Abstract
M.Ing.
Scaling problems in a heating or cooling system can be recognised by one or more of
the following symptoms. Reduction in heat transfer rate, in which the formation of an
insulating deposit on a heat transfer surface significantly reduces the cooling or heating
efficiency of the equipment. Reduced water flow, which results from a partial or complete
blockage of pipelines, condenser tubes, or other openings. Even a small build-up of scale
on a heat exchange surface reduces water flow. Scale may continue to build up in boilers
until heat transfer is so low that the metal overheats, permitting the tubes to rapture under
the operating pressure. Scale is usually found in water-handling equipment in which
water is heated, i.e. hot-water storage tanks, boilers, etc.
The magnitude of this problem may be appreciated by considering that scaling can
cause degradation, or complete failure in thermal and hydraulic performance which
increases initial and operating costs (Chan and Ghassemi, 1991). A fairly low-pressure
boiler, with only 0.6 mm of calcium sulphate scale on the tubes results in a 180°C
temperature drop. The cost involved due to heat transfer inefficiency and the removal of
scale, in Britain alone, is estimated at £1 billion per annum (Darvill, 1993). Poor
conductivity of a 25 mm thick CaC03 scale layer can decrease the heat transfer by 95%
(Glater eta/., 1980), whereas a Si02 scale layer 0.5 mm thick results in a 90% decrease in
heat transfer (Grutsch and McClintock, 1984).