Gas Engines

When considering the possibility of using internal combustion drivers, evaluate process requirements and costs. If a low-cost, gaseous fuel is available, gas engines and gas turbines may surpass other drivers in economical installation and operation. In the initial process design stage, a method of establishing the cost of purchase, installation, and operation for drivers is needed.

After the type of driven equipment is selected, the horsepower requirements must be estimated. Unless the horsepower requirements happen to fit one of the gas turbine sizes available, a gas engine will be needed.

The large, heavy-duty, integral engine-compressor has long been the workhorse of the gas compression industry. The in-line or V-shaped power cylinder, horizontal compressor cylinder configuration is familiar.

Other variations in compression using the reciprocating engine driver are the reciprocating compressor frame and the centrifugal compressor.

The separate engine-driven reciprocating compressor frame arrange merit will have approximately the same first cost as the integral type but slightly higher installation costs. The principal advantage of the separate engine-driven frame is that more compressor cylinders may be used by adding throws to the frame. Integral types are limited in this respect, and many throws are necessary in high-pressure or multiple-service applications.

While not common, engine-driven centrifugal compressors are used. This combination is desirable for low-ratio applications and in fuel-cost situations where the high engine efficiency is attractive. The difference in rotating speeds (engine 300-600 rpm, compressor 3,000-5,000 rpm +) requires the use of a speed increaser.

The primary advantage of the gas engine is the fuel rate (efficiency) (see Figure 7-18). Again, this is a plot of an average of the manufacturers' published data. It can be seen that rates vary from 6,500-7,000 Btu/ bhp-hour for the large units to 8,500-9,000 Btu/bhp-hour for the smaller ones. On this basis, large gas engines have thermal efficiencies of 35-39%.

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