Heat exchangers performances: biofouling effectBack
Monitoring of commercial heat exchangers is done by tracking or evaluating the overall heat transfer coefficient. The overall heat transfer coefficient tends to decline over time due to fouling.
By periodically calculating the overall heat transfer coefficient from exchanger flow rates and temperatures, the owner of the heat exchanger can estimate when cleaning the heat exchanger is economically attractive.
The individual heat transfer coefficient or film coefficient, in thermodynamics and in mechanics is the proportionality coefficient between the heat flux and the thermodynamic driving force for the flow of heat (i.e., the temperature difference, ΔT):
q : heat flux, W/m2 i.e., thermal power per unit area, q = dQ/dA
h : individual heat transfer coefficient, W/(m2*K)
ΔT : difference in temperature between the solid surface and surrounding fluid area, K
A simple method for determining an overall heat transfer coefficient for a shell and tube heat exchanger considers of all the resistances to heat transfer, combined in a single formula. (Note that this method only accounts for conduction within materials, it does not take into account heat transfer through methods such as radiation). The formula is as follows:
U = the overall heat transfer coefficient (W/(m2*K))
γ = the thermal conductivity of the material (W/(m*K))
h = the individual convection heat transfer coefficient for each fluid (W/(m2*K))
De = tube external diameter (mm)
Di = tube internal diameter (mm)
Rf = Additional resistance given by fouling (variable and increasing-W/mK)
Oftentimes during their use, heat exchangers collect a layer of fouling on the surface which, in addition to potentially contaminating a stream, reduces the effectiveness of heat exchangers. In a fouled heat exchanger the build-up on the walls creates an additional layer of materials that heat must flow through. Due to this new layer, there is additional resistance within the heat exchanger and thus the overall heat transfer coefficient of the exchanger is reduced.
Condenser tube with residues of bio-fouling (cut open)
The following graph shows a typical fouling resistance effect on the heat transfer:
Oil/water tubes heat exchanger
The red line shows the decreased overall coefficient without any cleaning method of the circuit: without cleaning, it takes only 0.4mm of fouling to decrease the heat transfer coefficient from 177 W/mK to 158 W/mK.
In other terms (green line), keeping clean the heat exchanger (avoiding fouling formation) reduces the operating (energy) costs of around 10%, for 0.4mm thick fouling.