The overall heat-transfer coefficient of a heat exchanger under operating conditions is reduced by fouling

Fouling and Heat Transfer

The overall heat-transfer coefficient of a heat exchanger under operating conditions is reduced by fouling 

During operation with most liquids and some gases a dirt film gradually builds up on the heat-transfer surface. The deposit is referred to as fouling.
The increased thermal resistance of the deposit can generally be obtained only from actual tests or experience. The fouling factor can be determined from the relation

R_d = 1 / U_d - 1 / U   (1)

where 

R_d = fouling factor - or unit thermal resistance of the deposit (m²K/W)

U_d = thermal conductance of heat exchanger after fouling (W/m²K)

U = thermal conductance of clean heat exchanger (W/m²K)

(1) can also be expressed as:

U_d  = 1 / (R_d + 1 / U)

Typical Fouling Factors

• Alcohol vapors : R_d = 0.00009 (m²K/W)
• Boiler feed water, treated above 325 K : R_d = 0.0002 (m²K/W)
• Fuel oil : R_d = 0.0009 (m²K/W)
• Industrial air : R_d = 0.0004 (m²K/W)
• Quenching oil : R_d = 0.0007 (m²K/W)
• Refrigerating liquid : R_d = 0.0002 (m²K/W)
• Seawater below 325 K : R_d = 0.00009 (m²K/W)
• Seawater above 325 K : R_d = 0.0002 (m²K/W)
• Steam : R_d = 0.00009 (m²K/W)