Frames, compressors

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. 

A unique feature of the reciprocating compressor is the possibility of multiple services on one compressor frame. On a multistage frame, each cylinder can be used for a separate gas service. For example, one cylinder may be dedicated to propane refrigeration, while the balance of the cylinders may be devoted to product gas. 

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

The reservoir is the lube oil storage tank. In some of the packaged compressors, it is built into the package base, and, in some standardized compressors, it is built into the compressor frame. In a reciprocating... 

Figure 10-2 shows a high-speed compressor frame and cylinders. The upper compressor is called a two throw machine because it has two cylinders attached to the frame and running off the crank shaft. The lower compressor is a four-throw machine because it has four cylinders attached to the frame. The number of throws refers to the number of pistons. 

Figure 10-2. High-speed reciprocating compressor frames and cylinders. [Courtesy of Dresser-Rana Company.)...

High-speed units are normally separable. That is, the compressor frame and driver are separated by a coupling or gear box. This is opposed to an integral unit where power cylinders are mounted on the same frame as the compressor cylinders, and the power pistons are attached to the same drive shaft as the compressor cylinders. 

The compressor frame, shown in Figure I i -2, is a heavy, rugged casting containing all the rotating parts and on which the cylinders and crossheads are mounted. Ail frames are rated by the compressor manu-... 

I Figure 11 -2. The compressor frame is the rugged casting thot contains the rotating I ports and on which the cylinders and crossheods are mounted. [Courtesy of, Dresser-Rand Company,]... 

A distance piece provides the separation of the compressor cylinder from the compressor frame as shown in Figure 11-9. At the top of the figure is a standard distance piece. 

The combined rod load should be checked anytime the gas loads are approaching the maximum rating of the compressor frame or anytime rod reversal is marginal or questionable. 

Forced coolant systems using a mixture of glycol and water are the most common for natural gas compressors. Normally, the compressor cylinder cooling system and compressor frame lube oil cooling system is combined. A single pump is used to circulate the coolant through the cylinders and the lube oil heat exchanger and then to an aerial cooler where the heat is dissipated. 

Late in the field life it is desirable to compress the 100 MMscfd for the example field downstream of the separator from 800 psig at 100°F to l.(XX) psig. An engine-driven separable compressor is available from surplus. The engine is rated for 1,600 hp at 900 rpm. Horsepower is proportional to speed. The compressor frame has six 7-in, bore by 6.0-in. stroke double-acting cylinders with a minimum clearance of 17.92%, a rod load limit of 25,000 lb, and rod diameter of 1.75 in. Assume k = 1.26, Z, -0.88, and Zd = 0.85. 

Select a compressor frame as in Table 12-9B and note the average polytropic efficiency listed, speed, and head/stage. 

Select the compressor frame from Table 12-9B for a typical example, not attempting to promote any particular manufacturer (each manufacturer has its own tables). Based on the inlet volume and the required discharge pressure, select frame size. 

Calculate the compression ratio, Pd/Ps = P2/P1 = Pc-From Table 12-9B for the compressor frame selected, select polytropic efficiency, Cp, and using Figure 12-65A, determine adiabatic efficiency, e d. 

Precompressor. The example presented here in Figure 1 uses precompression. Since compressors are normally the most expensive pieces of equipment in the entire plant, the process is usually designed to fit the closest compressor frame size available. 

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