Two-Stage Scroll Compressors

Scroll Compressor Inside the Condenser

Two-Stage Scroll Compressors

Two-Stage Scroll Compressors - For many years HVAC manufacturers used two compressors in their higher efficiency equipment to stage the air conditioner or heat pump for added efficiency. They would use a small compressor for the lighter loads or demands and then when the air conditioner or heat pump needed more demand the small compressor would shut down and the larger compressor would kick on to satisfy the demand.

The HVAC controls were often complicated and the equipment was difficult to troubleshoot when there was an air conditioner problem. There were all sorts of ways to accomplish staging including one manufacturer that would sell one whole unit that was actually two condensing units manufactured as one piece of equipment. The air handler for that condenser has a dual slab coil for each stage of the air conditioner. A two-stage cooling thermostat would kick the second compressor when the demand called for additional cooling. Then new technology became available that changed how staging was done for higher efficiency models.

Two-Stage Scroll Compressors

Then HVAC manufacturers began using scroll compressors and later an engineer figured out that the design of the scroll compressor can easily be modified to allow for a simple way to stage the compressor for low and high demand situations whenever it is needed. A standard single stage compressor will kick on when the thermostat calls for heating or cooling.

Heating for a heat pump in heat mode and cooling for both the air conditioner and heat pump for the cooling season. The controls are not complicated and a single compressor can now be used for staging the air conditioner or heat pump.  An engineer figured out that you can easily put a plate in the compressor. The plate has two holes drilled in it. One big hole for high demand and one small hole for low demand.

A solenoid is used to shift that plate for whatever mode is called for from the thermostat depending on the demand. So now nearly every HVAC manufacturer is using the two-stage compressor for staging in their higher efficiency equipment.

Life After Two-Stage Scroll Compressors

There will come a day soon when modulating compressors are the king of the day just as two-stage scroll compressors are now. The compressor will have an ECM motor controlling the speed of the compressor and the modulation will be somewhere from 25% all the way to 100% of capacity. The whole HVAC system of the future will be able to modulate based on demand.

These types of systems are already available in commercial refrigeration and in mini-split air conditioners and heat pumps. The systems will be very efficient and save lots of money on utility bills which what the two-stage scroll compressors do now. The modulating systems will offer more efficiency than ever in the future.

Modulating vs Two-Stage vs Single-Stage Cooling Systems

The following chart demonstrates the temperature swing characteristics of a modulating system, a two-stage system, and a single-stage system. The modulating system maintains more even temperatures closer to the desired set point while the two-stage system has a little more temperature swing and the single stage has the biggest temperature swings simply because of how it operates. A single stage, on a call for cooling, will operate at 100% of capacity while the two-stage and the modulating systems have stages based on demand. An example of a modulating air conditioning system is a VRV or VRF system. The V stands for Variable. The R stands for refrigerant and the last V stands for volume. A VRF is exactly the same thing with a different name. It stands for Variable Refrigerant Flow. These are much more complex systems but they deliver more comfortable temperatures with added efficiency over the two-stage or single-stage systems. We have a planned future article where we plan to devote the entire article to the modulating cooling systems.

Modulating vs Two-Stage vs Single-Stage Cooling

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Two-Stage Scroll Compressors

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