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  1. #1
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    Jun 2005
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    I was curious if the compressor in a typical home air conditioning unit runs at a consistent pressure? I am trying to get an idea for the efficiency of the unit, and how much more efficient would the system be if I were able to cool the pressurized coolent further. I am considering a heat exchanger with cool water from a river, or well, to suppliment the normal air exchange.

    So, on a hot day, use the blower to cool it down to the 90 degree temp outside, and then go through the water cooler to see if you can run it down to below 70. How much more efficient would that make the system? Would my compressor run at a lower pressure? Or just kick on less frequently?

    Am I correct to assume that the units with a higher SEER efficiency rating run under a higher pressure? And therefore have a higher temp difference across the system?


  2. #2
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    Originally posted by feet1st
    Am I correct to assume that the units with a higher SEER efficiency rating run under a higher pressure? And therefore have a higher temp difference across the system?

    actually, higher seer units run at a lower pressure. they will have larger condensor coils, which cool the refrigerant more, allowing it to become a liquid at a lower pressure.

    [Edited by craig1 on 06-24-2005 at 03:31 PM]

  3. #3
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    Nov 2004
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    Originally posted by feet1st
    I was curious if the compressor in a typical home air conditioning unit runs at a consistent pressure?
    (No.)
    I am trying to get an idea for the efficiency of the unit, and how much more efficient would the system be if I were able to cool the pressurized coolent further. I am considering a heat exchanger with cool water from a river, or well, to suppliment the normal air exchange.
    (You can only subcool so far without returning to saturation point...uh....I think that's right.Pleas correct me if I am thinking wrongly.)

    So, on a hot day, use the blower to cool it down to the 90 degree temp outside, and then go through the water cooler to see if you can run it down to below 70. How much more efficient would that make the system?
    (You are on the right track in you thinking...however I dont know if this is really practical.The genuises at goodmans figured it all up and I just believe 'um)

    Would my compressor run at a lower pressure? Or just kick on less frequently?
    (Yes I think you would run at lower pressure because more of condenser space would be available for desuperheat and condensing,but I am no expert.I think you have to get your liquid pressure back up,for several reasons.Where the hell is R.J.Dalton?
    The more I think about it, the less I know.I hope to see an simple,concise answer.)

    Am I correct to assume that the units with a higher SEER efficiency rating run under a higher pressure? And therefore have a higher temp difference across the system?


  4. #4
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    Jan 2004
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    Instead of using water from a stream to cool the discharge gas.

    Have a desuperheater installed and heat your water with it.
    Basically free hot water in the summer.
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  5. #5
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    I will put this in as few words as possible without getting too technical.

    When the objective is to obtain the highest operating efficiency of the refrigerant cycle as possible you keep the suction pressure as high as possible while maintaining the desired evaporator temperature and keep the high side pressure as low as possible while condensing the refrigerant and creating enough pressure differential to allow the metering device to work.

    Raising the suction pressure and lowering the head pressure greatly increases the volumetric efficiency of the compressor.

    In a nutshell that is it. Keep the pressure differential between the two pressures as minimal as is practical. Cooling the condenser with water instead of air is one way of doing just that and is in fact done all the time.

    Water cooled condensers using cooling towers are a good example. No matter how hot the outside ambient air temperature gets, the evaporating water from the tower keeps the circulating water cool enough to properly cool the condenser. In hot dry climates this is great as long as water is available and inexpensive.

    I could get more technical but I promised to keep it simple.

    Norm

  6. #6
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    Keeping the pressure differential to a practical minimum not only decreases the compressor operating cost and power consumption it also increases compressor BTUH capacity.

    In other words, with an air cooled condenser, the hotter the outside air ambient temperature, (when you need cooling capacity the most)the higher the head pressure, the more it costs to operate and the lower the operating capacity.


  7. #7
    Join Date
    May 2004
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    Ala boat a/c systems. No condensing fans. Small pump circulates seawater through an exchanger that cools the hot gas.

  8. #8
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    If the heat exchanger was downstream of the condensing coils, the caution would be not to subcool the liquid to the point where a normal pressure differential across the metering device was lost.

    Also, one would have to weigh any efficiency gains against the cost to install a subcooling heat exchanger and the ongoing cost to provide that exchanger with a cooling medium, such as pumped in water.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  9. #9
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    Jun 2005
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    The shophound as got it! That is the basis for the question. The objective is to use a ram pump or sling pump and have the river water pumped using the power of the river, rather than electricity.

    Then, I want to power the compressor with the falling water, see my other post:
    http://www.hvac-talk.com/vbb/showthr...threadid=78906

    So, I'd essentially have a river cooled house, using a combination of river powered, and cooled systems to achieve that. Including a large (5,000 gallons??) holding pond at the top of the hill to help even out flow rates etc.

    And I said to "suppliment" the air exchange, because I'd plan to still leave that in place, both so I put less thermal pollution into the river, and as a backup in case my water cooling system is non-functional at some point for any reason. However, if the water cooling system alone works well enough (better?), I might turn off the blower.

    So, the question really was more directed towards... would it be worth it? How much more efficient would the cooling system be, if I could water cool it with say a 60 degree liquid? As opposed to say 90 degree air? Does anyone know the basic formulas to estimate that for a home-sized unit?

    I almost think the bragging rights alone might be worth it

    The responsiveness on this message board has been great! Thanks to all.

  10. #10
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    I don't know the formula, but I would think that you would raise the seer by 4 to 6 points.

    A 3 ton would need about 15 gal a minute for the condenser, or 540 an hour.

    But I think the water volume to run the compressor might be alot more then you thing.

    Just a guess, but in excess of 200 gal a minute for a 3 ton compressor.
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  11. #11
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    Oct 2001
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    What about the feasibility of using a fine mist of water sprayed on the condensing coils to improve cooling through evaporation? Years ago I read about doing this to help cool your car radiator when doing mountain driving. The idea then was to use the windshield wiper fluid reservoir and pump to direct a spray of water over the cars radiator. Would it be worth it to wire the second stage cooling circuit of the thermostat to a solenoid valve that would mist the condenser coils? How would you calculate the increased capacity of the unit relative to lowered coil temperature?

  12. #12
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    Originally posted by holabr
    What about the feasibility of using a fine mist of water sprayed on the condensing coils to improve cooling through evaporation? Years ago I read about doing this to help cool your car radiator when doing mountain driving. The idea then was to use the windshield wiper fluid reservoir and pump to direct a spray of water over the cars radiator. Would it be worth it to wire the second stage cooling circuit of the thermostat to a solenoid valve that would mist the condenser coils? How would you calculate the increased capacity of the unit relative to lowered coil temperature?
    Not a good idea. As the water evaporates on the condenser coil plates it will leave mineral deposits, thereby decreasing the heat transfer efficiency of the coils rather than increasing it.

    Feet1st, frankly I can't see how you'd get enough gpm through a heat exchanger using gravity flow. Perhaps if the exchanger sat far below the source of water and the exiting water didn't have to go uphill, it might do a little. It's an intriguing idea...would be interesting to see some numbers crunched to return some theoretical data on performance. You'd have to allow for pressure drops via the exchanger, friction resistance of all piping and equivalent lengths of all fittings, etc.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  13. #13
    Join Date
    Jun 2005
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    5
    I live in Minnesota. Summer is generally very humid here, so irregardless of the mineral deposits issue, the mist won't see much evaporative cooling

    Yes, I'm starting to see that boatloads of water are going to be needed to deliver sufficient power to the compressor... although if I use the same water for cooling the coils I guess I might have to turn the compressor LESS

    The holding pond and gravity feed is the plan. Simply because it helps assure water is available when such a unit wants to kick on. I mean I can use an undersized river powered pump to fill the pond at night, and then open a valve and gravity feed the A/C during the day, as needed. Details on how to automate that are only worried about after the big picture discussion here.

    I'm thinking if I can't power my fridge, or one of the two A/C units that serve the home, then it's probably not worth the effort. And when I started running what numbers I have and understand, on doing the power output of a turbine for electricity, I could see a lot of water would be needed to get 500-1000watts. Turning the compressor directly will be much more efficient, but ultimately is still going to be a lot of water.

    I haven't closed on the place yet, so haven't been able to survey. I'm guessing that my hill (and planned holding pond) is about 30 feet above the typical water level of the river, and then the A/C unit would be roughly in the middle of the two. So I'd have about 15 feet of head behind the gravity feed. Although, if a water wheel is the drive, I don't guess the head is really going to matter. More the diameter of the wheel and the capacity of the buckets.

    I suppose the alternative would be to simply add a heat exchanger to gravity feed cooler water past the coils just after they've run through the existing coils and just accept the improved efficiency of the existing traditionally powered compressor. And if I can run enough water, that's cool enough, perhaps I can do without the fan on the existing coils.

    Thanks again for all the insight and ideas. I really appreciate it.

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