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Thread: Condenser question
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11-09-2002, 12:47 AM #1
Condenser question
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
Steve
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11-09-2002, 03:33 AM #2
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.Prof Sporlan
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24-09-2002, 06:49 AM #3
Density of Air with a Higher RH?
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?
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24-09-2002, 05:58 PM #4
I was thinking the same thing, Leo.
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24-09-2002, 11:12 PM #5but with moisture in the air the density or weight of the air would be less.
Say we have 90°F DB, 80°F 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 90°F DB, 65°F 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 90°F DB, 80°F WB state has the higher value: 0.249 versus 0.244, which means the 90°F DB, 80°F 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 90°F DB, 80°F WB state has more moisture in the air.Prof Sporlan
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10-10-2002, 02:01 PM #6
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.
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05-01-2003, 04:57 PM #7
RH% affect condensing capacity ?
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.
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05-01-2003, 05:25 PM #8
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.
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05-01-2003, 05:29 PM #9
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 Regards. Andy
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05-01-2003, 06:02 PM #10
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.
Steve
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05-01-2003, 07:00 PM #11
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.
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06-01-2003, 12:38 AM #12
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.
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06-01-2003, 03:04 AM #13
Condensor capacities
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 !!! )
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 ?!
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07-01-2003, 04:17 AM #14Originally 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...
Originally posted by Marc O'Brien
How would the thermal conductance change, if any exists, effect the condenser performance?
In theory it would also put a greater load on the condenser fan motor because the air is heavier.(admitantly miniscule)Steve
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08-01-2003, 03:49 AM #15
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 75°F dry bulb, zero RH, we have v = 13.476 ft3/lb. Density is: 1 / 13.476 = 0.07421 lb/ft3.
75°F 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.Prof Sporlan
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08-01-2003, 05:01 AM #16
I said it was a crack. Thanks for all the replys guys
Steve
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08-01-2003, 08:31 PM #17
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!:D
Dean
Subzero*psia
Extinction is simply proof of failure to adapt.
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