Crank end

A packing is required on double-acting cylinders to provide a barrier to leakage past the rod where it passes through the crank end cylinder closure. The same arrangement is needed at the head end if a tail rod or tan-... 

End clearance is required to keep the piston from striking the compressor head or crank end. Some small clearance is also required under suc-... 

Double acting cylinder (sum of head end and crank end displacement)... 

A single-acting, head end cylinder will not have load reversal if suction pressure is applied to the crank end. Similarly, if discharge pressure is applied to the head end of a single-acting, crank end cylinder, load reversal will not occur. 

Single Acting Compression of gas takes place only in one end of the cylinder. This is usually the head end (outboard end), but may be the crank end (inboard end or end of cylinder nearest crankshaft of driving mechanism). See Figures 12-2A, 12-3A, and 12-4A. 

Double Acting Gas compression takes place in both ends of the cylinder, head end and crank end, Figures 12-21, 12-2J, and 12-4B. 

Note that the crank end (Figure 12-2F) always has the piston rod running through it, while the head end usually does not, but may, if a tail rod (Figure 12-2L) is used. 

Ap = cross-sectional net area of piston, in. If cylinder is head-end, Ap is total area of piston if cylinder is crank-end, Ap is net area of piston area minus rod cross-section area, s = stroke length, in. 

The displacement of the head end and crank end of the cylinder must be added for the total displacement. 

The displacement of the crank end is less than that of the head end by the volume equivalent to the piston rod displacement. For a multistage unit, the piston displacement is often only given for the first stage. ... 

For double-acting cylinders, the clearance at the head end should be calculated separately from that of the crank end, because for small cylinders, the volume occupied by the piston rod is significant when cylinder unloading is considered. 

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. 

Piston displacement (double acting cyl.) = 731 cfm each Piston displacement head end = 374 cfm Piston displacement crank end = 357 cfm Clearance, (cylinder data) = 11.1%... 

Discharge is at the head end of the cylinder suction is at crank end (toward crankshaft). 

The far end of the cylinder is called the head end. The end of the cylinder nearest the central shaft is called the crank end. I have shown the valves only on the head end, to simplify my description of the compressor s operation. 

The piston continues its travel toward the cylinder head. At some point it stops, and reverses its direction. This point is called bottom dead center, indicated by the dotted line in Fig. 29.1. Of course, bottom dead center cannot coincide with the end of the cylinder. The piston would have to travel past the valve ports for this to occur. If the piston travels past the discharge port, the compressed gas could not be pushed out of the cylinder into the discharge line. So bottom dead center must line up with the crank-end edge of the valve ports, as shown in Fig. 29.1. 

As soon as the piston reverses its direction of travel, the pressure of the gas inside the cylinder drops. The gas pressure drops below the discharge line pressure, and the spring-loaded discharge valve slams shut. The piston continues its travel toward the crank end of the cylinder. The pressure of the gas inside the cylinder continues to fall, but the suction or intake valve remains closed. 

At some point, as the piston approaches the crank end, the gas pressure inside the cylinder falls below the pressure in the suction line. The suction, or intake, valve now springs open, and gas is drawn out of the suction line and into the cylinder. This portion of the intake stroke continues until the piston returns to top dead center. 

Turning the wheel at the back end of the cylinder counterclockwise, pulls back a large internal plug in the head. Now, when the piston starts to withdraw toward the crank end of the cylinder, there is more gas left inside the cylinder to expand. The greater the volume of gas inside the cylinder when the piston is at bottom dead center, the closer the piston is to top dead center before the intake valve opens. The delay in the opening of the intake valve reduces the amount of gas drawn into the cylinder. This reduces the number of moles of gas compressed by the piston. Compression work also diminishes and the driver horsepower or amp load drops. 

Unfortunately, valve disablers have a detrimental effect on the adiabatic compressor efficiency. This means that, even though no gas may be moving through the crank end of a cylinder, the piston is still doing work on the gas inside the crank end of the cylinder. If you would like proof, place your hand on the valve cap on such a disabled cylinder. The high temperature you will feel is wasted compression work going to useless heat. I have measured in the field that, after a cylinder end is completely disabled, it is still converting 20 percent of the former compression work to heat. 

Let us assume that a reciprocating compressor has two cylinders working in parallel. Each cylinder has both a crank-end section and a head-end suction, where gas is compressed. In effect, we have four small compressors working in parallel. The inlet and outlet pressures, and hence the compression ratio, for each of these four minicompressors, is the same. The relative efficiency for each minicompressor is then... 

The Beta Scan plot obtained from the No. 2 cylinder crank end is shown in Figure 25-9. This plot shows the valve installed in this cylinder end was experiencing a 25%-30% loss in compression work. 

The two-stage compressor shown in Figure 25-10 is restricted, due to its mechanical configuration, to adjustment of the head-end (i.e., suction) cylinder only. Therefore, if it is necessary to unload a compressor by disabling one stage, it is best to disable the crank end. If the head end is removed... 

Not running at rated speed Horsepower higher at increased suction pressure Two-stage machine not operating in tandem Valve parts left in crank-end when operating on head-end... 

Table 25-1 shows that for the compressors discussed, the temperature rise for the individual cylinders varied from 28°F for the No. 1 cylinder crank end to 42°F for the No. 2 cylinder crank end. The key point of this table is that compression efficiency varies inversely with temperature rise. As both the suction and discharge pressures were the same for all cylinder ends, the reason for the variable temperature rise was different efficiencies of compression. Because the work performed by the piston at each cylinder end was about the same (except for No. 2 cylinder head end which had a bad unloader), the observed temperature increases were inversely proportional to the gas flows. This means that if the No. 1 cylinder crank end was moving 30 MMscfd of gas, then the No. 2 cylinder crank end was moving only 20 MMscfd and the No. 1 cylinder head end was moving 23 MMscfd. 

The compressor valve inefficiency corresponding to the excessive discharge temperature from the No. 2 cylinder crank end, could have been due to a variety of problems ... 

Ah yes, but the valve plates on the intake valves on the crank end have been removed." The worker paused to inspect the control panel and continued, "See, there is no pressure rise across the second stage of the machine."... 

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