## DYNAMIC COMPRESSORS – RPM AND BREAK HORSEPOWER

The ratio of compression, defined as the ratio of the discharge absolute pressure to the absolute suction pressure, is an indicator of the amount of pressure that the compressor adds to a gas or gas mixture. At any particular RPM, a dynamic compressor adds certain head (pressure) to the gas. The total head/pressure developed depends on the compressor design, its RPM, and the amount of gas flow at suction. At a fixed RPM and inlet CFM of gas, the head developed by a centrifugal compressor is the same irrespective of the weight of gas. The head developed by a centrifugal compressor does not depend on the density of the gas and it is possible to convert the feet of head into psi (pounds per square inch) and vice versa.

The density of a gas does not affect the head developed, but affects the discharge pressure of the compressor.

Example:

Two identical compressors handle 200 cfm of air and hydrogen at 12,000 rpm and as per the compressor characteristics; the differential head developed is 20,000 feet at discharge conditions. What will be the discharge pressure under these conditions? (Densities at discharge conditions for air and hydrogen are 0.65 and 0.15 lb/ft3, respectively.)

Case 1.

Discharge head for air = 20,000 ft: Density = 0.65 lb/ft3

Discharge pressure = head × density = 20,000 × 0.65 lb/ft2

= 13,000 lb/ft2

= 90.30 lb/in.2 = 6.35 kg/cm2 g

Case 2.

Discharge head for hydrogen = 20,000 ft: Density = 0.15 lb/ft 3

Discharge pressure = head x density = 20,000 × 0.15 lb/ft2

= 3,000 lb/ft2

= 20.83 lb/in.2 = 1.46 kg/cm2g

From the above example, it is clear that for the same CFM flow, RPM and the head developed the discharge pressure is high for a high-density gas than for a low-density gas.

BHP refers to the break horsepower that is required by the compressor shaft to achieve the desired compression ratio. Because the gas density fluctuates very often in industrial systems, a centrifugal/dynamic compressor tends to change power consumption while in operation.

The ratio of compression, R, is the absolute discharge pressure divided by the absolute suction pressure. R is an indicator of the amount of pressure that the compressor adds to the gas.

At a given RPM, a dynamic compressor adds a certain head to the gas.

The total head added depends on:

The design of the compressor

The amount of gas flow and

The operating RPM (speed)

As RPM increases, the total head developed by the compressor increases.

At a fixed RPM and CFM, the compressor attains approximately the same feet of head, regardless of the weight of the gas handled.

The head developed by the compressor does not depend on the density of the gas being handled.

The feet of head or the meter of head can be converted into PSI or kg/cm2 equivalent.

Similarly, PSI or kg/cm2 can be converted into feet or meters, respectively.

When a compressor at a given RPM is handling a heavier gas, the work it does over a pound of gas is the same as the work done on a pound of lighter gas.

Head represents the amount of foot pounds or kilogram meter of work done per unit weight.

A compressor at a given RPM handles two different gases. The gas that requires a larger volume per unit weight is the lighter gas.