Efficiency Of compression

There are economic and operational reasons for considering an additional stage of compression. The addition of a stage of compression requires an additional scrubber, additional cylinder or case, and more complex piping and controls. In addition, there are some horsepower losses due to additional mechanical friction of the cylinder or rotating element and the increased pressure drop in the piping. This horsepower loss and additional equipment cost may be more than offset by the increased efficiency of compression. 

The power requirement is then that for compression of the gas from pressure Pi to P3 and for imparting the necessary kinetic energy to it. Under normal conditions, however, the kinetic energy term is negligible. Thus for an isothermal efficiency of compression 17. the power required is ... 

The feed enters the evaporator at 295 K and the concentrated liquor is withdrawn at the rate of 0.025 kg/s. The concentrated liquor exhibits a boiling-point rise of 10 degK. Heat losses to the surroundings are negligible. The nozzle efficiency is 0.95, the efficiency of momentum transfer is 0.80 and the efficiency of compression is 0.90. 

The efficiency of compression may be defined as the product of the cylinder efficiency and the true volumetric efficiency, or it is the work done in a complete cycle to compress isothermally (without clearance) a given volume of free air, divided by the work actually expended in compressing the same volume of free air. 

Actual Values of Efficiencies.—Tests of piston compressors show extreme variations of mechanical efficiency from 76 to 97 per cent, with approximate averages for the more common sizes of 85 per cent. The true volumetric efficiency of piston air compressors will vary from 80 to 97 per cent, and the cylinder efficiency for water-jacketed compressors from 80 to 85 per cent. This will result in efficiencies of compression varying from 64 to 82 per cent. 

Multiplying the mechanical efficiency by the efficiency of compression will show variations from 48 to 79 per cent. That is, the energy required to compress a certain amount of air isothermally is only from 48 to 79 per cent of that actually expended in the steam cylinder. Tests show that with the great majority of compressors this product ranges from 50 to 60 per cent, while with some of the best compressors under test conditions it will reach as high as 78 per cent. 

Fig. 25 shows the action of a Root s pump. Gas inlet and outlet are separated by only a narrow gap which enables a back flow of gas from the exhaust region to the inlet region. Although the efficiency of compression (maximum compression... 

The compression ratio (P2/P1) of the compressor is usually given by the manufacturer and therefore the temperature of the air leaving the compressor is easily found from (2.13). If the efficiency of compression is known e.g. 90% and that of the turbine ri, is known e.g. 85% then a better estimate of the output energy can be calculated. In this situation T2 becomes l2e and T4 becomes T4e, as follows -... 

Table 25-1 shows that for the compressors discussed, the temperature rise for the individual cylinders varied from 28°F for the No. 1 cylinder crank end to 42°F for the No. 2 cylinder crank end. The key point of this table is that compression efficiency varies inversely with temperature rise. As both the suction and discharge pressures were the same for all cylinder ends, the reason for the variable temperature rise was different efficiencies of compression. Because the work performed by the piston at each cylinder end was about the same (except for No. 2 cylinder head end which had a bad unloader), the observed temperature increases were inversely proportional to the gas flows. This means that if the No. 1 cylinder crank end was moving 30 MMscfd of gas, then the No. 2 cylinder crank end was moving only 20 MMscfd and the No. 1 cylinder head end was moving 23 MMscfd. 

Let us consider one more physical phenomenon, which can influence upon PT sensitivity and efficiency. There is a process of liquid s penetration inside a capillary, physical nature of that is not obvious up to present time. Let us consider one-side-closed conical capillary immersed in a liquid. If a liquid wets capillary wall, it flows towards cannel s top due to capillary pressure pc. This process is very fast and capillary imbibition stage is going on until the liquid fills the channel up to the depth l , which corresponds the equality pcm = (Pc + Pa), where pa - atmospheric pressure and pcm - the pressure of compressed air blocked in the channel. 

Seawater Distillation. The principal thermal processes used to recover drinking water from seawater include multistage flash distillation, multi-effect distillation, and vapor compression distillation. In these processes, seawater is heated, and the relatively pure distillate is collected. Scale deposits, usually calcium carbonate, magnesium hydroxide, or calcium sulfate, lessen efficiency of these units. Dispersants such as poly(maleic acid) (39,40) inhibit scale formation, or at least modify it to form an easily removed powder, thus maintaining cleaner, more efficient heat-transfer surfaces. 

Thermocompression Evaporators Thermocompression-evap-orator calculations [Pridgeon, Chem. Metall. Eng., 28, 1109 (1923) Peter, Chimin Switzerland), 3, II4 (1949) Petzold, Chem. Ing. Tech., 22, 147 (1950) and Weimer, Dolf, and Austin, Chem. Eng. Prog., 76(11), 78 (1980)] are much the same as single-effect calculations with the added comphcation that the heat suppied to the evaporator from compressed vapor and other sources must exactly balance the heat requirements. Some knowledge of compressor efficiency is also required. Large axial-flow machines on the order of 236-mVs (500,000-ftVmin) capacity may have efficiencies of 80 to 85 percent. Efficiency drops to about 75 percent for a I4-mVs (30,000-ftVmin) centrifugal compressor. Steam-jet compressors have thermodynamic efficiencies on the order of only 25 to 30 percent. 

The thermal efficiency of the process (QE) should be compared with a thermodynamically ideal Carnot cycle, which can be done by comparing the respective indicator diagrams. These show the variation of temperamre, volume and pressure in the combustion chamber during the operating cycle. In the Carnot cycle one mole of gas is subjected to alternate isothermal and adiabatic compression or expansion at two temperatures. By die first law of thermodynamics the isothermal work done on (compression) or by the gas (expansion) is accompanied by the absorption or evolution of heat (Figure 2.2). 

It follows that the efficiency of the Carnot engine is entirely determined by the temperatures of the two isothermal processes. The Otto cycle, being a real process, does not have ideal isothermal or adiabatic expansion and contraction of the gas phase due to the finite thermal losses of the combustion chamber and resistance to the movement of the piston, and because the product gases are not at tlrermodynamic equilibrium. Furthermore the heat of combustion is mainly evolved during a short time, after the gas has been compressed by the piston. This gives rise to an additional increase in temperature which is not accompanied by a large change in volume due to the constraint applied by tire piston. The efficiency, QE, expressed as a function of the compression ratio (r) can only be assumed therefore to be an approximation to the ideal gas Carnot cycle. 

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