Subject: Latent Heat

From: [email-masked] (JayKJay)
Date: 29 Jul 1995 11:14:50 -0400

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

Thanks

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From: [email-masked] (paul milligan)
Date: 29 Jul 1995 16:48:27 GMT

In article <[email-masked]>,
   [email-masked] (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. 

     Just to piss you off, 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.

Paul

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From: [email-masked] (Gerry O'Brien)
Date: 30 Jul 1995 15:19:53 GMT

In article <[email-masked]>, [email-masked] says...
>Could someone please e-mail me a definition (easy to understand) of latent

Latent heat is quite simply known as hidden heat.  It is the heat that is 
either added or removed in order to cause a change of state.  From gas to a 
liquid or liquid to a solid or vice versa.  It is the greatest heat load the a 
system will encounter.

During a change of state, you will notice no increase or decrease in 
temperature of the medium.  The temp change comes only after the change has 
occured.  To see this for yourself, try ice and water mixed together.  You will 
read a 32 deg. F temp until either the water is all frozen ot the ice is all 
melted.

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From: [email-masked]
Date: 1 Aug 1995 05:51:15 GMT

>   [email-masked] (JayKJay) writes:
     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.
     Take a pot of water and put it on a red hot burner.  As the water heats you can measure the change in temperature
      with a thermometer.  The heat that you measure is sensible heat.  Once the water reaches its boiling point, you'll 
     notice no change in the water's temperature until it is all vaporized.  The fact that you canot see the water heating 
     up does not mean there is no more heat being added.   This unmeasureable change in heat content is called latent
     heat.  I hope this helps you understand latent heat better.

     Karl

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From: [email-masked] (paul milligan)
Date: 2 Aug 1995 01:52:10 GMT

In article <[email-masked]>, [email-masked] wrote:
~~>>   [email-masked] (JayKJay) writes:
~~>>  Could someone please e-mail me a definition (easy to understand) of latent
~~>>  heat?
~~>>  
~~>>                                                                         
~~>>  Thanks
~~>>  
~~>>>>>
~~>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 <<then>> 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.

	BTW, if you read my entire post, I made the exact point you stress
later on.  After having gotten the basic concept across, I then refined
the definition a bit more accurately.

Your friend,
Paul

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From: John Cain <[email-masked]>
Date: Thu, 03 Aug 1995 10:33:16 GMT

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

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.   
                            
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