heat transfer and humidity

[UncleBob]

this is probably a simple question, but its been on my mind lately, and although I think I know the answer, I want to hear thoughts on it.

When you have a heat exchanger, such as a radiator, does it work better with high ambient humidity or low ambient humidity? IE, if you are in 90* F weather in Atlanta and its 100% humidity, vs Pheonix AZ, 90*F weather with 0% humidity, which one will have more efficient heat exchange, and by how much? Other factors?

[TheSilentChamber]

Water is a better thermal conductor than air, so I would say the more water vapor in the air the faster it would asorb heat from the radiator, but I'd never seen any factual evidence to support that. I'm sure curtis can help, hes the chemist around these parts.

Although I live in texas, and its been being ~105 with 90's% humdity and it sure doesnt feel like it takes the heat from my body very fast, more like a hot wet blanket draped over me.

[534BC]

That's a good question, and a good answer too. I also wonder about the barometric pressure. I'll go along with silents answer, but in the conditions we drive in it may be a very insignificant difference.

[corning_d3]

I found some interesting stuff...Its about drying pallets, but they test high and low humidity oven drying times..

Two basic schedules were used in the study, a steaming schedules (trials 1 and 3) and a dry heat schedule (trial 2). For the steaming schedule (trial 1 and 3), the heating valve of the kiln was shut off. Setting the wet bulb temperature to 160 degrees Fahrenheit controlled the high-pressure steam spray in the kiln. The wet bulb reached setpoint in 25 to 30 minutes. The actual wet bulb depression was one or two degrees. As Table 1 indicates, it took approximately 70 minutes for the conventional stringer pallets to be treated using these conditions (trial 1), 100 minutes for the block pallets to be treated (trial 1) and up to 16 hours (941 minutes) for pallet loads of cants to be treated (trial 3).
For the dry heat schedule (trial 2), the dry bulb was set at 180 degrees Fahrenheit and the wet bulb was set at 160 degrees Fahrenheit. The maximum recorded wet bulb was 150 degrees Fahrenheit. The steam spray was valved off during this trial. It took approximately 85 minutes for the conventional stringer pallets to be treated using these conditions, and up to 165 minutes for the block pallets to be treated.


As the data indicates, the higher humidity air (the steaming schedule) is more efficient at transferring energy.

[KiwiBacon]

Yes the heat capacity of air changes significantly with humidity. Your best source of info on this will be HVAC (heating, ventilation, air-conditioning) references.

For example at 20 C and humidity of 0%, it takes 20kJ to heat 1 kg of air by 1 deg C.
At 20 C and humidity of 80%, it takes 50kJ to heat 1kg of air by 1 deg C.

[beef_bourito]

Originally Posted by TheSilentChamber

Although I live in texas, and its been being ~105 with 90's% humdity and it sure doesnt feel like it takes the heat from my body very fast, more like a hot wet blanket draped over me.

that's because your body cools down by sweating. when water evaporates it absorbs heat energy from its surroundings, in this case it's your skin, but since it's more humid the water can't evaporate as easily or as quickly. this means that you won't cool down as much.

as fr the heat exchanger, all previous posts make sense, there's no real reason for me to post any more (or look for any more) evidence to support it.