Compressor ratio, overall

The differential pressure is expressed in terms of overall compressor ratio, Rj, which is defined as ... 

Flash gas compressors typically have an overall compressor ratio in the range of 5 to 20. 

The use of large compressors is probably more prevalent in oil field facilities than in gas field facilities. Oil wells often require 1 Tee pressure and the gas that flashes off the oil in the separator must be compressed in a flash gas compressor. Often a g i lift system is nei help lift the oil to the surface. As described in Volume 1, a gas li pressor must compress not only the formation gas that is produce. . .. v,. r oil, but also the gas-lift gas that is recirculated down the well. Gas lift i npressors are chaiacterized by both high overall compressor ratios and 1 atively high throughputs. 

First, calculate the overall compression ratio (R = Pa/Pj). If the compressor ratio is under 5, consider using one stage. If it is not, select an initial number of stages so that R < 5. For initial calculations it can be assumed that ratio per stage is equal for each stage. 

The remaining parameter is the compressor ratio P. The pressure ratio is a design parameter for the compressor component so it can be varied for optimum overall cycle thermal efficiency. 

Figure 4-12 presents an overall picture of the compressor duty ranges and machine comhinations applied in a typical nitric acid plant, including capacities and pressure ratios. The diagram represents typical... 

To obtain a more accurate relationship between the overall thermal efficiency and the inlet turbine temperatures, overall pressure ratios, and output work, consider the following relationships. For maximum overall thermal cycle efficiency, the following equation gives the optimum pressure ratio for fixed inlet temperatures and efficiencies to the compressor and turbine ... 

Intercooling can be used to hold desired temperatures for high overall compression ratio applications. This can be done between stages in a single compressor frame or between series frames. Sometimes economics rather than a temperature limit dictate intercooling. 

Calculation of the specific work and the arbitrary overall efficiency may now be made parallel to the method used for the a/s cycle. The maximum and minimum temperatures are specified, together with compressor and turbine efficiencies. A compressor pressure ratio (r) is selected, and with the pressure loss coefficients specified, the corresponding turbine pressure ratio is obtained. With the compressor exit temperature T2 known and Tt, specified, the temperature change in combustion is also known, and the fuel-air ratio / may then be obtained. Approximate mean values of specific heats are then obtained from Fig. 3.12. Either they may be employed directly, or n and n may be obtained and used. 

Fig. 3.13 shows the overall efficiency for the [CBTJic, plant plotted against the i.sentropic temperature ratio for various maximum temperatures Tj (and 6= Ty/Ti, with T, = 27°C (3(X) K)). The following assumptions are also made polytropic efficiency, rjp = 0.9 for compressor and turbine pressure loss fraction in combustion 0.03 fuel (methane) and air supplied at 1 bar, 27°C (3(X) K). 

For the ISTIG cycle, Fig. 6.18 shows thermal efficiency plotted against specific work for varying overall pressure ratios and two maximum temperatures of 1250 and 1500°C. Peak efficiency is obtained at high pressure ratios (about 36 and 45, respectively), before the specific work begins to drop sharply. Note that the pressure ratios of the LP and HP compressors were optimised within these calculations. 

Macchi et al. provided a similar comprehensive study of the more complex RWI cycles as illustrated in Fig. 6.19, which shows similar carpet plots of thermal efficiency against specific work for maximum temperatures of 1250 and 1500°C, for surface intercoolers. The division of pressure ratio between LP and HP compressors is again optimised within these calculations, leading to an LP pressure ratio less than that in the HP. For the RWI cycle at 1250°C the optimisation appears to lead to a higher optimum overall pressure ratio (about 20) than that obtained by Horlock [5], who assumed LP and HP pressure ratios to be same in his study of the simplest RWI (EGT) cycle. His estimate of optimum pressure ratio... 

Harvey et al. gave a parametric calculation of the thermal efficiency of this plant, as a function of turbine inlet temperature, the reformer pinch point temperature difference and the pressure level in the reformer (the compressor overall pressure ratio, r). 

To optimize the compressor design to minimize the overall power consumption, the compressor should have nearly equal ratios of compression in each of the stages. Thus, the total pressure ratio r across the compressor (i.e., input pressure to output pressure prior to after cooling) is... 

Increasing the air density increases the GT inlet air mass flow. For a given stoichiometric fuel-to-air-ratio and a given combustion temperature, increased air mass flow allows increased fuel flow, resulting in increased GT power output. Additionally, compressor efficiency increases with decreased air temperatures, resulting in less parasitic compressor shaft work consumed and greater net turbine power output. Therefore, TIC increases net incremental power output faster than incremental fuel consumption, resulting in improved overall fuel efficiency (reduced heat rate) see Fig. 24-64. 

Many of the design principles of a hydride tank also hold for a hydride compressor. It must be an effective heat exchanger. The more effective the heat exchange, the shorter is the cycle time and consequently the smaller the hydride inventory required. Ideally, the hydride should have a high slope to the Van t Hoff plot to produce maximum compression with minimum temperature excursion. Beds of different hydrides can be coupled in series to provide staged compression and thus achieve very high overall compression ratios with modest temperatures. The ability to tailor hydrides, as shown earlier, is very helpful to compressor optimization for specific applications (i.e., available heat sources, H2 input pressure, and desired output pressure). 

Compressor Type Inlet Flow Rate 1000 m% Compression Ratio Maximum Temperature K Overall Efficiency r ... 

For those cases where the permeability of reactant A is in between those of the two products, B and C, both the conversion and extent of separation increase with increasing permeation rate or permeation to reaction rate ratio (Table 11.9). The corresponding optimal compressor load (recycle flow rate to feed flow rate) also increases with the rate ratio. The top (permeate) stream is enriched with the most permeable product (i.e., B) while the bottom (retentate) stream is enriched with the least permeable product (i.e., C). It is noted from Table 11.9 that the optimal compressor loads for achieving the highest conversion and extents of separation can be quite different and a decision needs to be made for the overall objective. 

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