Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Misting systems for HVAC

  1. Dec 20, 2011 #1
    Hi Guys,

    I'm fairly new here, but I can't solve my problem (or I did but than I found a miracle)

    I'm trying to optimize a HVAC system, especially for dry and hot areas. Now I was thinking about the use of a misting system in front of the condensor as done by for instance these guys; https://www.mrmistsystems.com/new/store/document/ac_pre_cooling/Coolnsave_Tulane_Study.pdf

    However, I wanted to calculate the actual savings myself, so started to build a model;

    Basic working principle HVAC

    In the ideal vapor-compression cycle, refrigerant enters the compressor as a saturated vapor. As the refrigerant is compressed, it increases in temperature and pressure. After the compressor the refrigerant passes through the condenser. Heat energy (QH) is exchanged with the surrounding environment causing the refrigerant to cool and become a saturated liquid. Next, the refrigerant passes through the expansion valve causing the temperature and pressure to decreases. Because of the reduction in temperature and pressure, the refrigerant enters the evaporator as a saturated mixture. As the refrigerant passes through the evaporator, it absorbs heat energy (QL) from the environment that it is trying to cool. The refrigerant exits the evaporator as a saturated vapor and returns to the compressor to begin the process all over again.


    For the sake of the end product, we are taking the air surrounding the condensor as a starting point for the calculation. Since the effect of pre-cooling has to be shown.

    A general rule of thumb is that the saturation temperature of the refrigerant at the condensor pressure should be at least 10-15 degrees above the temperature of the environment. From that criterion, the condensor pressure can be chosen.

    The second important input number is the refrigerant mass flow rate of the system. Since, in this case it will not be variable for the different temperatures

    Thirdly we have to calculate the heat removal by the evaporator, which can be described as:

    Qe = Vair * d-air * Cpair * ΔT incoming air

    Qe = heat removal in evaporator
    Vair = volume of the air passing by the evaporator, dependent on refresh rate (m3)
    d-air = density of air at room temperature (kg/m3)
    Cp air = specific heat of air (kJ/kg*K)
    ΔT = temperature between the outside air and the inside air

    Based on the heat removal and the refrigerant mass, one can calculate the refrigerant conditions at evaporator outlet;

    Qe = mref * (hevap - hconl)

    Qe = heat removal in evaporator
    mref = mass flow refrigrator
    hevap = enthalpy at evaporator outlet
    hconl = enthalpy of saturated liquid at condensor outlet, since in the throttle only pressure is reduced; h condensor liquid ≈ h evaporator inlet

    hence; hevap = Qe/mref + hconl

    Compressor power

    W = n / (n-1) .Pevap . Vevap . (Pcon/Pevap)^( (n-1)/n) ) - 1

    W = compressor power
    Pevap = evaporator outlet pressure
    Vevap = Volume at evaporator outlet
    Pcon = condensor inlet pressure
    n = polytropic factor, normally between 1.25-1.41

    For the throttling valve: % flash = (hf high pressure - hf low pressure) / (hfg low pressure)


    With these formulas I end up with savings of about 12% for a 2 degree (Kelvin) drop of outside temperature, this feels off to me.

    What am I doing wrong?
  2. jcsd
  3. May 23, 2012 #2
    Are you really trying to figure why you should mist the air, or if you should? Hot and dry climates are practically asking for Water Cooled Towers or at least Hybrid Dry Coolers. But if you have to stay dry, most dry condensing units will meet their rated capacities with 115 degree ambient air.

  4. May 26, 2012 #3
    Sudden (unexpected) hot whether waves sometimes force to do this on the equpment not designed for high temperatures. But to keep without big reason the condensation pressure lower than designed may turn wrong. (However many would argue it would not.) Besides, for sure, the condenser fins get fouled from the water.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook