Fouling Factors in Heat Exchangers

August 17, 2016

The fouling factors to be used in the design of heat exchangers are normally specified by the client based on their experience of running the plant or process. If uncontrolled, levels of fouling can negate any benefits produced by careful heat exchanger design. The fouling factor represents the theoretical resistance to heat flow due to a build-up of a layer of dirt or other fouling substance on the tube surfaces of the heat exchanger, but they are often overstated by the end user in an attempt to minimise the frequency of cleaning. In reality, if the wrong fouling factor is used, cleaning may actually be required more frequently.

Fouling mechanisms vary with the application but can be broadly classified into four common and readily identifiable types.

Common Types of Fouling

• Chemical fouling: when chemical changes within the fluid cause a fouling layer to be deposited onto the tube surface. A common example of this phenomenon is scaling in a kettle or boiler caused by “hardness” salts depositing onto the heating elements as the solubility of the salts reduce with increasing temperature. This is outside the control of the heat exchanger designer but can be minimised by careful control of the tube wall temperature in contact with the fluid. When this type of fouling occurs it must be removed by either chemical treatment or mechanical descaling processes (wire brushes or even drills to remove the scale or sometimes high-pressure water jets).
• Biological fouling: this is caused by the growth of organisms within the fluid which deposit out onto the surfaces of the heat exchanger. Again this is outside the direct control of the heat exchanger designer, but it can be influenced by the choice of materials as some, notably the non-ferrous brasses, are poisonous to some organisms. When this type of fouling occurs it is normally removed by either chemical treatment or mechanical brushing processes.
• Deposition fouling: this is when particles contained within the fluid settle out onto the surface when the fluid velocity falls below a critical level. To a large extent this is within the control of the heat exchanger designer, as the critical velocity for any fluid/particle combination can be calculated to allow a design to be developed with minimum velocity levels higher than the critical level. Mounting the heat exchanger vertically can also minimise the effect as gravity would tend to pull the particles out of the heat exchanger away from the heat transfer surface even at low velocity levels. When this type of fouling occurs it is normally removed by mechanical brushing processes.
• Corrosion fouling: this is when a layer of corrosion products build up on the surfaces of the tube forming an extra layer of, usually, high thermal resistance material. By careful choice of materials of construction the effects can be minimised as a wide range of corrosion resistant materials based on stainless steel and other nickel-based alloys are now available to the heat exchanger manufacturer.

Corrugated Tubes

The use of corrugated tubes has been shown in be beneficial in minimising the effects of at least two of these fouling mechanisms: deposition fouling because of an enhanced level of turbulence generated at lower velocities, and chemical fouling. Chemical fouling is reduced because the enhanced heat transfer coefficients produced by the corrugated tube result in tube wall temperatures closer to the bulk fluid temperature of the working fluids.

Fouling Factors 
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.

The link below lists typicall fouling factors. 

Fouling Factors

Fouling is a function of the system and the equipment used.  Parameters which influence the fouling rate in a system include: the type of fluid, the type of heat exchanger, the temperatures, the velocities and the materials of construction.  The actual fouling rate is different for each application. 

There are numerous resources for fouling factor values.  The best source is from existing operating facilities.  When production data is available, it can be used to calculate the actual fouling factors for the system.  Together with the maintenance and cleaning history, this provides the best resource for selecting the fouling factors for a particular application.

However, when production data is not available, one must rely on other sources.  Over the years, typical fouling factors have been compiled for various systems and fluids.  The tables below show some of these typical fouling factors:

Process Fluids	Fouling Resistance
	(ft2-°F-hr/BTU)
Oils
Fuel Oil #2	0.002
Fuel Oil #6	0.005
Transformer Oil	0.001
Engine Lube Oil	0.001
Gases and Vapors
Acid gas	0.002 - 0.003
Ammonia vapor	0.001
Chlorinated hydrocarbons vapors	0.001 - 0.0015
Chlorine Vapor	0.002
CO2 vapor	0.001
Compressed Air	0.001
Hydrogen	0.0005
Hydrogen (saturated with water)	0.002
Light Hydrocarbon vapors (clean)	0.001
Natural Gas	0.001 - 0.002
Natural Gas Flue Gas	0.005
Nitrogen	0.0005
Polymerizable vapors (with inhibitor)	0.003 - 0.03
Refrigerant Vapors (oil bearing)	0.002
Solvent Vapor	0.001
Stable Overhead Products	0.001
Steam (non-oil bearing)	0.0005
Steam (exhaust, oil bearing)	0.0015 - 0.002
Liquids
Ammonia Liquid	0.001
Ammonia Liquid (oil bearing)	0.003
CO2 Liquid	0.001
Chlorinated hydrocarbons liquid	0.001 - 0.002
Chlorine Liquid	0.002
DEG & TEG Solutions	0.002
Ethylene Glycol Solutions	0.002
Gasoline	0.002
Heavy Fuel Oils	0.005 - 0.007
Heavy Gas Oil	0.003 - 0.005
Hydraulic Fluid	0.001
Kerosene	0.002 - 0.003
Light Gas Oil	0.002 - 0.003
Light hydrocarbon liquid (clean)	0.001
MEA & DEA Solutions	0.002
Naphtha and Light Distillates	0.002 - 0.003
Organic Heat Transfer Liquids	0.002
Refrigerant Liquids	0.001

                                                       

Water	Fouling Resistance		Fouling Resistance
	(ft2-°F-hr/BTU)		(ft2-°F-hr/BTU)
water temperature	125°F or less		over 125°F
water velocity	3 ft/s or less	over 3 ft/s	3 ft/s or less	over 3 ft/s
Boiler Blowdown Water	0.002	0.002	0.002	0.002
Brackish Water	0.002	0.001	0.003	0.002
Condensate	0.0005	0.0005	0.0005	0.0005
Cooling Tower Water (treated)	0.001	0.001	0.002	0.002
Cooling Tower Water (untreated)	0.003	0.003	0.005	0.004
City or Well Water	0.001	0.001	0.002	0.002
River Water (minimum)	0.002	0.001	0.003	0.002
River Water (average)	0.003	0.002	0.004	0.003
River Water (muddy or silty)	0.003	0.002	0.004	0.003
Sea Water	0.0005	0.0005	0.001	0.001
Treated Boiler Feed Water	0.001	0.0005	0.001	0.001