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  1. #1
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    Discharge pressure increases.....why?

    Hello, I'm trying to expand my understanding of residential systems and have a question that hopefully someone here can answer. I'm looking for the chain of events that cause discharge pressure to increase as ambient air temperature increases. As an example, take a system that is functioning normally at 85 degrees. As the ambient air heats up to say 95 what series of events happen in the system to cause the high pressure to increase?

    Thanks.

  2. #2
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    back to the books. look at your gas laws.
    get your posts up & go pro. the ed forums will be your friend

  3. #3
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    The laws of physics.

    Do you have an entrophy chart.

    If so look at it.

    And if your in the trade, or trade school. Get your post count up, and apply or pro membership.
    So you have access to forums where this and other technical things are discussed in the pro forums.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  4. #4
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    Quote Originally Posted by nutohvac View Post
    Hello, I'm trying to expand my understanding of residential systems and have a question that hopefully someone here can answer. I'm looking for the chain of events that cause discharge pressure to increase as ambient air temperature increases. As an example, take a system that is functioning normally at 85 degrees. As the ambient air heats up to say 95 what series of events happen in the system to cause the high pressure to increase?

    Thanks.
    Increase in outdoor ambient is only part of the reason why head pressure goes up. Heat gain will increase to the structure from an 85 degree day to a 95 degree day when attempting to hold indoor setpoint below outdoor ambient. If the evap fan suddenly quit on a 95 degree day, would head pressure stay the same as before the evap fan quit? Which direction would you expect it to go with an evap fan failure?
    • 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.

  5. #5
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    May 2009
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    Thanks ShopHound. Now we're getting somewhere. I'm not looking for a theoretical answer like "carnot says". I'm looking for a real world explanation.

    The compressor doesn't have a temperature sensor that causes it to directly increase the pressure when the temperature increases. The discharge pressure increase is in response to some physical changes elsewhere in the system. There may be more than one. But what?

  6. #6
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    Jun 2005
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    SW Wisconsin
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    Of course, both the outdoor temp going-up & the heatload or humidity level going through the indoor evaporator coil going up affects the condenser pressure.

    A big indoor factor affecting an increase in the outdoor heatload & pressure is the latent or humidity load.

    At same room temp, but at a 35% RH load compared to a 80% RH humidity there's a big difference in head pressure heatload.

    Always look at the (IWB) Indoor Wet Bulb Temp.
    I messed up that chart of an old, -low EER A/C.
    See how the condenser Temp/Split goes up as the WB goes up:

    Last edited by udarrell; 05-14-2009 at 01:54 PM. Reason: outdoor temp

  7. #7
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    Quote Originally Posted by nutohvac View Post
    I'm not looking for a theoretical answer like "carnot says". I'm looking for a real world explanation.
    Boyle says PV=nRT

    or, in the real world, Pressure is proportional to Temperature.

  8. #8
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    Quote Originally Posted by nutohvac View Post
    Thanks ShopHound. Now we're getting somewhere. I'm not looking for a theoretical answer like "carnot says". I'm looking for a real world explanation.

    The compressor doesn't have a temperature sensor that causes it to directly increase the pressure when the temperature increases. The discharge pressure increase is in response to some physical changes elsewhere in the system. There may be more than one. But what?
    The pressure-temperature relationship is key. Once you understand it, the light will go on. Remember sea level water boiling temperature, and that of Denver, Colorado, fully a mile above sea level. They are not the same, and atmospheric pressure is why.

    As pressure affects temperature, so temperature affects pressure. It is symbiotic, if you will. Some of the best instructional material concerning the vapor compression refrigeration cycle has been written by the Refrigeration Service Engineers Society (RSES) and is available at their website, rses dot org. RSES also has elearning available on this subject for the computer savvy/book averse.

    You would benefit greatly from instruction in refrigeration cycle theory (the word theory should never be paralleled with "hypothetical". We cool and heat homes every day with "theory"), even if its from a different source than RSES.
    • 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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    Western PA
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    Quote Originally Posted by sktn77a View Post
    Boyle says PV=nRT

    or, in the real world, Pressure is proportional to Temperature.
    As I interpret the OP, this is the answer that he wants.

    The discharge pressure increases in response to tenperature so that the vapor can condense.

    If the condensing temp is 85 degrees (for example) and the temp climbs to 90, it will no longer condense.

  10. #10
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    Quote Originally Posted by jpsmith1cm View Post
    As I interpret the OP, this is the answer that he wants.

    The discharge pressure increases in response to tenperature so that the vapor can condense.

    If the condensing temp is 85 degrees (for example) and the temp climbs to 90, it will no longer condense.
    Exactly...you need a temp. difference so that the condensing temp. is above the outside ambient temperature so you can have heat transfer. With a high-efficiency system that might be roughly 20F. above ambient. So let's say we have a system operating operating with an outside ambient of 80 and the corresponding condensing saturation temperature (CST) of 100F at 196 (R-22). It has its temp. difference of 20 and it's shedding its heat. Now hold everything constant, except now the outside temperature increases to 100 (for illustrative reasons). Now the saturation temp. of 100 is the same as the ambient temp. and no heat transfer can take place. So...the heat is recycled thru the system, and more heat is added by the evaporator. So now the refrigerant contains more total heat and will continue to add heat until the CST automatically increases until it reaches that 20 F. difference it needs to shed the heat - in this case about 120 which corresponds to about 260 PSI (R-22), and reaches equilibrium again.

  11. #11
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    central illinois
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    temp rise pressure rise
    work to live not live to work.

  12. #12
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    May 2007
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    Dry as a bone Tucson
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    series of events

    Quote Originally Posted by nutohvac View Post
    Hello, I'm trying to expand my understanding of residential systems and have a question that hopefully someone here can answer. I'm looking for the chain of events that cause discharge pressure to increase as ambient air temperature increases. As an example, take a system that is functioning normally at 85 degrees. As the ambient air heats up to say 95 what series of events happen in the system to cause the high pressure to increase?

    Thanks.
    Your metering device and system charge dictate the overall pressures thru the system.The compressor raises pressure to overcome the restiction so gas/liquid can flow.
    Some Talk, Some Do
    "keeping condensing pressures low and evaporator pressures high"
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  13. #13
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    Australia
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    Quote Originally Posted by ACFIXR View Post
    Your metering device and system charge dictate the overall pressures thru the system.The compressor raises pressure to overcome the restiction so gas/liquid can flow.
    I think your over simplifying the answer. There are more factors and as previously stated you always have a pressure tempreature relationship.

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