The design and rating of Plate Heat Exchangers (PHE) entails the classic engineering trade-off between heat transfer and pressure drop. Countless studies have characterised the thermal and hydraulic performance of a wide variety of PHE and formulated fundamental correlations for fluid film coefficients, h, and friction factor, f [1]. However such academic results appear seldom to inform the site engineer regarding the best design and operating practices of heat exchangers in the New Zealand dairy industry. In this paper we present the thermal analysis of a site hot water preheater located at one of New Zealand’s largest dairy factories. Using fundamental heat exchanger theory, the analysis also looks at how to increase the heat exchanger’s duty and the coupled effect on pressure drop and pumping cost.