cooling load calculation for car - Page 2
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  1. #14
    one part of cooling load calculatin for automobile is conduction via body and glass. we can calculate with following equation q=U.A.CLTD which U=overall heat transfer coefficient; A=surface area; CLTD value can obtain from ASHRAE table. my question is, if we use another material of body which not obtain in ASHRAE table, how we can calculate CLTD value?

  2. #15
    Join Date
    Apr 2012
    Posts
    467
    Its so variable we have door panels may have a little foam or just may be plastic. Floors with carpet exhaust heat coming through the floor while stationary.A roof in direct sunlight.Color of the vehicle.A firewall behind the dash that may be 140 f..The list goes on and on. Air infiltration..its alot and is real.When we are not on recirculate and have the fan off air will blow in anyway through the vents. A car is not "tight" go run it in the creek and see how long it takes to sink. If it stayed up for 30 seconds you are doing pretty good.
    Its any ones guess of what BTU is needed and all will be wrong.
    Go big/ for the quick pull down for when you jump in the car and its 160F inside and 105 outside.
    If its 100F out and you are in stopped in traffic with the sun beating down,reach up and turn off the air you will want it back on within 3 minutes I guarantee it. Heat gain is huge in a metal box with lots of glass.The sloped windshield is where a huge chunk of the gain is.
    The bottom line is not gain but what it will take to condition x amount of fresh air coming in. So select the cfm you want there are usually 4 or 5 vents I would say 150 to 200 cfm is about right..then see how much capacity it takes to cool that volume of fresh air coming in to between 45 and 50F coming out the vents...Its going to be alot if its hot out with alot of humidity.May actually take a couple ton.You can get by with less on recirculate but still dont cut yourself short.Heat gain is very high in a vehicle.
    People say well I can cool my entire bedroom with 5000 btu and have extra capacity.
    True but when it kicks off the temp doesnt raise back up in two minutes.
    Or would 5k be enough if that bedroom was 160 f when you turned that unit on in that bedroom and expected to be comfortable in 5 minutes or less.
    Go big you will thank me later.Its a car not a house.

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