Clearance is the volume remaining in a cylinder end when the piston is at the end of its stroke. This is the sum of the volume between the head of the cylinder and the piston, and the volume under the valve seats. The total clearance is expressed in percent of the total piston displacement, normally between 4 and 30%.
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.
End clearance is required to keep the piston from striking the compressor head or crank end. Some small clearance is also required under suctlon and discharge valves so that the valves can be removed and reinstalled.
These clearances are called fixed clearances and can be adjusted by:
* Removing a small portion of the end of the compressor piston
* Shortening the projection of the cylinder heads into the cylinder
* Installing spacer rings between cylinder head and body or under the valves
Variable clearance that can be changed very readily can be built into the cylinder. Figure 11-19 is an example of a fixed volume clearance pocket mounted on the cylinder. This type is separated from the cylinder by a valve that can be opened and closed from the outside.
Fixed clearance can also be added to the outer end of the cylinder by adding a fabricated clearance bottle with the desired volume. To change the performance of the cylinder the clearance can be changed by shutting down the compressor, unbolting one bottle, and installing another bottle with a different volume. It is very easy in that respect to add clearance and subtract clearance from a cylinder if the cylinder is set up to receive clearance bottles.
More flexibility can be obtained with a variable volume clearance pocket such as that shown in Figure 11 -20. This is a plug built into the outer cylinder head. When moved, the clearance volume of the outer end of the cylinder changes.
Clearance is normally expressed as a percent or fraction of cylinder displacement. It is given by:
Single acting cylinder (head end clearance)
