plate outperforms shell and tube , shell in coil because of plate with high thermal conductivity, heat can be transferred over the plate well. Despite the lack of contact between hot and cold water, it can take the heat. The three types of heat exchangers and heat exchange in the area as well, so it does not affect the trial. It will depend on the nature of the heat exchanger. And flow rate The constant flow of hot water, then add cold water up the flow rate of heat exchange is reduced.