How to Make Air Moist | Explained

[n0_3sc]

I've seen and heard people do this:

"Placing a cup of water in front of any heater prevents the air from getting 'too dry' "

Can someone explain this??

 

[russ_watters]

I'm not sure what you want explained. Air contains water vapor. In winter, the air is drier. By evaporating water, you make the air more moist. Among other things, when air is more humid, there is less static electricity, less dust, and your skin won't dry out.

 

[Danger]

Essentially, the water evaporates and increases the relative humidity. While I generally hate humidity (after 13 years in the Detroit area), it can be beneficial in some regards, as Russ pointed out. On the other hand, it can make extremes of temperature almost unbearable.

 

[n0_3sc]

So its the heat that physically evaporates the water...
Also doesn't water require a relatively high amount of heat energy to evaporate it ?
Even so, is the evaporation rate enough to make the surrounding air noticeably moist?

 

[russ_watters]

Yes, it takes a lot of heat to evaporate water. No, using a cup of water in front of a heater will not make a noticeable difference in a room's humidity.

 

[arildno]

I would recommend that you buy a humidifier, instead of staring at a cup of water atop your television set.

 

[FredGarvin]

Yes, it takes a lot of heat to evaporate water. No, using a cup of water in front of a heater will not make a noticeable difference in a room's humidity.

I guess that depends on your frame of reference. Water will evaporate without additional heat added.

 

[Danger]

a cup of water in front of a heater will not make a noticeable difference in a room's humidity.

Perhaps not consciously noticeable, but there's definitely a difference in the environment. At my mother's place, the furnace was originally a gravity type coal-burner converted to gas. The ducts run horizontally under the floor in the living room, with one outlet in the centre of the room and a cold air intake off near the wall. Mom pulled the floor grate up and put a margarine tub in the duct, which she keeps filled with water. It has to be refilled every couple of days. She can tell when it's empty without looking (she forgets to fill it sometimes) because she starts getting nosebleeds, breathing problems and static shocks.

 

[russ_watters]

I guess that depends on your frame of reference. Water will evaporate without additional heat added.

Well, no it won't - whether evaporating it or boiling it, it is the same 2400 kJ/kg heat of vaporization. The temperature won't necessarily change (caveat below), but it requires input heat.

Caveat: if there is no input of heat to the system, then the temperature of the system will drop. That's what an evaporative cooler does for homes in a desert: if you take 100F air at 10% RH and evaporate water into it, you can end up with 75F air at 50% RH.

So that means if you run a humidifier in the winter, your house will be more comfortable at the same temperature, but your heating bill will be higher.

 

[russ_watters]

Perhaps not consciously noticeable, but there's definitely a difference in the environment. At my mother's place, the furnace was originally a gravity type coal-burner converted to gas. The ducts run horizontally under the floor in the living room, with one outlet in the centre of the room and a cold air intake off near the wall. Mom pulled the floor grate up and put a margarine tub in the duct, which she keeps filled with water. It has to be refilled every couple of days. She can tell when it's empty without looking (she forgets to fill it sometimes) because she starts getting nosebleeds, breathing problems and static shocks.

That's an extremely small amount of added humidity to be able to stop a nosebleed. Whole house humidifiers have outputs on the order of 10 gallons per day. If I keep my bedroom door closed, my room humifier will add maybe 20%rh to my room on a cold day and use a gallon or two a day.

http://www.lowes.com/lowes/lkn?action=productDetail&productId=185806-40586-DS2001C&lpage=none

 

[FredGarvin]

Well, no it won't - whether evaporating it or boiling it, it is the same 2400 kJ/kg heat of vaporization. The temperature won't necessarily change (caveat below), but it requires input heat.

Caveat: if there is no input of heat to the system, then the temperature of the system will drop. That's what an evaporative cooler does for homes in a desert: if you take 100F air at 10% RH and evaporate water into it, you can end up with 75F air at 50% RH.

So that means if you run a humidifier in the winter, your house will be more comfortable at the same temperature, but your heating bill will be higher.

That's what I meant by frame of reference. It requires energy to evaporate, but it will simply get it from it's surroundings. One wouldn't necessarily have to provide that energy. It would get it from the ambient surroundings. It's unimportant. I have no idea why I brought that up.

 

[DaveC426913]

That's an extremely small amount of added humidity to be able to stop a nosebleed. Whole house humidifiers have outputs on the order of 10 gallons per day.

I live in a house that's about 90 years old and is heated by radiators. Each rad has a reservoir that can be hung on it to fill with water. The reservoir holds maybe a litre of water. (No, we never actually used them.)

 

[DM]

It requires energy to evaporate, but it will simply get it from it's surroundings. One wouldn't necessarily have to provide that energy.

Only depending on the temperature outside. Water will not evaporate in cold temperatures, hence you would have to physically provide that specific latent heat of vaporisation.

 

[russ_watters]

Water will evaporate in cold temperatures - even air below freezing. Regardless, we are talking about air inside, which is always about the same temperature, so water evaporates more readily in winter than in summer.

 

[DM]

Water will evaporate in cold temperatures - even air below freezing. Regardless, we are talking about air inside, which is always about the same temperature, so water evaporates more readily in winter than in summer.

This is true if you apply induced heat energy to the water. I'm addressing vaporisation when it's only influenced by the "ambience". Excluding cold temperatures because I personally wanted to solely focus on the kinetic energy of air molecules.

 

[DaveC426913]

Ice will directly sublimate to vapour even in the dead of winter.

 

[DM]

I agree but I'm only addressing the ambience in terms of heat energy, such as in cooking, causing water molecules to vaporise (hot temperatures only). I'm aware that objects such as ice turn to vapour. In all, it's just best to avoid this scenario otherwise it'll become too complicated.

 

[russ_watters]

So then what are you talking about? Your point seemed pretty clear and concise to me...

 

[DM]

So then what are you talking about? Your point seemed pretty clear and concise to me...

My point didn't address cold temperatures. How do air molecules provide heat energy to the water molecules in cold temperatures? They do not have enough kinetic energy. Therefore I was solely addressing vaporisation in hot temperatures only.

Don't assume the water molecules (or even ice molecules) are colder than the surrounding air, this could lead a small percentage of kinetic energy from the air molecules to contribute to vaporisation.

 

[russ_watters]

My point didn't address cold temperatures.

I don't want to belabor this, but I do want the information in here to be correct. You said:

Water will not evaporate in cold temperatures...

...which certainly is "address"ing cold temperatures, and just plain isn't true. And it seems like you still believe it to be true, because:

How do air molecules provide heat energy to the water molecules in cold temperatures? They do not have enough kinetic energy.

...that's the same as saying "water will not evaporate in cold temperatures" (now talking about the mechanism) and it still isn't true.

This is getting more into chemistry, but I'm pretty sure the mechanism of evaporation (or sublimation, which Dave brought up) is that air molecules with high kinetic energy (which there are, even in a cold environment because the atmosphere is one big billiards table, with a bell curve of kinetic energies for individual particles) will "knock off" molecules from the surface of the water/ice. At the same time, some water molecules with a low kinetic energy will be deposited on the surface of the ice. Depending on temperatures and relative humidity, there may be either a net inflow (precipitation) or outflow (evaporation, sublimation) of water molecules.

So even if you have water just above freezing or ice somewhat below it, at the same temperature of the air above it, the air above it will (depending on humidity) provide the heat necessary to cause sublimation or evaporation.

Don't assume the water molecules (or even ice molecules) are colder than the surrounding air, this could lead a small percentage of kinetic energy from the air molecules to contribute to vaporisation.

In fact, because of evaporation/sublimation, the temperature of water or ice in equilibrium will always be slightly lower than the temperature of the air (very slightly, but still...). The molecules that get "knocked off" first are the ones with the highest kinetic energy, lowering the average kinetic energy of the water/ice.

 

[DM]

You've just disected both of my paragraphs in half, which of course leads you to comment on something that is not pertinent to this case.

My point didn't address cold temperatures. How do air molecules provide heat energy to the water molecules in cold temperatures? They do not have enough kinetic energy. Therefore I was solely addressing vaporisation in hot temperatures only.

Don't assume the water molecules (or even ice molecules) are colder than the surrounding air, this could lead a small percentage of kinetic energy from the air molecules to contribute to vaporisation.

The above post depends on both paragraphs, with the latter being the assumption and hence the former being the case.

Now, if a cube of ice is warmer than the surrounding air, will the air molecules have sufficient energy to cause "sublimation"? The way I see it and correct me if I'm wrong is that heat energy is transferred to the outside because the water molecules possesses a further amount of internal energy.

I know that I caused a few moments of confusion by stating that "water will not vaporise in cold temperarures" but I had this assumption in mind:

Don't assume the water molecules (or even ice molecules) are colder than the surrounding air, this could lead a small percentage of kinetic energy from the air molecules to contribute to vaporisation.

Therefore my point didn't address cold temperatures in the case where an ice cube is colder than the surrounding air. This naturally leads to vaporisation, regardless of the cold temperature (except 0 Kelvin).

 

[DaveC426913]

All molecules of air and/or water are not at equal temperatures (i.e. do not have equal kinetic energy).

1] Just because the air is "cold" does not mean air molecuiles do not have sufficient energy to impart to water molecules to cause them to vapourize.
2] Even the water molecules can impart energy to their neighbours, causing the neighbour to vapourize, regardless of air temp.


I think that you are looking at the vapourization of water as too black and white. Maybe you should restate your question.