Short #62 – Impacts of Variable & Staged Compression

In this short podcast episode, Bryan covers multi-stage or variable-speed compressors. He also explains the impacts of staged compression. Multi-stage (or variable-capacity) compressors can come in many different forms, but they all have one thing in common: they can adjust their capacities.

We typically rate equipment for its maximum capacity. However, when you vary the capacity, you get turn-up or turn-down; the refrigerant mass flow rate increases or decreases. When a unit turns down the capacity, the output decreases; the blower should also reduce its CFM output accordingly. While the compressor staging can vary, the coils and metering device stay the same, so the system must handle staged compression. We sometimes have to pay extra attention to the metering device to make sure the system operates as it should.

When we decrease the compressor capacity, the suction pressure goes up while the head pressure goes down; the pressure differential depends on the refrigerant flow. You’ll also run a lower condensing temperature and higher evaporating temperature. However, if the blower adjusts its CFM output with the turn-down, these effects will be less significant. With a higher evaporator temperature, we can expect a warmer evaporator coil, which will decrease dehumidification.

Since our compression ratio will be lower, you can expect some efficiency gains during a turn-down. You can also expect lower amp draws. We can control capacity and reduce it without having to worry about short cycling.

When you turn up a compressor, as you can on some ductless systems, you can expect the opposite effects of a turn-down: higher head pressure and lower suction pressure.

Bryan also discusses:

• Variable-capacity compression in ductless systems
• Approach temperature
• Turn-down rate on equipment

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