Once through is used where the water will not be re-circulated. Water is cooled down to fill a batch or spray over a product. Although most applications require the water to be re-circulated but in some cases the water is used to clean a vegetable then discarded because you don't want to re-circulated the dirty water on the clean vegetables. Typically food processers use once through cooling. Each system is different processes range from 35F-70f water...
Btu/hr = 500 x gpm x Tin-Tout. Temperature drop and flow will determine chiller capacity (Btu/hr). Chiller capacity will be effected by increasing or decreasing flow or temperature drop.