Heat transfer equation

[mech212]

In heat transfer equations sometimes they use Q=m*Cp*deltaT = m* delta H ,, and sometimes uses Q=U*A*delta T ,, so when to use each equation ? in heat exchangers they use the first equation then the second one so i got confused a little , AT last does the two equations equals each other ?

 

[IRstuff]

One equation is heat capacity and the other is heat flow. If you can't even tell the difference simply by dimensional analysis, then you need to do some serious studying. You could start with:

http://web.mit.edu/lienhard/

http://www/ahtt.html,

but even the Wikipedia articles would help.

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[mech212]

ok , actually i searched more in a heat exchanger text book and both equations determines the rate of heat transfer , i don't know from where you get the heat capacity which is only the term " Cp " not the equation i wrote !!

 

[willard3]

Time to do some study ,mech 212. This is not a heat transfer class.

 

[IRstuff]

Did you not write: "In heat transfer equations sometimes they use Q=m*Cp*deltaT ?"

The equation is mass * heat_capacity * change_in_temperature, which is the Joule heat content of a material whose result is joules.

Your second equation results in watts.

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[25362]

The first equation gives the heat gain or loss of each of the heat exchanging streams.
The second equation gives the heat transferred in the heat exchanger.

 

[davefitz]

If there is only a small pressure loss across the HX, then the heating process can be approximated as isobaric ( constant pressure) , such that Cp*delta T= delta H. This is part of thermodynamics.
At a small section of the HX tubing, the local rate of heat transfer Q=Ua*A*deltaT, where Us= overall heat transfer coefficient that includes the inside + outside convective HT coefficient + fouling + tube conductivity. The HX heat transfer is best determined using compact heat exchanger theory ( aka e-NTU method)

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[dbill74]

The first equation is the heat gain/loss from the fluid on one side of the heat exchanger. With two (2) fluids this needs to be calculated twice, once for hot side, once for cold side. deltaT is the change in temperature of the fluid on one side of the HX; so T_hot-in - T_hot-out. Similar for cold side.

The second equation, Q=U*A*delta T, is the rate of heat flow through the walls of the heat exchanger. U*A is a function of the physical properties of the HX itself (not the fuild on either side), delta T in this equation is the temperature difference between the hot and cold sides.

So which equation(s) to use? The equation(s) that solves for your unknown.

 

[takiyasamsama]

Mech212,

Q = m*Cp*dT; this one is use for sensible heat where no phase change happens. Single phase heat exchanger would use this all the time.
Q = m*dH; this one is use when latent heat is involve. Mainly for condensers and evaporators, delta-H is your latent heat value.

You can find latent heat using HTRI or if you already have the latent heat then simply multiply those with your mass flow rate. Be sure to check for units as well.

Hope this helps.

 

[Rspain]

Hi,

One is a heat balance, it will tell you the maximum energy you can move from one stream to another (with infinite area)
The other is a speed equation, it will help you determine the speed of energy transfer (heat flux), thus makes possible to size the heat exchanger

Regards,
Roberto