One of the inaccuracies involved in estimating power from Bhp/MM equations, or curves, stems from friction horsepower losses. For example, say a compressor has a 12-inch stroke frame with a maximum rating of 4000 Bhp. Also assume that the frame and cylinders have a mechanical efficiency of 93% (basis of Equation 300-13). Friction horsepower would be 280 HP (4000 ´ 0.07) for the frame. Now, say this frame is only loaded to 2000 Bhp for a certain application. The friction HP is about the same (i.e., 280 HP) so now that friction loss is 14% (280 ¸ 2000) of load and the mechanical efficiency is 86%.
This example may be somewhat out of the ordinary, but it points out the fact that significant error can arise from not accounting for frame loading. It is impractical to develop a loading factor, because it varies from one design to the next. Friction HP also varies with the number of throws and sizes of cylinders. The vendor should be contacted if there is serious concern about the friction HP contribution to overall compressor power.
Figure 300-12 shows a theoretical PV Diagram and the effects of changes in clearance, pressure ratio, and k value on the area (work) of the compression cycle. The figure shows that increasing the clearance decreases the volumetric efficiency; hence, the ICFM is reduced. Therefore, clearance is an appropriate variable to use to control the capacity.