PDA

View Full Version : Does high altitude affect a car's air conditioner?


Laelaps
09-01-2012, 09:36 AM
I live in the OK flatland. My wife and I drove our 2009 Honda Accord (w/about 7000 miles on it) up to the Sangre de Christo mountains in New Mexico on a fly fishing and horseback riding trip.

The air conditioner went out at some point. The fan was blowing fine, and cooler air (cooler than outside) was blowing, but it wasn't cold.

Then on the way home, after about six or seven hours on the road, (just outside of Oklahoma City) it got cold again.

Since it got cold again, it wasn't a freon problem. It's obviously not a bad fuse.

Does high altitude screw with a car's air conditioner?

levous
09-01-2012, 10:00 AM
could it have just been that the temp was higher in NM then in OK? If the system wasn't set on recirculate, then the air wouldn't feel very cold inside. Of course, you many tell me, "no duh, sherlock!" :)

OlAndrew
09-01-2012, 10:39 AM
Ummm...what that compressor is compressing is the r134a or whatever gas is used inside the A/C unit. Compressing it makes heat, which is radiated to the atmosphere through the condenser, a sort of radiator in front of the engine's radiator. The condensed gas becomes a liquid, which is piped to an evaporator, which is like another little radiator inside the car's cabin with a blower. It evaporates and cools the air being blown through the evaporator.

Modern auto A/C units will only put out about 20 degrees below the outside airtemp, so if its 100 outside, your A/C may only put out 80 degree or so air. If the condenser is blocked by dirt or trash or something, if the enigine is running unusually hot, or it the gas pressure inside the A/C is low, you won't even get your 20 degrees.

rob2001
09-01-2012, 11:35 AM
Air conditioners have a "compressor" as part of their cooling mechanism. In higher altitudes, air is thinner. There is less air to compress and thus, cool.




Not.

tiktok
09-01-2012, 11:45 AM
Basically there are three effects of altitude:
(http://energyexperts.org/EnergySolutionsDatabase/ResourceDetail.aspx?id=2319)
The energy-holding content in air at higher altitude is greater than at sea level per pound of dry air. For example, a 78 deg F saturated air (78 Wet Bulb and 78 Dry Bulb) at sea level has 41.586 BTU/lb dry air vs 46.374 BTU/lb dry air at 5000 ft altitude, or +11.5%. This means that the same cooling tower could cool a greater amount of water at altitude at the same temperature conditions PER POUND OF DRY AIR. For example, for the conditions of 95 Inlet Temp to 85 Outlet Temp at 78 Wet Bulb, a counterflow cooling tower with 4 ft of 0.75 inch cross-fluted fill material, the equilibrium Liquid-to-Gas ratio at sea level would be about 1.588, or it will cool 1.588 lbs of water per lb of dry air. At 5000 ft altitude this L/G ratio would increase to 1.873 lb water per lb dry air, a 17.9% increase.

The air is less dense at higher altitude and therefore it takes more cubic feet of air per lb of dry air. The same 78 deg F saturated air at sea level takes 14.010 ft^3/lb dry air vs 16.953 ft^3/lb dry air at 5000 ft altitude, or +21.0%. In a similar manner the density is reduced from 0.0729 lb mixture/ft^3 to 0.0605 lb mixture/ft^3, or 17% less, thereby reducing the air pressure drop (and fan horsepower) through the tower FOR THE SAME AIR FLOW VOLUME.

The electric fan motors have reduced cooling capability at higher altitudes due to the less dense air. This means that at very high altitudes, you may not be able to load the motor to the full nameplate load and full motor temperature rise. However, a compensating factor is that at altitude, the ambient temperature is normally less than at sea level. Large field erected cooling tower customers may limit the fan motor power to 90% for longevity of the motor, so the reduced motor capability may not be an issue.
Now, what is the bottom line of combining these three factors? The first factor is positive and the second factor is negative on performance. The overall effect is positive and generally small, 3-8% at 5000 ft altitude, but will depend on the actual temperature conditions.

rob2001
09-01-2012, 11:53 AM
Basically there are three effects of altitude:
(http://energyexperts.org/EnergySolutionsDatabase/ResourceDetail.aspx?id=2319)
The energy-holding content in air at higher altitude is greater than at sea level per pound of dry air. For example, a 78 deg F saturated air (78 Wet Bulb and 78 Dry Bulb) at sea level has 41.586 BTU/lb dry air vs 46.374 BTU/lb dry air at 5000 ft altitude, or +11.5%. This means that the same cooling tower could cool a greater amount of water at altitude at the same temperature conditions PER POUND OF DRY AIR. For example, for the conditions of 95 Inlet Temp to 85 Outlet Temp at 78 Wet Bulb, a counterflow cooling tower with 4 ft of 0.75 inch cross-fluted fill material, the equilibrium Liquid-to-Gas ratio at sea level would be about 1.588, or it will cool 1.588 lbs of water per lb of dry air. At 5000 ft altitude this L/G ratio would increase to 1.873 lb water per lb dry air, a 17.9% increase.

The air is less dense at higher altitude and therefore it takes more cubic feet of air per lb of dry air. The same 78 deg F saturated air at sea level takes 14.010 ft^3/lb dry air vs 16.953 ft^3/lb dry air at 5000 ft altitude, or +21.0%. In a similar manner the density is reduced from 0.0729 lb mixture/ft^3 to 0.0605 lb mixture/ft^3, or 17% less, thereby reducing the air pressure drop (and fan horsepower) through the tower FOR THE SAME AIR FLOW VOLUME.

The electric fan motors have reduced cooling capability at higher altitudes due to the less dense air. This means that at very high altitudes, you may not be able to load the motor to the full nameplate load and full motor temperature rise. However, a compensating factor is that at altitude, the ambient temperature is normally less than at sea level. Large field erected cooling tower customers may limit the fan motor power to 90% for longevity of the motor, so the reduced motor capability may not be an issue.
Now, what is the bottom line of combining these three factors? The first factor is positive and the second factor is negative on performance. The overall effect is positive and generally small, 3-8% at 5000 ft altitude, but will depend on the actual temperature conditions.



Still, the compressor isn't compressing air.

High altitude means less air and less ability for the condenser to transfer heat . Humidity may also be a factor on what the air coming out of the vents 'feels' like.