We are using chemicals to feed into cooling water system; however,
Calcium sulfate scale will arise around the tube side of heat
exchanger.  The material of the tube is copper or carbon steel shell. 
As Calcium sulfate scale is very rigid to remove cleanly, if anyone
could offer effective method to resolve the said scale problem.

Strangely, http://www.gxsugar.com/homepage/Eng/E2.htm

(a page about sugar processing) seems to indicate that
hot sugar water ("juice") might do the trick:

"Calcium sulfate has a quite high solubility in hot water
and juice ..."


Search: "removing calcium sulfate"

://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22removing+calcium+sulfate%22

Very common industrial problem. Most inhibitors are phosphorus-based,
and one-shot cleanouts tend to be EDTA-based. Suggest that you contact
NALCO ... they are usually more than happy to help, and have a variety
of products on the market for just this type of application. (I work
in mining.. no stranger to this problem).

CHeck out http://www.ondeo-nalco.com/industries/mining_and_minerals/scale_control.html

Hot one Normal sulfuric acid.

For removing calcium sulfate and other water-borne scales, from MS and
copper surfaces, CITRIC ACID SOULTION will be more beneficial then
other acids, as it is less corrosive.