by either reducing the pulse rate or closing off the valve, and a liquid or two-phase refrigerant mixture may enter the temperature-controlled coil  12  via the second control valve  32  at a desired pulse rate. The mass flow rates of hot and cold refrigerant are controlled to achieve a mixture of a desired temperature. The temperature-controlled coil  12  may act as an evaporator in a manner well known to those of ordinary skill in the art, with at least a portion of the coil  12  cooling down to a temperature well below the dew-point of the load space air such that a portion of moisture in the load space air is condensed and removed from the system. This method will continue whenever dehumidification is desired. If heating of the air in the load space  14  is desired, separate heaters (not shown) in the chamber may be used to heat the air without adding moisture to the dehumidified air. 
     In another construction, shown in  FIG. 2 , a cascade refrigeration system  36  for low-temperature cooling includes a high stage refrigeration system  38  and a low stage refrigeration system  40 . The high stage system  38  cools the low stage system  40  via a cascade heat exchanger  42 . 
     The high stage refrigeration system  38 , which operates in a manner well known to those of ordinary skill in the art, includes a high stage compressor  44 , a high stage air-cooled or water-cooled condenser  46 , a solenoid valve  48 , and a cascade heat exchanger  42  in heat-transfer communication with the low stage refrigeration system  40 . An expansion valve  50  is located at the inlet to the cascade heat exchanger  42 . 
     The low stage refrigeration system  40  includes a low stage compressor  54  in fluid communication with the cascade heat exchanger  42  and a coil  12  located in a load space  14 . A liquid line  56  fluidly connects the cascade heat exchanger  42  to the coil  12  and may also include an expansion valve or other expansion device (not shown). An injection line  52  carrying liquid refrigerant from the condenser  42  includes a solenoid valve and an expansion valve to selectively cool superheated vapor refrigerant returning to the compressor. Under some conditions, the superheated vapor leaving the coil  12  may cause the compressor  54  to overheat, thus the injection line cools the superheated vapor by selectively allowing some liquid refrigerant to expand. The cascade system operates in a manner well understood by those of ordinary skill in the art, except for the portion of the system that is the invention, as described below. 
     In accordance with the present invention, a superheated vapor line  58  fluidly connects the low stage compressor  54  to the coil  12  (which is more appropriately termed the “temperature-controlled coil” as explained above) and includes a first control valve  30 . The liquid line includes a second control valve  32 . The first and second control valves  30 ,  32  are controlled by a chamber controller  34  to regulate the mixture of superheated vapor and liquid or two-phase refrigerant that enters the temperature-controlled coil  12 . The temperature-controlled coil  12  is located within a test chamber  10  and is in heat-transfer communication with the load space  14 . 
     The chamber controller  34  of the second construction operates in two modes: temperature mode and temperature/humidity mode. In each mode, the flow of refrigerant through the first and second control valves  30 ,  32  is regulated to achieve a mixture of superheated vapor and liquid or two-phase refrigerant that is appropriate to maintain the load space  14  at a temperature or temperature/humidity set-point inputted by a user. The modes are the same as previously described in the first construction of the invention. 
     In previous designs of a cascade system for temperature/humidity control of test chambers, a high stage evaporator was located in the test chamber load space  14 . In accordance with the present invention, the specialized high stage cooling circuit on the high stage refrigeration system  38  is removed from the chamber&#39;s temperature-transitioning environment  14 . This removal of mass reduces the thermal load and improves temperature transition performance. The refrigerant circuiting and modes of operation are also simplified. Fewer circuit components are required, increasing reliability of the equipment and reducing costs. This design also improves efficiency and increases the heat dissipation capacity of the equipment at high relative humidity conditions without compromising other modes of operation. 
     In an alternate construction, instead of merging the liquid line with the superheated vapor line and controlling the mixture of refrigerant, a heat exchanger may provide heat transfer communication between the liquid and superheated vapor lines in order to provide a temperature-controlled refrigerant to the coil  12 . 
     Thus, the invention provides, among other things, an apparatus and method for controlling the humidity and temperature of a live load test chamber. Various features and advantages of the invention are set forth in the following claims.