Boiling water in a pan science question
 

OK, so I keep a large kettle on my stove filled with water at a low temp to help humidify the house. (Yeah it is very low tech.)

My question is as the water boils away, just steams actually so I suppose it is technically just evaporating at an elevated temp., will it evaporate faster as the level in the pan gets lower or at a constant rate?

If I remember my jr.high science correctly I think once the water all gets up to temp. heat added equals water evaporated....one unit of heat will evaporate one unit of water, regardless of the volumn of the body of water.

So for me it would go down at (lets say 1/4" per hour) a constant rate whether the pan is half empty or half full (just kidding) or basically full.

Tom W

It will take longer to heat up if the pan is full, but once it is heated, the energy that you put in determines how much water will evaporate and not the level of the water.

We recently installed a foam-drum humidifier in the forced-air return duct, and it is working wonders. The cats hardly shock us any more.

My folks had one of those foam drum humidifiers. Worked well, when it wasn't all gunked up with calcium, and whatever else you find in hard water.

We have a softener which works well. Without it, I would hate to be the one to clean out the humidifier.

We have an additional RO filter for drinking water, and I'm seriously considering hooking the humidifier up to it.

As the water reduces while heated in the pot, the quotient/ratio of heat-energy vs heated element changes, so the water will evaporate at an accelerated pace as its volume reduces at the same supplied heat level.

The only ways to slow the acceleration is to turn the heat back or put a lid on ... :D ... or cool off the heated element by adding cold water.

Just watch that you don't over-humidify the house during cold weather.
If your windows are sweating with moisture, think of all of the places where warm air meets cold exterior walls...
At my last house, I was running two Holmes cool air wick-type (?) humidifiers feeding them 3 or so gallons a day of RO water, keeping indoor humidity at or above 40% RH, and there was some mold found behind some insulation in the garage during the home inspection when I went to sell that house. Now (at the next house) I keep my central heater's humidistat at the level for -20 deg F :rolleyes:

I say it will evaporate more quickly as the water level drops. This assumes that the level of heat input remains constant and that the pan is of relatively constant diameter. Based on those assumptions, the surface area exposed to the air remains constant, but the amount of heat lost through the sides of the pan will decline, causing the water temp to rise.

But, range top burners are regulated. Granted, they're not regulated well, but it's true.

To the original poster, don;t forget that surface area is also a factor. 1 gallon of water at boiling temperature will dissipate faster in to the air in a 24" diameter pot than it will in a 12" diameter one.

My, there are all kinds of variables in this one!:D

It should evaporate faster as the level falls because of the steady amount of heat applied to less liquid. Less heat will be lost from the pan itself when the water level is lower. Also it will be closer to the heat source and will boil off faster with that same quantity of heat to less water.

The pan is large, about 14"d and about 12" tall with parallel sides. I run the burner on nearly the lowest setting so that if it runs out of water it won't burn the pan.

Yes, the sides of the pan radiating heat will affect the loss some, but I am thinking for the most part heat in will equal water evaporated, more or less.

Thanks for the responses.

I have to go check my water now.

Tom W

Tom:

If you have radiant heaters, just setting the pan next to the air flow will get the moisture into the air and will save you the additional expense of burning the moisture into the air via your stove...

100% moisture will migrate to lower percentages of moisture...i.e.: that pan should evaporate into your room(s) if the home is fairly dry...and as Robert Roe pointed out, too much IS NOT A GOOD THING...

Let nature handle it for you...and since you're using your home's heating system...let it work a little harder for you...let that heated air pull the moisture from the pan(s) and insert the moisture...

Beats you paying the electric/gas company any more money than they're entitled too!

Besides, the way you're doing it...it's almost the same as sticking a fan in front of an open refrigerator/freezer to cool your home, or using the oven/fan combo to heat the place...the latter is, usually, responsible for burning down a few homes/apartments during the winter, each year...

:D

Don't worry guys, I am well aware of the consequences of too much moisture. I have the stove on very very low and it barely steams....about like a hot bowl of soup.

Thanks for all the input.

Tom W

Energy cannot be created or destroyed. So the same amount of energy has to go somewhere. Since there is less water, and being that the energy cannot just dissappear, all things remaining equal, the water will evaporate faster.

Correct!

The Latent Heat of Vaporization of water is a constant = 970btu/lb

If your burner is putting out a constant btuh into the water, the water will evaporate at the same rate regardless of level.

HUH?

Bingo. Some other terms to know: sensible heat, superheat, subcool. This will pay off with your HVAC repair bills.