Efficiency, compression isentropic

Similar to volumetric efficiency, isentropic (adiabatic) efficiency T is the ratio of the work required for isentropic compression of the gas to work input to the compressor shaft. The adiabatic efficiency is less than one mainly due to pressure drop through the valve ports and other restricted passages and the heating of the gas during compression. 

The operating cost for air compression is basically the electric utility needed ftir ihe isentropic compression. Electric energy needed to compress air may be calculated using Eq. (2.29). The isentropic efficiency of the compressor is taken as 60% and the electric energy cost is 0.06/kWhr,... 

A third objective is similarly obvious. If compression and expansion processes can attain more isentropic conditions, then the cycle widening due to irreversibility is decreased, cr moves nearer to unity and the thermal efficiency increases (for a given t). Cycle modifications or innovations are mainly aimed at increasing (by increasing or decreasing a)-... 

Consider next a similar recuperative cycle, but one in which the compression process approximates to isothermal rather than isentropic, with the work input equal to the heat rejected (this may be achieved in a series of small compressions of polytropic efficiency Tjp, followed by a series of constant pressure heat rejections). It may then be shown that the thermal efficiency of this cycle is given by... 

The factor [ I + c — < PilP )l-Y] is called the theoretical volumetric efficiency and is a measure of the effect of the clearance on an isentropic compression. The actual volumetric efficiency will be affected, in addition, by the inertia of the valves and leakage past the... 

Methane is compressed from 1 bar and 290 K to 10 bar. If the isentropic efficiency is 0.85, calculate the energy required to compress 10,000 kg/h. Estimate the exit gas temperature. 

Equation B.28 is useful to estimate the polytropic coefficient n if the inlet and outlet pressures are known, along with an estimate of the isentropic efficiency. Knowing the polytropic coefficient allows the outlet temperature for a real gas compression to be estimated from Equation B.26. 

Equation B.14 is the work required for an ideal adiabatic (isentropic) compression. To account for inefficiencies in the compression process and the mechanical inefficiency, the isentropic compression efficiency is introduced ... 

Introducing the isentropic compression efficiency gives Equation B.15, the same result for a reciprocating compressor. In a reciprocating compressor, the net effect of the cycle is a flow process, even though intermittent nonflow steps are involved. 

Whereas reciprocating compressors are normally designed on the basis of adiabatic work (together with an isentropic efficiency), centrifugal compressors are usually designed on the basis of polytropic work. By analogy with Equation B.15, the work required for a poly tropic compression is given by ... 

An engine operates on an Otto cycle with a compression ratio of 8. At the beginning of the isentropic compression process, the volume, pressure, and temperature of the air are 0.01 m, llOkPa, and 50°C. At the end of the combustion process, the temperature is 900°C. Find (a) the temperature at the remaining two states of the Otto cycle, (b) the pressure of the gas at the end of the combustion process, (c) the heat added per unit mass to the engine in the combustion chamber, (d) the heat removed per unit mass from the engine to the environment, (e) the compression work per unit mass added, (f) the expansion work per unit mass done, (g) MEP, and (h) thermal cycle efficiency. 

An ideal Diesel cycle with a compression ratio of 17 and a cutoff ratio of 2 has a temperature of 313 K and a pressure of 100 kPa at the beginning of the isentropic compression process. Use the cold air-standard assumptions and assume that k= A. Determine (a) the temperature and pressure of the air at the end of the isentropic compression process and at the end of the combustion process, and (b) the thermal efficiency of the cycle. 

Air is compressed from 14.7 psia and 500°R isothermally to 821.8 psia, heated isochorically to 2500° R, and then expanded isentropically to 14.7 psia in a Wicks cycle. Determine the heat added, heat removed, work added, work produced, net work, and cycle efficiency. 

The Otto cycle is a spark-ignition reciprocating engine consisting of an isentropic compression process, a constant-volume combustion process, an isentropic expansion process, and a constant-volume cooling process. The thermal efficiency of the Otto cycle depends on its compression ratio. The compression ratio is defined as r= Fmax/f min- The Otto cycle efficiency is limited by the compression ratio because of the engine knock problem. 

When friction is present, the problem is handled with empirical efficiency factors. The isentropic compression efficiency is defined as... 

Figure 7.27. Efficiencies of centrifugal and reciprocating compressors, (a) Polytropic efficiencies of centrifugal compressors as a function of suction volume and compression ratio (Clark Brothers Co.), (b) Relation between isentropic and polytropic efficiencies, Eqs. (7.22) (7.23). (c) Isentropic efficiencies of reciprocating compressors (De Laval Handbook, McGraw-Hill, New York, 1970). Multiply by 0.95 for motor drive. Gas engines require 7000-8000 Btu/HP.

Finding Work of Compression with a Thermodynamic Chart Hydrogen sulfide is to be compressed from 100°F and atmospheric pressure to SOpsig. The isentropic efficiency is 0.70. A pressure-enthalpy chart is taken from Starling (Fluid Thermodynamic Properties for Light Petroleum Systems, Gulf, Houston, TX, 1973). The work and the complete thermodynamic conditions for the process will be found. 

Volumetric Efficiency. For practical reasons, the gas is not completely discharged from a cylinder at each stroke of a reciprocating machine. The clearance of a cylinder is filled with compressed gas which reexpands isentropically on the return stroke. Accordingly, the gas handling capacity of the cylinder is less than the product of the cross section by the length of the stroke. The volumetric efficiency is... 

Ethylene is to be compressed from 5 to 75 atm in three stages. Temperature to the first stage is 60°F, those to the other stages are 100°F. Pressure loss between stages is 0.34 atm (5psi). Isentropic efficiency of each stage is 0.87. Compressibilities at the inlets to the... 

In a compression process, the isentropic work, as given by Eq. (7.27), is the minimum shaft work required for compression of a gas from a given initial state to a given discharge pressure. Thus we define a compressor efficiency as... 

Irreversibilities in the compressor and turbine greatly reduce the thermal, efficiency of the power plant, because the net work is the difference between the work required by the compressor and the work produced by the turbine. The temperature of the air entering the compressor TA and the temperature of the air entering the turbine, the specified maximum for Tc, are the same as for the ideal cycle. However, the temperature after irreversible compression in the compressor Ts is higher than the temperature after isentropic compression T B, and the temperature after i never - ible expansion in the turbine TD is higher than the temperature after isentropic expansion T d. 

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