How to calculate a heat exchanger in 10 minutes?

24-06-2019

As a customer, how do I know if the proposal of my supplier is close to what my process needs?

Two parameters can be calculated: capacity and heating surface. Below you can find a brief description on how to do this yourself. Be warned that this is a strongly reduced and conservative calculation method, meant to be fast and to dimension approximately. In reality a lot more parameters need to be considered so the exact outcome may be different.

Capacity P = Q x Cp x (T2-T1) x rho

With:
P = Capacity in [J/s] of [W]
Q = Flow in [m³/s]
Cp = specific heat [J/kg.K]
T2 – T1 = temp @ inlet heat exchanger – temp @ outlet HE [K]
Rho = density in [kg/m³]

You could check and make the calculations for both hot and cold side, with P1 = P2 as a must.

Heating surface = P / (k x dTavg)

(“How many tubes at which length do we need?”)

with ‘k’ thermal heat conductivity coefficient in [W/m².°C]
Some typical values for k: (conservative + estimations!)
Steam – water:    1000
Water – water:       700
Water – oil:           300
Steam – oil:          500
Steam – air:        25 to 150

dTavg = average temperature difference between cold and hot side [°C], see example

Small example:

Suppose we want to heat up 5 000 L hot water from 70°C to 90°C in 1u.
In our department we have saturated steam available @ 5 barg (saturation temperature = 159°C)

P = (5 x 1000 x 4,186 x 20)/3600 = 116 kW
dTavg = [(159-70) + (159-90)] / 2 = 79°C
HS = 116 000 / [(1000)*(79)] = 1,47 m²