The concepts behind different heat transfer equations?

[Ciocolatta]

Heya,

I'm a little confused about the heat equations and the difference in concepts and applications.

So there's:

Q = UA delta T <- I've used this for general heat transfer, like conduction through walls...

Q = UA delta T(log mean temperature difference) <- I've used that for calculating the overall area required for a heat exchanger, using log mean temp diff if i can assume rate of temperature change throughout is proportinoal and there's no phase change.

Q = mC delta T <- i already forgot what i use this for. its just like...how much energy i put in for sensible heat change right?

Q = (latent heat of vaporisation) x mass/time <- i used that to calc. the amount of heat consumed/released when fluid vaporises/condenses

how else can they be used for, how are they different and what other applications?
thanks!

 

[timthereaper]

Well, your question requires way more explanation than I think you realize. It's very hard to give you a concise and, more importantly, a complete answer. To be honest, to understand how to properly use the equations you posted, it takes a couple of classes in thermodynamics and heat transfer. If you've already had that, then maybe we can give you a refresher.

1) This equation isn't used for conduction, unless your U=k/delta_x. This would be more accurate for convection, if your U is the convection heat transfer coefficient.

2) You're right on that one.

3) It's accurate for heat transfer into solids, if c_p = c_v roughly.

4) You've got that one right.

As far as other applications, you can't really use them for something else other than heat transfer. However, those equations have kept engineers busy for a long time, so they're pretty good as they are.