What is Fouling in Chillers and how chillers are sized as per the Fouling Factor?

fouling like in any water pipe or any tank  in general....chemical treatment is always added to avoid that in chiller.....the chemicl type and amount is determine by a specilties ,not by the hvac engineer

this is accumulation of contamination in the form of solid or soluble material\\salt\\any chemical  which may affect the performance.

The chiller manufacturer provides the fouling of factor of chiller under period of time so the selection can be done on that basis.

The deposition and accumulation of unwanted materials such as scale, algae, suspended solids, and insoluble salts on internal or external surfaces of the pipes of the equipment like boilers and heat exchangers

accumulation of unwanted foreign material like algae, suspended solids, and insoluble salts effecting the over all flow rate of the system is called fouling 

Fouling is scale or foreign material build–up that reduces heat transfer. Most packaged chillers for  Air–conditioning have sealed loop systems. There is little opportunity for fouling within the chiller barrel, because the cooling fluid is sealed from external environmental factors. Process chillers in plastics molding and other industrial applications often have open chilled water tanks or open processes where the water can foul badly and added capacity must be available to compensate for the fouling.