If a machine is described as a screw compressor, its compression ratio is most accurately stated as which value?

If a machine is described as a screw compressor, its compression ratio is most accurately stated as which value?

• A. 20:1
• B. 4:1
• C. 6:1
• D. 8:1

Answer: (A)

The key idea is what we mean by compression ratio: it’s the discharge pressure divided by the suction pressure (absolute). A screw compressor is designed to deliver a relatively large pressure rise in a single pass, producing a high continuous compression of the gas. Because of this design, the pressure lift it achieves tends to be quite large compared with some other compressor types, especially in ammonia systems that require higher condenser head pressure. Among the given values, a ratio around 20:1 reflects that higher pressure lift typical of screw machines in practice, making it the best choice. The other options represent smaller ratios that are less characteristic of a screw compressor in this context, where the machine is expected to deliver a substantial discharge-to-suction pressure rise.

The key idea is what we mean by compression ratio: it’s the discharge pressure divided by the suction pressure (absolute). A screw compressor is designed to deliver a relatively large pressure rise in a single pass, producing a high continuous compression of the gas. Because of this design, the pressure lift it achieves tends to be quite large compared with some other compressor types, especially in ammonia systems that require higher condenser head pressure. Among the given values, a ratio around 20:1 reflects that higher pressure lift typical of screw machines in practice, making it the best choice. The other options represent smaller ratios that are less characteristic of a screw compressor in this context, where the machine is expected to deliver a substantial discharge-to-suction pressure rise.