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Steve25
11-09-2002, 12:47 AM
Does a condenser become more effeicent when the humidity is higher and temp remains the same?

Example:

90 deg F OA, @ 40%rh, = 280# head

90 deg F OA, @ 90%rh,= 278#head ??????

Or would head stay the same but increase subcooling?

The only reson I ask is a condesner has a set CFM moving accross it. Air containing more moisture has more BTU per CFM thus 1 CFM would do more work at a constant temp. However I think the effect may be minimal if at all. BUT in thery is this true?

Steve

Prof Sporlan
11-09-2002, 03:33 AM
The Prof isn't necessarily a heat exchanger expert, but he does have a handle on psychrometrics...

Air which has a higher density should allow a condenser coil to exchange more heat... and air with a greater RH will be more dense than air with a lower RH.

Leo Mihm
24-09-2002, 06:49 AM
ProfSporlan
Density of air is relevant to weight. but with moisture in the air
the density or weight of the air would be less. or do I have it all wrong?

Dan
24-09-2002, 05:58 PM
I was thinking the same thing, Leo.

Prof Sporlan
24-09-2002, 11:12 PM
but with moisture in the air the density or weight of the air would be less.
The Prof suspects you're thinking the moisture in the air takes up some volume, thereby leaving less volume for the air, and therefore leaving you with less dry air mass, which would be correct thinking, BTW :)

Say we have 90F DB, 80F WB air entering the condenser. Per PMTHERM, it has a humidity ratio of 0.020, which means there is 0.02 lbs water per lb air, and a density of 0.070 lb dry air/ft3.

If we go to more desert conditions, say 90F DB, 65F WB, we get a humidity ratio of 0.007, and a density of 0.071 lb dry air/ft3. This makes sense as we have less water in the air.

Again, the Prof isn't a heat exchanger expert, but if we were to look at the specific heats (Cp) of the air at both conditions, we would find the 90F DB, 80F WB state has the higher value: 0.249 versus 0.244, which means the 90F DB, 80F WB state will absorb more heat per degree change. This is no doubt due to the fact that water has a higher specific heat than dry air, and the 90F DB, 80F WB state has more moisture in the air.

DaBit
10-10-2002, 02:01 PM
My experience is that high RV decreases the ability to remove heat. I noticed this when testing the effectiveness of some radiators. With the same heat input (electrically generated), water<->ambient temp differences were higher at high RV days.

bersaga
05-01-2003, 05:57 PM
Steve,

If you are familiar with psychrometric charts, you'll notice that for a constant dry bulb temp ( as per your example), you vary the RH% ( say between 40% and 90%) :

At 90%RH - High absolute humidity or moisture content and also high enthalpy

At 40% RH - Lower moisture content and lower enthalpy

I believe any benefit you get from the high enthalpy ( ability to absorb heat ) will be neutralised with the high moisture droplet content ( that would retard heat transfer) - which is greater ? I can't say for sure.

That's why the dry bulb temperature is the main determinant of the condensor capacity. Heat transfer of heat exchangers are governed by the LMTD and in this case , the refrigerant temperature is constant (condensing temperature) with the other variables being incoming air DB and outgoing air DB.

;)

Andy
05-01-2003, 06:25 PM
Hi,
what about evaporative effect or the latent cooling element, some extra heat will be removed at higher RH% due to this effect I would think?
Regards. Andy.

Andy
05-01-2003, 06:29 PM
Hi,:)
also as the professor points out, water has a higher heat capacity than air so for every CFM you pass over the condenser, more cooling will be created the higher the water content.

Hope this is clear:confused: Regards. Andy

Steve25
05-01-2003, 07:02 PM
I will have to dig up a chart and look at that. Andy said my point well, a lbs of water will absorb more heat than a lbs of air. But the consensus seams to be that it is so minimal you will not see a drop in head or a gain in subcooling.

Dan
05-01-2003, 08:00 PM
I have seen aftermarket products that use a damp media before the condenser air inlet, and these devices do appear to lower condensing temperatures. But they are also lowering the air inlet temperature due to evaporative effect.

Spraying water on a coil, or an evaporative condenser, would be another thing entirely since it is changing the state of the moisture on the coil surface and the effect is much more obvious.

Thus, I imagine that the wetter the air the greater the condensing capacity, but if a change of state is not occurring it has to follow specific heat rules and the difference should be hard to notice in the normal ranges of RH that we see.

So I agree with Steve's original thoughts, after winding my way through the thread.:)

Dan
06-01-2003, 01:38 AM
Hmmm. Perhaps, the more moisture, the less dense so the fan can move more air? If so, would this mean that there is a small advantage on top of another small advantage? Avagadro's law has always befuddled me.

How can we have the same amount of molecules in a cubic foot of moist air compared to dry air if the water vapor molecules are smaller? Hmmm.

bersaga
06-01-2003, 04:04 AM
Hi guys !

The moment Steve posted a query of varying RH% effects on an air-cooled condensor , we should look at a pseudo- evaporative condensor !

Strictly speaking, this is the scenario :

1. Air-cooled condensor - capacity is proportional to temp. diff. between condensing temp. and entering air DB

2. Water-cooled condensor - capacity is proportional to temp. diff. between condensing temp and incoming water temp.

3. Evaporative condensor : (more complicated!) the capacity depends on the levels of temperatures as well as the temperature difference - this is because the major heat transfer mechanism in the evap. cond. is due to the vaporisation of water from the condensor tubes and this rate is proportional to the difference of water vapour pressure of the liquid water on the tube and the water vapour pressure in the saturated air that surrounds the tube.

For a given condensing temperature , the condensing capacity increases with a reduction in wet-bulb temperature.

Now , taking the scenario of 90F 90%RH and 90F 40%RH, the wet bulbs are 88 F and 71 F respectively. So theoretically, the air with the lower RH% should enhance condensor capacity. But again as I mentioned earlier, this is only possible when the tubes are wetted ( when it rains maybe !)

Now on to evaporative cooling : As for evaporative cooling , almost saturated air ( 90F 90%RH) is not a good candidate as opposed to ( 90F 40%RH) - this is a constant enthalpy process on the psychrometric chart and if you plot these 2 points on the chart and draw constant enthalpy lines to the saturation curve, you will realise how much more capacity the air at 90F 40% RH has from an evporative cooling point of view. (This is why misting systems do not work well in high humidity countries e.g. Malaysia as opposed to the Australian Outback !!!:D )

In summary, for an air-cooled condensor, it's the LMTD using DB that matters unless you are wetting the condensor finnned tubes.

Does this make sense , anyone ?!

Steve25
07-01-2003, 05:17 AM
Originally posted by Marc O'Brien

Get hold of 1000 people and make them stand really close to each other in a really compact formation, then turn on some of that great culture-rich American music by eminem and ask the 1000 to start dancing. Well, very quickly they would no longer be a compact formation, they would also start to warm up. This might be a straight forward way to analogise how it is that smaller molecules can occupy the same space... with a little more imagination, of course...

Nice, all the great music to come from the states and this is what we are known for :)


Originally posted by Marc O'Brien

How would the thermal conductance change, if any exists, effect the condenser performance?

Let me take a crack at this. I will use 1 cubic foot as a constant volume for example. 1 cf of air at 1% rh has less mass than 1 cf of air at 98% rh. O2 and N2 both already being superheated gases have very little ability to absorb heat compared to H2O. (a glass of water holds more heat than a glass of air). Therefore when the N2 and O2 molecules are displaced by H2O in the 1 cf of air it has the ability to absorb more heat plus any latent effect gained by evaporation of the H2O, if any. I would say thermal conductance does change because a less efficient vapor/gas is replacing a more efficient vapor/gas. Fan CFM is constant, so is volume over the coil.

In theory it would also put a greater load on the condenser fan motor because the air is heavier.(admitantly miniscule)

Prof Sporlan
08-01-2003, 04:49 AM
Density of moist air can be simply defined as follows:

density = (Ma + Mw) / V = (1/v)(1 + W)

where:

Ma = mass of dry air
Mw = mass of water
V = volume
v = specific volume in unit mass of dry air
W = humidity ratio

Humidity ratio is defined as :

W = MWw/MWa * xw/xa

where:

MWw = molecular weight, water = 18.01528
MWa = molecular weight, air = 28.9645
xw = no moles, water
xa = no moles, air

therefore:

W = 0.62198 * xw/xa

Fortunately, one only need to look up humidity ratio and specific volume on a psych chart to confirm Marc's assertment.

Actually, ASHRAE's published tables on thermodynamic properties of moist air in their Fundamentsals Handbook makes this easy.

For example 75F dry bulb, zero RH, we have v = 13.476 ft3/lb. Density is: 1 / 13.476 = 0.07421 lb/ft3.

75F dry bulb, 100 percent RH, we have v = 13.882 ft3/lb. Density: is 1 / 13.882 = 0.07204 lb/ft3.

Marc's understanding of Avogadro appears to be adequate... :)

Of course, air density is not the only criteria in determining heat removal capability of air. Specific heat must also be considered, which the Prof had noted in a previous post, increases with increased RH. Without thinking too hard on the subject, as the Prof is enjoying one of his finer brews, the increased specific heat of humid air is probably offset with its reduced density.

Steve25
08-01-2003, 06:01 AM
:D I said it was a crack. Thanks for all the replys guys

subzero*psia
08-01-2003, 09:31 PM
Theoretically, it does improve the heat transfer. If you have a higher RH, the specific heat of the air 'approaches' the specific heat of water because it does have a higher concentration or density.... which will also effect the load on the condenser fan motor.... but I would suppose that it too would be negligable. Gain alittle here and lose it there......




GEEEEEZE! I just went back and read the second page on this.... there I go jumping ahead again and all for nought! :p