The compressor frame, shown in Figure 11-2, is a heavy, ragged casting containing all the rotating parts and on which the cylinders and crossheads are mounted. All frames are rated by the compressor manufacturers for a maximum continuous horsepower, speed (rpm), and rod load. The rated horsepower is determined by the maximum horsepower that can be transmitted through the crankshaft to the compressor cylinders. The rod load is the force imposed on the piston rod by the pressure differential between the two ends of the piston.
Each frame is designed for a maximum number of cylinders. The frame itself does not indicate the number of stages or the duty of the compressor. An individual frame can be used for many different sizes of compressor cylinders and for a wide range of applications. Frames are typically classified as separable (balanced-opposed) or integral-type, as shown in Figure 11-3.
Separable (balanced-opposed) frames are characterized by an adjacent pair of crank throws 180° out of phase. The frame is separate from the driver. Integral-type frames are characterized by having compressor cylinders and power cylinders mounted on the same frame and driven by the same crankshaft.