Reciprocating Compressors – Capacity Changes

The following methods can be used to change the capacity of the machine:
• Change speed
• Increase or decrease clearance of cylinders
• Re-bore unlined cylinders and change pistons
• Re-line cylinders and change pistons
• Replace cylinders

Speed change is rarely a viable option. Obviously, increasing speed will not be an option if the frame is already running at the maximum rated speed. Change in the speed of a motor-driven unit requires replacement of the driver. If the driver is a variable speed machine, there is a chance that something can be done, but the system must be carefully studied. The torque and power ratings of the driver, compressor, couplings, and gear (if any) should be checked, and a torsional analysis of the system conducted. A review of the pulsation dampers and piping design may also be required.

For small changes in capacity, it may be possible to alter the clearance of the cylinders. It will usually be very difficult to significantly increase capacity in this manner unless the cylinders were originally over-clearanced. However, it might be possible to use this method in combination with other methods. Ordinarily, capacity can be reduced quite easily by adding clearance, with spacers or clearance pockets, or by reducing piston length.

Unless the cylinders are at maximum diameter, unlined cylinders can be re-bored, sometimes by a substantial amount, to increase displacement. Similarly, liners can be replaced in lined cylinders to increase or decrease displacement. There is no guideline for the amount of displacement change that can be achieved. Consult the manufacturer. If changes are contemplated, a computer study of the combined rod loads should be made. Although a large change would not be expected, the manufacturer should review the torque-effort diagram.

In rare cases, where it is desired to increase capacity and power, the crankshaft may be a limiting factor. Crankshafts have maximum horsepower-per-throw and total horsepower (torque) ratings.

There is no way to upgrade the maximum allowable working pressure of a cylinder, unless it was under-rated for the original application. It is worthwhile asking the manufacturer whether the cylinder’s present nameplate is the actual maximum rating.

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