## Heat transfer equation

## Heat transfer equation

(OP)

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 ?

## RE: Heat transfer equation

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## RE: Heat transfer equation

## RE: Heat transfer equation

## RE: Heat transfer equation

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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## RE: Heat transfer equation

The second equation gives the heat transferred in the heat exchanger.

## RE: Heat transfer equation

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

## RE: Heat transfer equation

_{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.

## RE: Heat transfer equation

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.

## RE: Heat transfer equation

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