Compression Cycles

Natural gas Hquids are recovered from natural gas using condensation processes, absorption (qv) processes employing hydrocarbon Hquids similar to gasoline or kerosene as the absorber oil, or soHd-bed adsorption (qv) processes using adsorbants such as siHca, molecular sieves, or activated charcoal. Eor condensation processes, cooling can be provided by refrigeration units which frequently use vapor-compression cycles with propane as the refrigerant or by... 

If the compression cycle approaches the isothermal condition, pV = constant, as is the case when several stages with intercoolers are used, a simple approximation of the power is obtained from the following formula ... 

Vapor-Compression Cycles The most widely used refrigeration principle is vapor compression. Isothermal processes are realized through isobaric evaporation and condensation in the tubes. Standard vapor compression refrigeration cycle (counterclockwise Ranldne cycle) is marked in Fig. ll-72<7) by I, 2, 3, 4. 

Although the T-s diagram is veiy useful for thermodynamic analysis, the pressure enthalpy diagram is used much more in refrigeration practice due to the fact that both evaporation and condensation are isobaric processes so that heat exchanged is equal to enthalpy difference A( = Ah. For the ideal, isentropic compression, the work could be also presented as enthalpy difference AW = Ah. The vapor compression cycle (Ranldne) is presented in Fig. H-73 in p-h coordinates. 

The helical and spiral-lobe compressors are generally similar and use two imermeshing helical or spiral lobes to compress gas between the lobes and the rotor chamber of the casing. The compression cycle begins... 

Earlier in the chapter, when the compression cycle was described, a portion of the indicator. Path 3-4, was referred to as the expansion portion of the cycle. The gas trapped in the clearance area expands and partly refills the cylinder taking away some of the capacity. The following equation reflects the expansion effect on capacity and is referred to as the theoretical volumetric efficiency Ev,. 

Figure 4-2. The compression cycle of a helical-lobe compressor.

The terms pressure ratio and volume ratio are used interchangeably in the literature on these machines. To prevent confusion, volume ratio t , is defined as the volume of the trapped gas at the start of the compression cycle divided by the volume of the gas just prior to the opening of the discharge port. Pressure ratio is defined, in Equation 2.64, as the discharge pressure divided by the suction pressure. Their relationship is given in the following equation. 

Figure 4-22. Pressure-volume diagram of the compression cycle of a sliding vane compressor.

To start, convert the flow to values estimated to be the compressor inlet conditions. Initially, the polytropic head equation (Equation 2.73) will be used with n as the polytropic compression exponent. If prior knowledge of the gas indicates a substantial nonlinear tendency, the real gas compression exponent (Equation 2.76) should be substituted. As discussed m Chapter 2, an approximation may be made by using the linear average ut the inlet and outlet k values as the exponent or for the determination of the polytropic exponent. If only the inlet value of k is known, don t be too concerned. The calculations will be repeated several times as knowledge of the process for the compression cycle is developed. After selecting the k value, u,se Equation 2.71 and an estimated stage efficiency of 15 / to de clop the polytropic compression exponent n. 

Another reason for the interest in heat-driven cycles is their ability to produce higher temperature outputs than vapour compression cycles. There are industrial... 

Fig. 5a. Vapour compression cycle Fig. 5b. Basic adsorption cycle...

For theoretical cycle work performed in an adiabatic compression cycle (nonideal fluid) ... 

The term refrigeration refers to the gas coming into contact with evaporator coils on a dx vapor-compression cycle, coils on an absorption cycle, vortex... 

A cylinder is a pressure vessel that holds the gas during the compression cycle. There are two basic types ... 

Equipment diagram for refrigeration using a basic vapor compression cycle. 

Figure 2, the pressure-enthalpy plot of the standard vapor compression cycle, traces the state of the refrigerant through the refrigeration system. (Enthalpy represents the energy of the refrigerant as... 

The standard vapor compression cycle on the pressure-energy diagram. 

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. 

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