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Old 06-23-17, 09:13 AM   #1
Sccoupe
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Default Multi-compressor heat pump.

Doing lots of reading on single stage vs. 2 stage vs. inverter heat pumps. There are a few youtube videos on adding a VFD to a split system compressor.

The thought is, for a diy setup, why not a multi-compressor system. So, if we need 4 tons of heat/cooling, why not use 4, one ton compressors that are cheaply and easily found and stage compressor use based on need?

I guess the inverter and 2 stage units are using a wide range TVX to keep the evaporator from freezing when the compressor is running at lower speeds (or with only 1 of the 4 compressors running in our hypothetical scenario)?

Thoughts on possible pitfalls?


EDIT: In the search, the terms tandem compressor and EXV have come up. I like the sound of electronic expansion valve.

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Old 06-23-17, 10:03 AM   #2
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I'm not an expert on the subject. But I know in the commercial side of things they have tandem compressors or sometimes even more.. For example we have a 40ton chiller and work that has 4 10ton compressors in it..

I'm actually contemplating a similar system for my house. Two 1 ton compressors. I don't think anyone on this site has done anything of the likes but it is a thing
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Old 06-23-17, 11:36 AM   #3
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Do you know if there are any refrigerant check valves in that 40 ton machine or is it just 4 compressors piped in parallel?

Doing more reading on these electronic expansion valves. I really like this idea because a system can be built with the proper sensors and EXV and then the rest of the playing is in the software (the easy part for me).


There is some good info in here. http://web.ornl.gov/~jacksonwl/hpdm/...Orlando_V7.pdf


My brain gets a little out of control, but you could even use one VFD drive to run 3 different 110V compressors. The VFD I have converts 220v single phase to 220V 3 phase. As I understand each phase is 110V to ground giving 3 different 110V controlled sources. Add relays to control on/off to each compressor. One relay on and ramp up to needed capacity. If max capacity is reached on the single compressor, ramp down, switch a second relay and ramp back up to needed capacity, etc. Add in the EXV and it sounds like fun!
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Old 06-24-17, 01:24 AM   #4
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The large lab chiller setups I have seen are just multiple smaller chillers in parallel. There's a PLC or similar device load balancing them but they're otherwise independent. The reason being that if one breaks down, the remaining units take the load with no impact on operations. (In business, unplanned downtime is expensive!)
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Old 06-24-17, 07:39 AM   #5
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I have seen larger commercial A/C systems that will run 2 or 3 compreasors. It keeps the machine from short cycling, but still allows it to cool even during the highest demand.

With a compressor ran off an inverter coupled with an electronically controlled expansion valve all you need is 1 compressor.
For my house I have a 2 ton inverter split for 500 to 700 square feet, which is way oversized. It works great. No short cycling and we can fire up the stove during the day and not heat up the house.

Last edited by oil pan 4; 06-24-17 at 11:42 AM..
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Old 06-24-17, 09:19 AM   #6
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How does one select a condenser and evaporator if using a variable speed compressor (or multiple compressors) and EXV? If the compressor slows down (or 2 of 3 compressors shut off), then the EXV needs to restrict flow to match. Would this not starve and freeze up the evaporator that is now too large for the slowly spinning compressor that now looks like a smaller compressor? Or does slowing the condenser fans down also handle that?
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Old 06-24-17, 11:46 AM   #7
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The electronic expansion valve needs to restrict flow to maintain low side pressure as the compressor rate goes up and down. When the pressure drop is too great that's when you get freeze up.
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Old 06-24-17, 10:55 PM   #8
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Quote:
Originally Posted by Sccoupe View Post
How does one select a condenser and evaporator if using a variable speed compressor (or multiple compressors) and EXV? If the compressor slows down (or 2 of 3 compressors shut off), then the EXV needs to restrict flow to match. Would this not starve and freeze up the evaporator that is now too large for the slowly spinning compressor that now looks like a smaller compressor? Or does slowing the condenser fans down also handle that?
If you are employing an EXV, it is rated for a certain maximum capacity, which varies with the refrigerant being used. The control system opens and closes the EXV automatically after it is programmed. Typically, there are a few electronic sensors that let the control see what is happening in the system. Compressor capacity, blower speeds, and thermostat setpoint can all change wildly, but the EXV control will adjust automatically.
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Old 06-25-17, 08:59 AM   #9
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You could also use a mechanical expansion valve.
Some of the newer air conditioning units have gone back to mechanical.
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Old 06-25-17, 09:41 AM   #10
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I like TX valves because they are not complicated. They do what they do or they don't. They are also not static or lightning sensitive and don't need to be programmed or fiddled with. There are many different types of controller and driver systems for electronic valves and they aren't cheap. I have no idea which one is better than the other for your application because I have not messed with any of them.

My thrifty nature won't allow me to take the plunge,. Lately i've been thinking about using the control and drive motor inside broken front load washing machine units to power a compressor. That would be in my price range for an experimental system.

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