Outlet Temperatures

[coopsy]

How do I calcuate an outlet temperature of the hot fluid. I know the in/out temps of the cold fluid, the inlet temp of the hot fluid, the mass flowrates of both and the heat capacities of both.

 

[25362]

When speaking of common heat exchangers as used in chemical plants, given some pre-conditions are fulfilled, the enthalpy balance should be pretty easy. These are: if

a. there is no phase change such as vaporization, condensation, or crystallization, in which you need to include latent heats,
b. the fluids are single phase all through, e.g., no suspensions, emulsions, froths, and the like,
c. the H/E unit is thermally insulated so as to consider heat losses or gains truely negligible,
d. the heat capacity, Cp, values, are reasonably constant across the H/E (a condition not always applicable),
e. there is no chemical reaction taking place,
f. the heat transferring unit is operated in a steady state condition,

the enthalpy balance:

[(mass flow rate)(delta t)(Cp)]₁=[(mass flow rate)(delta t)(Cp)]₂

should allow you to find the missing temperature.

Temperatures do not change linearly across the exchanger. If it is a parallel-flow unit the outlet temperature of the warm fluid t_2,outwill always be higher than that of the cold fluid t_1,out. If, on the other hand, it is a truely countercurrent unit, there could be a crossing between those two. Obviously, never could the warmer fluid come out cooler than the incoming cold fluid.