Subject : Latent Heat - Defined & discussed

From: (paul milligan) (JayKJay) wrote:

~~>Could someone please e-mail me a definition (easy to understand) of latent heat?

Latent heat is the moisture content in the air.

There is water vapor in the air. If you want to take it out, you must condense it out by passing it across a surface which is cold enough ( at or below dew point ) to cause water to form ( like the mirror in the bathroom when you take a shower, or a window on a winter day.

This condensation occurs at 100 % relative humidity. When the air can no longer hold any greater concentration of water vapor, the vapor will change to a liquid. When a vapor changes to a liquid, it gives up heat. This is latent heat, because the energy came from changing vapor into liquid, not from lowering the temperature. All refrigeration units have some effect on both latent and sensible heat, assuming the evaporator is below dewpoint.

The proportion will vary according to the amount that the coil temperature falls below dewpoint, and the time the air (with water vapor) gets to sit on that coil and lose energy.

In other words, the temperature ( sensible ) does not change, but rather the energy is given up by the process of changing state from a gas to a liquid.

Sensible heat is that heat which you can sense ( standard temperature readings ). Latent heat is that heat which you can not sense ( the heat energy that it took to turn that water into vapor in the first place ), although the energy is still there.

When you remove water from the air without changing the temperature, your RELATIVE humidity goes down. You have changed the RELATIVE amount of LATENT HEAT (moisture) to SENSIBLE HEAT ( temperature ).

When you raise the ( Sensible ) temperature without adding water vapor ( latent heat ), your RELATIVE humidity goes down, because the RELATIONSHIP between temperature and water content has changed, in favor of temperature.

If you add BOTH LATENT and SENSIBLE heat, your RELATIVE humidity may stay the same, if you have maintained the same proportions.

All of this slides up and down the scale based on PRESSURE ( this is how refrigeration works ), but for normal purposes, assume 14.7 lb atmospheric pressure.

The total heat energy of LATENT and SENSIBLE present is called ENTHALPY, expressed in BTU's / pound.


From: (paul milligan) In article <3vkfcj$>, wrote:

~~>Contrary to what our friend Paul may think, latent heat is not moisture content in the air. Latent heat is the amount of heat required to bring about a change of state in a substance.

Our friend Paul thinks you are not exactly on target. You refer to the application of sensible heat to a condensable substance which is currently in it's liquid state, or vice versa ( heat removal from vapor ).

Once it is in it's gaseous state, it <> is referred to as 'containing latent heat', which is the heat of vaporization. The amount of heat applied to it was sensible when it was applied or removed. Once it is contained in the gaseous state of the substance, it becomes latent.

While I agree that the statement I made about it being 'moisture in the air' was technically loose, I think it conveyed the point to those who may not be familiar with the subject, which was my intent.

Your friend,


From: John Cain

Latent heat gains increase the moisture content of the air in the conditioned space but DO NOT affect the the dry-bulb (as read from a standard thermometer) temperature.

To maintain a constant relative humidity within the conditioned space, the air must be supplied at a lower moisture content than that of the air within the space.

Latent gains are given by: Ql=Ma(Gr-Gs)Hfg

where: Ql= Latent heat gains in kW

Ma= Mass flow rate of supply air in kg per second

Gr= Room moisture content kg/kg

Gs= Supply air moisture content kg/kg

Hfg= Latent heat of evaporation kJ/kg

Add the above to previous threads and you will have a reasonable understanding of the definition of latent heat.

| John Cain I.Eng ACIBSE MASHRAE EMail: |

| BSC Building Services Ltd Tel: 01908 231128 Fax: 231129 |

From: lewharri

I've noted many of the responses that you have received to your original question, and as far as I can tell, they are all correct. But taken together, the term "latent heat" may now be even more confusing, unless you already knew quite a lot before you asked.

I sometimes teach psychrometrics to people who are new to air conditioning. I explain latent heat this way:

(I'm assuming that your question comes from trying to understand "latent heat" in an air conditioning context.)

Any mixture of air and water vapor contains heat. Part of that heat is represented by the "sensible temperature" of the air.(Sensible heat can be measured by a normal with a dry sensing bulb.) The other part of the heat in the air is its "latent" heat. Latent heat is the energy that was used to evaporate the mass of water that the air now contains. So if the air now contains a great deal of water vapor, its latent heat is high. Conversely, if the air is rather dry, its latent heat is low. The sum of the sensible and latent heat of the air is called its "enthalpy", sometimes called its "total heat".

Latent heat is often used as A/C engineering jargon for the moisture content of the air. The usage is correct, but sometimes confusing.

I hope that I assumed correctly that you are asking about air conditioning technology. If that is not your interest, this explanation, while correct, may be confusing in other contexts such as crystalization, melting or condensation. If we knew the context of your question, these explanations could probably be more helpful.

Best Regards,

Lew Harriman

Mason-Grant Company

Back to Interesting Threads Index