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  1. #1
    Join Date
    Dec 2011
    Location
    pensacola fl.
    Posts
    88

    humidity and evap. relationship

    Does a higher humidity level than normal in a home cause the evap. pressure to increase to a higher than normal level also? For example if there were a known problem with excessive indoor air flow.

  2. #2
    Join Date
    Sep 2008
    Posts
    582
    I'd expect that the pressure differences would be negligible. what you will have however is greater latent/less sensible. YMMV.

  3. #3
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Higher indoor humidity will cause the evaporator pressure to run higher than it would with lower indoor humidity and the same air temperature.
    It will also cause an increase in superheat in fixed metered systems, and a decrease in the temperature drop through the coil.

    Higher humidity = higher heat load.
    There is a big difference in the heat content of air that is 80 and 30%RH, and air that is 80 and 60%RH.

    This is why charging and performance charts use the wet bulb temperature of the air entering the coil.
    If more government is the answer, then it's a really stupid question.

  4. #4
    Join Date
    Jun 2010
    Location
    Western Nebraska
    Posts
    246
    Quote Originally Posted by bluecool View Post
    Does a higher humidity level than normal in a home cause the evap. pressure to increase to a higher than normal level also? For example if there were a known problem with excessive indoor air flow.
    Either variable would add load to the evap coil. Increased load will increase superheat which will increase pressure inside evap coil.

    Take Mark's example above. 80 deg @ 30% equals about 26.6 BTU per lb of dry air and associated moisture, or Enthalpy. 80 deg @ 60% equals about 34.2 BTU of Enthalpy. Evap load would increase by 22% or so with the same entering drybulb temp.

  5. #5
    Join Date
    Dec 2006
    Location
    Goodyear, AZ
    Posts
    197
    Temperature difference of an evaporator coil will vary with the total heat of the air entering the evaporator and the load on the condenser and air velocity.

    This temperature will vary from 10 F to 30 F depending on total heat of the air entering the evaporator. External factors affect coil performance. Principal among these are the circulation, velocity, and distribution of air in the cooled space and over the coil. These factors are closely related and in many cases is dependent one on the other.

    With a fixed orifice

    72 Dry bulb 72 Wet bulb leaving air is 63 Dry bulb
    72 Dry bulb 68 Wet bulb leaving air is 59 Dry bulb
    72 Dry bulb 62 Wet bulb leaving air is 54 Dry bulb
    72 Dry bulb 58 Wet bulb leaving air is 52 Dry bulb
    72 Dry bulb 55 Wet bulb leaving air is 50 Dry bulb

    A TXV system is +2 to 3 of a fixed orifice.

    Check Air, Superheat and Subcooling

  6. #6
    Join Date
    Oct 2011
    Location
    between here and over there
    Posts
    453
    Quote Originally Posted by rdholder View Post
    Temperature difference of an evaporator coil will vary with the total heat of the air entering the evaporator and the load on the condenser and air velocity.

    This temperature will vary from 10 F to 30 F depending on total heat of the air entering the evaporator. External factors affect coil performance. Principal among these are the circulation, velocity, and distribution of air in the cooled space and over the coil. These factors are closely related and in many cases is dependent one on the other.

    With a fixed orifice

    72 Dry bulb 72 Wet bulb leaving air is 63 Dry bulb
    72 Dry bulb 68 Wet bulb leaving air is 59 Dry bulb
    72 Dry bulb 62 Wet bulb leaving air is 54 Dry bulb
    72 Dry bulb 58 Wet bulb leaving air is 52 Dry bulb
    72 Dry bulb 55 Wet bulb leaving air is 50 Dry bulb

    A TXV system is +2 to 3 of a fixed orifice.

    Check Air, Superheat and Subcooling


    what do you mean by: "A TXV system is +2 to 3 of a fixed orifice"?

  7. #7
    Join Date
    Dec 2006
    Location
    Goodyear, AZ
    Posts
    197
    The temperature is from a fixed orifice system that was being testing. When testing a TXV system found that the temperature is + 2 to 3 degrees of a fixed system. I think it is because as I fill the coil with refrigerant lowering the superheat the air temperature goes up. The 13 SEER system tested were at 450 cfm per ton not 400 cfm of the fixed orifice 12 SEER tested.

  8. #8
    Join Date
    Jun 2010
    Location
    Western Nebraska
    Posts
    246
    Quote Originally Posted by rdholder View Post
    The temperature is from a fixed orifice system that was being testing. When testing a TXV system found that the temperature is + 2 to 3 degrees of a fixed system. I think it is because as I fill the coil with refrigerant lowering the superheat the air temperature goes up. The 13 SEER system tested were at 450 cfm per ton not 400 cfm of the fixed orifice 12 SEER tested.
    With a TXV system you aren't filling the evap, the TXV is. I've found very few TXV evap coils in residential applications that didn't try darn hard to maintain about 18* SH.

    With the info you provided on the TXV metered evap temps (fluctuating greatly with evap load) it sounds like a searching metering device.

  9. #9
    Join Date
    Dec 2006
    Location
    Goodyear, AZ
    Posts
    197
    Quote Originally Posted by westval View Post
    With a TXV system you aren't filling the evap, the TXV is. I've found very few TXV evap coils in residential applications that didn't try darn hard to maintain about 18* SH.

    With the info you provided on the TXV metered evap temps (fluctuating greatly with evap load) it sounds like a searching metering device.

    When a fixed orifice superheat is above the superheat of a TXV when there is a heavy load on the evaporator coil above 68 degrees wet bulb or the outside air is below 80 degree dry bulb a fixed the superheat will be above 16 degrees at that time a TXV will open adding more refrigerant to the coil rising the refrigerant temperature. An AC system with a TXV superheat is 14 degrees +-2 degrees or 12 to 16 degrees at the evaporator. I have seen an increase of superheat for 4 to 6 degrees because of the refrigerant line in a hot attic. I have work on AC were the superheat was set for 18 degrees. For every 1 degree the evaporator temperature go up the efficiency go up by 1.5% to 2%.

    Therefore air volume change across the coil will increase or decrease the refrigerant temperature. That increases or decreases the efficiency and capacity of a system. Increasing the heat by increasing the airflow or increasing the evaporating coil size by 10% will give a decrease of water removed from the air by 2% to 4%, but there will be a 4% to 8% capacity increase. This will result in a sensible heat increase in the system from a 72% to 74% and a latent heat decrease from 28% to 26%. An increase of the refrigerant temperature of 1F will increase compressor efficiency by 1.5% to 2%.
    The reason for the increase in heat and the increase in refrigeration temperature is due to the increasing size and/or airflow of the overall coil. Over size an evaporating coil or increase airflow, decreases the water removing latent heat capacity of a coil.

    Decreasing heat by a decrease of airflow or an evaporating coil size by 10% will result in a 2% to 4% increase of water removed from the air, and will cause a 4% to 8% capacity decrease. The capacity decrease results in a decrease of sensible heat from 72% to 70% thereby causing latent heat increases of 28% to 30%. A decrease of the refrigerant temperature of 1F will decrease compressor efficiency by 1.5% to 2%.

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