and the COP is


should not be confused with efficiencies applied to devices that operate along a process line. This efficiency is defined as

actual energy transfer ideal energy transfer for a work-producing device and ideal energy transfer

actual energy transfer duced in a heat engine, and W is the work required to drive the refrigerator. A heat engine produces useful work, while a refrigerator uses work to transfer heat from a cold to a hot region. There is an ideal cycle, called the Carnot cycle, which yields the maximum efficiency for heat engines and refrigerators. It is composed of four ideal reversible processes; the efficiency of this cycle is

These represent the best possible performance of cyclic energy conversion devices operating between temperature extremes, TH and TL. The thermodynamic efficiency for a work-consuming device. Note these definitions are such that n < 1. These efficiencies are convenience factors in that the actual performance can be calculated from an ideal process line and the efficiency, which generally must be experimentally determined. Table I.5 shows the most commonly encountered versions of efficiencies.

I.2.5 Power and Refrigeration Cycles

Many cycles have been devised to convert heat into work, and vice versa. Several of these take advantage of the phase change of the working fluid: for example, the Rankine, the vapor compression, and the absorption cycles. Others involve approximations of thermodynamic processes to mechanical processes and are called air-standard cycles.

Table I.5 Thermodynamic Efficiency

Heat engines and refrigerators:

Engine efficiency n = w/qh < nCarnot = (TH - TL)/ TH < 1

Heat pump C°.p. p' = Qh/w< P Carnot = TH/(TH - TL) > 1

Refrigerator cap. p = Ql/w< pCarnot = TlJ(TH - TL), 0 < P < », (QH/QL)Carnot = Th - Tl

Process efficiencies nad, turbine = ^actual, adiabatic/^isentropic nad, compressor = wisentropic/^actual, adiabatic nad, nozzle = K.E.actual, adiabatic/K.E.isentropic

0 0

Post a comment