Thread: Micro Heat Pump
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Old 12-22-19, 11:13 PM   #9
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Originally Posted by jeff5may View Post
Sorry, been busy working all the time. Let me just spout off some ideas for you and let you go from there.

With the capacity you have, it's going to take a long time to heat up much water. A cap tube is optimized for the original rig. A water cooler has a super long recovery time anyway. Nevertheless, a 1/6 hp setup will move around 2000 BTU per hour if it works correctly. A 1KW electric element will provide up to 3400 BTU per hour. I imagine the compressor would use somewhat less than 200 Watts, after a jolt at turn on and pressure ramp up. I would definitely run some sort of suction line accumulator, so the compressor can't suck up any liquid when it starts every cycle.

I'm skeptical about the short cold tube hx also. I could see it either not having enough surface area to move the heat at a reasonable dT, or being forced to run at a heavenly dT and turning into a 2 foot popsicle. Regardless, you want it to be efficient enough to move the compressor heat but way not cold enough to freeze the water at your flow rate. With such a short, narrow water jacket, I would definitely not be trying to cool the water below maybe 40 degF.

Also, I would start off using r134a, as its properties are well known. Setting superheat for a small system is touchy. A little bit of charge can make a bigger difference than one would assume. Much like a dehumidifier, a gram or two makes a huge difference in operating parameters.
Part of the reason for the short HX is space limitation and wanting to keep it at ground level with no elevation changes. I'm hoping to keep pumping height as minimal as possible since it will be feed directly by a well. Standing water level is 13ft and it will return back to a thermal cold storage well about 30 feet away. I will post a picture when I get caught up from holiday busyness.

On other thing I forgotten to mention, the well water first goes through a radiator before the heat pump. I run a blower on the radiator at night when temps drop below 40F. The water typically goes from 55F to 45F out the exit when its running. It works very well at keeping the greenhouse above freezing. I'm hoping to still run it on the 45F water when they are both running.

The grow beds don't need much heat and I thought this would be a good starter build before I get into my next two heat pumps. I have a 500w and 1000w scroll compressors that I want to build some heat pumps with.
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