REFRIGERATION PLANT

Abstract: An evaporative heat exchange unit has a refrigeration condenser (12) with water passing over the condenser to remove heat therefrom. Air is caused to move by fan (23) in countercurrent to the water flow, the air being drawn from an inlet (28) and exhausted from a discharge outlet (24). The discharge outlet (24) communicates with the inlet (28) by an air duct (27). An air damper (29) is located in the duct adjacent the outlet (24) and a second air damper (31) is located in the air inlet (28). Wet bulb temperature of the air passing to the fan (23) is sensed by the sensor (32). The sensor reading is used to control the relative positions of the air dampers (29, 31) to cause more or less of the exhaust air to pass from the discharge outlet (24) to the air inlet (28) as required to maintain a desired wet bulb temperature.

Description: REFRIGERATION PLANT BACKGROUND OF THE INVENTION This invention relates to improvements in refrigera¬ tion plant and relates particularly to refrigeration plant having an evaporative condenser cooled by an induced or force draught air movement. BACKGROUND ART In many refrigeration plants, condensing of the compressed refrigerant gas is effected in an evaporative condenser. The hot, compressed refrigerant gas passes to the evaporative condenser whereby heat and mass transfer occurs and the latent heat of condensation in the refrigerant is removed by heat transfer through the walls of the evaporative condenser. In commercial and industrial plants, water is caused to pass over the evaporative condenser and air is introduced below the condenser, using either a forced or induced draught arrangement. Some of the water passing over the condenser is evaporated by the latent heat of condensation of the refrigerant and the induced or forced air draught enhances the heat transfer and removes the evaporated water. The heat removed from the refrigerant gas results in an increase in the enthalpy of the air passing over the evaporative condenser. The hourly rate of heat removal may be calculated as follows: Q = M x (h2-hl) , where Q = heat removed per hr. M = total weight air supplied to the condenser by the forced or induced draught fans, per hour. h2 = enthalpy of air leaving the condenser, hi = enthalpy of air entering the condenser. The enthalpy of air entering the condenser is largely determined by the wet bulb temperature of the air , which is a measure of its relative humidity. Because heat will only flow from a higher energy level to a lower energy level, it follows, that the higher is the wet bulb temperature of the entry air, the higher the refrigerant condensing temperature has to. be to effect heat exchange. It also follows that the higher is the wet bulb temperature of the entry air, the less heat exchange occurs for a given refrigerant condensing temperature. It is also known that the higher is the condensing temperature of the refrigerant gas, the higher is the condensing pressure. Further, the higher is the condensing pressure, the higher is the energy consumption of the compressor and, therefore, the greater the heat which must be rejected as the total heat rejected in the condenser. The total heat is the sum total of the latent heat of evaporation of the refrigerant plus most of the compression energy less the difference in enthalpy between the refrigerant condensing temperature and the evaporating temperature of the water, if no liquid refrigerant subcooling takes place. A system operating under the above conditions normally works well provided that the ambient air temperature is not greatly less than the freezing point of water. However, as the ambient temperature in colder climates frequently falls below -10°C, the ambient air has such a low wet bulb temperature as to freeze some of the water cascading through the evaporative condenser. It has previously been proposed, to overcome problems associated with low ambient air temperature, to cease water flow through the evaporative condenser and to operate the. system with a dry condenser, i.e., the heat exchange process changes from an evaporative process to a simple sensible heat transfer process at the surface of the tube forming the condenser. The rate of heat exchange of a dry coil operation is only between 5 and 10% compared to that of an evaporative heat exchange process. As a consequence, the temperature difference between the condensing refrigerant and the dry cooling air has to increase significantly to handle the heat being rejected in the condenser. Therefore, the refrigerant condensing pressure will tend to be much higher with dry air cooling than with evaporative cooling and, therefore, the compressors in the refrigerant system use substantially more energy than they need to use. It is not uncommon for refirgeration plants in cold climates to use more refrigeration compressor energy in winter than in summer for this reason. For example, in a system used for cooling hot beef, the temperature of the cool room is gnerally not lower than -1°C (+30°F) and frequently the outside temperature in winter in cold climates is -20°C (-4°F) . In such conditions, the condensing temperature is frequently +32°C (+90°F) whilst in spring, summer and autumn, the condensing temperature would be the same with ambient air temperatures as high as +30°C to 35°C (86 to 95°F) . It is therefore desirable to provide an improved refrigeration system and process which facilitates evaporative cooling even when ambient air temperature is substantially lower than freezing (0°C) . SUMMARY OF THE INVENTION According to one aspect of the invention there is provided a refrigeration system comprising an evaporative condenser, means for passing water over the condenser, means for passing air over the condenser, air ducting means to enable discharge air to be returned to an air inlet, and control means to control the flow of returned air. Preferably, in one form of the invention, the control means includes one or more dampers in the air ducting means, the dampers being operated in conjunction with sensed wet bulb temperature of the air entering the system so as to be able to control the amount of returned air to obtain the desired wet bulb temperature. - Preferably, two sets of dampers are provided, one at the end of the air ducting means adjacent the air discharge from the evaporative condenser and the second in the ambient air inlet. The two sets of dampers may be interconnected so that, as one set opens to allow air to pass into the air ducting means, the other closes thus restricting the entry of ambient air to the system. DESCRIPTION OF THE DRAWING Figure 1 schematically illustrates a heat exchange system foran evaporative condenser of a refrigeration system. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, there is shown a heat exchange unit for the condenser of a refrigeration system. The condenser 12, illustrated schematically, generally comprises one or more series or coils of metal tubes through which relatively hot, gaseous refrigerant passes. The gaseous refrigerant enters at 14 and, after condensing, liquid refrigerant flows out at 16 to the evaporator of the refrigeration system (not shown) . Water is caused to cascade ovet the condenser 12 to assist the heat removal from the refrigerant. The water is sprayed onto the top of the condenser 12 through water sprays 17 which are supplied from the pump 18 which draws water from the sump 19. Alternatively, the water may be supplied from a tank (not shown) which enters the water circuit at 21. With this arrangement, the sump 19 drains to the tank through the outlet 22. Make-up water to replace that which is lost through evaporation is supplied through the ball valve 20. Air is caused to flow over the condenser coil 12, preferably in countercurrent to the water flow. In the embodiment illustrated, the air is forced by a fan 23 which draws in ambient air through the ambient air inlet 28 and forces the air over the condenser 12 to an outlet 24 where the air is discharged to atmosphere at 25. A droplet eliminator 26 is located above the water sprays 17 to remove, or substantially remove, water droplets carried in the air stream. In accordance with the invention, the heat exchange unit is fitted with an air duct 27 extending between the air outlet 24 and the air inlet 28. The upper or inlet end of the air duct 27 i.e. that end adjacent the air outlet 24, is provided with a damper mechanism 29 which enables- the inlet to the air duct 27 to be opened any desired amount, or closed, as required. Similarly, another damper mechanism 31 is provided to control the amount of ambient air entering the air inlet 28 and passing to the fan 23. Both damper mechanisms 29 and 31 comprise a plurality of shutters 30 pivotal about their longitudinal axes between substantially horizontal positions, at which the damper mechanisms are fully open, and substantially vertical positions at which longitudinal edges of adjacent shutters 30 are contacting and the respective damper mechanisms 29 and 31 are fully closed. Other forms of damper mechanisms or air flow controllers may be used in the performance of the invention. Both damper mechanisms 29 and 31 are controlled by a modulating motor 4 activated by an electrical, electronic or solid state control device 9. The control device 9 receives an input signal from a sensor 32 positioned in the air stream in front of the fan 23, the sensor being able to sense either air wet bulb temperature, total air enthalpy, or both. The modulating motor 4 is connected to the damper mechanisms 29 and 31 by links 5, 6, 8 and 7 so that both dampers 29 and 31 operate together to direct more or less air, as required, from the air discharge 24 to the fan 23. Thus, if the sensor 32 senses that the air wet bulb temperature, or the total air enthalpy, is too low, the modulating motor 4 is actuated so as to simultaneously close the damper mechanism 31 a small amount and open the damper mechanism 29 a similar amount whereby some of the air drawn in by the fan 23 is drawn from the air outlet 24. Conversely, if the sensor 32 senses that the air wet bulb temperature, or the total air enthalpy, has risen above a predetermined level, the damper mechanisms 29 and 31 are actuated by the modulating motor 4 to reduce the amount of air drawn through the air duct 27 from the air outlet 24 to the fan 23. . The lowest, desirable value of fan entry air wet bult temperature or enthalpy is set at such a value as to consistently eliminate freezing of water on the fan 23, casing, condenser 12 and condenser sump 19. Thus, the sensor 32 and control device 9 are pre-set to maintain a minimum air wet bulb temperature or total air enthalpy, below which the damper mechanisms 29 and 31 are actuated to recirculate air from the air outlet 24 to the fan 23. With the arrangement of the present invention, higher condensing refrigerant pressures are avoided thus reducing the energy consumption of the refrigerant compressor which would normally be required. Therefore, the present invention is able to save substantial compressor energy. Although the embodiment illustrated is a forced draught heat exchange unit, the invention is readily adapted for use with an induced draught heat exchange unit. The invention may be incorporated on existing evaporative condenser systems by the provision of external air ducting and appropriate damper mechanisms. Alternatively, the invention may be incorporated into the design of heat exchange units to more easily facilitate the return of discharge air to the fan intake. The control system of the present invention will maintain the desired wet bulb temperature within relatively narrow limits which may be set for maximum efficiency of the heat exchange unit. It will be seen that the use of the present invention avoids the need to disconnect the water supply to the water sprays 17 to prevent freezing of the water in the water circulation system. Thus, the heat exchange unit is able . to operate at maximum efficiency irrespective of ambient air temperature.

Claims: * Claims . 1. An evaporative heat exchange unit for a refrigeration system having a refrigerant condenser, means for passing water over the condenser, and air flow means to cause air to pass over the condenser, the air flow means having an ambient air inlet upstream from the condenser and an air discharge downstream from the condenser characterized by air ducting means extending between the air discharge and the air inlet to enable at least some pf discharged air to be returned to the air inlet, and control means to control the flow of returned air. 2. An evaporative heat exchange unit according to Claim 1, characterized in that said control means includes a sensor to measure the wet bulb temperature of the inlet air to the air flow means. 3. An evaporative heat exchange unit according to Claim 1, characterized in that said control means includes at least one air damper means to control the flow of return air through the air ducting means. 4. An evaporative heat exchange unit according to Claim 3, characterized in that said control means has a first air damper means at the air discharge end of the air ducting means and a second air damper means at the air inlet, the first air damper means controlling the amount of air entering the air duct means and the second air damper means controlling the amount of ambient air entering the air inlet. 5. An evaporative heat exchange unit according to Claim 3, characterized in that said control means further includes modulator motor means connected to said at least one air damper means to move the air damper means between an open position and a closed position in response to sensed wet bulb temperature of the air entering said air flow means. 6. An evaporative heat exchange unit for a refrigeration system having a casing, a refrigerant condenser within the casing, water spray nozzles above the condenser to spray water onto the condenser, a forced or induced draft fan to force air through the casing passed the condenser in countercurrent to the flow of water over the condenser, the casing having an ambient air inlet and a discharged air outlet characterized in that an air duct extends between the air outlet and the air inlet, a first air damper is mounted in the end of the air duct adjacent the discharge air outlet and a second air damper is mounted in the air inlet, sensor means mounted in the air inlet adjacent the fan to measure wet bulb temperature of inlet air, and damper control means to control the operation of the first and second air dampers in response to the sensor means whereby at least some of the air discharged at the discharge air outlet is selectively returned to the air inlet. 7. An evaporative heat exchange unit according to Claim 6, characterized in that said damper control means comprises a motor connected to -each air damper by links and operable to move each air damper simultaneously between open and closed positions to vary the amount of returned discharge air to the air inlet in accordance with measured wet bulb temperature of the inlet air. 8. -An evaporative heat ■ exchange unit substantially as hereinbefore described with reference to the drawing.

Inventors: VISSER KLAAS, VISSER, KLAAS
Applicants: VISSER KLAAS, VISSER, KLAAS
Application year: 1985
Main classification: F25B39
IPC codes: F25B39, F28B1, F28D5