Compressors isentropic efficiency

Compressor Isentropic efficiency T)c = Isentropic enthalpy ri.se/enthalpy rise... 

Table 23.3 summarizes the conditions at each state point. (Points 2s and 4s represent points the conditions for isentropic processes input properties in the table are underlined.) Point a is at atmospheric conditions (the dead state). Point 1, the compressor inlet, is reached after the atmospheric air undergoes a throttling process (constant enthalpy) and experiences a 0.2 psi pressure drop, and point 2s would be achieved in an isentropic compressor. The ratio of the reduced pressure equals the ratio of the actual pressures for an isentropic process. In point 2, the enthalpy is calculated from the compressor isentropic efficiency. In the combustion chamber, the pressure drops 2 psi and the temperature is increased to 1800°F. Point 4s and 4 are analyzed similarly to 2s and 2. Finally, the pressure drops to point 5 where the air enters the atmosphere. The accompanying h-s (or T-s) diagram shows the cycle and includes all of the pressure drops and turbine and compressor inefficiencies. 

Compressor isentropic efficiency at 75% (commonly called adiabatic)... 

W is gas flow rate, in Ib/min Es is compressor isentropic efficiency, in fraction. 

It should be noted that the inside diameter decreases with increasing pressure since outer diameter was heid constant. This approach maintains consistency of the piping system arrangement but results in increased hydraulic resistance. Turbine and compressor isentropic efficiency were assumed to decrease with increasing pressure as shown in Table 6-4. 

The sensitivity of Brayton turbine and compressor isentropic efficiency to overall plant performance was examined for a range of efficiencies 2% from their baseline values Turbine and compressor efficiencies were assumed to vary simultaneously as listed in Table 6 5. 

The turboalternator has many characteristics that determine the operational efficiency of the Brayton power cycle. Irreversibilities in the component which affect the system efficiency include the turbine and compressor isentropic efficiency, bearing power losses, windage losses, EM PR alternator losses, thermal management power losses (pump), and ambient heat loss to space. These losses will be described in more detail in the sub-component sections below. Reference 9-11 describes the significance of these parasitic losses to the overall power system efficiency. 

For the actual systems, compressor work will be higher than for ideal for the isentropic efficiency and other losses. In the case of hermetic or accessible compressors where an elec trical motor is cooled by the refrigerant, condenser capacity should be ... 

From this relationship, it is obvious that polytropic efficiency is the limiting value of the isentropic efficiency as the pressure increase approaches zero, and the value of the polytropic efficiency is higher than the corresponding adiabatic efficiency. Figure 3-6 shows the relationship between adiabatic and polytropic efficiency as the pressure ratio across the compressor increases. Figure 3-7 shows the relationship across the turbine. 

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,... 

From the study of uncooled cycles in Chapter 3, we next move to consider irreversible cycles with compressor and turbine isentropic efficiencies, tjc and r/p, respectively. 

By introducing isentropic efficiencies of the turbine ni and compressor the turbine work output is given as ... 

The isentropic efficiency (see Appendix B) is a function of the machine design and pressure ratio (Pout/Pm) A first estimate of the isentropic efficiency of a reciprocating compressor can be obtained from ... 

Estimate the compressor efficiency. Compressor manufacturer s data should be used where possible. If this is not available, then for a reciprocating compressor, the isentropic efficiency can be estimated from Equation 13.17 ... 

The compressor efficiency (in this case, isentropic efficiency) can now be calculated and in turn the polytropic coefficient. This allows the outlet temperature to be calculated, together with the compressor power. 

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 ... 

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.

Separately driven centrifugal compressors are adaptable to low-pressure cryogenic systems because they can be coupled directly to steam turbine drives, are less critical from the standpoint of foundation design criteria, and lend themselves to gas turbine or combined cycle applications. Isentropic efficiencies of 80 to 85% are usually obtained. 

Compressor efficiencies are usually expressed as isentropic efficiencies, i.e., on the basis of an adiabatic reversible process. Isothermal efficiencies are sometimes quoted, and design calculations are simplified when isothermal efficiencies are used. In either case, the efficiency is defined as the ratio of the power required for the ideal process to the power actually consumed. 

Isentropic efficiency of a compressor rjc is the ratio of isentropic work to actual work input... 

To size a conq)ressor requires calculating the power needed for compression. This can be done by assuming an isentropic compression and then correcting the result by dividing by an isentropic efficiency. The power can also be calculated by assuming a polytropic compression, and then correcting the result by dividing by a polytropic efficiency. Both methods will be considered. The isentropic method is also used for blowers and vacuum pumps, but the polytropic method could also be used if data were available. First, we need to derive relationships to calculate the compressor power. 

For an adiabatic gas compressor, the efficiency with respect to isentropic operation r] is a measure of internal irreversibilities so is the dimensionless rate of entropy generation Sg/R = Sfi/fiR. Assuming tliat the gas is ideal with constant heat capacities, show that r] and Sg/R are related through the expression ... 

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