Refrigeration horsepower requirement

The following information was used in olefin plant case studies to determine if the ethylene/propylene cascaded refrigeration systems had enough horsepower for various plant operations. The propylene was condensed against cooling water at 110°F and the ethylene was condensed against propylene at -20°F. For comparison, the horsepower requirements for each refrigerant alone are also shown. 

Example. With the example cascaded system at an evaporator temperature of -100°F, the horsepower requirement is 6.2hp/ton refrigeration. A ton of refrigeration is equal to 12,000 BTU/hr. 

Determine the COP, horsepower required, and cooling load of a Carnot vapor refrigeration cycle using R-12 as the working fluid and in which the condenser temperature is 100°F and the evaporation temperature is 20°F. The circulation rate of fluid is O.llbm/sec. Determine the compressor power required, turbine power produced, net power required, cooling load, quality at the inlet of the evaporator, quality at the inlet of the compressor, and COP of the refrigerator. 

Example. A 50 ton two-stage propane refrigeration system has a 40°F evaporator temperature and a 120°F refrigerant condensing temperature. Find the gas horsepower requirement and condenser duty. 

Having established the cooling curve for the gas being liquefied, it becomes necessary to calculate the refrigeration system associated with it. Or, if only an estimate of refrigeration horsepower is required, reference may be made to a correlation of brake horsepower as a function of refrigerant evaporating temperature. Such correlations have been previously published [4, 10]. 

Looking at this low temperature refrigeration as to power requirement, one expander horsepower removes its heat equivalent to 2,545 Btu/lir, as eompared with 12,000 Btu/hr, about 4.7 times as mueh. This is refened to as a ton of refrigeration. Thus, the turhoexpander must develop 4.7 hp to generate a ton of refrigeration however, it delivers 4.7 hp haek as power. 

This quick but accurate graph shows the design engineer how much horsepower is required for mechanical refrigeration systems, using the most practical refrigerant for the desired temperature range. 

Thus, if a gas mixture exerts 100 psia total pressure and is composed of 20% by volume (mol%) propane and 80% by volume butane, the partial pressures are 20 and 80 psia for propane and butane, respectively. The liquid in equilibrium with this mixture of vapors would have a lower percentage of propane and a higher percentage of butane. If this mixture is used as a refrigerant, the low-boiling component (propane) reaches equilibrium with a higher concentration in the condenser (as liquid) and increases the total pressure in the condenser. This requires more head and more horsepower at the compressor. 

For final design horsepower and equipment selection, the usual practice is to submit the refrigeration load and utility conditions/requirements to a reputable refrigerant system designer/manufacturer and obtain a warranted system with equipment and instrumentation design and specifications including the important materials of construction. Always request detailed operating instructions/controls and utility quantity requirements. 

The tubes in the condenser required for subcooling steal heat-transfer surface area required for condensation. In effect, the condenser shrinks. This makes it more difficult to liquefy the refrigerant vapor. The vapor is then forced to condense at a higher temperature and pressure. Of course, this raises the compressor discharge pressure. And, as we have seen in the pressure section, this increase in compressor discharge pressure invariably reduces the compressor s capacity and may also increase the horsepower needed to drive the compressor. 

Similarly, any fouling will reduce efficiency, and 5 mil (5 thousandths of an inch) of a scale or deposit may well reduce efficiency by 20 to 30%. A small amount of scale can greatly increase refrigeration compressor horsepower or cooling water pumping requirements. 

The steam turbines require one-third the energy of An electric motor. Each refrigeration unit has a different horsepower/ton characteristic, which also depends upon ambient conditions. There are hard constraints on compressor loads and cost penalties (soft constraints) on electrical load. Steam, refrigeration, compressed air, and electrical loads to the plant vary continually. While the author does not suggest that the optimum operating and control strategy is known, he does imply that a computer control system is the only way to operate the plant in an optimum manner. 

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