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

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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.
Yes . Latent heat of evaporation or condensation .
 
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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?
It absorbs heat which is rejected at the condenser.
Lots of strange words. Basically it's a chemical reaction in a closed system. Nobody really mentioned pressure, but that's why there's high pressure and low pressure. The refrigerant is a gas. Under high pressure, it turns into a liquid. When it phase changes to a gas, it takes in heat when transitioning to a gas and provides the cooling effect. The system then cools the down the gas and converts it back into a liquid so you can repeat the whole cycle. 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.
 
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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.
Just open the windows for the first few miles to help exhaust the hot air . Simple and easy . Also , keep in mind , tooling along at 30 MPH , the compressor & A/C are not operating at max capacity . You can down shift 1 gear & bring up the engine RPM & it will help . Of course , will burn more gas . When your speed increases , shift back into top gear .
 
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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?
It absorbs heat which is rejected at the condenser.
Lots of strange words. Basically it's a chemical reaction in a closed system. Nobody really mentioned pressure, but that's why there's high pressure and low pressure. The refrigerant is a gas. Under high pressure, it turns into a liquid. When it phase changes to a gas, it takes in heat when transitioning to a gas and provides the cooling effect. The system then cools the down the gas and converts it back into a liquid so you can repeat the whole cycle. 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.
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.
 
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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.
Yup, and it is a set amount based on system capacity. With variable compressors the system adjust to some extent, but it can never exceed its max capacity. The reason outside temperature makes a difference is because the condenser has less temperature difference to work with when it's hot outside. Humid air actually makes it more efficient because water will have a cooling effect on the condenser. So whatever latent heat the evaporator absorbs from moist air, is basically cancelled out on the condenser side.
 
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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.
 
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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.
The energy required for a phase change is a set amount, it doesn't vary with outside air temp. The energy stays the same, but the time varies. The system works on temperature difference to exchange heat. The greater the temp difference, the faster heat can be exchanged, and the smaller the temp difference, the slower the heat exchange rate. The system works as "hard" in any scenario because work does not involve time. Now, if we're talking power, then yes there will be a difference.
 
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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.
 
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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.
 
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Originally Posted by BMWTurboDzl
... How does humidity increase the demands on the system?
Cooling capacity expended condensing moisture can not be used to cool the air. You wouldn't want to cool the air without removing the moisture, anyway!
 
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Originally Posted by kschachn
... The heat absorbed at the evaporator is rejected at the condenser.
... along with additional energy put into the system via the compressor.
 
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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.
 
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Originally Posted by KrisZ
Humid air actually makes it more efficient because water will have a cooling effect on the condenser.
I was scrolling around looking for this. To an extreme, if it's raining, those droplets going through your grille will take a lot of heat away. Could plausibly even set up a mister. One needs lots of airflow (through a great fan), lots of conductivity (thin fins) and lots of molecules of (air, water, whatever) to take the heat away from the metal so the freon inside can re-condense.
 
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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.
 
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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
 
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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.
Animals? Dogs and cats don't sweat,
If you aren't an animal what are you, a vegetable or a mineral? Through the bottoms of their paws. wink
 
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Originally Posted by RayCJ
If there is adequate airflow over the condenser, humid outside air or rainy conditions can improve efficiency.
Rain can cool a hot condensor and improve efficiency. Please explain how humid outside air can improve condensor efficiency, especially given the facts I stated above. And for simplicity, let's assume the A/C is running in a normal mode with sufficient airflow inside and out, and the only variable is 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.
Rain can cool a hot condensor and improve efficiency. Please explain how humid outside air can improve condensor efficiency, especially given the facts I stated above. And for simplicity, let's assume the A/C is running in a normal mode with sufficient airflow inside and out, and the only variable is humidity.
Humid air has a slightly bigger heat capacity, so in theory the same volume can hold more heat. That's why when you try to cool it, it takes a bit more energy vs the dry air. In practice it makes little difference because vehicles have enough cooling overcapacity that it doesn't affect them. Car AC systems are designed the same way. Unlike home AC systems where overcapacity actually hurts occupant comfort, in vehicles these systems have to be a lot larger because they have to deal with a variety of climates and situation. Plus the system can be better regulated. The user can blend in some heat if needed, switch to outside air or recirculate, regulate fan speed etc.
 
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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.
Rain can cool a hot condensor and improve efficiency. Please explain how humid outside air can improve condensor efficiency, especially given the facts I stated above. And for simplicity, let's assume the A/C is running in a normal mode with sufficient airflow inside and out, and the only variable is humidity.
First: Hmmmm... 1 sentence from my explanation is expounded upon without the support of the sentences around it; then, a pointed question is asked with multiple caveats, conditions and exceptions designed to regulate the answer. In short, I provided a general answer; 1 sentence was picked-out and that 1 sentence is then expected to unlock the mysteries of the universe. Hmmmm.... So, all things being fair, lets NOT for simplicity, assume the A/C is running in a normal mode with sufficient airflow inside and out, and the only variable is humidity. Let's instead look at a general case of a hot fluid running through some pipes and also assume that moist air is traversing across the outside of those pipes. At this stage, we know nothing about the relative temperatures of the elements in question. We know however that in some circumstances, moisture from the air will be near (and/or make contact with) the external surface of the pipes and that moisture might evaporate. -And I say "Might" evaporate because I've said nothing about the relative temperatures involved. In the cases when the conditions cause the moisture to evaporate, an endothermic process takes place; meaning, it takes energy from somewhere (the hot pipes) to bump the energy in the external moisture to the evaporation point. (Believe it or not, the moisture does not even need to make direct contact with the surface of the external pipes for the evaporation to happen). In this example, if there were no moisture involved, there would be no endothermic reaction and the pipes would not lose as much heat. THIS (without caveats) is how humidity can improve the condenser efficiency.
 
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