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  1. #1
    Join Date
    Jul 2017
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    Confused Fan Coil cooling capacity, rated and non-rated condition

    Dear ALL,

    I am a student who is taking a class for heat transfer, which touched slightly on the HVAC topic, and I am kind of confused about an equipment performance on non-rated condition.

    Lets say I now have a fan coil, which its catalogue stated that it has a total nominal cooling cap. of 4.17kW, in which 3.01 is sensible. This result is based on a rated condition of air inlet at 27oC db/19.5oC wb, chilled water 7/12oC.

    What will be the cooling capacity of such equipment if the air inlet temperature is different? As it is the same equipment, some physical properties should remain unchanged. Is it the cooling cap.? The SHR?
    What will happen to the chilled water in/out temp?

    I guess some parameter must be fixed in the control system (eg chilled water in/out temp.?) but I cant get a sense of it in my heat transfer textbook. I think this must be a very basic question but I just cant wrap my head around this. Can someone light me up on this issue?

    Regards,

    KumaKid

  2. #2
    Join Date
    Aug 2001
    Location
    forney texas
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    My take. If the unit in question is designed for a 16-20 degree Fahrenheit split, it should give that same deltaT regardless of water temp and ambient temps, as long as it is functioning as designed


    Sent from my iPhone using Tapatalk

  3. #3
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
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    Heat transfer can get complicated. Your question is actually a good one. The capacity of the coil doesn't change as long as chilled water flow and inlet temperature remain at design. To know the total heat exchanged you need to consider the inlet air conditions. The amount of sensible and latent cooling varies with inlet conditions but the total capacity does not. Since the stat is monitoring dry bulb temperature there are conditions where the only cooling done will be sensible. This happens because if the dry bulb is higher than set point and the RH is very low the coil will only cool until the stat is satisfied and the only cooling done is sensible. The reverse happens with high humidity except the amount of latent heat can basically over load the coil and stat set point will not be satisfied.
    The chilled water outlet temperature will vary with the load and flow if the flow varies, which it usually does.
    Hope this helps.

  4. #4
    Join Date
    Jun 2017
    Location
    Rockledge, Florida.
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    If your system has a positive displacement compressor (which is likely the case) the refrigerant volumetric flow rate will remain relatively constant with changing air/water side conditions. If the system has a TXV the evaporator superheat should also remain relatively constant but most other system parameters will change. If you want to figure out the actual capacity you’ll have to relate volumetric flow to mass flow using refrigerant density (Capacity = mass flow rate x change in enthalpy). Generally speaking if your inlet air temperature goes up the delta T between air and refrigerant will increase causing better heat transfer and improving the ability of the evaporator to absorb heat. That heat has to be rejected by the condenser so if the water flow rate is held constant the temperature of the water will increase and the end result will be an overall increase in system cooling capacity. Without knowing the flow rates, geometry, and heat transfer coefficients the actual change in capacity is unknown.

  5. #5
    Join Date
    Jun 2003
    Location
    Texas
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    Quote Originally Posted by QwikProducts View Post
    If your system has a positive displacement compressor (which is likely the case) the refrigerant volumetric flow rate will remain relatively constant with changing air/water side conditions. If the system has a TXV the evaporator superheat should also remain relatively constant but most other system parameters will change. If you want to figure out the actual capacity you’ll have to relate volumetric flow to mass flow using refrigerant density (Capacity = mass flow rate x change in enthalpy). Generally speaking if your inlet air temperature goes up the delta T between air and refrigerant will increase causing better heat transfer and improving the ability of the evaporator to absorb heat. That heat has to be rejected by the condenser so if the water flow rate is held constant the temperature of the water will increase and the end result will be an overall increase in system cooling capacity. Without knowing the flow rates, geometry, and heat transfer coefficients the actual change in capacity is unknown.
    spot on lol

  6. #6
    Join Date
    Jan 2004
    Location
    PA
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    Quote Originally Posted by QwikProducts View Post
    If your system has a positive displacement compressor (which is likely the case) the refrigerant volumetric flow rate will remain relatively constant with changing air/water side conditions. If the system has a TXV the evaporator superheat should also remain relatively constant but most other system parameters will change. If you want to figure out the actual capacity you’ll have to relate volumetric flow to mass flow using refrigerant density (Capacity = mass flow rate x change in enthalpy). Generally speaking if your inlet air temperature goes up the delta T between air and refrigerant will increase causing better heat transfer and improving the ability of the evaporator to absorb heat. That heat has to be rejected by the condenser so if the water flow rate is held constant the temperature of the water will increase and the end result will be an overall increase in system cooling capacity. Without knowing the flow rates, geometry, and heat transfer coefficients the actual change in capacity is unknown.
    If the air inlet temp increases and the RH decreases, the SHR will increase. How much, is unknown without either onsite testing, computer simulation, or manufacture testing/simulation.
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