Reusing a compressor to build a small freezer

[boomchke]

Hi Folks - I'll apologize in advance if this isn't the right place - but I've honestly tried everywhere else and mostly been turned away because "I'm not a professional HVAC guy". Anyways - I thought I'd try here and see if folks could help me out and my end goal is related to making ice so I figured HomeBrew was worth a shot (and I've been a reader for a long time and there are so many helpful folks here!).

So. I'd like to build a small freezer with the end goal of cooling/freezing an aluminum plate. Im trying to build a machine to build clear ice and to do that, you need to directionally freeze the water. Most machines that do this have a freezer plate that water in a bag sites on top of and it freezes from the bottom up. So my initial thought was to take a small chest freezer, make a hole in the top of the it, and expose the water through the hole to the freezing air. I ran into a number or problems but what it really boiled down to was that it was entirely inefficient and took up WAY too much space.

So I thought to myself that I could make a much smaller freezer cavity and just put the evaporator lines in a much smaller insulated space which I would then cover with an aluminum plate to expose to the water.

As luck would have it - a neighbor has a small freezer they are getting rid of since one of their kids was defrosting the inside of it and nicked an evaporator line which drained all the refrigerant. So I have a compressor that I know works, I believe I can use the other pieces from the freezer (metering device, etc) and just add in my own custom evaporator and condensor lines made out of coper.

So first questions first - am I nuts? I've done done some initial reading and it seems fairly simple to recharge a system. It also seems possible to solder new pipes onto the compressor. The compressor says right on it what kind of refrigerant it takes - but there are some pieces Im missing in terms of how much refrigerant it would take (seems to be more about pressure than volume) and some other odds and ends.

Is this a feasible idea? Have folks built freezers/fridges by using a "found" compressor?

Any help or pointers is appreciated.

 

[ajdelange]

Sounds like an interesting project but I think you are going to have to do quite a bit of tweaking to get it working well. First some questions
1) Can you use the existing condenser?
2) What type of metering device do you have?
3) What's the refrigerant?

Obviously things are going to be a lot easier if you can use the existing condenser as it is matched to the compressor. Then you would only need to make the evaporator. That might be a block of aluminum with some channels drilled through (you'd have to get a machine shop to do that for you unless your home shop is pretty good) and tapped to accept nipples and tee's to make up some sort of manifold. If you can recover the distributor from the old evaporator then perhaps you wouldn't need a manifold at the feed end - just adapters between the distributor's output tubes and the channels. The really nice thing about that would be that you could definitely use the metering device you have whatever it is.

Assuming the refrigerant to be R134a you can be pretty loose about how you handle it as its not an HCFC and is thrown about pretty casually (the propellant in lots of "aerosol" cans is R134a). Where I am really going with this is that when you get your system together you need to get ALL the air and moisture out of it unless you don't care about early failure. The accepted way of doing this is to evacuate with a vacuum pump but you probably don't want to buy one of those. I'm thinking you might do a half assed job by flushing it with refrigerant but I don't really feel comfortable recommending that. Some hobbyist types get another compressor and use that as a vacuum pump so that's another possibility.

These systems are brazed rather than soldered so they can withstand higher pressure. Assuming that your ambient is 80 °F and you achieve subcooling of 10 °F that means you will be condensing at about 90 °F which implies high side pressure of 104 psig. I don't think I'd be comfortable with just plumbing solder at that level. And note that if the ambient goes up 10° that the head pressure will go up to 124 psig. Brazing rod is an alloy of copper, silver and phosphorous so it ain't cheap and to get it to melt you really need an oxyacetylene system though some get by with oxygen/propane or MAPP gas.

As for charging - that is done by temperature and pressure. You will have to install pressure monitoring points at the input to the compressor and at the output of the condenser. You will need to get a refrigeration gauge set. This has dials that indicate the pressure they are reading AND adjacent to each pressure index, a temperature. What you need to do here is evacuate your system and then charge it (not running) with R134a to perhaps 15 psig (both high and low side gauges will read about the same with the compressor off). Now start the compressor. High side pressure will go up and low side pressure will go down. Slowly add refrigerant until low side pressure is about 18 psig. This will give you an evaporator temperature of about 20 °F (which you will read from your gauge next to 18 psi). Now check the temperature of the suction line near where it enters the compressor. You want that to be at least 10 ° higher than the evaporation temperature. Do this with load on (water in the freezing compartment) and without. Now check the high side pressure and line temperature. The temperature that you read from the high side gauge is the condensing temperature and you want the temperature of the line leaving the condenser to be about 10 ° lower than this. Tuning the system is a matter of getting the evaporating pressure to a value that corresponds to temperature below freezing at a refrigerant flow rate that results in enough superheat (difference between suction line temperature and evaporating temperature) that insures no liquid enters the compressor and enough sub cooling (difference between condensation temperature and condenser out line temperature) to be sure there is at least some liquid in the condenser.

Now the caveat. I have never built such a system. In fact I have never really worked with R134a except to top up the refrigerant in my cars. I pulled the numbers above out of... well you know where I got them) so take everything with a grain of salt. If you have never worked on refrigeration systems there is clearly a lot for you to learn. Obviously there are several ways you can hurt yourself here. This is why no one will talk to you on any of the refrigeration forums. BE CAREFUL if you decide to undertake any of this.