Compressor horsepower, determining

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 COP, the compressor horsepower required, turbine power produced, net power required, and cooling load for the cycle. 

Horsepower is the work done in a cylinder on the gas by the piston connected to the driver during the complete compression cycle. The theoretical horsepower is that required to isen-tropically (adiabatically) compress a gas through a specified pressure range. The indicated horsepower is the actual work of compression developed in the compressor cylinder(s) as determined from an indicator card. Brake horsepower (bhp) is the actual horsepower input at the crankshaft of the compressor drive. It does not include the losses in the driver itself, but is rather the actual net horsepower that the driver must deliver to the compressor crankshaft. 

This is approximately correct because the mechanical losses in the compressor are only about 1-3%. The head determined from Figure 12-67 can be used for the polytropic gas horsepower relation given previously. 

Shaft horsepower is greater than gas or hydraulic (hpg) horsepower due to the effects of bearings, seals, and windage (or wheel friction losses). Although these losses can be determined individually, the sum of all amount to 1-3% of rated gas (hpg) horsepower on large- to medium-size compressors. ... 

Unlike centrifugal pumps and compressors, few criteria can be used directly to determine mixer effectiveness and efficiency. However, product quality and brake horsepower are indices that can be used to indirectly gauge performance. 

A compressor feeds ethylene to a pipeline, that is 500 ft long and 6 in. in diameter. The compressor suction pressure in 50 psig at 70°F, the discharge pressure is 800 psig, and the downstream pressure at the end of the pipeline is 300 psig. For each of the two following cases, determine (1) the flow rate in the pipeline in scfm and (2) the power delivered from the compressor to the gas, in horsepower ... 

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. 

The key to reading Fig. 8.9 and these accompanying tables for the Fa cost in dollars is of course finding the horsepower of the compressor. It is therefore necessary to determine the required horsepower from the common given variables, which normally are as follows ... 

ILLUSTRATIVE EXAMPLE 15.9 Determine the hydraulic, brake and installed horsepower required to compress a reactor feed of5000 Ib/h of ethane vapor at 20 psia and 75°F, having a density of 0.105 Ib/ft, to 200 psia. Large compressors typically have efficiencies in the range of 75% to 85% employ a median value of 80% in the calculation. 

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