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Ac compressor with liquids?

  1. Apr 14, 2006 #1
    I need to pump a refrigerant in its liquid state (from low to high pressure), and I am wondering if a normal rerfrigerator compressor is capable of doing this. I don't know the internal specifics; if it somehow depends on the compressability of the fluid it's pumping. I would prefer to use one of those compressors that are in car ac systems if possible. (I'm trying to build a heat engine)

    Thanks,
     
  2. jcsd
  3. Apr 14, 2006 #2
    You could just run your fridge backwards and turn it into a heat pump :wink: For the most part, the liquid should be treated as an incompressible liquid. I would *think* the pump *should* work just fine. But thats just a blind guess.
     
  4. Apr 14, 2006 #3
    An engine, not heat pump :). The fluid can flow, it just has to be liquid inside the pump iteself to reduce the volume needing to be pumped...

    Also if someone knows where I might find approximate flow rates of car ac compressors I'd be really appreciate.
     
    Last edited: Apr 14, 2006
  5. Apr 14, 2006 #4

    Cliff_J

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    Longstreet - why does the volume matter? If you are going to perform work on something by compressing it and trying to force it to the liquid portion of a temperature/pressure phase diagram at a higher pressure, you are starting with a gas.

    A typical A/C compressor would likely not work unless there are one-way check valves on its output so the pistons wouldn't be trying to compress a liquid inside a fixed volume. And being near incompressible, not a good situation.

    If you have a liquid, use a liquid displacement pump like an oil pump that could be inter-meshing gears or a gerotor. A power steering pump would have a built-in valve to prevent its pressure from exceeding maybe 1000-1200psi and they come in many differing levels of flow from 2 GPM to 7 GPM or higher.
     
  6. Apr 14, 2006 #5
    The smallest quantity of liquid entering in a recip. can damage the piston (this is generally called as 'liquid stroke' and audible by loud knocks inside the compressor) and is avoided in all cases as far as refrigeration and air condition systems are concerned. Scroll compressors can take care of (accidental) small liquid quantities, but not for a continuous service.

    You should use positive displacement pumps (either piston or gear). Propellant storage facilities of aerosol can filling plants use piston pumps.
     
  7. Apr 14, 2006 #6
    Thanks. I hadn't thought of power steering, and the flow rate is even in the range that I need. My plan is basically a low temperature steam engine. So, it needs to be liquid pumped into the higher pressure boiler (still under 100psi though). It's just an experiment using used or off the shelf parts.
     
  8. Apr 14, 2006 #7

    Danger

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    Re: Cliff's suggestion. Don't oil and power steering pumps rely upon the pumped fluid for lubrication? I'm just wondering how they'll stand up to pumping water.
     
  9. Apr 14, 2006 #8
    It is liquified butane, instead of water. Not sure if that makes it any better lubricant.
     
  10. Apr 15, 2006 #9

    Cliff_J

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    Houston, we have a problem. Butane is flammable and you are going to pressurize it? We might all be causal about a butane lighter, but they are dangerous as well and much lower pressure. Its bad enough when you can find examples of steam engine explosions, but one with a flammable....I think it doesn't need explanation.

    If you're going to do a steam engine with water, use a water pump. Not the kind on a car engine, that's only good for 15psi, try something like McMaster.com and get a real water pump like on page 305.
     
  11. Apr 16, 2006 #10
    Butane is used in refrigerators as a replacement for ozone depleting chemicals, which I imagine run at over 3x the pressures I plan to run at. And obveously it would need to be evacuated like any other refrigerator system before putting the butane in. I wonder if a pressure washer pump has a near vacuum quality seal. those things go at several thousand psi.

    I am also working on an alternate design without needing a compressor at all. It might still need a small pump but the pressure differences would not be very large. Basically it would have two buffers, alternating with one filling with refrigerant from the condenser, and one emptying to the boiler (see first image).

    Or, maybe by heating and cooling the buffer (see second image).
     

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    Last edited: Apr 16, 2006
  12. May 11, 2006 #11
    Longstreet

    Did you get any progress on this? I have seen Butane as a preferred material for this sort of purpose. With such a low boiling point less heat is needed than for say water/oil at the same pressures.

    The butane liquid on the hot side steams to a high enough pressure to drive a turbine/steam engine/condensor backwards. I am still at the theoretical stage but would love some real world examples using low tech materials and especially the long term stability and lubrication.

    Matthew
     
  13. May 24, 2006 #12
    It looks like the seals in whatever pump you decide to use will be the crucial element. Most high pressure pumps that I am aware of use the pressure to assist with the seal, so I am not sure whether any of the pumps suggested will work in a near vacuum situation.

    It may be a case of trial and error unless you can get the specs on a particular pump. There are a lot of hydraulic pumps used in various situations and industries. Not all of them are huge and expensive.

    The pump from a pressure washer would be a real blast:smile: if it will seal under low pressure situations. There are even some small electric pressure washers that turn out about 1200 psi.

    Hmm how about a pump that is driven by a magnetic coupling?? No seals are needed.
     
    Last edited: May 24, 2006
  14. May 30, 2006 #13
    Haven't built it yet, though I have the main parts (air motor, heat exchangers, water pump). I'll definatly post when I get it assembled.
     
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