wetbulb versus drybulb

David G · Jul 25, 2020

Humans are evaporative cooled, so the dewpoint (wetbulb) temperature really matters to our ability to cool. Does cooling on mechanical devices also suffer as the relative humidity rises? It was 90 degrees and 90% relative humidity today while I was out mowing.

rustred · Jul 25, 2020

that's enough to make me pant like an old dog. i hate heat. did you have a wobbly pop on each side of you in ice?

David G · Jul 25, 2020

Got to think about something while mowing, I could not decide so asked opinions.

tomturkey · Jul 25, 2020

My memory is a bit foggy after 50 years, but when I was at Sheppard Air Force Base, Texas. If the wet bulb temperature was 88" degrees f. all outside activities were to be curtailed or delayed. Got us out of PT a few afternoons, stayed in the ac barracks. gobble

Glen SW Wi · Jul 25, 2020

A piece of metal would not be affected by the humidity. It is because of the evaporation of sweat that causes the relationship. Sweating is a cooling process and since metal has no liquid to evaporate there is no cooling.

Glen SW Wi · Jul 25, 2020

They shut down outside activities because with high humidity your sweating isn't working as effectively at evaporating and thus the cooling process wasn't working as well. I had the same down time when I was at Lackland for basic.

As an aside, I used to always have a argument with my brother in law, an Airforce trained meteorologist and later NOA weather forecaster, as to weather metal could get colder than the ambient temperature. I always maintained that chill factor has no effect on objects that don't generate heat. Chill factor has to do with rate of heat loss and a piece of iron can only get as cold as the ambient (outside)temp. Then again what do I know about it? I was just a technical instructor in ballistic missile propulsion systems (Atlas and Thor)in the Air Force.

Janicholson · Jul 25, 2020

Evaporation from hay drying lowers the temp (of the hay). cooling effects of a lake/pond/river. is increased when humidity is low. So there you have it muggy is cess cool than dry heat of the same place on the thermometer. Jim

tomturkey · Jul 25, 2020

thank u for your service to our Great Nation. gobble (retired Chief, E-9)

Glen SW Wi · Jul 25, 2020

Have a nephew who retired at that grade also. Me, I just got to E-4 and got out. Thank you for your service also.

paul · Jul 25, 2020

I was unloading small squares, only 100 or so, yesterday, it was over 100 on their humidity heat index. I only did 10-15 bales and cane in and had a water.

Whew.

If something is wet with rain, dew, frost, snow, and the moisture evaporates off of it, that surface will cool down.

It is t exactly like us sweating as we replace the moisture, but there is an effect.

It won’t affect your hot engine or machine, as the moisture is already off it.

Paul

MarkB_MI · Jul 25, 2020

Humid air is less dense than dry air; this affects engine performance, although not by that much. I suspect the less-dense air doesn't cool as well, but due to the reduced horsepower it's probably a wash.

David G · Jul 25, 2020

That is good to hear, kind of took the snot out of me, did not want to overheat engine.

Adrian Billheimer · Jul 25, 2020

Yes, I learned in operating industrial ammonia refrigeration, that high heat and humidity results in reduced evaporation and reduced heat transfer at the condenser, therefore reduced ability to maintain refrigeration, (freezing ability) in our spiral freezers. I can prove this by the percentage the compressors run at during the hot/humid times vs. cooler/dryer days. Ya, I know very technical, but, shows the point.

Bill(Wis) · Jul 25, 2020

While attending the US Army Ranger School at Fort Benning, Georgia in August of 1958, the entire post was on heat alert which meant we only had to run through the bayonet assault course (the most deviously cruel course ever devised) nine times. The raw recruits got to do it once and remembered it for life. We caught a lot of flack from our instructors over that because the cold weather students got to do it thirteen times.

JMOR · Jul 25, 2020

(quoted from post at 21:27:21 07/25/20) That is good to hear, kind of took the snot out of me, did not want to overheat engine.

lso, wetbulb is not dewpoint.

wore out · Jul 25, 2020

(quoted from post at 20:14:16 07/25/20) Yes, I learned in operating industrial ammonia refrigeration, that high heat and humidity results in reduced evaporation and reduced heat transfer at the condenser, therefore reduced ability to maintain refrigeration, (freezing ability) in our spiral freezers. I can prove this by the percentage the compressors run at during the hot/humid times vs. cooler/dryer days. Ya, I know very technical, but, shows the point.

Reading between the lines a little here, it sound like your condensers or cooling towers were cooled with water spray along with airflow?

buickanddeere · Jul 25, 2020

Wet bulb and dry bulb mean nothing to the airflow over radiator fins .

4520BW · Jul 26, 2020

You are correct. At work we strip all the humidity out of the combustion air supply then reheat and add humidity to the correct standard. If the engine is for epa certification the standards must be held within a couple percent.

Adrian Billheimer · Jul 26, 2020

Yes they are.

teddy52food · Jul 26, 2020

If you pour water through the radiator with the engine running, it will cool it faster
than air alone. The dirt track racers did it a lot. Cleaned the dirt from it too.

wetbulb versus drybulb

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