START UP HEATING 2

[roker]

Hello,
The reaction part of a plant should be heated before starting production with the gas to be treated at the plant.
The heating is performede by circulating nitrogen with a blower through the equipment (catalytic reactors, heat exchangers, vessels and the piping) in a closed loop.
The system should be heated from 20 C to about 200 C by hot nitrogen which is heated by thermal oil at 335 C in part of the heat exchangers which are in the loop of equipment to be heated.
The heating is an unsteady process, what are the guidelines for calculating the time it will take to heat up the system.
regards,
roker

 

[speco]

Roker,

Batch heating calculations are pretty straightforward. However, the trick is to include the all the components of the heat load in the calculation of the batch size or mass. That is, any piping, vessels or other equipment which are heated along with the nitrogen should be treated as part of the batch, but converted to an equivalent mass of nitrogen, based on the specific heats of those components.

The elements you need to know to calculate the batch heating time are:

1. The equivalent mass of nitrogen in the batch as described above.

2. The surface and average heat transfer coefficient of the exchanger.

3. The (average) mass flows and specific heats of both hot and cold streams.

4. The inlet temperature of the hot fluid (usually a constant). That's the hot oil in this case.

5. The initial and final temperatures of the (cold) process stream.


Kern's book, Process Heat Transfer, has a calculation procedure for this. I believe there is a calculator at cheresources.com to help with this.

Regards,

Speco (

http://www.stoneprocess.com)

 

[reena1957]

Dear Roker,
Here is a short cut calculation procedure:
Full credit to Speco, as you have to have all the data required as per him, the neat nitrogen mass flow rate below is the cold side flow rate mentioned by him. Here are the steps. My units are all metric Engg.
1. Calculate the total heat load taking the total equivalent Nitrogen Mass (as per step 1 of Speco above), Av Nirogen sp heat and the initil and final temperatures - in Kcals.( Beware:No time units)
2.Using the mass flow rates of Nitrogen(Neat)and hot oil flow rates, and inlet hot oil temperature(335), calculate the Hot oil exchanger outlet temperatures and Nitrogen temperatures(once outlet and once inlet). This involves solving one heat transfer eqn and one heat balance eqn(heat gained by Nitrogen = heat lost by oil)with two unknown temperatures. This has to be done twice once at the start of heating and once at the end of heating process corresponding to the 20 deg c inlet and 200 deg c outlet of Nitrogen.
3. Now calculate the LMTD at the beginning and at the end of heating process for the exchanger.
4. Now calculate the time averaged LMTD by using LMTD(initial) and LMTD(final):
LMTD(initial) - LMTD(final)
LMTD(time Avg)= ----------------------------
{ LMTD(initial) }
ln { ------------- }
{ LMTD(final) }

Heat load (calculated in step 1
5. Time required = ---------------------------------
U . A(exchnager) .LMTD(time Avg)


Voila! Now you are done.
Best wishes

 

[roker]

Dear speco,

Thank you, could you please advise the chapter in Kern's book.

regards,
roker

 

[roker]

Dear reena1957,

Thank you, I suppose that by "neat" you mean the actual flow not the equivalent flow?

regards,
roker

 

[reena1957]

Yes Yes - it is the Nitogen flow rate through the exchanger. The other equivalent thing is the mass of equivalent Nitrogen. Actually real system Nitrogen mass may be about 5% of the total mass.
best wishes