Question

A pipe of outside diameter 200 mm is lagged with an insulating material

of thermal conductivity 0.06 W m–1 K–1 and thickness 75 mm. The pipe

carries a process fluid at a temperature of 300 °C and the average

temperature of the outer surface of the lagging is 45 °C.

(a) Estimate the rate of heat loss per metre length of pipe.

(b) Explain why the thermal resistance of the pipe wall can be ignored.

4. A pipe carrying superheated steam at 300 °C has an outside diameter of

120 mm and is lagged with two layers of insulating material. The first

layer (adjacent to the outer pipe wall) is 25 mm thick and has a thermal

conductivity of 0.072 W m–1 K–1. The second layer (covering the first

layer) is 20 mm thick, has a thermal conductivity of 0.051 W m–1 K–1 and

an outside temperature of 28 °C.

Estimate the rate of heat loss per metre length of pipe (assume the thermal

resistance of the pipe wall is negligible).

Answer 1

a)

Rate of heat loss/ metre = 171.57 W/m

b) For calculating heat flow across an insulated pipe, Thermal resistance(R_T) must be calculated for every layer of insulation.

Thermal resistance varies with the insulation thickness of each layer, their respective thermal conductivities, outer pipe diameter and the temperature of the pipe.

R_T = ln (r₂/r₁)/2LK

Where,

r2 = outside diameter + thickness of insulation

r1 = outside diameter

L = length of the pipe

K = thermal conductivity of the pipe.

In rate of heat transfer calculation, R_T takes into account all the parameters of the pipe.

Q=(T_o -T₁)/R_T

T_o - outside temperature

T_{1 -- inside temperature}

₄₎

_{K1 -- thermal conductivity of the 1st layer of insulation.}

_{K2 --- thermal conductivity of 2nd layer of insulation.}

rate of heat loss/ metre = 190.43 W/m