5 replies to this topic

Hello,

I have to design a cylindrical fired heater which is a reboiler for a splitter column.
My problem is to calculate the pressure (not pressure drop) of flue gas in radiant, convection, air preheter and chimney. This is necessary for pressure instrumentation. I have calculte the chimney draught but I don't know how to establish the value of pressure of flue gas.

Thanks in advance.

[latexman]

With atmospheric pressure at the flue exit, isn't it just the backpressure caused by the flow of flue gas to the exit? I.e. the pressure drop caused by the flue gas flow.

Thank you for your answer.

I was thinking at this subject, and I have another question:

Why it is necessary to start from the exit of flue gas and not with the bottom of the heater? The outside pressure is the same. I would start to subtract the pressure drops from the bottom to the top of the heater to have a lower pressure on top than at bottom, flue gas having an ascensional drive fom high pressure (bottom) to lower pressure (top).

[marthin_was]

Dear Ramo,

First, I'm sorry if I got your questions wrong.

You ask about: how to calculate pressure at certain point? for instance: at the stack

My response: you have to understand that pressure reading in the certain position is a reading of back pressure from the outlet until that point. Back pressure is always calculated from the exit until the inlet. In your case, the exit point is atmosphere and the inlet is the flue gas coming out from heater

thanks,
Marthin

[ethylene]

my 2 cents opinion

flue gas pressure drop thru out the heater will be change of atmospheric pressure by total heater height minus different pressure between atmospheric and radiant box at bottom of heater.

then plot the pressure curve for both flue gas and atmospheric from bottom to stack. absolute pressure for flue gas at each section will be atmospheric pressure (at each section level) minus the difference of the curves. the maximum pressure (compared to atmospheric) should be at bridgewall.

[rupirocks]

hey hii ramo,
i have been calculating the fluid side pressure and the delta P by a certain technique.u can try this out.while designing the heater at the first case, u assume a certain DELTA P.now,for each pipe starting from the last one in radiant section,u can the find the enthalpy of the process fluid at the terminal point.from enthalpy-temp-press chart u can find out temp and press.friction factor of the pipe is known to you. by simple pressure drop equation,u can find the skin pressure drop of that pipe.u continue to find it for all the tubes.the add up all pressure drops.if the cumulative pressure drop is near to ur accepted one then the design is accepted.and subtracting the calculated DELTA P from the inlet pressure gives u the pressure at the outlet of the heater.

regards

Rupirocks