To improve the efficiency of the centrifugal refrigerant compressor, which action is beneficial?

• A. Lowering the evaporator pressure
• B. Lowering the condenser pressure
• C. Raising the evaporator pressure
• D. Raising the condenser pressure

Answer: (B)

In a centrifugal compressor, the energy needed per unit mass is tied to the pressure rise the refrigerant must undergo from the evaporator (suction) to the condenser (discharge). Lowering the condenser pressure reduces the discharge pressure, so the compressor doesn’t have to impart as much head. With a smaller pressure ratio to achieve, the compressor does less work for the same mass flow, which lowers input power and raises the refrigeration cycle’s COP. The cooler condensing temperature that comes with lower condenser pressure also helps with heat rejection and can improve overall efficiency.

Lowering the evaporator pressure would increase the required head (the compressor would have to raise pressure more), which tends to reduce efficiency. Raising evaporator pressure reduces head somewhat but can compromise cooling capacity or system constraints. Raising condenser pressure increases the head further, which worsens efficiency.