Lets assume an exchanger that is 100% efficient (no heat loss). Duty (Q) and LMTD is known. Your overall heat transfer coefficient (U) is dependent on your transfer coefficients on the shell and tube side, and the resistance of the pipe walls (we're going to ignore entrance effects and fouling) . In order to calculate the transfer coefficients for the tube and shell sides, you need your Reynolds number in order to calculate Nusselt Number. The Reynolds Number is based on the diameter of your pipes and shell. So from a design perspective, if Q = U*A*LMTDHow would you calculate the surface area needed without knowing the specs (diameters) of your tubes and shell? And without any software such as ASPEN, HTRI, etc. The only information I have available are flowrates, temperature data, and thermal properties of the fluids.