There is a variety of compressors to choose from for refrigeration services. The most common are centrifugal, reciprocating, and screw compressors. The type of refrigerant used also influences the compressor choice. Small, up to 200 hp, halocarbon-type refrigeration systems normally use reciprocating compressors which have the crankcase vented to the compressor suction.
This type of compressor can also be used for propane, but the problem of propane solubility in the oil at higher temperatures requires special lubricating oil and a crankcase heater.
Refrigeration load also influences the compressor selection. Centrifugal compressors are not normally economical below about 500 hp with motor drivers, or about 800 hp with turbine drives. Above 1,000 hp, and particularly where the load is close to an even multiple of this number, the use of centrifugal compressors becomes more economical. For lower horse powers, reciprocating, screw, and rotary compressors are commonly used.
At the normal process temperatures encountered in gas processing, a three or four wheel centrifugal compressor is normally required for refrigeration service. This offers the opportunity of utilizing multiple inter stage flash economizers and permits multiple chiller temperature levels for further reductions in horsepower. Centrifugal compressor capacity is controlled by speed variation or suction or discharge pressure throttling. Discharge throttling can cause surge. It is also possible to recirculate refrigerant discharge vapors to the compressor suction during operation at lower loading in order to avoid surge problems. Such recirculation results in wasted horsepower and is one of the primary drawbacks to utilizing centrifugal units.
Process temperatures generally dictate two stage compression in a reciprocating machine. This affords the opportunity for one inter stage economizer, and also one additional level of chilling. In a conventional refrigeration system, the first stage cylinder is normally quite large as a result of the low suction pressure. The economizer also reduces first stage volume, cylinder diameter, and consequently rod load. Capacity adjustment is accomplished by speed variation, variable clearance on the cylinders, valve lifters, and recirculation of refrigerant vapor to the suction. As with centrifugal compressors, recirculation does result in wasted horsepower. It is also possible to throttle the refrigerant suction pressure between the chiller and compressor in order to reduce cylinder capacity. However, suction pressure control can result in wasted horsepower and the possibility of below atmospheric suction pressure, which should be avoided.
Screw compressors have been used in refrigeration systems for many years. They can be employed with all refrigerants. The limitation for suction pressure is about 3 psia with standard discharge pressures at 350 psig. Discharge pressures of over 750 psig are also available. Screw compressors are gaining popularity in the gas processing industry. Screws can operate over a wide range of suction and discharge pressures without system modifications. There are essentially no compression ratio limitations with ratios up to 10 being used. They operate more efficiently in the 2 to 7 ratio and are comparable in efficiency to reciprocating compressors within this range. Automatic capacity control can provide capacity adjustments from 100% down to 10% with comparable reduction in power requirements. Screw compressors normally operate at 3600 rpm direct coupled to motor drives. However, they can operate over a range of speeds from 1500 to 4500 rpm. Engine drives, gas turbines, and expanders can also be used as drivers.
There is a limited application for large rotary compressors. This is the low-temperature field in which the rotary serves the purpose of a high volume low stage or booster compressor. These booster compressors are applied at saturated suction conditions ranging from –125°F to –5°F with R-12, R-22, ammonia, and propane refrigerants. Available units range in horsepower from 10 to 600 hp and in displacement from 60 to 3600 cfm in a single unit.
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