(b) Heat flux
Heat flux is defined as the heat
removed from the process fluid by the cooling water per unit area per unit
time, and can be calculated by the following equation:
The higher the heat flux, i. e. the heat load, the greater the possibility of scale
or corrosion problems in heat exchangers.
(c) Over-all heat transfer coefficient
Over-all
heat transfer coefficient is the index of thermal efficiency, referred to as
U-value, and can be determined by the following equation:
The U-value is decreased by fouling
with scale, corrosion products, and slime during the operation of heat
exchangers. The decrease in U-value is more pronounced with heat
exchangers of high U-values under the same level of fouling.
(d) Fouling factor
Fouling
factor indicates the degree of fouling by scale and other deposits in the heat
exchangers. The factor is calculated by
the following equation:
The fouling factor is also
determined by the thermal conductivity and the thickness of the
fouling
material using the following equation:
Heat exchangers are commonly designed with fouling
factors (designed r2) in the range of 0.0002 to 0.0006 m2 · hr· oC
/kcal, depending on the expected fouling tendency of the cooling water.
Therefore, r2 in service must
be maintained below the designed r2.
The
permissible scale thickness is estimated from the designed r2 and l2. The l2 can be estimated to a certain degree
from the cooling water quality and the chemical treatment program. Figure 3.8 shows
an example of the relations between values of r2 and L2.
To be continued in Part 4...
