Does air conditioning remove more heat from humid air?

[cockpitvisit]

Hello,

assume I have an air condition unit which is not very strong and thus always runs at full power.

If I let water evaporate in the room without adding heat (e.g. by hanging lots of wet towels), will this improve the heat removal by the aircon unit and thus lower the room temperature, or will there be no difference?

Evaporating water would add a new heat transport mechanism - it would take away the heat of vaporization from the air and deposit that heat in the aircon unit where the air is cooled down and the vapor condenses back to water. So from this point of view, it should improve the cooling effect. But from the other side, the aircon unit would receive slightly cooler and more humid air - would this increase or decrease its efficiency?

 

[DaveC426913]

1] Humid air transfers heat better. A warm humid room will feel hotter than a warm dry room.
2] One of the big things that AC does to help a space feel cooler is to remove a significant amount of moisture from the air.

So, adding moisture will counteract your attempts to feel cooler.

 

[russ_watters]

1] Humid air transfers heat better. A warm humid room will feel hotter than a warm dry room.
2] One of the big things that AC does to help a space feel cooler is to remove a significant amount of moisture from the air.

So, adding moisture will counteract your attempts to feel cooler.

I'm not sure #1 is what you meant to say. A humid room feels warmer (and clammy) because higher humidity inhibits the evaporation of sweat. #2 is correct and thus the conclusion is correct: adding humidity to the room makes the AC work harder to accomplish its task.

 

[lychette]

1] Humid air transfers heat better. A warm humid room will feel hotter than a warm dry room.
2] One of the big things that AC does to help a space feel cooler is to remove a significant amount of moisture from the air.

So, adding moisture will counteract your attempts to feel cooler.

the question asks will the temperature be higher in a humid room or a dry room...not what will it 'feel like'

 

[DaveC426913]

the question asks will the temperature be higher in a humid room or a dry room...not what will it 'feel like'

Good point. I made an assumption, based on the knowledge that A/C is generally used to make people feel more comfortable, that this was about subjective temperature.

If the OP returns, perhaps s/he can clarify the intent.

 

[russ_watters]

the question asks will the temperature be higher in a humid room or a dry room...not what will it 'feel like'

If you add humidity to the room it will end up warmer because you are making the AC work harder.

 

[lychette]

If you add humidity to the room it will end up warmer because you are making the AC work harder.

I don't understand what you mean by the phrase 'you are making the AC work harder'
An AC unit removes heat energy from a room and transfers it to the outside.
The AC does not know if the air in the room is humid.
Removing heat energy will lower the temperature in the room.
Cooler air will cause moisture to condense so the humidity will decrease.
Condensing vapour will tend to raise the temperature.
the AC continues removing heat !

 

[russ_watters]

I don't understand what you mean by the phrase 'you are making the AC work harder'
An AC unit removes heat energy from a room and transfers it to the outside.
The AC does not know if the air in the room is humid.
Removing heat energy will lower the temperature in the room.
Cooler air will cause moisture to condense so the humidity will decrease.
Condensing vapour will tend to raise the temperature.
the AC continues removing heat !

The AC unit has to remove a certain amount of heat from the room to match the heat flow into the room through the walls, windows, etc. If you add moisture to the room, you increase the amount of heat the air conditioner has to remove from the room because in addition to making the air colder, it also has to remove more heat to squeeze out the moisture you added.

 

[lychette]

The AC unit has to remove a certain amount of heat from the room to match the heat flow into the room through the walls, windows, etc. If you add moisture to the room, you increase the amount of heat the air conditioner has to remove from the room because in addition to making the air colder, it also has to remove more heat to squeeze out the moisture you added.

'squeeze out the moisture'..?
I think that you are confusing the role of air conditioner with that of dehumidifier. Air conditioner is designed to lower temperature, dehumidifier uses the same principle of lowering temperature with the aim of making water condense.

 

[Ketch22]

Either way, a dehumidifier creates a reduced temp zone in a room and puts the excess heat back into the room,thus no apparent change in temp. An ac unit just puts the heat outside. Water has a specific heat required to vaporize isothermally. It take same amount of heat flow into the water to create vapor as it does out of the vapor to cause condensation into liquid. As the AC unit moves heat somewhere else it will also cause some condensation which is isothermal. That portion of the energy is expended, however, caused no reduction in temp.

 

[Fervent Freyja]

'squeeze out the moisture'..?
I think that you are confusing the role of air conditioner with that of dehumidifier. Air conditioner is designed to lower temperature, dehumidifier uses the same principle of lowering temperature with the aim of making water condense.

Incorrect. Please know what you are talking about before posting. Most modern air conditioners also remove humidity from the air : http://www.davidgrayheatingandair.com/articles-2013/how-do-air-conditioners-remove-humidity.aspx

So, like russ posted, adding humidity will force the air conditioner to work harder, thereby keeping temperatures higher than it would be without.

I personally perceive your responses on this thread to be attacking (nit-picking) other members. I advise you not to do that again.

 

[OmCheeto]

...lots of wet towels), will this improve the heat removal by the aircon unit and thus lower the room temperature, or will there be no difference?

This reminds me a bit of the "Instant Vacuum Dryer" thread.
After thinking about this, I think that wet towels may actually improve the efficiency of the A/C unit.

Say you have wet towels that hold 1 kg of water.
As the water evaporates, the the towels will cool off, cooling off the environment. 2,265 kilo-joules of free swamp cooling!
But, the extra moisture will cause the A/C unit to have to expend that much energy to re-condense the moisture: Zero sum (theoretically...)
But one thing I noticed about my brand new window A/C unit, is that the condensate is routed to the outside section, and the hot radiator actually sits in the pool of cool condensate.*
So as that water evaporates outside, it will act as a kind of swamp cooler, and carry that energy away, making the unit consume less energy.

*I actually thought they had forgotten to put a drain hole in my old A/C unit, and was going to drill a hole in the bottom. Fortunately, it died of heat stroke last year before I got around to it, as when I saw my new unit didn't have one either, I decided that they must be designed that way. Then I scratched my head for a few moments, wondering why, and then said to myself; "Ah ha! Brilliant!"

 

[lychette]

Incorrect. Please know what you are talking about before posting. Most modern air conditioners also remove humidity from the air : http://www.davidgrayheatingandair.com/articles-2013/how-do-air-conditioners-remove-humidity.aspx

So, like russ posted, adding humidity will force the air conditioner to work harder, thereby keeping temperatures higher than it would be without.

I personally perceive your responses on this thread to be attacking (nit-picking) other members. I advise you not to do that again.

advice duly noted

 

[lychette]

Incorrect. Please know what you are talking about before posting. Most modern air conditioners also remove humidity from the air : http://www.davidgrayheatingandair.com/articles-2013/how-do-air-conditioners-remove-humidity.aspx

So, like russ posted, adding humidity will force the air conditioner to work harder, thereby keeping temperatures higher than it would be without.

I personally perceive your responses on this thread to be attacking (nit-picking) other members. I advise you not to do that again.

you response makes physics sense...I am not familiar with the phrase 'squeeze out the moisture' from another response.

 

[russ_watters]

'squeeze out the moisture'..?
I think that you are confusing the role of air conditioner with that of dehumidifier. Air conditioner is designed to lower temperature, dehumidifier uses the same principle of lowering temperature with the aim of making water condense.

Nope, I'm not confusing the two. As you say, they do basically the same thing. So if you add enough moisture to the air to exceed the moisture content of the air conditioner's supply air, you force the air conditioner to remove that moisture.

Maybe it would be helpful if you picked some conditions and we worked through the energy requirements.

 

[marcusl]

There is a quantitative answer to this question, folks. The latent heat of vaporization for water is approximately 2.3 kJ/g (with a slight temperature dependence). You can calculate the amount of water vapor present in, say, a liter of air--it depends on the relative humidity, temperature and pressure--and compare the energy required to condense that vapor into water to that required to cool the air by a certain amount (1K, for instance). The AC unit can remove only a certain amount of energy per unit time, and energy removed from water vapor cannot be removed from (and therefore cannot reduce the temperature of) air. I leave the calculation to those interested, but note that the latent heat quoted above is so large compared to the specific heat of air (1 J/g-K) that it can will significantly reduce the air temperature drop at high humidity.

 

[OmCheeto]

I leave the calculation to those interested...

Ok. I did one calculation.
My 736 watt window A/C unit would take 51.3 minutes to remove 1 kg of moisture from the air.
Assuming it did nothing else, and COP of 1.

Does this mean that I would have wasted 629 watt hours of energy, as there was no cooling?

I think one problem with this thread, is that there are (at least) two separate questions:

1. Does air conditioning remove more heat from humid air? (The title)
2. ...would this increase or decrease its efficiency?

I think the answer to #1 is yes.
And #2? My guess is, that because the A/C unit has to run for nearly an hour, with less than 100% efficiency, just to remove the water, the answer is "decrease".ps. I just went outside and measured my A/C's "condensate pool" volume. It should hold almost exactly 1 kg of water.

 

[russ_watters]

I think one problem with this thread, is that there are (at least) two separate questions:

1. Does air conditioning remove more heat from humid air? (The title)

All else being equal, the answer is yes.

2. ...would this increase or decrease its efficiency?

I think the answer to #1 is yes.
And #2? My guess is, that because the A/C unit has to run for nearly an hour, with less than 100% efficiency, just to remove the water, the answer is "decrease".

I think #2 is a "wrong question". To an engineer, "efficiency" is heat out divided by energy in: almost all air conditioners are more efficient when the air is moist because the heat transfer in condensation is very effective.

But I don't think that's what the OP wants to know. The right question is: does it take more or less energy to cool moist air. It takes more.

 

[russ_watters]

But one thing I noticed about my brand new window A/C unit, is that the condensate is routed to the outside section, and the hot radiator actually sits in the pool of cool condensate.*
So as that water evaporates outside, it will act as a kind of swamp cooler, and carry that energy away, making the unit consume less energy.

Does the coil just sit in the water or is the water dripped or sprayed onto the coil?

I think such a feature is required in Europe and Japan, but I haven't seen it much in the US.

I have run tap water to sprayers on the condenser of my split system AC unit, which gives up to about a 20% efficiency boost (half output increase, half input reduction), but it is only economical when it is hot out due to the water cost. Using the condensate is of course free and I may try to do that.

 

[NTL2009]

...
I have run tap water to sprayers on the condenser of my split system AC unit, which gives up to about a 20% efficiency boost (half output increase, half input reduction), but it is only economical when it is hot out due to the water cost. Using the condensate is of course free and I may try to do that.

I tried this on my old A/C, and it actually reduced efficiency! I was perplexed, and then I realized the fins were so close together, that the water had bridged the gaps and reduced the air flow! Very counter-intuitive at first.

The other concern is that minerals in the tap water might build up and coat the coils/fins and reduce thermal conductivity over time. The condensate should be mineral free, and only contain a bit of dust/dirt from the air, but I would think that would tend to wash away?

 

[rbelli1]

Does the coil just sit in the water or is the water dripped or sprayed onto the coil?

Most of the window units I have seen splash water on the condenser using a ring built into the fan.

BoB

 

[OmCheeto]

Does the coil just sit in the water or is the water dripped or sprayed onto the coil?

Both the old(2004) and new(2015) unit have the hot coil sitting in the drip pan.

My old A/C unit​

The rust band is about 1/2 in tall.

I think such a feature is required in Europe and Japan, but I haven't seen it much in the US.

I have run tap water to sprayers on the condenser of my split system AC unit, which gives up to about a 20% efficiency boost (half output increase, half input reduction), but it is only economical when it is hot out due to the water cost. Using the condensate is of course free and I may try to do that.

Free is a good price.

On a goofy side note, I did a calculation this morning, trying to determine what's going on at the molecular level.
Somehow, my evaporated water molecule ended up at a temperature of 5427°F.
I decided that I had forgotten how latent heat works, and found; "″Sensible heat″ measures change in a body's internal energy that may be ″sensed″ with a thermometer. ″Latent heat″ measures change in internal energy that seems hidden from a thermometer – the temperature reading doesn't change." [ref: wiki, Latent Heat, Usage]

I think I will have to do more studying, as even that doesn't make sense.

 

[256bits]

Assuming it did nothing else, and COP of 1.

Assuming your living space is saturated steam, yeah it would be able to condense only liquid water.

But, in real life the water vapour becomes part of the air.
Dry air would have the 1kJ/kg⋅C. With added moisture in the air, with the ballpark normal relative humidity and room temperature, the Cp goes up a little to around 1.02. Before condensing out the water vapour to a liquid, the air has to be cooled down to the dew point temperature, at which there is 100% relative humidity. Cooling the air further condenses some water vapour.

 

[256bits]

http://www.engineeringtoolbox.com/air-psychrometric-table-d_1574.html
From that chart 70 deg F air saturated contains 110 grain of water vapour per pound of dry air.
50% relative humidity would contain 55.
Since 1 grain = 1/7000 of a pound---> 55/7000 is approx. 1/100 pounds of water vapour per pound of dry air.

 

[OmCheeto]

Does the coil just sit in the water or is the water dripped or sprayed onto the coil?
...

Another thing I noticed, is that there is a u-tube of copper tubing, coming out of the condenser coil, that sits in the pool.

My old A/C unit.
​

I'm not willing to take apart my new A/C unit, to see if it has this tube, as it's supposed to be 88°F today.

But I am really curious about all this, so I constructed a temporary shade for my A/C unit a few minutes ago.

One less variable is always a good thing.
ps. Fun thread. I've pretty much figured out what most of the components in my old A/C unit do. There are a few things not shown in the diagrams I've seen. ("What the heck is this little micro tubing?")
pps. And given that it was only 60*F this morning, and couldn't get my old A/C unit to turn on, I burned myself after heating the temperature sensing probe, and touching it to see if that was the problem. Ouch.
And then I electrocuted myself trying to rewire the thing, to bypass the temperature sensor.
Apparently, I hadn't had enough coffee.

 

[OmCheeto]

Assuming your living space is saturated steam, yeah it would be able to condense only liquid water.

As I stated earlier, I'm trying to analyze the problem piecemeal.

But, in real life the water vapour becomes part of the air.

In real life, as I discovered yesterday, this is very complicated.

Dry air would have the 1kJ/kg⋅C. With added moisture in the air, with the ballpark normal relative humidity and room temperature, the Cp goes up a little to around 1.02. Before condensing out the water vapour to a liquid, the air has to be cooled down to the dew point temperature, at which there is 100% relative humidity. Cooling the air further condenses some water vapour.

Synopsis of the 11 hour experiment I did yesterday:

Humidity dropped from 76% to 67% inside the house, within the first hour, and then did not change.
I am confident that the humidity actually dropped, as the condensate pool went from dry to full in less than an hour.
I am not confident in the actual numbers, as I made my own psychrometer.

I was hoping the condensate tray would remain empty long enough, such that I could make a comparison of temperatures between empty vs full tray, but it filled up really fast.

The Th & Tc temperatures, for both the evaporator and condenser, were unremarkable.

          Condenser          Evaporator
time      Th        Tc       Th       Tc       notes
12:41     97.1      79.7     63.1     44.3     start of experiment
16:59     102.7     85.8     62.8     42.7     maximum condenser temps
22:47     91.4      73.3     60.7     38.8     end of experiment

They only answer the question:
Q: Does Om's A/C work?
A: Yes.
​

I would describe a few more complicating details of the experiment, but my friends have conspired to talk me into another solution.
Friends; "It's going to be 97°F today. What's the solution to this problem"?
Om; "Sit in the river".
Friends; "Be over by 10".
Om; "I'm getting ready, as we speak".

 

[OmCheeto]

...1/100 pounds of water vapour per pound of dry air.

I found that ratio also, at http://www.kwangu.com/work/psychrometric.htm
From that number, I determined that my house contained, atmospherically, 3.5 kg of water at the start of the experiment.

 

[256bits]

I am confident that the humidity actually dropped, as the condensate pool went from dry to full in less than an hour.I am not confident in the actual numbers, as I made my own psychrometer.

Interesting stuff.
A homemade psychrometer can be a hair attached to a dial or marker.
I never made one, but now curiosity may make me do what I should have done years ago when I had adequate hardware. Ha Ha

 

[OmCheeto]

Interesting stuff.
A homemade psychrometer can be a hair attached to a dial or marker.
I never made one, but now curiosity may make me do what I should have done years ago when I had adequate hardware. Ha Ha

It appears that I made the modern equivalent of the 1861 model.
I folded a paper towel in half over my hand dish washer tool drip dryer, wetted it, and stuck a very light RTD probe between the sheets.

As long as the surface tension was able to hold the two halves together such that the RTD didn't fall out, I figured it was working.

ps. After some more observations, I'm having severe reservations regarding the "conventional wisdom" of the primary purpose of the "slinger ring". Although I'm in firm belief that the condensate pool increases the efficiency of a window A/C unit, I think the slinger blade serves more to try and clean up the pool.

 

[256bits]

It appears that I made the modern equivalent of the 1861 model.

Hygrometer, or the preferred name in the 1700's the wiki says by some, the hygroscope.

But then I see you went all high tech with a modification of the wet bulb-dry bulb technique.

 

[lychette]

It appears that I made the modern equivalent of the 1861 model.
I folded a paper towel in half over my hand dish washer tool drip dryer, wetted it, and stuck a very light RTD probe between the sheets.

As long as the surface tension was able to hold the two halves together such that the RTD didn't fall out, I figured it was working.

ps. After some more observations, I'm having severe reservations regarding the "conventional wisdom" of the primary purpose of the "slinger ring". Although I'm in firm belief that the condensate pool increases the efficiency of a window A/C unit, I think the slinger blade serves more to try and clean up the pool.

I love the hand powered surfactant fluid dispenser and the debris removal tool BUt...' the temperature sensing bulb'...too 20Century

 

[OmCheeto]

Hygrometer, or the preferred name in the 1700's the wiki says by some, the hygroscope.

But then I see you went all high tech with a modification of the wet bulb-dry bulb technique.

High tech, and low brow.
I build a portable version yesterday morning, and took several measurements.
They made no sense.
So I redesigned it.

location      % RH      mg h2o/kg air      temp (°F)       notes

model 1               
kitchen        61       10.70               74             furthest point from A/C unit. should have the highest moisture content
LR south       58        9.80               72             where I sit and stare at data. should have a median moisture content
MBR south      70       10.10               67             where the A/C unit is located. should have the lowest moisture content

conclusion: there was something wrong with the folded paper towel design
theory: the huge plane of moisture was creating a localized increase in humidity
          
model 2                 
kitchen        63       11.70               75                 
MBR south      60        9.00               69           
Back Porch     44       13.70               91
kitchen        60       11.50               76
MBR south      62        9.30               69
conclusion: "relative humidity" is an annoying concept. look at the "Back Porch" numbers: lowest relative humidity, yet the highest moisture content by mass.

ps. I love doing A/C experiments, as I have a legitimate reason to run my 8000 BTU unit, 24/7.