Cylinder clearance

Reciprocating compressor capacity may easily be adjusted by changing compressor speed, changing compressor cylinder clearance, unloading compressor cylinder inlet valves, recycling gas from unit discharge to unit suction, or a combination of these methods. All these methods may be accomplished either manually by the operator or automatically by the control panel. 

The use of speed control and/or a recycle valve is covered in Chapter 10. Our discussion in this chapter will concentrate on cylinder inlet valve unloaders and changing cylinder clearance. 

As the piston starts its suction stroke, the gas that remains in the cylinder in the fixed and added clearance areas expands until the pressure in the cylinder is equal to the pressure in the line outside of the cylinder. The greater the clearance, the longer it takes for the suction valves to open and the less new gas enters the cylinder. Therefore, less gas will be compressed as cylinder clearance is increased. 

The capacity of the cylinder is a function of piston displacement and volumetric efficiency. This is in turn a function of cylinder clearance, compression ratio, and gas properties. 

C = cylinder clearance, percent of piston displacement Z5 = compressibility factor at suction, psia Z(j = compressibility factor at discharge, psia k = ratio of specific heats, Cp/C ... 

For double-acting cylinders, % clearance is based on total clearance volume for both the head end and crank end of the cylinder X 100 divided by the total nel piston displacement. The head and crank end % clearance values will be different due to the presence of the piston rod in the crank end of the cylinder. The % clearance values are available from manufacturers for their cylinders. The values range from about 8% for large 36-in. cylinders to 40% for small 3-and 4-in. cylinders. Each cylinder style is different. 

Note that this requires or assumes that compressor cylinder clearance Vp be established. For studies, it may be assumed and the effects calculated. Values range from 5-35% clearance on actual cylinders. Special designs are used for smaller or larger values. 

Reading chart Figure 12-18C, E, = 84.5 at assumed 7% cylinder clearance. 

For suction drums, AV is evaluated using the suction volumetric efficiency, E . E = cfm at suction conditions/PD at the selected rpm, or E can he obtained from curves of volumetric efficiency versus the compression ratio for varying cylinder clearances. 

A twin-cylinder, single-acting compressor, working at 5 Hz, delivers air at 515 kN/m2 pressure, at fee rate of 0.2 m3/s. If the diameter of the cylinder is 20 cm, the cylinder clearance ratio 5% and the temperature of the inlet air 283 K, calculate the length of stroke of the piston and the delivery temperature. 

Equation (8.5) does not account for the volumetric efficiency (VE) loss of reciprocating-type compressors. Volumetric efficiency is simply the clearance allowed in the compressor cylinder head in which this compressed gas volume is allowed to mix with the inlet gas to be compressed in the next compression stroke. Thus this compressed gas in the cylinder clearance is recycling, which makes the output of the compressor less efficient, the larger this clearance volume. Volumetric efficiencies usually range fron 6 to 20%. An average reciprocating compressor VE should be 10%. For conservative cost calculating, use 15% of overall GHP for the VE efficiency. 

The centrifugal-type compressor is advantaged here, since it doesn t have a VE efficiency (cylinder clearance) to account for. There are no cylinders in a centrifugal-type compressor. More and more major oil/gas producers are therefore installing centrifugal compressors in their... 

Plastic piston skirts have long been a development topic the first succesful example was the use (in diesel engines) of inserts in carbon fibre reinforced, internally lubricated PES by Floquet Monopole in France. Two of these in each skirt have the effect of reducing the piston-to-cylinder clearance, with consequent reductions in noise, fuel consumption and pollution, and improved performance (see Fig. 6.20). 

The best way to unload a reciprocating compressor is to increase the cylinder clearance. When the piston completes the end of its stroke, the space between the piston face and the cylinder head is called the clearance. Increasing the clearance reduces the scfd flow of gas to a compressor running at a constant speed and a fixed suction pressure. The effect of adjusting the cylinder clearance is proportional to the suction stroke volumetric efficiency (Evs). Evs is calculated as follows ... 

Physically, the cylinder clearance is adjusted with a device similar to a rising-stem valve handle. Turning the handle clockwise pushes the adjustable cylinder head toward the piston, which increases the flow of gas through the compressor. This is the exact opposite of what normally transpires when we close a valve by turning the valve handle clockwise (see Fig. 25-1). 

For a constant speed driver such as motor, capacity of a reciprocating compressor can be controlled by Unloading inlet valve and/or unloaditig pockets or reservoirs at the cylinder. Lift suction valve (suction valve unloading) will prevent compression and open pocket to cylinder (clearance unloading) will increase cylinder clearance volume. Either action will reduce compressor capacity. If gas demand is intermittent, on-ofT control of motor can be used to control capacity. 

Clearance volume of a reciprocating conqxessor is usually expressed as a percent of piston displacement and referred to as percent clearance or cylinder clearance, C. 

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