Chemical Engineering - Heat Transfer - Discussion

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 de-scaling 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. This is once again 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. This is to a large extent 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.

But option (A) also must be an answer because fouling factor is a dimensionless quantity too.

Yes, I agree @Shalini, option A is also correct.

The fouling factor has the unit (m^2.K)/W hence consists of dimension.

The fouling factor, Rf, as well as the deposition rate, d, and the removal rate, r, can be expressed in the units of thermal resistance as m2. K/W or in the units of the rate of thickness change as m/s or units of mass change as kg/ m2.s. Distributed randomly all over the surface.

Unit of fouling factor = m^2K/kcal.