Abstract: Aimming at fouling and the changes of fluid temperature and flow exist during the long use of heat-exchangers,three parameters, NTU , θ and R ,are defined to characterize the heat transfer properties of different heat-exchangers in view of the second law of thermodynamics.The studies are performed from both theory and an example of how three parameters influence heat exchange.The conclusions are derived to guide the high-efficiency operation of heat exchangers.Heat transfer decreases as NTU or θ or R decreasing.And fouling affects less the heat exchangers with bigger R than those with smaller R ,while fouling affects the heat transfer independent of θ .