Heat Required to maintain correct temperature

[Mangan]

Hi
My background is HVAC but I was asked to provide a thermal efficency report on some process pipework. My problem is I need to find out how to calculate the amount of heat required to maintain Bitumen in a 5,000 litre tank at 160 celcius.
I have the heat required to maintain the temperature but I believe I need to know how much heat my heat loss is from the tank.
The tank is lagged with 200mm fibreglass on the sides and roof.
Similarly I have jacketed pipework to the tanks and I am trying to find out how much heat is required to maintain the bitumen at 160 celcius. The thermal oil will keep the bitumen hot but how much heat do I lose.

Any help will be appreciated.

 

[athomas236]

hello,

it will be necessary to know the tank shape and dimensions together with the ambient temperature before such calculations can be made.

athomas236

 

[Mangan]

Hello athomas

The Tank is cylindrical, it measures approx. 15 metres in diameter by 35 metres high. I must take worst external ambient temperature at minus 3 celcius.

Thank you in advance.

 

[nbucska]

Mangan:
Your tank is about 6,190,000 liters, not 5,000. One cubic
meter is 1000 l.

If the tank exists, measure outside temp and cooling rate
from a higher temp. Find/measure specific heat of bitumen
and calculate from these data -- use sufficient safety factor and temp. control.




<nbucska@pcperipherals.com>

 

[Mangan]

Apologies for incorrect data
The Tank is cylindrical, it measures approx. 15 metres in diameter by 35 metres high.

It actually contains 5,000 Tonnes of bitumen not litres.
The extra capacity of the tank is for safety reasons. The bitumen is stored at approx. 160 celcius, and the tanks are not allowed to be filled to their full capacity.

bitumen content = 5,000 Tonnes
min. ambient temp. -3 celcius(worst case)
max. ambient temp. 25 celcius
installation site (sea side?) Harbour/quay side
max. wind. The area can be stormy approx 90mph in worst case

Any Help on calculating the thermal requirements on Jacketed pipework heated by thermal oil and how do I calculate the termal load on existing pipework.

Thank you

 

[athomas236]

Using the methods described in ASTM Standard Practice C680-82, my calculations show the following heat losses.

At ambient of 25C and wind speed of 0.5mph (according to Standard for still air) heat losses are 5.98MW from sides and 0.17MW from the top. Making the approximation that the loss from bottom of the tank is same as from the top then the total loss is 6.32MW.

At ambient of minus 3C and wind speed of 90mph heat losses are 8.47MW from sides and 0.24MW from the top, giving approximate total loss of 8.95MW. You should think carefully about designing for this worst case because these conditions of low temperature and high wind speed may not occur at the same time. In addition these conditions may only occur for sort time periods which will not significantly affect the temperature of the tank contents.

Hope this helps.

athomas236

 

[25362]

I have a question to athomas236: have you considered that the 200 mm fiberglass insulation to walls and roof would reduce the external temperature to just a few (say 20) degrees Celsius above air ? In that case, could it be possible that the heat loss would be one magnitude smaller than estimated ?

 

[athomas236]

Hello 25362,
You are right I did make a mistake in my calculations. I calculated the heat loss in W/m^2 as shown below then multiplied this by area m^2 and temperature difference degree C to give the loss in W, should, of course, multiplied heat loss W/m^2 by area m^2.
Regards,
athomas236

Calculations are shown below

Case = Fibreglass insulation at 25C/0.5mph
Diameter of tank or vessel D m 15.00
Height of tank Ht m 35.00
Hot surface temperature Th C 160.00
Cold surface temperature Tc C 29.85
Ambient temperature Ta C 25.00
(Th+Tc)/2 C 94.92
(Tc+Ta)/2 C 27.423
(Tc-Ta) C 4.846
Wind speed Vel mph 0.500

Cold surface emissivity Em 0.20
Radiation co-efficient Hrad W/m^2C 1.232
Convection co-efficient Hconv W/m^2C 4.306
Overall surface co-efficient Hoa W/m^2C 5.538
Conductivity - fibreglass Tcond W/mC 0.0412

CONDITIONS WITH 200mm INSULATION
Cold surface heat loss Qc W/m^2 26.84
Cold surface resistance Rc m^2 C/W 0.1806
Insulation resistance Ri m^2 C/W 4.8494
Overall resistance Roa m^2 C/W 5.0299

TOTAL HEAT LOSS
Sides of tank HLsides kW 44.27
Top and bottom HL t&b kW 0.63
Total heat loss HL Total kW 44.90