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Re: Does high humidity affect the cooling or A/C systems? [Re: kschachn] #5163433 07/17/19 12:56 PM
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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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Re: Does high humidity affect the cooling or A/C systems? [Re: atikovi] #5163583 07/17/19 03:49 PM
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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.


"You keep asking questions PandaBear and you'll end up a vegetarian like my wife" - Camu Mahubah
Re: Does high humidity affect the cooling or A/C systems? [Re: KrisZ] #5163641 07/17/19 04:57 PM
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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.

Re: Does high humidity affect the cooling or A/C systems? [Re: atikovi] #5163646 07/17/19 04:59 PM
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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.


Various musings: http://hangfire.net
Re: Does high humidity affect the cooling or A/C systems? [Re: atikovi] #5163743 07/17/19 07:13 PM
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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

Re: Does high humidity affect the cooling or A/C systems? [Re: BMWTurboDzl] #5163744 07/17/19 07:13 PM
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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

Re: Does high humidity affect the cooling or A/C systems? [Re: RayCJ] #5164231 07/18/19 11:26 AM
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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.


Various musings: http://hangfire.net
Re: Does high humidity affect the cooling or A/C systems? [Re: HangFire] #5164251 07/18/19 11:53 AM
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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.

Last edited by KrisZ; 07/18/19 11:59 AM.

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Re: Does high humidity affect the cooling or A/C systems? [Re: atikovi] #5164326 07/18/19 01:15 PM
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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.

Last edited by PandaBear; 07/18/19 01:18 PM.

"You keep asking questions PandaBear and you'll end up a vegetarian like my wife" - Camu Mahubah
Re: Does high humidity affect the cooling or A/C systems? [Re: HangFire] #5164674 07/18/19 08:39 PM
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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.

Re: Does high humidity affect the cooling or A/C systems? [Re: atikovi] #5165022 07/19/19 07:57 AM
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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.


Various musings: http://hangfire.net
Re: Does high humidity affect the cooling or A/C systems? [Re: HangFire] #5165123 07/19/19 09:24 AM
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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.

Last edited by KrisZ; 07/19/19 09:32 AM.

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Re: Does high humidity affect the cooling or A/C systems? [Re: HangFire] #5165517 07/19/19 04:03 PM
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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

Re: Does high humidity affect the cooling or A/C systems? [Re: atikovi] #5165753 07/19/19 07:53 PM
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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.


The act of preventative maintenance, in and of itself, is FAR MORE important than brand/grade/base choices among lubes and filters.
- under maintaining something is akin to abuse/neglect; that can kill equipment by shortening the lifespan
- over maintaining something has never been proven to be anything but a waste of time and money
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