Originally Posted by mk378
It takes energy to condense water. High humidity significantly increases the demand on an A/C refrigeration system. Think of what it would take to boil the water as it comes out of the drain pipe.
Does high humidity affect the cooling or A/C systems?
- Thread starter atikovi
- Start date
Yes .
Latent heat of evaporation or condensation .
Originally Posted by kschachn
Originally Posted by BMWTurboDzl
Q: So does the refrigerant absorb the latent heat as if converts from a liquid into a gas?
Originally Posted by 4WD
Read some recommendations that after a car is super heated in the sun … don't start out on recirculating AC
Maybe that's from the days of window cranks … ?
pretty easy to vent the hot air and I'm gonna use my windshield screen.
Originally Posted by Wolf359
Originally Posted by kschachn
Originally Posted by BMWTurboDzl
Q: So does the refrigerant absorb the latent heat as if converts from a liquid into a gas?
Originally Posted by kschachn
What? There is no chemical reaction whatsoever. It is completely physical.
And what I said is correct. The heat absorbed at the evaporator is rejected at the condenser.
Yes. A proper chart includes humidity along with temperature to determine correct pressures. They would not include this info if it were irrelevant. Has a direct effect on expected evaporator and vent temperatures.
Originally Posted by Wolf359
The hotter it is, the harder the whole system has to work. Humidity might have the effect where it takes longer to cool or just takes more energy to covert the gas back to a liquid.
Several people have provided correct answers to this question on general terms but I thought I would run some calculations to find out the magnitude of the humidity load in specific terms for anyone interested in the details.
As has been mentioned, when the A/C cools humid air it eventually reaches the dew point of the air (100% RELATIVE humidity) and further cooling condenses water out of the air as the temperature drops. At the final A/C exit temperature the air is still at 100% RELATIVE humidity although the lbs water / lb air have decreased on absolute terms.
On a typical humid summer day ambient air might be 90 deg F and 50% relative humidity. According to the relative humidity tables, this air would contain 0.015 lbs water per lb of air. After running through the A/C the temperature might be lowered to 50 deg F but would be at the 50 degree dew point and therefore 100% relative humidity. The humidity tables show 50 degrees and 100% relative humidity air will contain about 0.007 lb water/ lb air. Thus 0.008 lbs of water have to be condensed for each lb of air cooled from 90 deg 50% humidity to 50 deg A/C outlet conditions.
Referring to the steam tables we find that the heat of evaporation/condensation in this temperature range requires the transfer of about 1050 BTU of heat for each lb of water. Our .008 lbs of water would therefore require the removal of 8.4 BTU of heat to condense it.
Turning to the heat removal required for just the air, other tables show that cooling a lb of dry air at these conditions requires 0.17 BTU of heat transfer for each degree of temperature change. Our 40 degree temperature drop in the A/C thus requires 0.17* 40 or 6.8 BTU of cooling for just the air.
So in the end starting with 90 degree and and 50% humidity and an A/C outlet of 50 degrees, a little over half (55%) of the A/C capacity goes toward removing the humidity. This is why cars in humid areas can never get really cool without going to recirculate mode. The humidity charts are very nonlinear, so this number is only correct at these conditions, but I would say that any time humidity is near 50%, at least half the cooling load on the A/C will be for humidity removal.
For human cooling purposes removing humidity and lowering temperature both decrease how much sweating is needed to cool yourself, so even though the AC outlet temp isn't as low, its still more comfortable.
On the plus side, high humidity air has slightly more heat capacity than dry air so you cars cooling system is working a fraction better.
Originally Posted by BMWTurboDzl
... How does humidity increase the demands on the system?
Originally Posted by kschachn
... The heat absorbed at the evaporator is rejected at the condenser.
In a nut shell, we all agree that evaporation will take away heat, right. You turn it around and you can agree that condensation will inject heat to a system right?
So if you have hot humid air, when you cool it some of the water in the air will be condensed into liquid water, that is the opposite of evaporation, and that means you need to take away these generated heat. That means the AC has to work harder.
For the same inlet and outlet temperature, the AC has to run harder to cool down a humid air, because more of it will condense into liquid water.
You cannot just cool down humid air without condensing some water, because of thermodynamics and relative humidity.
Originally Posted by KrisZ
Humid air actually makes it more efficient because water will have a cooling effect on the condenser.
Three indisputable points of physics.
Humid air is less dense than dry air at the same temp and pressure... it may feel more dense, but it is less.
Less dense air has a slower rate of thermal transfer across the coils.
Any cooling of condensed water on the coil before it drips off is cooling of air that does not happen, because the A/C only moves heat at a given maximum for whatever setting its on.
Put those all together, and it takes longer for an A/C system to cool a given volume of humid air than dry air, all other factors being equal.
There's 2 sides to this.
Evaporation takes place inside the evaporator and makes the coils cold. Room air passes over the evaporator coils and the heat from the room air gets transferred into the cold coils. At a practical level, the evaporator coils can frost-up and thus reduce the efficiency of the transfer process. Humid air is also less dense than dry air so, even if the evaporator does not frost-up, the evaporation process is not as efficient and less cold will get transferred to the room air.
The condenser coils transfer the heat from the refrigerant to the outdoor (atmospheric) environment. Some condenser coils (for example in boat/marine applications) are designed to be submerged in rapidly flowing water to carry away the heat. Automotive condenser coils need to work when they are wet or dry. They rely mainly on air flow (fans or forward speed of the vehicle). Efficiency of the condenser coils can vary depending on the amount of airflow, the temperature gradients between outside air and coil temperature and corresponding dew-points. If there is adequate airflow over the condenser, humid outside air or rainy conditions can improve efficiency. If the dew points are high and there's not much air circulation, condenser transfer efficiency decreases.
There's no easy answer to this because thermodynamic theory gets clouded by reality. In the absence of sufficient air flow, high outdoor humidity decreases the efficiency of the condenser coil transfer process. In some less common cases, high humidity and airflow can help the condenser process. Finally, high room humidity can decrease the efficiency of the evaporator coil transfer process.
(In a nutshell, humidity is evil in most circumstances).
Ray
Originally Posted by BMWTurboDzl
Originally Posted by atikovi
Originally Posted by Dave9
Not directly. Pulling moisture from the air is just an intended side effect. It does make it more comfortable to animals because their perspiration evaporates easier, but this is not an extra load on the system unless you set the A/C to a lower temperature in order to have comfort by pulling more moisture out of the air, instead of set to a specific temperature... so, yes and no, depends on how you look at it.
Originally Posted by RayCJ
If there is adequate airflow over the condenser, humid outside air or rainy conditions can improve efficiency.
Originally Posted by HangFire
Originally Posted by RayCJ
If there is adequate airflow over the condenser, humid outside air or rainy conditions can improve efficiency.
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We need to look at the thermodynamics side of things. There is no difference between car and home AC assuming both are in re-circulation (no outside air exchange).
Phase change takes away or inject heat, the humid air inside the house will takes more energy to cool than dry air of the same temperature.
The outside air going through the condenser, you have to look at the heat capacity of air between different relative humidity, but in general, yes, humid air will take away more heat on the HOT side of the AC, aka the condenser side or the OUTSIDE.
Remember, condensation / evaporation of water is a lot more energy intensive than the heating or cooling of humid air.
I don't need to look at the heat capacity of humid air, to answer than cooling a room or a car takes more energy on a humid day. Keeping the same commercial freezer / refrigerator in an air tight room would be a different scenario because the inside humidity won't change based on the outside humidity.
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Originally Posted by HangFire
Originally Posted by RayCJ
If there is adequate airflow over the condenser, humid outside air or rainy conditions can improve efficiency.