ENTHALPY DIFFERENCE -VS- SPECIFIC HEAT : SIZING OF HEAT EXCHANGER

[takiyasamsama]

Hi,

As title referred, I understand that for a single phase (sensible heat) it is preferred that specific heat method is used to determined the duty. However, when the flow is a slug flow I found that even a minor fraction of vapor would effect the calculated duty when using specific heat method. In that case, I'll turn to enthalpy difference to calculate the duty.

My question is this, I knew that software like HTRI would use enthalpy difference method when calculating for duty if pre-installed media is used, in fact for a single phase (sensible heat), the enthalpy difference method could be used as well. So which one is better to determine the duty regardless of the type of flow and phase it is?

Thanks!

 

[Caloooomi]

Do you get different answers?

I personally use enthalpy difference as I can pull specific enthalpies for pure components from REFPROP.

 

[takiyasamsama]

Difference by few hundreds despite requiring to design 10& - 15% slightly higher.

Let say using enthalpy method I got around 2350 kW and using specific heat method I got around 2100 kW.

On top of that, my supplier sizing software does not have features where you can design using enthalpy method so it's quite difficult to design for application with slug flow even sometimes when the amount of vapor is low, we simply ignore and design it as full liquid form (zero (0) vapor fraction), it works but less accurate.

 

[Rspain]

Dear Taki,

Specific heat and enthalpy are the same thing, an Calorific energy balance. Please refer to definition of specific heat (at constant pressure) = variation of Enthalpy with respect to temperature when pressure is held constant. Being Enthalpy a state function, the actual path (pressure remaining constant) does not matter when calculating Enthalpy variations of a pure substance between two points.

Cp (T2 - T1) = H2 - H1

No difference at all.

The possible outcome differences may arise from unexact estimates/assumptions: for instance, assuming specific heat remains constant over the temperature range.

If Enthalpies are experimental, use enthalpies.

BR;
Roberto