I asked a friend of mine who works in HVAC at a truck company "Do you happen to know how much heat the AC system can remove from the truck cab in a given time period" and he said the following:
20,000 Btu/hr (minimum)
Outside (fresh) air, Panel Vents
90 degF Ambient
50% relative humidity
We actually do a little better than that
This is, of course, in reference to the AC system on a big truck. A car almost certainly is a bit lower than that.
At full whack an automotive AC sucks in 5-7 kW at the drive belt, so I suspect that there isn't much in it between a truck and a car. This is not too surprising, our requirement is driven by the speed with which we can pull down a 60 deg C interior to something reasonable, the truck will be defined more by the steady state performance. One of the least attractive jobs around is the evaluator who has to get into a stinking hot car and then time how long it takes to get to 'comfortable' (you can't just measure cabin temp because rate of airflow and some other stuff affects the comfort rating). Cheers
Comfort factor -
is the heat (infra-red radiation) from glass, roof, interior surfaces a big part of it?
I've often wished I could either suck the hot air from above the dash and exhaust or cool it, or provide some kind of heat shield. (dual pane glazing?)
Very much so. Radiant heat is a huge issue, as is the general heat capacity of the interior. Our HVAC system also has a little sunlight sensor, which cranks the demand temp down a bit when the sun shines. Cheers
Note that the 20,000 BTU/hr works out to just less than 8 hp delivered, and assuming a reasonable compressor efficiency, this puts you back to the 15 - 20 horsepower draw from the compressor that I've always used as a rule of thumb for automotive A/C.
This seems quite a bit more than the suspiciously low 1kW = 1.4 HP Greg keeps referring to.
Rob45 - how do you get to 15-20 hp draw - surely the AC system draws less power than the heat it shifts - it should draw perhaps 2 to 3 hp for a 20kBTU rating. Or where am I wrong here?
