Water vs. air as a heat-transfer medium

DanHolohan

How much more heat will water carry as compared to air? What do you think?

Steamhead

Steam can move the most BTUs, because of its latent heat.

Henry

Simple, if your heat loss is 80,000 BTU then wether you put out 80,000 BTU with water or air, it is 80,000 BTU.

SWEI

The specific heat capacity of air depends on its temperature, moisture content, and the altitude at which you measure it.

Per unit of volume, water has about 3200 times the specific heat capacity of dry air (at 77°F.) By weight (isobaric mass) the difference is only about 4x.

DanHolohan

Let me put it another way. How much faster will water transfer heat, as compared to air.

DanHolohan

Try this: Blow up a balloon and hold a match to it. It will explode instantly because the air inside the balloon can't move the heat away from the surface of the balloon very quickly.

Now take another balloon and fill it with water. Hold a flame to it. The balloon won't explode because the water is very good at carrying the heat away from the surface of the balloon.

I'm wondering about how much faster water actually moves the heat away from the surface of the balloon, as compared to air.

DanHolohan

And isn't that amazing? Thanks, guys.

SWEI

It gets even trickier once you throw latent heat into the equation...

jumper

Water in balloon has more heat capacity so maybe that's why it takes longer to destroy water balloon than air filled one?

Harvey Ramer

DanHolohan said:

Let me put it another way. How much faster will water transfer heat, as compared to air.

Water will transfer heat 24.17 times faster than air. That is, and I quote from Engineering Toolbox "the quantity of heat transmitted through a unit thickness of a material - in a direction normal to a surface of unit area - due to a unit temperature gradient under steady state conditions".

The thermal conductivity of water is .58 (W/m K) And the thermal conductivity of air is .024 (W/m K) Watts per meters kelvin.

http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

In the case of the balloon, there are 2 things going on. First the water against the balloon wall in contact with the flame has, as Kurt pointed out, much greater heat capacity than air. That, in itself would do little good if it couldn't pass it along to it's neighbor though. It doesn't sound that grand but I'm saying water conducts heat 24.17 times faster than air does.

And copper conducts heat 16,708.33 times faster than air. That is mind boggling.

Larry_52

Put this in proper context, water is a liquid and air is a gas (obvious). So how many more molecules are in contact with the ballon surface? To put this in equal terms you would need to use 100% dry steam versus air. Then transfer would be on equal terms, and saturated dry steam has slightly less conductivity then ambient air. Of course though steam starts at a handicap as it is already 212 degs which means it is less dense or less molecules per unit volume.

Harvey Ramer

Larry said:

Put this in proper context, water is a liquid and air is a gas (obvious). So how many more molecules are in contact with the ballon surface? To put this in equal terms you would need to use 100% dry steam versus air. Then transfer would be on equal terms, and saturated dry steam has slightly less conductivity then ambient air. Of course though steam starts at a handicap as it is already 212 degs which means it is less dense or less molecules per unit volume.

True, in order to get exact figures on both elements, they have to be in the same phase. But also the same temperature.

I was thinking that Dan probably meant the difference with water being in liquid state and air being gaseous.

Gordy

Hatterasguy said:

Harvey Ramer said:

Water will transfer heat 24.17 times faster than air. That is, and I quote from Engineering Toolbox "the quantity of heat transmitted through a unit thickness of a material - in a direction normal to a surface of unit area - due to a unit temperature gradient under steady state conditions".

.

You've confused thermal conductivity with specific heat.

Specific heat is used to compare the relative capability of a material to change temperature with the addition of energy. This is the parameter that governs the capability to heat a building.

Thermal conductivity simply shows the capability of the material to dissipate heat to its surroundings. It's not relevant to the original question.

When @Dan rephrased the original question it fits perfectly Hat. " How much faster will water transfer heat as compared to air."

DanHolohan

Yes, that's what I was looking for. Thanks.

Gordan

Transfer heat to what?

In the context of space heating, I think it's difficult to claim that hydronic systems have quicker recovery than forced air. Except, perhaps, hydro-air. Forced convection is fast.

With hydro-air, there are two heat exchanges as opposed to "scorched air." On the other hand, hydro-air is likely more efficient from the distribution point of view.

It's difficult to make useful comparisons absent context.