I have been doing a lot of reading about DIY chilling systems and looking at systems that immerse a coil in a box of water and pump water in and out etc. Most of them are rather primitive, but there were several conversations about using a mini split air conditioning system to chill water. The mini split units have two sides, the outdoor unit is the compressor and condenser that vents the heat into the outdoors and the inner unit is the fan and evaporator that provides cooling and in some units heat as well. One of the benefits of a mini split unit is that the heat produced from the chilling process is outdoors. The refrigerant pipes are routed through the wall to the indoor unit.
So I was surprised that I saw a mini split unit on craigslist that had been there 20 days. It was located nearby and seemed reasonably priced so I emailed. We negotiated a price at exactly half of the new cost. That may seem high but this unit had several things going for it. It is 9000 btu’s about a 3/4 HP chiller size. It is 115 volt so I don’t have to run a separate 220 line to run it, and I can justify hacking it a bit if I don’t pay full price. I apparently am an extremely trusting customer since I could’t see it working before I bought it. I told the man who sold it that I was trusting him and he said after all, I now knew where he lives.
The seller told me the refrigerant was “sucked down” into the outside condenser when it was disconnected. I read the disconnect process online and that the way these units are disconnected so it sounded right. The air conditioner comes with 16′ of refrigerant pipe that seems to be in good condition; 1/4″ and 3/8″ copper pipes. The 1/4″ delivers liquid refrigerant (this unit uses 410a) and the 3/8″ releases the vapor back to the condenser unit outside to be turned back into liquid. The valve for the lines designates a high and low side.
Mini Split System Frigidaire
My intention is to create a chiller inside using this refrigerant. I have several resources about how to build your own chiller, including a Reefkeeping.com article from 2007 where someone used two minisplits to chill water using a “barrel chiller”. This is a container that water flows through that also has a refrigerant coil inside it.
Barrel Chiller for Aquarium Setup
It is almost impossible to find a barrel chiller for sale online. S&W Wilson Inc. carries them but the 3/4 HP size is $555 without shipping!
Chiller Barrel
Another article about creating a DIY chiller uses an indoor dehumidifier and a home built coil. It also has a link to a 1 ton coil that sells for about $60. But the author said that coil was too large for his application. He also describes how to build a refrigerant coil to immerse into water to chill it.
One of the coolest things about this article is a great short explanation of the refrigeration cycle.
The first half of the tube cycle is called the high side. Here the pressure and temperature are high. The refrigerant condenses from gas to liquid here. It then passes, in liquid form, through the long, thin, curled up, copper tube called “the capillary tube”. The capillary tube’s function is to hold back the liquid so that the pressure on the high side stays high. Some larger chillers use a thermostatic expansion valve instead of a capillary tube. The official name in both cases is “metering device”. On the other side it enters the low side where pressure and temperature are very low. Here it evaporates into gas. Now the point of all this is of course to get the cold section in contact with aquarium water and the hot section in contact with air so that heat is transferred from the water to the air. The part that is in contact with flowing air is called the condenser, and the part in contact with water is called the evaporator.
Refrigeration Explanation
I have an expensive water to water heat exchanger for the wood boiler system that I was going to use for the chilling system too. But there are refrigerant to water heat exchangers as well. A study of ebay offerings revealed an Alfa Laval AC10-16 Stainless Brazed Plate Heat Exchanger 650 PSI for R410A that has to have brazed connections for the refrigerant. The two refrigerant lines are 1/4″ and 3/8″ but they don’t specify the water connections. I believe they are 1/2″ which is what my pump is too. I looked up the specs and the CX-10 is rated for 1/2 HP. For about $60 shipped this is a much more reasonable investment than a barrel chiller.
Ebay also has a Coaxial Coil heat exchanger that seems to match the requirements for an evaporator. The one ton size is matched to 9500 btu on this chart.
Coaxial Coil Sizing
The one ton size is available for about $180 but the lines are 3/8″ and 5/8″ so it seems too large for the mini-split condenser. I didn’t find the smaller models for sale.
It also seems as though instead of blowing air through the indoor evaporator unit, I could have the water chilled by it by imitating the process used in a commercial chiller. The indoor unit is composed of a fan, an evaporator coil and the electronic controls. I found a generic parts diagram on Smart Clima which is a chinese site that also shows barrel water chillers but not with pricing to order online.
Mini Spilt Indoor Air Unit Structure
Indoor unit parts
1. Base
2. Cross Flow Fan Axletree
3. Cross Flow Fan
4. Cross Flow Fan Fixed Plate
5. Electric Heater
6. Evaporator Assembly
7. Room Temperature Sonde Frame
8. Louver
9. Outlet Part
10. Screw Cover
11. Middle Frame
12. Filtering Net
13. Face Plate
14. Step Motor
15. Armor Tubing
16. Plate
17. Motor Cabinet
18. Motor Platen
19. Motor
20. Electric Box Small Coverplate
21. Electric Box Cover
22. Electric Control Plate
23. Electric Box
24. Press Tube Plate
25. Wall-Mounting Frame
26. Remote Controller
I would rather not destroy the indoor unit to create the chiller though. That way if the chiller design does not work, I would still have the mini-split air conditioner to install.
As I see it I have four choices, in order of expense:
- Purchase the Alfa Lavel heat exchanger for the refrigerant and use it as the indoor unit. (Learn to braze.)
- Purchase the titanium coil for the refrigerant and build a container for it that allows water to flow through and chill. (Learn to braze too.)
- Purchase the Coaxial Coil and just reduce the connections to 3/8″ and 1/4″ (Probably would need to braze these connections as well.)
- Figure out how to use the existing evaporator to chill water. (This is most expensive if I take apart the interior unit as its initial cost is higher than the other choices.)
I will also have to figure out how to control the chilling process too. Perhaps with an aquarium temperature controller. But these are fun problems to have, right?