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View Full Version : latent heat question please help me if you can thanks.

jason bowling
07-14-2004, 10:02 PM
I need to know what latent heat is if there is someone out there that can fill me in I would really apreciate it. Thank you jason

bruiseandy
07-14-2004, 10:06 PM
heat added to a substance that causes a change in state but not temperature. When you heat water all the heat added until it hits 212 degrees is sensible you can see a temp rise. Once heat is added after that the water cannot get hotter then 212 degrees so it changes state to a vapor. That is latent heat.

jimgpe
07-15-2004, 03:33 PM
Latent heat is the heat of evaporation - in the case of HVAC, the evaporation of water. Think of it this way.

If you want to boil a pot of water, you and I both know you need to put it on the stove and heat it. The heat requiredt to raise the water temp to 212 is sensible heat - you can feel the temperature rise.

The heat required to flash it to steam at 212 is latent heat.

If you could capture all the resultant steam and re-condense it back into liquid water at 212F you would get all that heat back. This is essentially what you are doing when a cooling coil sweats - the coil is absorbing the heat required to condense the humidity in the air back to liquid water.

The heat exchanged to make water into steam or steam into water at a constant temperature is latent heat.

jackmm1
07-15-2004, 04:03 PM
Jason-Latent heat (known as hidden heat) takes place in 2 components in the refrigeration cycle. The evaporator(latent heat of evporatization)liquid changes to a gas and in the condenser (latent heat of condensation)a gas changes back to a liquid. Latent heat tranfer is where most of your work is done in a sys and most Btu's removed. Not in the case of senible heat, which is heat that can be measured.

jason bowling
07-15-2004, 05:02 PM
Thanks guys I think that I got it. Laten heat is Heat where you can change the state of somthing for ex. liquid to steam without raising the temp. This is because that the temp can not rise any more with out the state of the subject changing. Essentially you are adding sensible heat to start with and at a certain temp. where you can not add anymore sensible heat to something before it changes into the next state the heat that changes it is latent. Have I understood you all corectly. Thanks for the info. I do not ever want to stop learning want to be the best I can be.

kool it man
07-16-2004, 09:15 AM
This site explains it relatively well...:cool:

http://daphne.palomar.edu/jthorngren/latent.htm

NormChris
07-16-2004, 11:57 PM
Here you go. This short article on Superheat & Subcooling covers Latent heat as well. In order to really understand Latent heat correctly you need to know something about Superheat and Subcooling. These three concepts are the very heart of HVAC theory. Get these three down well and you have the basis for understanding all the rest.

Norm

glgto
07-17-2004, 09:44 PM
basic terms heat you cant measure with a thermometer

NormChris
07-17-2004, 09:52 PM
Originally posted by glgto
basic terms heat you cant measure with a thermometer

Actually, you can measure latent heat effects using a wet bulb thermometer.

glgto
07-17-2004, 09:55 PM
now my wheels are turning explain please

glgto
07-17-2004, 10:03 PM
something link dry steam and wet steam?

glgto
07-17-2004, 10:04 PM
like not link sorry

NormChris
07-17-2004, 10:06 PM
If you look on a psychrometric chart you will see that the wet bulb lines match the lines of enthalpy perfectly. If you know the wet bulb temperature of air you know the btu/lb of air. Measuring the difference in wet bulb temperatures between the return and supply air will tell you how many btu/lb the evaporator removed from the air. Thus the following equation is used to prove the actual operating capacity in Btuh of any cooling system.

Btuh = 4.5 X CFM X delta Btuh

You get the delta Btuh from the enthalpy differences of the return and supply air by measuring the wet bulb temperatures of the return and supply air and reading the enthalpy for each wet bulb temperature. Therefore, for air you can determine the latent heat involved when a change of state takes place in air.

Norm

glgto
07-17-2004, 10:23 PM
i had a brain fart thanks for the reminder i was trying to vision someone standing over a boiling pot saying well i got the wb temp of the air now how do i get the wb temp of the water????

NormChris
07-17-2004, 11:56 PM
Yup, as the water evaporates off the wet wick of the wet bulb themometer the temperature drops due to the latent heat cooling effect and the wet bulb depression is an indication of how much latent heat was involved. So, latent heat can be measured at least with air.

jason bowling
07-18-2004, 03:23 PM
Guys this is awsome. There is so much that you can learn from you fellows. I have been doing residential hvac for 4 years now. I have always wanted to be a service tech, but I have a nack for installing and have been to valuable to my employers as an istaller to be moved to service. I just recently had a job interview with a commercial service company and they are considering hiring me. This is the opprotunity that I have been wating for. I will use this site for info and appriciate all of the good knowledge that you fellow can share thank you. Jason

NormChris
07-18-2004, 03:35 PM
Welcome aboard Jason. There are a number of very experienced and articulate HVAC people here willing to help you. Post your questions. Also, use the search function as many questions have been asked before.

In addition, look at the 'For Your Interest" area on this site and you will find some informative yet to the point articles.

Norm

jason bowling
07-18-2004, 04:17 PM
Thank you norm for all of the help and encouagement. All I want to be is well rounded and the best I can be in my field. I also want to learn things the right way not just the way that some of the half hiny techs in my town this is the wright way. Thanks again.