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LAB MANUAL FOR PHT
CRITICAL RADIUS OF INSULATING MATERIAL
EXPERIMENT 3
CRITICAL RADIUS OF
INSULATING MATERIAL
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LAB MANUAL FOR PHT
CRITICAL RADIUS OF INSULATING MATERIAL
EXPERIMENT NO: 03 DATE:
AIM: Determine critical radius of insulating material
THEORY:
Whenever there exists any hot cylinder of pipe (e.g. steam piping) it is customary to put the
insulation around the pipe. Naturally, one would expect that thicker the insulation, the lesser will
be the heat loss from pipe. But it is not the case always.
As stated above, addition of insulation does not always reduce heat loss, As thickness of
insulation increases, heat loss also increases to certain limit called critical radius of insulation.
Addition of insulation there after reduce the heat loss.
To determine the value of critical radius at q max, the equation is given as,
k .r = 1/h0. r2/1
or ( r ) critical radius = k/h0
.where, k = conductivity of insulating material
.h0 = convective heat transfer coefficient
Hence, for a particular insulating material for a particular value of h0, if outer radius is less than
rcr, than addition of insulation will increase the heat loss till the actual radius equals to rcr.
.Further addition of insulation reduces the heat loss
:SPECIFICATIONS
:Test pieces .1
mm O. D., G. I. pipes, provided with cartridge heaters inside and plaster of paris insulation of 33
different thickness outside. The thickness of
TP1 = 21 mm (dia. 75 mm)
TP2 = 33.5 mm (dia. 100 mm)
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LAB MANUAL FOR PHT
CRITICAL RADIUS OF INSULATING MATERIAL
TP3 = 8.5 mm (dia. 50 mm)
TP4 = 3.5 mm (dia. 40 mm)
Controls and measurements .2
i) Dimmerstats: 2 amps. Capcity, 4 nos. to supply independent input to heaters in the test pieces
ii) Voltmeter:0-250 V
iii) Ammeter: 0-3 A
iv) Selector switches: 4 Nos. to indicate heater input (in volts and amps.) in downward position
.and to disconnect the meters in upward position
:v) Mutichannel digital temperature indicator
vi) Thermocouples located one each at inner and outer multichannel digital temperature
indicator
vii) Thermocouples located one each at inner and outer radius of insulation for all test pieces
.and a thermocouple to note the ambient temperature
:EXPERIMENTATION
To study the critical radius phenomenon and to show experimentally that heat loss across the
.insulation is maximum at critical radius, provided outside heat transfer coefficient is constant
:EXPERIMENTAL PROCEDURE
.Connect the supply plug to the socket .1
Check that all the dimmerstats are at zero position .2
.Initially, all the selector switches should be in upward direction .3
Switch ‘ON’ the main switch and the corresponding heater switches. Operate the .4
corresponding dimmerstat one by one and adjust the input to the heaters such that all the heater
.surface temperatures come to be the same
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LAB MANUAL FOR PHT
CRITICAL RADIUS OF INSULATING MATERIAL
The unit will reach steady state within around two hours, note down the heater input and .5
.temperatures
:OBSERVATION TABLE
Sr. Input
Test piece Ti C T0 C
.No Voltage Current
1 TP1
2 TP2
3 TP3
4 TP4
Ambient temperature (T C) = _____________ C
:CALCULATIONS
Heat input, q = V * I watts .1
Surface area (A) of .2
TP1 = 0.070 m2
TP2 = 0.094 m2
TP3 = 0.047 m2
TP4 = 0.0376 m2
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LAB MANUAL FOR PHT
CRITICAL RADIUS OF INSULATING MATERIAL
,Convective heat transfer coefficient .3
Q= h0 A (Ts – T) W
Critical radius, rcr = k/h0 (where , k = 0.48 w/m k) .4
’GRAPH: Plot the graph of ‘heat input’ v/s ‘actual radius
:CONCLUSION
i) Critical radius for h0 = _____________ was found to be __________ m
ii) Heat lost (i.e. heat input) was maximum for test piece of critical radius
RESULT :
