There are many types of positive displacement compressors, but only the three major types will be discussed in this book. These are the blower, reciprocating compressor, and screw compressor. Blowers and screw compressors share some common features and a common history, while reciprocating compressors are noticeably different.
The blower is a positive displacement compressor that was invented and patented in 1860 by Philander H. Roots and Francis M. Roots two brothers from Connersville, Indiana. Initially it was intended as a gas pump for use in blast furnaces. The blower design consists of two figure-eight elements or lobes. These lobes are geared to drive in opposite directions .
Since the rotary lobes need to maintain clearance between each of the lobes a single stage blower can achieve only limited pressure ratio differential—typically 2.0. However, it is capable of compressing large volumes of gas at efficiencies up to 70%. Therefore, blowers are frequently used as boosters in various compression applications.
A typical two-lobe blower operating sequence is shown below. Note that the upper rotor or lobe is turning clockwise while the lower lobe is turning counterclockwise
Position #1: gas enters the lower lobe cavity from the left as compressed gas is being discharged from this cavity to the right and simultaneously gas is being compressed in the upper lobe cavity
Position #2: the upper lobe cavity is about to discharge it’s compressed gas into the discharge line.
Position #3: gas enters the upper lobe cavity from the left as compressed gas is being discharged from this cavity to the right and simultaneously gas is being compressed in the lower lobe cavity.
Position #4: the lower lobe cavity is about to discharge it’s compressed gas into the discharge line.