Question

Calculate temperature change: 20 000 l/h cheese milk (V1) is to be heated from 4°C to 34°C by 30 000 l/h hot water (V2) at 50°C. Density (r) and specific heat (cp) for milk are about 1020 kg/m^3 and 3.95 kJ/kg, K and for water 990 (at 50°C) and 4.18.

(a) 20.5℃

(b) 21.5℃

(c) 18.8℃

(d) 19.5℃

The question was posed to me at a job interview.

This intriguing question comes from Heat Exchangers in portion Building Blocks of Dairy Processing of Dairy Engineering

Answer 1

Right choice is (d) 19.5℃

To explain: The temperature change for the hot water can then be calculated:

                     20 000 x 1 020 x 3.95 x (34 – 4) = 30 000 x 990 x 4.18 x Dt2

Dt2 = 19.5°C. The hot water temperature will drop by 19.5 from 50 to 30.5°C.