Micro Heat Pump

gadget · 12-17-19, 11:24 AM

New member here and new to heat pumps. I do have some refrigerant experience with a freeze dry I built years back and feel confident about building a heat pump.

I have a greenhouse where I move heated water through an inground grow bed. I have been heating the water with wood and occasionally a 1K watt heating element. It would be nice to just use a small heat pump instead.

With the 1K watt heater it is able to increase the water temp in the tank above what I need over time. I have a small compressor that I believe is 1/10 HP. I'm hoping to be able to get about 300-400 watt equivalent heat output. It will be a water to water setup. I have a ground water source that can go up to 2 gallons a minute at 55F. Pumping height is 15-18 feet under load, so pumping wattage is low. Water would be returned to a cold storage well 50 ft away.

The pump is salvaged from a water cooler and was R134a. I would use the pump and cap tube. I would initially charge it with R134a and eventually 75/25 nButane/Propane.

For heat exchangers, I was thinking some 1/4 copper tubing coiled up inside about 20" or so of 1" PVC pipe. The well water and circulation water for the grow beds would pass through the PVC and around the copper coils placed inside.

I could use a temperature controller based off of the exit water temp to keep it from freezing. It may not be necessary with such a low power compressor

My first question is can this pump run 24/7 and survive? I use to run my freeze dryer pump for about 3-5 days strait. It used propane and mineral oil. It would get warm to the touch but not hot.

compressor and possible refrigerant

compressor

gadget · 12-17-19, 11:25 PM

I ran the pump briefly and it looks like it may be a 1/6 HP. It was using about 130 watts. I think it will be a good match for what I need.

Daox · 12-18-19, 05:22 PM

Sounds like a cool project. I look forward to seeing what you do. Unfortunately, I have no answers for you.

gadget · 12-19-19, 12:06 AM

Quote:

Originally Posted by Daox

Sounds like a cool project. I look forward to seeing what you do. Unfortunately, I have no answers for you.

No worries. No one's posted any thoughts yet and maybe that is a good thing cause no one sees any problems yet. I'm hoping to start on it tomorrow. I think I will turn this into a build thread.

It seems like more and more as time goes on allot of the DIY forums have less traffic. I think allot of the younger generations just are not into DIY projects

pinballlooking · 12-19-19, 08:21 AM

The other issue is a lot DIY do it on there spare time. It takes time to document you project. Even more time if you want others to be able to follow in your steps.
I think people don’t always take the time to document it. I wish more would take the time to document it and I thank the people that do take the time to document their journey.

I may not be doing this project now but a month from now all bets are off.

I wish I could offer insight on your project but that is not my lane as I am still learning. It does really interest me so I will be following along.

gadget · 12-19-19, 09:11 AM

Quote:

Originally Posted by pinballlooking

The other issue is a lot DIY do it on there spare time. It takes time to document you project. Even more time if you want others to be able to follow in your steps.
I think people don’t always take the time to document it. I wish more would take the time to document it and I thank the people that do take the time to document their journey.

I may not be doing this project now but a month from now all bets are off.

I wish I could offer insight on your project but that is not my lane as I am still learning. It does really interest me so I will be following along.

I will definitely be documenting it. I have done many other build threads. Some successful, some not.

One of my other areas of concern besides running the pump 24/7 is if the heat exchangers will work. And also, I have no idea how to size or calculate anything related to refrigeration. I think I got lucky on my freeze dryer but I have no idea how efficient it was. It did work well though.

SDMCF · 12-20-19, 03:24 AM

Quote:

Originally Posted by gadget

For heat exchangers, I was thinking some 1/4 copper tubing coiled up inside about 20" or so of 1" PVC pipe

I have some difficulty visualizing this so maybe I am not correctly understanding your design. Can you really coil 1/4 copper tubing within a 1" pipe? How much water would then be inside the 1" pipe and outside the 1/4 tube? Is that enough to act as a heat exchange medium, especially over just a 20" length?

jeff5may · 12-21-19, 11:14 PM

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.

Here's an easy to use cap tube app from danfoss:
https://www.secop.com/solutions/comp...ube-selection/

gadget · 12-22-19, 10:13 PM

Quote:

Originally Posted by jeff5may

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.

jeff5may · 12-25-19, 08:34 AM

The important thing to know here is the heat capacity of running water. 1 BTU =1 pound of water changing 1 degF. A gallon of water weighs close to 8 1/3 pounds, so the harvestable heat is 8 1/3 BTU per degF per gallon. This will tell you how much water (at the very least) you need to flow to move your heat.

The heat flow follows Fourier's law of thermal conduction. If you buy copper tubing, the manufacturer publishes tested specs for it. Do a little math and find out if you are in the ballpark as far as surface area and flow rate.

12-17-19, 11:24 AM	#1
gadget Helper EcoRenovator Join Date: Dec 2019 Location: Northern Utah Posts: 49 Thanks: 1 Thanked 2 Times in 2 Posts	Micro Heat Pump New member here and new to heat pumps. I do have some refrigerant experience with a freeze dry I built years back and feel confident about building a heat pump. I have a greenhouse where I move heated water through an inground grow bed. I have been heating the water with wood and occasionally a 1K watt heating element. It would be nice to just use a small heat pump instead. With the 1K watt heater it is able to increase the water temp in the tank above what I need over time. I have a small compressor that I believe is 1/10 HP. I'm hoping to be able to get about 300-400 watt equivalent heat output. It will be a water to water setup. I have a ground water source that can go up to 2 gallons a minute at 55F. Pumping height is 15-18 feet under load, so pumping wattage is low. Water would be returned to a cold storage well 50 ft away. The pump is salvaged from a water cooler and was R134a. I would use the pump and cap tube. I would initially charge it with R134a and eventually 75/25 nButane/Propane. For heat exchangers, I was thinking some 1/4 copper tubing coiled up inside about 20" or so of 1" PVC pipe. The well water and circulation water for the grow beds would pass through the PVC and around the copper coils placed inside. I could use a temperature controller based off of the exit water temp to keep it from freezing. It may not be necessary with such a low power compressor My first question is can this pump run 24/7 and survive? I use to run my freeze dryer pump for about 3-5 days strait. It used propane and mineral oil. It would get warm to the touch but not hot. compressor and possible refrigerant compressor

12-18-19, 05:22 PM	#3
Daox Administrator Join Date: Aug 2008 Location: Germantown, WI Posts: 5,525 Thanks: 1,162 Thanked 374 Times in 305 Posts	Sounds like a cool project. I look forward to seeing what you do. Unfortunately, I have no answers for you. __________________ Current project - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. & To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

12-19-19, 08:21 AM	#5
pinballlooking Super Moderator Join Date: Jun 2013 Location: SC Posts: 2,923 Thanks: 172 Thanked 564 Times in 463 Posts	The other issue is a lot DIY do it on there spare time. It takes time to document you project. Even more time if you want others to be able to follow in your steps. I think people don’t always take the time to document it. I wish more would take the time to document it and I thank the people that do take the time to document their journey. I may not be doing this project now but a month from now all bets are off. I wish I could offer insight on your project but that is not my lane as I am still learning. It does really interest me so I will be following along. __________________ Current project Aquaponics system , Passive Solar Greenhouse build To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. Solar Install 12.5 Kwh- To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. Mini Split installs - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. EV Chevy Volt - To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

12-21-19, 11:14 PM	#8
jeff5may Supreme EcoRenovator Join Date: Jan 2010 Location: elizabethtown, ky, USA Posts: 2,431 Thanks: 431 Thanked 619 Times in 517 Posts	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. Here's an easy to use cap tube app from danfoss: https://www.secop.com/solutions/comp...ube-selection/ Last edited by jeff5may; 12-22-19 at 11:21 AM.. Reason: Words

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12-17-19, 11:25 PM	#2
gadget Helper EcoRenovator Join Date: Dec 2019 Location: Northern Utah Posts: 49 Thanks: 1 Thanked 2 Times in 2 Posts	I ran the pump briefly and it looks like it may be a 1/6 HP. It was using about 130 watts. I think it will be a good match for what I need.

12-25-19, 08:34 AM	#10
jeff5may Supreme EcoRenovator Join Date: Jan 2010 Location: elizabethtown, ky, USA Posts: 2,431 Thanks: 431 Thanked 619 Times in 517 Posts	The important thing to know here is the heat capacity of running water. 1 BTU =1 pound of water changing 1 degF. A gallon of water weighs close to 8 1/3 pounds, so the harvestable heat is 8 1/3 BTU per degF per gallon. This will tell you how much water (at the very least) you need to flow to move your heat. The heat flow follows Fourier's law of thermal conduction. If you buy copper tubing, the manufacturer publishes tested specs for it. Do a little math and find out if you are in the ballpark as far as surface area and flow rate.