# Heat TransferWatch

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#1
Hey guys, was hoping you could help me with a heat transfer question i have, any help would be much appreciated!

A reactor vessel is made of thin mild steel of 2mm wall thickness and contains hot reactants held at 120oC. It is mounted on an external platform and exposed to wind and temperatures as low as -15oC. It is to be insulated to reduce heat loss and to reduce the external surface temperature of the plant using 150mm fibre insulation with a conductivity of 0.15 W/mK. The external convective heat transfer coefficient to the ambient air at -15oC is assumed to be 7 W/m2K.

Calculate:
i) the reduction in heat flux (W/m2) from the vessel, and
ii) determine if the external surface temperature of the insulation will be above freezing point. State clearly any assumptions you have

My answer is below, but i am not sure it is correct?

WITHOUT INSULATION (i've taken the value of k from literature to be 52 w/m^2k)
U=1/(1/h+x/k)
U = 1 / (1/7 + 0.002/52)
U = 6.998

delta T = 135degrees C

q = 6.998 * 135 = 944.746

WITH INSULATION
U= 1/( (1/h) + (x/k) + (x2/k2) )
U= 1/( (1/7 + 0.002/52 + 0.15/0.15)
U = 0.87497

delta T = 135 degrees C

q = 0.87497 x 135 = 118.121

reduction in heat flux = 826.625

For part ii):

I used the equation:

q = h (Ts - Tf)
118.121 = 7 (Ts - -15)
16.874 = Ts + 15
Ts = 1.874 --> therefore above freezing point!

Thank you very much, any help would be much appreciated, I will rep you forever if you help!!
0
5 years ago
#2
(Original post by crazy_smurf)
Hey guys, was hoping you could help me with a heat transfer question i have, any help would be much appreciated!

A reactor vessel is made of thin mild steel of 2mm wall thickness and contains hot reactants held at 120oC. It is mounted on an external platform and exposed to wind and temperatures as low as -15oC. It is to be insulated to reduce heat loss and to reduce the external surface temperature of the plant using 150mm fibre insulation with a conductivity of 0.15 W/mK. The external convective heat transfer coefficient to the ambient air at -15oC is assumed to be 7 W/m2K.

Calculate:
i) the reduction in heat flux (W/m2) from the vessel, and
ii) determine if the external surface temperature of the insulation will be above freezing point. State clearly any assumptions you have

My answer is below, but i am not sure it is correct?

WITHOUT INSULATION (i've taken the value of k from literature to be 52 w/m^2k)
U=1/(1/h+x/k)
U = 1 / (1/7 + 0.002/52)
U = 6.998

delta T = 135degrees C

q = 6.998 * 135 = 944.746

WITH INSULATION
U= 1/( (1/h) + (x/k) + (x2/k2) )
U= 1/( (1/7 + 0.002/52 + 0.15/0.15)
U = 0.87497

delta T = 135 degrees C

q = 0.87497 x 135 = 118.121

reduction in heat flux = 826.625

For part ii):

I used the equation:

q = h (Ts - Tf)
118.121 = 7 (Ts - -15)
16.874 = Ts + 15
Ts = 1.874 --> therefore above freezing point!

Thank you very much, any help would be much appreciated, I will rep you forever if you help!!
Seems ok.
I did these using a slight variation of the method and get the same answers.

I don't know how confident that makes you though.
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