The use of fouling factors is a common method to account for the expected fouling tendency of a process and maximize the runtime between cleaning for shell and tube heat exchangers.  Fouling factors are selected to represents the fouling expected just before cleaning time.  This method results in the heat exchanger performing better when clean and just meeting the heat transfer requirements when fouled. 

Both physical and economical considerations must be reviewed when selecting the appropriate fouling factors.  Physical considerations that influence the selection of the fouling factors are:

• nature of fluid
• type of fouling
• fluid temperature
• wall temperature
• material of construction
• fluid velocity

Economic considerations that influence the fouling factors selection are:

• Initial purchase cost of the exchanger
• Variations of cost with size
• Frequency of cleaning required
• Cost of cleaning (including loss of production)
• Fluid pumping cost
• Desired net return on investment

The initial purchase cost increases with increasing fouling resistance while cleaning and down-time expenses generally decrease with increasing fouling resistance.  However, using large fouling factors can lead to more fouling than anticipated and result in more frequent cleaning.  Selecting the optimum fouling factors involves satisfying conflicting goals.