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I don't have literature...just a picture
Perhaps someone will come along with useable information.
New Guest
I'm trying to design a Refrigeration system for a car. I need help in determining the cooling load. Is there any literature available that helps to calculate heat transfer coefficient of various car body parts? Any pdf that has extensive calculation procedure is very much helpful.
thanks in advance.
Professional Member
I don't have literature...just a picture
Perhaps someone will come along with useable information.
Professional Member*
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Professional Member
I saw that truck too, somewhere on the LIE though.
Professional Member*
Lol that's right by where I seen it.. lol
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Regular Guest
This should take care of what you need
Professional Member*
Nicee..Originally Posted by ArthurHagar
This should take care of what you need
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New Guest
are you have been do your cooling load calculation? I'm trying to calculate cooling load, too. do you have any literature?
Professional Member
What size and color of car and how much window area? That makes a big difference.
New Guest
I am going to design air conditioning for electric vehicle. So I must calculate the cooling load. I think CLTD method is the simplest method. Is there any difference between vehicle and building cooling load calculation using CLTD method? Because there is only a little references to calculate automotive cooling load.
Professional Member
The A/C compressor in the Prius is 1.8kW max and it gets plenty cool. Compare the total window area of the car you're designing for to that of the Prius, and assume it's a linear correlation. (That falls apart if the "car" is very large, like a RV or bus.) You'll probably be implementing variable speed so oversizing isn't as bad as it is for a constant speed system.
BTW, for systems of that size, it is worth designing a 2 phase inverter to allow the use of cheap (relatively) and common "single phase" compressors. I know quite a bit about the Shannon Liu quadrature drive that actually does some elaborate downconversion and FFT to detect the compressor speed, but it was designed for reciprocating refrigeration compressors. A reciprocating A/C compressor should work, but I don't think a rotary or scroll compressor would have enough torque ripple to make the signal.
Professional Member
A vehicle can be a pretty tough calculation.
A vehicle believe it or not requires a much larger capacity system than what many people think.
Think of it like this...park the car in the sun with the windows up and the windshield facing the sun..put your thermometer on the steering wheel and come back in three hours this is the temperature you will be fighting rolling down the road on that trip with your windows up actually a little less as the wind cools the car somewhat but this is exactly what you are fighting in slow traffic.Heat gain is tremendous and far exceeds the outside temperature.
Ever notice that when you turn the AC off it warms rather quickly?...its the sun coming through the windows.
Also an automobile is something we want to pull down quick...you may have to get in the hot car 4 times a day.
While I am not a fan of over sizing,with an automobile you need to think in terms of how big a system can I put in this thing without it being to noticeable in regard to pulling the vehicle down,thats the bottom line seriously.
This is not a steady state application...people will be entering a passenger compartment that is very very hot...think in terms of how fast you can cool the cabin and not the steady state.If you think in terms of steady state you will be a very very long time cooling that cabin. A proper system has alot of reserve..I mean alot..once we get the cabin cool we can turn the fan to one or two and even begin to cycle
The best aftermarket unit I have seen was in a hard top Jeep I took for a test drive.It had rotary knobs like a room AC the temp was not controlled by pressure cycling but rather actual cabin temperature it self..in other words if you turned towards warmer the unit kicked off.
It was close to 100 the day I test drove it.Pull down was quick and it began to cycle {longer and slower cycles}.
It had a hi and low fan speed and hot/cold knob that was it,it fit under the dash and was not very big.Granted the compressor was belt driven off the engine the engine was a four cylinder but it did not unduly tax the engine. I do not know much about aftermarket all electric units though.I did like the fact it cycled according to cabin temperature I wish all units operated this way.
Cooling a vehicle is a brutal job...the sun beating in, air infiltration while we are doing 70 mph..windows cracked while people smoke,waiting at a stop sign with low airflow through the condenser,bug buildup on the condenser.
Also many people run on recirculate others on fresh air,anyway if you have 95f and fifty percent humidity and are running on fresh with 170 cfm coming out of the vents in order to have 45f vent temp you need close to 20 thousand btu to accomplish such. This can freeze you at night when the sun is not out, but typically just keep you comfortable when the sun is coming through the windows.
If you are on recirculate then you will be fighting essentially that temp you had when you left it parked with the windows up however you will lower the humidity pretty quick in the small space and it will cool better that cooling outside air and blowing it in.In a dry climate though you can be better off with fresh air. 95f and 15 percent humidity with that same 170 cfm running on fresh will take close to 10 thousand btu.
Professional Member
Just to add to above as I ran out of time for the edit.
Fresh air can be preferred a in dry climate when you have a car load of people...fresh forces air to the back.
Body odors are pushed out of the vehicle.
The system works much better with a window cracked open on fresh as compared to recirculated air,as positive cabin pressure reduces infiltration of outside unconditioned air. If you are going for the most efficiency however use recirculate.
As I stated prior a vehicle is a difficult one to figure as it is very complex.
The easiest way would be to park it in the sun and see what the gain is,but as said that still wouldn't be enough capacity as we want quick pull down times.
The very hot components in the car will be giving up heat to the air in the car while you are cooling it.
Just the vents alone can take a minute to cool off.
How long do you think it would take to cool a house that was say 160f inside and all of the furnishings as well if the system was designed to capacity? We can remove the heat from the air easy enough ,problem is it is getting reheated almost as fast as we are removing the heat.
For a car go big...think pull down time and not steady state
New Guest
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?
Professional Member
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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