Refrigeration horsepower

A commonly used unit of COP is (tons of refrigeration)/ (horsepower input). Some of the refrigerants suited to particular temperature ranges arc listed in Tables 1.10, 8.23, and 8.24. 

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. 

Example. A water-cooled unit with an evaporator temperature of-40°F will require 3 horsepower/ton of refrigeration. A ton of refrigeration is equal to 12,000 BTU/hr. Here are equations for these curves in the fonn ... 

Figure 1. Gas horsepower for single-stage propane refrigeration system.

Figure 9. Gas horsepower for two-stage propylene refrigeration system.

Figure 17. Gas horsepower for three-stage ethane refrigeration system.

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. 

Compressor horsepower is best determined using the horsepower calculation in Chapter 6. For refrigeration compressors, the horsepower can he approximated another way that may prove to be simpler. The compressor horsepower per ton of refrigeration load depends upon the evaporator and eondenser temperatures. See the seetion titled Estimating Horsepower per Ton in Chapter 11. 

Determine the approximate horsepower and condenser duty for a single-stage propane refrigeration system (see flow diagram of Figure 11-43). 

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. 

Figure 11 -35. Gas horsepower for single-stage propylene refrigeration system. (Excerpted by special permission Mehra, Y. R. Chemical Engineering, Jan. 15,1979. McGraw-Hill, Inc., New York. All rights reserved.)...

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. 

Table 11-14 compares four basic systems using refrigerant 12, and generally indicates that the total horsepower can be reduced by using a multistage compression arrangement... 

Many compressors are designed and operated at a fixed condition in a process or refrigeration cycle. However, at least an equal number are designed and operated over a varying, or at the initial selection unknown, set of conditions of suction or discharge pressures. This situation is a reality and an economical necessity if the full horsepower of the compressor and driver combination is to be realized. 

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