Question

a)      A vertical plate of 700 mm wide and 120 cm height is maintained at a temperature of 95^oC in a room at 25^oC with an air conditioning with velocity of 36000 cm/min. Calculate the heat transfer coefficient and convective heat transfer rate.

(b)      Two concentric cylinders are located in a large room. Determine the view factors for the two cylinders when the length is 20 cm, the inner radius (r₁) is 5 cm and the outer radius (r₂) 10 cm. The whole system is in thermal equilibrium.

(c)      A heat exchanger uses oil (c_p = 1880 J kg^-1 K^-1) at an initial temperature of 205°C to heat up water with heat capacity of 1 kcal kg^-1 K^-1, and density = 0.998 g cm^-3, flowing at 0.00376 m³ min^-1 from 16°C to 44°C. The configuration of the heat exchanger is in countercurrent. The oil mass flow rate is 270 kg h^-1.
(1 cal = 4.18 J)

(i)       What is the heat transfer area required for an overall heat transfer coefficient of 0.34 kW m^-2 K^-1)?                                              

(ii)       Determine the number of transfer units (NTU).                                                       

(iii)      Calculate the effectiveness of the heat exchanger

Answer 1

b). Consider the following figure;

Then by summation law, i.e., Now, by law of reciprocity, i.e., Now, again by the law of summation,

c). Given: And, since mass flow rate = volume flow rate X Density, we have -

​​​​​​Then, Now, for counter flow,