Compressors isothermal compression

The Intercooled Regenerative Reheat Cycle The Carnot cycle is the optimum cycle between two temperatures, and all cycles try to approach this optimum. Maximum thermal efficiency is achieved by approaching the isothermal compression and expansion of the Carnot cycle or by intercoohng in compression and reheating in the expansion process. The intercooled regenerative reheat cycle approaches this optimum cycle in a practical fashion. This cycle achieves the maximum efficiency and work output of any of the cycles described to this point. With the insertion of an intercooler in the compressor, the pressure ratio for maximum efficiency moves to a much higher ratio, as indicated in Fig. 29-36. 

The theoretical required (isothermal) compression work in the compressor, which is assumed to operate isothermally at To, is... 

Isothermal compression is presented here to represent the upper limits of cooling and horsepower savings. It is the equivalent of an infinite number of intercoolers and is not achievable in the practical types of compressors described in this book. For an isothermal process. 

The discussion of the last section is then useful in considering the evaporative cycles. We shall see that the effect of water injection downstream of the compressor (and possibly in the cold side of the heat exchanger) may lead towards the [CBTJiXr type of plant, with increased cold side effective specific heat and hence increased heat exchanger effectiveness. Water injection in the compressor may lead to a plant with isothermal compression. 

The scope of coverage includes internal flows of Newtonian and non-Newtonian incompressible fluids, adiabatic and isothermal compressible flows (up to sonic or choking conditions), two-phase (gas-liquid, solid-liquid, and gas-solid) flows, external flows (e.g., drag), and flow in porous media. Applications include dimensional analysis and scale-up, piping systems with fittings for Newtonian and non-Newtonian fluids (for unknown driving force, unknown flow rate, unknown diameter, or most economical diameter), compressible pipe flows up to choked flow, flow measurement and control, pumps, compressors, fluid-particle separation methods (e.g.,... 

Although isothermal compression is desirable, in practice the heat of compression is never removed fast enough to make this possible. In actual compressors only a small fraction of the heat of compression is removed and the process is almost adiabatic. 

The schematic Ericsson cycle is shown in Fig. 4.27. The p-v and T-s diagrams of the cycle are shown in Fig. 4.28. The cycle consists of two isothermal processes and two isobaric processes. The four processes of the Ericsson cycle are isothermal compression process 1-2 (compressor), isobaric compression heating process 2-3 (heater), isothermal expansion process 3-4 (turbine), and isobaric expansion cooling process 4-1 (cooler). 

Expressions for the theoretical power requirements of gas compressors can be obtained from the basic equations of thermodynamics. For an ideal gas undergoing an isothermal compression (pv = constant), the theoretical power requirement for any number of stages can be expressed as follows ... 

The constant temperature process is a case when n=l, which is equivalent to isothermal compression, the constant pressure process n = 0 and the constant volume process n = Generally, it is impractical to build sufficient heat transfer equipment into the design of most compressors to convey the bulk of the heat of compression. Therefore most machines tend to operate along a poly tropic path that approaches the adiabatic. Most compressor calculations are based on the adiabatic curve [3]. 

Real compression processes operate between adiabatic and isothermal compression. Actual compression processes are polytropic processes. This is because the gas being compressed is not at constant entropy as in the adiabatic process, or at constant temperature as in the isothermal processes. Generally, compressors have performance characteristics that are analogous to those of pumps. Their performance curves relate flow capacity to head. The head developed by a fluid between states 1 and 2 can be derived from the general thermodynamic equation. 

For a given compression ratio and suction condition, the work requirement in isothermal compression is less than that for adiabatic compression. This is one reason why cooling is useful in compressors. 

All the foregoing concerned zero-clearance compressors, ones in which no gas is left in the cylinder at the end of the discharge stroke. For mechanical reasons it is impractical to build a compressor with zero clearance. So in real compressors there is always a small amount of gas in the top of the cylinder, which is repeatedly compressed and expanded. If the compression and expansion are reversible, either adiabatic or isothermal, then they contribute as much work on the expansion step as they require on the compression step, and thus they contribute nothing to the net work requirement of the compressor. For real compressors the compression and the expansion of the gas in the clearance volume contribute to the inefficiency of the compressor compressor designers make the clearance volume as small as practical. 

In this cycle, gas is isothermally compressed from point 1 to point 2. The heat of compression, which raises the gas temperature at the compressor outlet, is removed with cooling water in the aftercooler downstream of... 

These variables are used to determine costs associated with hydrogen compression for both types of gaseous storage using a multistage compressor. The energy required (kW) for isothermal compression is a function of the production rate as well as the inlet and outlet pressures... 

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