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In this paper, the transient response of Plate Heat Exchangers under time dependant temperature of the fluid on hot and cold sides has been studied. The effect of viscosity variations has also been considered. In addition the maldistribution of the fluid (unequal flow in different channels) has also been investigated. The governing equations have been solved using finite difference numerical methods. U flow arrangement has been considered in this simulation. It is shown that the transient response of a system of PHE significantly depends on the viscosity variations, as well as the maldistribution parameters. The effect of these parameters on the thermal efficiency of the PHE is also discussed