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I'm contemplating salvaging a used refrigerator compressor to use to aerate my pond. A new compressor actually intended for pond aeration is 600 to 900 bucks.

There are several good youtube videos on repurposing old compressors for DIY air compressors for low demand usage, and I know someone who used a reefer comp and a water tank to power the air stapler he used to build 300 bee hives. In that usage it was running 8 hours a day for several months.

More info: Checking a few replacement compressors at random they have a displacement of 7 to 12 cubic centimeters. If run at 1800 rpm, and using 10 cm^3 for the sake of round numbers, we get 18 liters a minute, or not quite 5 gpm. If perfectly efficient, this works out to about 17 watts.

Oil lubed air compressors typically put in about 1 mg/m^3, or about a mg/hour. 24 mg/day, about 0.6 g/month. Bacteria will eat oil. This may well be within the munch capacity. However oil separation filters are cheap, and will handle any aerosols. Not sure if oil vapour is a concern.

I have found that temperature is a concern. If I need to use an oil/water separator, I have to keep it from freezing. Given winter temperatures of potentially -40, this probably means keeping the compressor inside the house, or at keeping the trap inside the house.

Questions I don't know:

Will the compressor survive a 100% duty cycle?

How do I check the oil level -- it's no longer a closed system.

Fridge compressors typically have an output at about 120 psi. My pond is 14 feet deep so will take ~7 psi. Will operating so far under design be harmful?

Should I be asking this on Engineering.SE instead?

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    FWIW, the "repurposed air pump of choice" for aquaculture and serious aquaria is the automotive "smog pump" from a car - much better suited in terms of high volume and low pressure, where your refrigeration compressor will have high pressure ability but low volume. As far as I recall you'll also pay a severe penalty in power bill .vs. a more suitable pump operating efficiently. – Ecnerwal Dec 14 '20 at 0:12
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    How do I check the oil level? ... just test the water in the pond – jsotola Dec 14 '20 at 1:26
  • The fish in your pond may not like the compressor oil very much - not a closed system remember, so any oil you put in one end will sooner or later come out the other end... – brhans Dec 14 '20 at 3:39
  • For a low pressure , high volume application , radial compressors rather than reciprocals are normally more efficient. – blacksmith37 Dec 14 '20 at 15:50
  • @blacksmith hvac compressors are usually scroll type not radial or reciprocal. This is true for almost all refrigerators and most home ac units. – Ed Beal Dec 15 '20 at 0:30
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I used a old refrigerator compressor as a small shop compressor decades back with an used R12 tank. it works and is fine for shop air however I probably not do this with your fish. (Today refrigerant tanks have check valves to prevent this use and it’s not the safest method to use old propane tanks because of the thin metal used and no internal coatings in most cases).

Why not? The compressor will be lubricated with mineral oil or Poe/pag oil on newer compressors and the compressor is designed to pass oil, if using any type of compressor I would suggest oil-less or at a minimum have a trap to catch the oil. It will only take a few drops to coat the water and could be fatal to your fish, I have a small solar water pump on my pond. It has frozen solid 3 or 4 times but has continued to work when thawed I got it from harbor freight for under 30$ and it has worked well.

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Another consideration ; use a water pump to spray water into air . Or , what I do with my tiny pond ( 42" deep) is pump water up about 3 feet and let is run down an artificial stream ; mine is about 30' long with 3 drops of a few inches. It makes an interesting visual and some small fish like to play "salmon" and jump the drops to go to the top end and spawn . I have danios but they only survive down to about 50 F.

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  • I do agree with pumping water fountain or waterfall that works great and is inexpensive. I have goldfish and koi they survive the top freezing but I do break the ice so they can surface there metabolism slows with the cold water so we don’t supplement in winter because the excess food rots in there stomachs and they die (or we have been told this) After I quit feeding them in the winter I think I have only lost 2 in over 10 years. – Ed Beal Dec 14 '20 at 16:56
  • Not as cold here , I notice the fish slow or stop eating on their own as it gets cold. – blacksmith37 Dec 15 '20 at 3:56
  • I was not as lucky koi grab the fish food on instinct or I was told this and that’s why they were dying, after I quit feeding them in the winter they have done better even though my current home only has a small pond not the +10k gallon monster I had previously. – Ed Beal Dec 15 '20 at 14:27
  • Bubbles are MUCH more efficient. In addition to having a huge surface to volume ratio, in bulk they create a rising column of water.. – Sherwood Botsford Dec 15 '20 at 18:40
  • I didn't notice your low temperature , with ice on the surface ,air bubbles look like your only choice. – blacksmith37 Dec 15 '20 at 21:49

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