Reciprocating Compressors – Estimating Cylinder Size to Accommodate a Given Flow Rate

Compressor cylinders are made in certain classes defined by:

1. Maximum allowable working pressure (MAWP)
2. Stroke
3. Number of valves per cylinder
4. Diameter range

The diameter in a class is varied by changing the wall thickness of the inner cylinder, or liner. Each class of cylinder has essentially the same clearance volume regardless of the diameter. However, the percent clearance varies with diameter. Volumetric efficiency also varies since it is related to clearance.

For example, assume that a 12-inch-stroke cylinder (double-acting) has a 3-inch rod, a MAWP of 1000 psig, a speed of 360 RPM, a diameter range of nine to 11 inches, and a clearance of 15% when the diameter is 10 inches. Further assume that the application has a pressure ratio of three, a molecular weight of 20, and a k value of 1.25. Figure 300-11 shows how percent clearance and volumetric efficiency vary with diameter for this cylinder class.

The percent clearance varies from about 8% to well over 30% among the many classes of cylinders available. There is no rule of thumb to relate percent clearance to diameter with much accuracy. However, for rough estimates use:

• 20% for diameters up to eight inches
• 15% for eight to 20 inches
• 12% in excess of 20 inches

The common approach to cylinder sizing is to make an educated guess at Ev, and then solve for the displacement rate using Equation 300-4. Cylinder diameter can then be calculated using Equations 300-24, 300-26, 300-28, as appropriate. This approach may have to be repeated two or three times to arrive at a combination that satisfies a given inlet flow quantity, Q.

As cylinder diameters do not come in an infinite number of increments, it is customary to select the next largest increment. In multistage machines, depending on the size of increments, oversizing of an initial stage is sometimes balanced by slightly undersizing the subsequent stage, assuming the interstage pressure level is not fixed (by a sidestream for example).

Percent Clearance and Volumetric Efficiency Variance

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