Refrigeration utilizes the basic thermodynamic property of evaporation to remove heat from a process. When a refrigerant is evaporated in a heat exchanger, the medium that is in contact with the heat exchanger (i.e., air, water, glycol, food) transfers heat from itself through the heat exchanger wall and is absorbed by the refrigerant, resulting in the refrigerant changing from a liquid state to a gaseous state. Once the refrigerant is in a gaseous state, the heat must be rejected by compressing the gas to a high pressure state and then passing the gas through a condenser (a heat exchanger) where heat is removed from the gas by a cooling medium resulting in condensation of the gas to a liquid. The medium in the condenser that absorbs the heat in a cooling medium and is often water, air, or both water and air. The refrigerant in this liquid state is then ready to be used again as a refrigerant for absorbing heat.
In general, industrial refrigeration systems utilize large amounts of horsepower oftentimes requiring multiple industrial compressors. Due to this fact, industrial refrigeration systems typically include large centralized engine rooms and large centralized condensing systems. Once the compressors compress the gas, the gas that is to be condensed (not used for defrosting) is pumped to a condenser in the large centralized condensing system. The multiple condensers in a large centralized condensing system are often referred to as the “condenser farm.” Once the refrigerant is condensed, the resulting liquid refrigerant is collected in a vessel called a receiver, which is basically a tank of liquid refrigerant.
There are generally three systems for conveying the liquid from the receiver to the evaporators so it can be used for cooling. They are the liquid overfeed system, the direct expansion system, and the pumper drum system. The most common type of system is the liquid overfeed system. The liquid overfeed system generally uses liquid pumps to pump liquid refrigerant from large vessels called “pump accumulators” and sometimes from similar vessels called “intercoolers” to each evaporator. A single pump or multiple pumps may deliver liquid refrigerant to a number of evaporators in a given refrigeration system. Because liquid refrigerant has a tendency to evaporate, it is often necessary to keep large amounts of liquid in the vessels (net positive suction head (NPSH)) so the pump does not lose its prime and cavitate. A pump cavitates when the liquid that the pump is attempting to pump absorbs heat inside and around the pump and gasifies. When this happens, the pump cannot pump liquid to the various evaporators which starve the evaporators of liquid, thus causing the temperature of the process to rise. It is important to note that liquid overfeed systems are designed to overfeed the evaporators. That is, the systems send excess liquid to each evaporator in order to ensure that the evaporator has liquid refrigerant throughout the entire circuit of the evaporator. By doing this, it is normal for large amounts of liquid refrigerant to return from the evaporator to the accumulator where the liquid refrigerant in turn is pumped out again. In general, the systems are typically set up for an overfeed ratio of about 4:1, which means that for every 4 gallons of liquid pumped out to an evaporator, 1 gallon evaporates and absorbs the heat necessary for refrigeration, and 3 gallons return un-evaporated. The systems require a very large amount of liquid refrigerant in order to provide the necessary overfeed. As a result, the systems require maintaining a large amount of liquid refrigerant to operate properly.
Referring to FIG. 1, a representative industrial, two-stage refrigeration system is depicted at reference number 10 and provides for liquid overfeed where the refrigerant is ammonia. The plumbing of various liquid overfeed refrigeration systems may vary, but the general principles are consistent. The general principles include the use of a centralized condenser or condenser farm 18, a high pressure receiver 26 for collecting condensed refrigerant, and the transfer of liquid refrigerant from the high pressure receiver 26 to various stages 12 and 14. The two-stage refrigeration system 10 includes a low stage system 12 and a high stage system 14. A compressor system 16 drives both the low stage system 12 and the high stage system 14, with the high stage system 14 sending compressed ammonia gas to the condenser 18. The compressor system 16 includes a first stage compressor 20, second stage compressor 22, and an intercooler 24. The intercooler 24 can also be referred to as a high stage accumulator. Condensed ammonia from the condenser 18 is fed to the high pressure receiver 26 via the condenser drain line 27 where the high pressure liquid ammonia is held at a pressure typically between about 100 psi and about 200 psi. With reference to the low stage system 12, the liquid ammonia is piped to the low stage accumulator 28 via the liquid lines 30 and 32. The liquid ammonia in the low stage accumulator 28 is pumped by the low stage pump 34, through the low stage liquid line 36 to the low stage evaporator 38. At the low stage evaporator 38, the liquid ammonia comes in contact with the heat of the process, thus evaporating approximately 25% to 33% (the percent evaporated can vary widely), leaving the remaining ammonia as a liquid. The gas/liquid mixture returns to the low stage accumulator 28 via the low stage suction line 40. The evaporated gas is drawn into the low stage compressor 20 via the low stage compressor suction line 42. As the gas is removed from the low stage system 12 via the low stage compressor 20 it is discharged to the intercooler 24 via line 44. It is necessary to replenish the ammonia that has been evaporated, so liquid ammonia is transferred from the receiver 26 to the intercooler 24 via liquid line 30, and then to the low stage accumulator 28 via liquid line 32.
The high stage system 14 functions in a manner similar to the low stage system 12. The liquid ammonia in the high stage accumulator or intercooler 24 is pumped by the high stage pump 50, through the high stage liquid line 52 to the high stage evaporator 54. At the evaporator 54, the liquid ammonia comes in contact with the heat of the process, thus evaporating approximately 25% to 33% (the percent evaporated can vary widely), leaving the remaining ammonia as a liquid. The gas/liquid mixture returns to the high stage accumulator or intercooler 24 via the high stage suction line 56. The evaporated gas is then drawn into the high stage compressor 22 via the high stage compressor suction line 58. As the gas is removed from the high stage system 14, it is necessary to replenish the ammonia that has been evaporated, so liquid ammonia is transferred from the high pressure receiver 26 to the intercooler 24 via the liquid line 30.
The system 10 can be piped differently but the basic concept is that there is a central condenser 18 which is fed by the compressor system 16, and condensed high pressure liquid ammonia is stored in a high pressure receiver 26 until it is needed, and then the liquid ammonia flows to the high stage accumulators or intercooler 24, and is pumped to the high stage evaporator 54. In addition, liquid ammonia at the intercooler pressure flows to the low stage accumulator 28, via liquid line 32, where it is held until pumped to the low stage evaporator 38. The gas from the low stage compressor 20 is typically piped via the low stage compressor discharge line 44 to the intercooler 24, where the gas is cooled. The high stage compressor 22 draws gas from the intercooler 24, compresses the gas to a condensing pressure and discharges the gas via the high stage discharge line 60 to the condenser 18 where the gas condenses back to a liquid. The liquid drains via the condenser drain line 27 to the high pressure receiver 26, where the cycle starts again.
The direct expansion system uses high pressure or reduced pressure liquid from a centralized tank. The liquid is motivated by a pressure difference between the centralized tank and the evaporator as the centralized tank is at a higher pressure then the evaporator. A special valve called an expansion valve is used to meter the flow of refrigerant into the evaporator. If it feeds too much, then un-evaporated liquid refrigerant is allowed to pass through to the compressor system. If it feeds to little, then the evaporator is not used to its maximum capacity, possibly resulting in insufficient cooling/freezing.
The pumper drum system works in a nearly identical fashion to the liquid overfeed system, with the main difference being that small pressurized tanks that act as pumps. In general, liquid refrigerant is allowed to fill the pumper drum, where a higher pressure refrigerant gas is then injected on top of the pumper drum thus using pressure differential to push the liquid into the pipes going to the evaporators. The overfeed ratios are generally the same, as is the large amount of refrigerant necessary to utilize this type of system.
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