Question

A compressor design primarily depends on:

• A. Suction pressure
• B. Discharge pressure
• C. Pressure ratio
• D. Compressor size

Hint:

Always consider the pressure ratio when analyzing or designing compressors — it governs thermodynamic efficiency, stage requirements, and type selection.

Answer 1

The Correct Option is C

The design and performance of a compressor are most strongly influenced by the pressure ratio across the compressor.
The pressure ratio is defined as:
\[ \text{Pressure Ratio} = \frac{P_{\text{discharge}}}{P_{\text{suction}}} \]
This ratio determines how much the gas must be compressed.
A higher pressure ratio means the gas is being compressed to a much higher final pressure, which affects the choice of compressor type (e.g., reciprocating, centrifugal, or axial), the number of stages, intercooling requirements, and power consumption.
Let’s examine why the other options are not primary design parameters:
(1) Suction Pressure: While important, suction pressure alone doesn't define the compression task — it's only meaningful when compared to discharge pressure.
(2) Discharge Pressure: Similar to suction pressure, knowing only the discharge pressure is insufficient unless the suction pressure is known too.
(4) Compressor Size: The size of the compressor is a result of the design, not a defining parameter. It depends on the required mass flow rate and the pressure ratio.
Hence, for selecting the correct type, size, and number of compression stages, the most critical factor is the pressure ratio.