Does high humidity affect the cooling or A/C systems?

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Increased heat capacity is another reason humid air takes more energy than dry air to cool, though that factor is very small. Evaporation off the coils pales in comparison to drainage, and we are expending energy to cool the condensate as it drains off. Some non-automotive systems use the evaporator's condensate to cool the condensor coil, or the pipe to it. I know of no automotive system that does this, but maybe there are some out there.
 
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Originally Posted by HangFire
Increased heat capacity is another reason humid air takes more energy than dry air to cool, though that factor is very small. Evaporation off the coils pales in comparison to drainage, and we are expending energy to cool the condensate as it drains off. Some non-automotive systems use the evaporator's condensate to cool the condensor coil, or the pipe to it. I know of no automotive system that does this, but maybe there are some out there.
The reason the difference is small is because on the condenser side, water in humid air already went though a phase change, which is the most energy intensive process, and is now in vapor form which will absorb slightly more heat from the condenser vs dry air. On the evaporator side, however, water in humid air has to undergo a phase change from vapor to liquid, because the evaporator coils temperature is below the dew point. So it takes more energy for that. Now, if you are driving in rain during a hot day, water will undergo a phase change on both the condenser and evaporator, so in those kind of conditions the effect of humid air is cancelled out. That is why some home systems, like you mentioned, use the evaporator drain water to cool off the condenser. This way water goes though a phase change on both ends, not just one.
 
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Originally Posted by HangFire
Increased heat capacity is another reason humid air takes more energy than dry air to cool, though that factor is very small. Evaporation off the coils pales in comparison to drainage, and we are expending energy to cool the condensate as it drains off. Some non-automotive systems use the evaporator's condensate to cool the condensor coil, or the pipe to it. I know of no automotive system that does this, but maybe there are some out there.
I personally would not consider energy absorption from humidity as being an overall small factor. In some cases, it could be be the dominant means of condenser cooling. A great many factors go into this so, it's difficult to generalize -even for a lowly automotive AC system. There are many process that require very fast cooling. Humidity/mist systems tend to perform the best. Case in point is CNC machining. Flood cooling is OK and good for chip control but the parts still come-out hot. With mist systems, the parts come-out (quite literally) cold. Also, many industrial cooling systems (and some residential cooling condensers have retro-fit kits that) intentionally raise the humidity by releasing ultra-fine water mist onto the condenser coils. It basically doubles the efficiency of the condenser. (NOTE: Don't do this at home because chlorinated water will eventually break down the aluminum fins on your condenser). This form of mist/humidity cooling is how swamp boxes work in places like Arizona and New Mexico. It's been over 35 years since I did thermo calculations for a DoD wind tunnel at my Alma Mater and I haven't touched that work since then but, you might wish to look up enthalpy evaporation charts. In a split instant, you'll see that evaporative cooling is the cat's meow. As for this thread, I part with this: As far as automotive cooling systems are concerned, humid outside air can produce a significant increase on condenser efficiency if the vehicle is in motion (i.e. a large volume of air flow). There are many edge-cases where the factors of humidity, air flow, air temperature, coil temperature etc, decrease condenser efficiency. Ray
 

dnewton3

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Originally Posted by atikovi
With miserable humidity and dew points in the upper 70's I was wondering if that affects the cooling and A/C systems in any way. Is it harder for the radiator to give off heat when the humidity is high? What about the condenser?
Many people have given good answers, but I think some may have missed the general questions (there were actually two; one about the rad and one about the a/c) Overall, the entire topic for both questions is about thermal transfer of energy. Humidity in the air at the rejection point (the radiator of the car or the condenser of the car) does not get greatly altered in a practical sense. High vs low air moisture content does not shift the ability of those systems to reject heat out into the air flow. In theory you could calculate the minute differences, but in reality it does not make much difference. What really causes problems is the ambient temps. Really hot days make it a bit more difficult for the radiator cooling, but it REALLY makes it harder on the A/C system, because it drives up the head pressure significantly. That head pressure in turn needs more and more power from the compressor driver to achieve the task of generating the capacity via the vapor compression system. It does not alter the capacity of the system, but it does make for a much larger energy demand to create the needed dP for the vapor compression cycle to get to the saturated discharge state.
 
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