Does air temp have an effect on evaporation rate?

[Sugar Bear]

It seemed to me that when I boiled sap on Wednesday and it was 70 degrees out, my evaporation rate was slower. Would that be the case? I burned a lot of wood and had lots of boiling bubbles but little steam coming off the pans so I am wondering if my evaporation rate is slower because it is warm out.

[Haynes Forest Products]

Was the outside humidity high. When its raining out its a tough day of boiling ahead. I do believe the cooler the surface temperature is the higher the boiling rate.

[wnybassman]

When it is warm, the temp difference between steam and air is closer so you don't see the steam as much. I think barometric pressure plays a big role too, but I am no scientist.

[ecolbeck]

I dont think that temperature, relative humidity, or air pressure have any practical effect on evaporation rate. However, they could easily have an effect on the appearance of water vapor over your pan. I would guess that on a warm day with lower relative humidity there would be less steam appearing over the pan than a cold day with high RH even thought the actual amount of water vapor is the same in both cases. This could give the impression that evaporation rate has slowed when in fact it hasn't.

[mike z]

I know it slows way down when you're outside standing in the pouring rain boiling on a wide open system. Seems even slower if its 3:00am.:-|:cry:

[Haynes Forest Products]

The fact that Sugar Bear clearly said he felt his evaporation rate had slowed I'm going with it. I believe relative humidity as having a large effect. Evaporation rates are effected by humidity. I live in the Denver area when I get out a shower and dry off withing 5 min Im dry. When I get out of the shower in Door County Wisconsin over the 4th of July I wipe some of the water off and get used to it. That's the difference between 10% humidity verses 60%.

[Sugar Bear]

Just to add some more confusion to this household of conjecture.

Shut down the boiling Wednesday evening around midnight to get some sleep for my Thursday and fired back up at 6:30 PM Thursnight to finish off my last 15 gallons of sap. Sap seemed to go faster then it was going on Wednesday and Wednesnight. Was quite a bit cooler on Thursnight too. Less humid to I think.

Might change my name from Sugar Bear to Confused Bear.

Could change it to Happy Bear however.

[Sugar Bear]

It did not rain on Wednesday but on Monday the ground around my rig had 6 inches of snow on it. By Wednesday afternoon it was gone. So was my cooling snow piled around the barrels. The only way to walk around in the yard was with the good ole Tractor Supply swamp boots. On Thursday night I was able to walk around the yard in my moccasins.

Asked my girlfriend about this issue ... evaporation rate given atmospheric conditions ... and she said she thought I would evaporate faster on a cold dry night, but she was not certain.

She went to Yale on a scholarship and was a chemical engineering major. Took courses like Organic Chemistry at Yale and got good grades in them.

You would think they would actually teach them some solid science and make them able to confirm whether I am disillusional or not.

But no dice!

[Sugar Bear]

The more I ask questions and the more i experience the real world, the more I get confused.

[Sugar Bear]

I burned a heck of a lot of my firewood on Wednesday, the good dry Black Locust that was supposed to be used to keep my house warm.

[ecolbeck]

Evaporation rates at ambient temperature are greatly affected by relative humidity. As stated before, you will dry off more quickly after a shower in arid Denver than in a humid city. But this isn't what's going on in the sugar house.

What were discussing here is evaporation at the boiling point of water. The atmospheric effects of temperature, pressure , and humidity have no practical (i.e. measurable in a meaningful way) effect in this context.

[blissville maples]

Barometric pressure will likely have the biggest impact on boil. I believe high pressure creates a slightly slower boil.

[ecolbeck]

Higher atmospheric pressure will cause a higher boiling point. Sap will boil at 213F instead 212F (or whatever). However, his doesn't cause a "slower boil" in any meaningful sense. While technically it will take more energy to heat the sap to boil at a higher temperature (more wood/oil consumed), the differences are minute.


Here are some rough numbers.

In a 2x6 evaporator with 8 gallons of sap it will take an additional 16.5 kcal of heat to warm the sap the extra 1F (from 212 to 213F).

To evaporate that same 8 gallons of sap it will take 16,200 kcal of heat. It takes nearly 1000 times the energy to evaporate the water than it does to heat it that extra degree.


On a day with high atmospheric pressure you will have to add one extra firewood split for every 1000 that you burn in order to maintain your boiling rate.

[kingOFgEEEks]

Just to pile on with the questions - would the outside temperature and humidity have an effect on the sugar house reaching the 'draft point', where the steam from the boil finally sucks out the cupola?
I boiled Saturday night, and it was wet, drippy, and foggy outside. The smoke from my sugarhouse chimney, and my home chimney, kind of lazily drooped down and stayed on the ground. My sugarhouse was absolutely uninhabitable - I had to take a deep breath outside, then run in and quickly toss a couple splits on the fire, or top off my feed tank. And then, like a switch was flipped, my cupola started drafting, and the air in the sugarhouse cleared up significantly.

[Haynes Forest Products]

So there is a correlation between Barometric pressure and boiling rate and because you have water vapor that will exit the waters surface at a lower temperature and in reality at a lower visual boiling rate and because we think of the term boiling rate as the rate at which the water vapor leaves the liquid surface exiting the evaporator hence what we consider our rate to be.

Yes that was one sentence without any periods because its more dramatic if i talk real fast without stopping. Holy Molly. OK just funning with you.
In reality I do know what were all trying to say but cant put it to words. Some people are just better at it.

I will also say that when its late and there are a few guys standing around enjoying a cold one and some guy shows up with his mother in law and she asks What makes the syrup come out the valve I usually head out to the nearest tree and have a one on one talk with it and it helps if you have one hand on it as you chat.

[ecolbeck]

KingofGeeks that's an interesting question.

I think there would be a variety of factors

1. Temperature inversion. Cool air near the ground, warm air aloft. Warm air typically rises, but it can't in this situation
2. High humidity makes steam more visible. fog on top of fog in your case.
3. lack of wind to disperse steam/smoke.
4. Size of evaporator compared to volume of air needing to be vented. A small evaporator in a large space will not vent as well because it doesn't warm the space as quickly?
5. Open vs. closed sugar house. Open sugar house warms more slowly, slowing the development of the stack effect.

[kingOFgEEEks]

KingofGeeks that's an interesting question.

I think there would be a variety of factors

1. Temperature inversion. Cool air near the ground, warm air aloft. Warm air typically rises, but it can't in this situation
2. High humidity makes steam more visible. fog on top of fog in your case.
3. lack of wind to disperse steam/smoke.
4. Size of evaporator compared to volume of air needing to be vented. A small evaporator in a large space will not vent as well because it doesn't warm the space as quickly?
5. Open vs. closed sugar house. Open sugar house warms more slowly, slowing the development of the stack effect.

It was definitely an inversion the other night, so I'm sure that had something to do with it.
Additionally, my shack is still 3-sided. My plan is to start adding the 4th wall as I boil, but so far, I haven't done any daytime boils, so I haven't had the light to do any carpentry.
My experience so far with this new shack, is that the smoke and steam back up, until the pan starts a good full boil, and the shack warms up. I can believe that closing the 4th wall will speed this process up, and would help me get up to a good boil faster.
Thanks for the insight. I wonder if my evaporation rate will improve once I finish closing up the shack? My evaporator is a homemade 32" x 74" flat pan with dividers. I lit the fire under it at 7:30 Saturday night, and cooked off 35 gallons of sap in 5 hours, including warm-up and cool-down.

[ecolbeck]

I doubt finishing the shack will affect the evaporation rate. I would think that with a pan that size you would be getting at least 15gph? If you're not happy with the rate you might look into what's causing it to be slow.

[kingOFgEEEks]

I doubt finishing the shack will affect the evaporation rate. I would think that with a pan that size you would be getting at least 15gph? If you're not happy with the rate you might look into what's causing it to be slow.

I don't think it was too bad. It took about an hour to warm up, and an hour to cool down, so really, 30 gallons over 3 hours was the effective rate, plus 5 more during cooldown. The arch is far from optimal, but I did try to make some tweaks this year to get it better, and so far it's helping. Also, I need to put my big boy pants on, and go shallower than the 2" I'm running it at now.