Design of heat exchanger step :3 : Estimate

The 3rd Step in Heat exchanger design a preliminary estimate of the size of the exchanger is made,using a heat-transfer coefficient appropriate to the fluids, the process,and the equipment.

Generally we can get reference from various resources listed below, either expected heat flux (W/m2) or range of heat transfer co-efficient. based on this information, we can estimate the heat transfer area required.

But if the case is, you are doing for first time, and have no earlier experience in that type of heat exchanger, then better is make your self ready with all information e.g fluid properties which includes

1. Dynamic viscosity (for the temperature ranges)
2. Conductivity (for the temperature ranges)
3. Density (for the temperature ranges)
4. Boiling temperature
5. Specific heat

The grate equation (you can see in my earlier post) for heat duty is:
Q = U A LMTD & Q = m. cp. DT

where
Q = heat duty (kcal/hr)
U = over all heat transfer co-efficient (Kcal/hr.m2.oC)
A = heat transfer area (m2)
LMTD = Log mean temperature (degC)
cp = Specific heat of the fluid to be heated / or to be cooled (kcal/kg)
DT = temperature difference of any one of the fluid (degC)
m = mass flow rate of the fluid (kg/hr)

LMTD is calculated based on the temperature on both the side for both the fluid
it’s simple if you understand the following figure

& LMTD = (dt_o – dt_i) / ln(dt_o / dt_i)

Just Plot your temperature, take difference on each side, and do the arithmatics…

Some good refrences (Free google search results 🙂

http://www.chemsof.com/lmtd/lmtd.htm
http://en.wikipedia.org/wiki/Log_mean_temperature_difference
http://www.wlv.com/products/databook/databook.pdf