An economizer, one type of heat exchangers, is specifically named to carry out heat exchange between hot gas and water. It is considered in this work to provide heat recovery between exhaust gas and boiler's feed water. As it has been studied to obtain high heat recovery with several designs, here the economizer is devised with new approach to achieve the best heat recovery as well as economical applicability in an existing pasteurized milk plant. The new economizer is designed to divide an exhaust gas into two portions flowing up on the left and right of the economizes passing across aligned banks of tubes and then flowing down and up again in an unmixed-triple pass fashion. The pressure drop and dew point temperature of corrosive acid depended on the fuel's type are also taken into account. Moreover, the mathematical models based on the energy equation in partial differential equations of two-dimensional initial value problems have been developed to simulate the performance of the newly designed economizer. The observation data prove that the designed economizer can achieve the heat recovery of the exhaust gas up to 57% with the average of 38% and can save the consumption of liquefied petroleum gas of about 13%. © 2012 Curtin University of Technology and John Wiley & Sons, Ltd.