Aheat exchanger is to cool ethylene glycol (cp = 2.56kj/kg·◦c) flowing at a rate of 3.2kg/s from 80◦c to 40◦c by water (cp = 4.18kj/kg·◦c) that enters at 20◦c and leaves at 70◦c. determine (a) the rate of heat transfer (b) mass flow rate of water.

a)

b)

Explanation:

A)

Consider the energy balance just for the ethylene glycol; we can write an energy balance for this compound because it does not mix with any other.

The heat is negative for this balance because the energy in heat form is going out of the ethylene glycol.

B)

Consider the same energy balance, but for water:

The heat that the ethylene glycol losts is the same that the water receives, so, for this equation Q is the same quantity that we found above, but with a positive sign because it is entering to the water.