Patent Application
20080190121
The present invention relates to systems for conditioning enclosed spaces, and in particular to a unit cooler for refrigerating and dehumidifying a storage space and for eliminating moisture condensation therein.
Present governmental code, regulations, and guidelines do not allow any water to drip from overhead surfaces in food processing rooms down upon the production area. Additionally, those regulations have strict limits as to the temperatures at which refrigerated food must be maintained, 41° F. being the accepted upper limit. For decades, food processors have utilized traditional direct expansion refrigeration unit coolers as standard components in field erected, or built up, systems that refrigerate the processing rooms. The unit coolers are located within the processing room and include a cooling coil and fan for circulating and cooling the air within the room. The cooling coil is connected at a direct expansion device interior to the unit cooler and to a compressor located exterior of the processing room. Installation requires only two fluid connections and modest control.
These unit coolers are effective at temperature control, but not at humidity control. The direct refrigeration system maintains the relative humidity (% Rh) levels in the food processing room at about 90 to 100% Rh. These high Rh. levels present problems for food processors. From time to time, the overhead equipment and/or the overhead structure gets colder than the dew point of the air in the room, particularly during defrost cycles. In these situations, water from the air condenses on the overhead equipment and/or the overhead structure. The condensed water then drips down upon the production area. Such dripping raises the potential for food contamination. If a governmental or private inspector notes such dripping from the ceiling, they will typically require a cleanup and sanitation procedure in the food processing room. This shutdown results in waste disposal of the involved food and many labor hours of lost production time for the food processor. As a result this is an active interest in eliminating condensation through improved dehumidification.
One basic approach is currently used, wherein a separate desiccant dehumidification system is added to supplement the conventional unit cooler. While satisfactory for avoiding condensation problems, installation and equipment costs are high and a large increase in energy consumption, 50% to 200%, is incurred because the dehumidifiers use energy to operate and add heat to the refrigerated space. This in turn increases the cooling load on the refrigeration system and results in higher energy cost for the refrigeration system.
It would be desirable to provide a unit cooler for these cold room facilities that would provide both the requisite refrigeration and operate at humidity levels overcoming condensation problems. The use of serial refrigeration and reheat coils has been proposed for lowering excessive humidity conditions in ambient personal comfort conditions. Representative of such an approach is disclosed in U.S. Pat. Nos. 5,622,057 and 3,798,920 wherein a reheat coil, modulated in response to humidity conditions, is inserted fluidly before the cooling coil and downstream thereof in the airflow. The two coils are substantially the same. When dehumidification is required, the subcooling coil is operative until set conditions are attained and thereafter modulates within control limits. Such systems are accepted for applications above about 50° F., but are not approved by the manufacturers for high humidity conditions below this temperature. For temperatures below this level, the coil must operate at below freezing temperatures, resulting in progressive ice buildup requiring defrost cycles and varying the air flow to the reheat coil. This results in control instability leading to high maintenance costs, compressor failures, poor room temperature control, erratic air flow and poor room humidity control.
This present invention provides a unit cooler for cold rooms that passively provides temperature and humidity control without modulation. The unit cooler provides full time dehumidification and refrigeration in a factory assembled unit cooler that may be installed and operated without complexity for either the system installer or the system operator. The unit cooler uses directly and continuously serially connected refrigeration direct expansion (DX) and reheat/subcool (SR) coils to achieve lower % Rh room conditions without a large increase in energy consumption.
The foregoing advantages are achieved by a unit cooler employing design criteria that allows the refrigeration system to dehumidify and run through defrost cycles without additional controls or complexity or the difficulties of the prior art systems discussed above. The unit cooler provides a passive and natural balance of over-cool and reheat of air during refrigeration. This is achieved by providing a desired dew point for the supply air that avoids condensation, operating the DX coil at a temperature is achieve the dew point and operating the SR coil to achieve the desired supply temperature for maintaining the space at room design temperature.
In achieving this benefit, the DX coil is operated at a reduced saturated suction temperature (SST) about 3 to 9 degrees F. without significantly increasing compressor equipment or operating cost. By example, if a conventional system operates at +25° F. SST as a traditional DX system, the design suction temperature is lowered to ±20° F. SST. This represents an efficiency loss to the system of roughly 8%, the traditional DX system Energy Efficiency Ration (EER) being about 11.2, the present invention operates at EER of about 9.2, which is significantly recoupled in the present invention.
The unit cooler design criteria also requires that the SR coil gives energy back to the refrigeration system that has experienced a loss in design efficiency due to the above decrease in design SST. That energy is returned in the SR coil pass as the condensed liquid is refrigerated from saturation to a subcooled state, and allows for about 15 to 25% of compressor cooling capacity to be returned in liquid cooling. To accomplish this, the unit cooler is to be designed with the SR coil to have 15% to 35%, and preferably 20% to 30% as much primary coil surface and secondary coil surface as the DX coil. Above and below this range, passive operation cannot be attained and unreliable modulating operation would be required. This recovers much of the lost efficiency in the system, resulting in a system having an EER of only 3% or less than the traditional DX refrigeration system, and substantially less than the energy costs of separate units for dehumidification.
Generally it is preferred to deliver air to the cold room at roughly the same temperature, i.e. 41° F. or less, and flow rate as it would with a traditional DX refrigeration system to maintain the design temperature. This is accomplished because the 3 to 9° F. lower design SST creates a minimum air temperature at the DX coil that is about 2 to 8° F. lower than the tradition DX refrigeration system. This establishes a dew point with the passively integrated unit cooler that is about 2 to 8° F. lower than a traditional DX refrigeration system.
With these design criteria the unit cooler naturally delivers air with a relative humidity at 70% to 85%. For operation within this range, controls are not needed as the dehumidification unit cooler naturally balances out at these conditions. With this supply air relative humidity, the passively integrated unit cooler prevents water condensation dripping issues in the refrigerated space.
The ability to refrigerate and dehumidify at the same time, this invention has many applications outside of food processing rooms. Those applications include controlled humidity storage rooms for goods ranging from seeds to paper and from consumer goods to industrial goods.
Accordingly, it is an object of the present invention to provide a unit cooler providing a supply air at a humidity avoiding condensation in cold room facilities.
Another object is to provide a unit cooler refrigeration system that passively provides humidity control for cold room facilities.
A further object is to provide a unit cooler for preventing condensation problems in cold room facilities that may be installed and operated without increased complexity.
The above and other advantages of the present invention will become apparent upon reading the following written description taken in conjunction with the accompanying drawings in which:
A traditional DX refrigeration system for a unit cooler for conditioning of a refrigerated space such as a cold room or walk-in refrigerated or freezer room is shown in
The installation is simple requiring only two fluid connections at the preassembled unit cooler The controls for such a system are also basic, requiring a temperature control, and a defrost system for deicing the coil. Deicing may be effected, during system shut down conventionally through ambient thawing, or electric or gas assisted deicing. The room operates at saturated conditions at the design temperature, and deicing raises the air temperature allowing condensate buildup, liquid or ice, on the now colder room components resulting in undesired release onto food products therein.
The present invention is shown in
An integrated refrigeration and dehumidification unit cooler system 10 according to an embodiment of the invention is shown in
As shown in
The unit cooler 18 includes a fan 36 that draws return air from an inlet 38 in an air path serially through the coils 26, 28 to an outlet 40 for circulating conditioned supply air as indicated by the arrows and return to the inlet 32. As hereinafter described, the supply air is at a supply air temperature for maintaining temperature conditions at or below a design temperature and at a dew point preventing condensation on overhead surfaces notwithstanding defrost cycles and operational conditions.
The SR coil 28 uses coil air from DX coil 26 to subcool the refrigerant liquid to a temperature below saturation. The subcooled refrigerant liquid is moved through subcooled liquid line 30 to the expansion device 24, which receives refrigerant liquid in a sub-cooled state. With sub-cooled refrigerant liquid, the expansion device can do more cooling than it could with warm, saturated refrigerant liquid in the prior art system. The sub-cooling of the refrigerant liquid gives DX coil 26 the ability to do more cooling than it could with warm, saturated refrigerant liquid. The extra cooling capacity allows DX coil 26 to cool air entering the unit cooler to a lower temperature than it could with warm, saturated refrigerant liquid. In this case the colder air coming from DX coil 26 is still at 100% Rh. Once the DX coil 26 cools the air to this new lower level, the air is heated by SR coil 28. In this way the air ends up with a relative humidity below 100% Rh due to the reheating by the SR coil 28 to a temperature above the moisture saturation condition.
The unit cooler 18 refrigerates and dehumidifies the air by delivering air to the room 11 at a temperature that is still low enough to hold the room at the design temperature, but at a lower relative humidity. Obtainable relative humidity numbers for air leaving the SR coil 28 are in the range of 75% to 90%.
Because SR coil 28 drives lower air temperatures in DX coil 26, the temperature that unit cooler 18 delivers to room 11 is nominally the same after SR coil 28 as it would be in the prior art DX refrigeration system of
The differences between the present and prior art systems are apparent from the pressure enthalpy diagram of
The enthalpy diagram for the present invention, as in prior art system, the line 12 represents the change across the compressor and the line 14 the change across the condenser. The contribution of the SR coil to subcooling is denoted by line 28 wherein the refrigerant liquid is further cooled into the lower enthalpy sub-cooled zone. The change across the expansion device is denoted by line 18. Thus, the contribution of the DX coil to the change in enthalpy is increased as denoted by line 26 in comparison to line 122.
The contribution of the SR coil 28 to the dehumidification of supply air to the room is shown in the comparison of the systems in the psychometric charts of
In
The method for establishing the above operation for a refrigerated space will initially prescribe a supply air temperature for maintaining an upper permissible room temperature or less and also the required air flow volume. Based on operations within the room, a dew point range for the supply air is prescribed that will provide acceptable humidity conditions preventing condensation on overhead surfaces. The dew point thus establishes the operating dew point for the DX coil and the temperature rise from the SR coil. A compressor/condenser package is then selected for achieving the dew point. A SR coil size is determined to achieve the temperature rise at the air flow. The heat balance of the system is then determined. Because of the addition of the SR coil and consequent lowering of the fluid temperature to the expansion device and DX coil, a lower DX coil dew point may result requiring reiterations of these steps be undertaken as necessary to establish the interface between the coils to achieve performance within the design limits.
An example of the foregoing advantages is set forth in the following example.
A prior art system is operated with a compressor, Model No. 06DM316, from Caryle, providing a saturated suction temperature (SST) of 25° F. and a saturated condensing temperature (SCT) of 115° F. The net refrigeration effect of this refrigeration system is (nominal 4 Tons) 48,000 btu/hr of cooling, which is delivered to a refrigerated room through a system of
A system in accordance with
These examples demonstrate that the system in accordance with the present invention incorporating the integral sub-cool/reheat coil able to achieve dehumidification in a refrigerated space runs with 5.5% more power consumption than traditional DX refrigeration, but with a lower relative humidity by 14% RH. Such a reduction accommodates the temperature excursions typically encountered in food processing without presenting a condensation condition on in-place equipment. The modest power increase is substantially less, both in capital and operating cost, that a supplemental installation. The synergistic effects of the subcooling and reheating of the SR coil thus provides an effective solution for overcoming condensation problems in refrigerated storage applications.
Having thus described a presently preferred embodiment of the present invention, it will now be appreciated that the objects of the invention have been fully achieved, and it will be understood by those skilled in the art that many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the spirit and scope of the present invention. The disclosures and description herein are intended to be illustrative and are not in any sense limiting of the invention, which is defined solely in accordance with the following claims.
