Module 7. Mass transfer

Lesson 32

ASSIGNMENTS

Q. 1. Calculate the rate of heat flow through a plane homogeneous wall the thickness of which is considerably smaller than its width is considerable smaller than the its width and height, assuming the wall to be made from: (a) Steel (λ = 40 W/ m-°C); (b) Concrete (λ = 1.1 W/ m-°C); (c) diatomite brick (λ = 0.11 W/ m-°C). In all the three cases wall thickness = 50 mm. the temperature of the wall surfaces are maintained constant: t_w1 = 100°C and t_w2 = 90°C.

ANSWERS

For a steel wall q = 8000 W/m²;

For a concrete wall q = 220 W/m²;

For a wall laid from diatomite brick q = 22 W/m².

Q. 2. The rate of heat flow through a plane wall of thickness = 50 mm, q = 70 W/m². Determine the difference between the temperature of wall surfaces and the numerical values of the temperature gradient through the wall, if it is made from: (a) brass (= 70 W/m-°C) (b) red brick (= 0.7 W/m-°C); (c) cork (= 0.07 W/m-°C).

ANSWERS

For a brass wall t = 0.05°C and grad t = 1°C/m;

For a brick wall t =5°C and grad t = 100°C/m;

For a cork wall t = 50°C and grad t = 1000°C/m;

Q. 3. Determine the loss of heat Q, W, through a wall laid from red brick of length l = 5 m, height h = 4 m and thickness = 0.250 m, if the temperatures of wall surfaces are maintained equal to t_w1 =110°C and t_w2 = 40°C. The thermal conductivity of red brick = 0.76 W/m-°C.

ANSWERS

The loss of heat Q = 3920 W.

Q. 4. Determine the thermal conductivity of the material of a wall 40 mm thick, if at a temperature difference between surface temperatures Δt = 20°C and the rate of heat flow Q = 145 W/m².

 ANSWERS

The thermal conductivity λ = 0.29 W/m-°C.

Q. 5. It is necessary to insulate the flat surface so that the loss of heat from unit area of this surface per unit time will not exceed 450 W/m². The temperatures of the surface underneath the layer of insulation t_w1 = 450°C and the temperature of the external surface of the insulating layer t_w2 = 50°C.Determine the thickness of insulation for the cases:

a) The insulation is made from sovelite, for which

    λ= 0.09+0.0000874t, W/m-°C.

b) The insulation is made from asbestos cement, for which

    λ= 0.109+0.000146t, W/m-°C.

ANSWERS

(a) = 100 mm

 (b) = 130 mm