Plate type heat exchangers are, in many ways, superior to the original shell and tube design. As with any heat exchanger, you must calculate the operating temperatures so it works as desired.
Cold Leg Heat Transfer Coefficient

Calculating the temperature of your cold legs requires knowing the mass flow rate, cold inlet temperature, cold outlet temperature and specific heat of the fluid in the system. To calculate the heat transfer rate, multiply the mass flow rate by the specific heat of the fluid, by the difference of the outlet, minus the inlet temperature.
Hot Leg Heat Transfer Coefficient

Calculating the temperature of your hot leg is similar to the cold leg with a minor difference. You need all the same data, but the equation is figured by multiplying the mass flow rate by the specific heat of the fluid, by the difference of the inlet temperature, minus the outlet temperature.
Heat Transfer Coefficient

This calculation lets you know how efficient your heat exchanger is, plate type heat exchangers will generally be threeandahalf to four times more efficient than shell and tube designs. One over the overall heat transfer coefficient is equal to one over hot leg heat transfer coefficient, plus one over cold leg heat transfer coefficient, plus the fouling coefficient.