7 replies to this topic

[saurabh singh]

Dear all,

Currently I am working on a project to evalute the Efficiency of the fired heater , as we changed to NG from Fuel Oil.

The current firing rate of NG is 3500 Sm3/hr and combustion air flow is 45000 Nm3/hr. NG pressure is 0.6 kg/cm2g and the flow and pressure matches according to the heat capacity curve (by back calculations). My process side heat duty is 25 MM kcal/hr. This heater's convections section is common with another heater and flue gas of that heater (@ 800 deg.C) enters below the convection section of the concerned heater. The stack temperature is currently 280 deg.C.
Accounting for all the losses my Theoretical Heat Available for absorption by process side is 29.7 MM kcal/hr.
So I am not able to account for the 5 MM kcal/hr of heat.

In the turnaround we have changed the burner also. So my question is that is it possible that even with excess air available there can be chances of incomplete combustion to account for that 5 MM kcal. What can be the possible reasons for that??? Can some fault in burner designing cause that?

Regards

[Root]

Dear all,

Currently I am working on a project to evalute the Efficiency of the fired heater , as we changed to NG from Fuel Oil.

The current firing rate of NG is 3500 Sm3/hr and combustion air flow is 45000 Nm3/hr. NG pressure is 0.6 kg/cm2g and the flow and pressure matches according to the heat capacity curve (by back calculations). My process side heat duty is 25 MM kcal/hr. This heater's convections section is common with another heater and flue gas of that heater (@ 800 deg.C) enters below the convection section of the concerned heater. The stack temperature is currently 280 deg.C.
Accounting for all the losses my Theoretical Heat Available for absorption by process side is 29.7 MM kcal/hr.
So I am not able to account for the 5 MM kcal/hr of heat.

In the turnaround we have changed the burner also. So my question is that is it possible that even with excess air available there can be chances of incomplete combustion to account for that 5 MM kcal. What can be the possible reasons for that??? Can some fault in burner designing cause that?

Regards

Heater Efficiency=Total heat absorbed/ Toal Heat given*100
This quick estimation and for detail calculation difference is very small compre to above equation.
Toor

[JMW]

What oil is this? Is it a heavy fuel oil that requires heating?

[kkala]

My experience on fired heaters is quite limited, so below is only an opinion on the subject.
1. Escape of unburnt NG (say 580 Nm3/h) is unlikely, since this would have been after-burnt downstream of burners; possible consequences: corrosion on convection section fins, smoke, reduced draft, even visible flame from stack. So such a big gas loss would have been detected (Leadermann's book, title something like "Guide to Process equipment").
2. "This heater's convection section is common with another heater and flue gas of that heater (@ 800 deg.C) enters below the convection section of the concerned heater. The stack temperature is currently 280 deg.C."
Situation seems not clear (no downstream battery limit for the two heaters). Probably you can start with a heat balance of both heaters working simultaneously. Or with the other heater out of service (there must be isolation dumper on other heater's flue gases to the concerned heater).
3. An elementary balance: NG in (assumed as pure CH4) 3310 Nm3/h, combustion air in 45000 Nm3/h, flue gases out 280 oC. LHV of CH4 is 8550 kcal/Nm3, so:
Heat in 3310x8550 kcal/h = 28.3 MM kcal/h .
Enthalpy of flue gases out (approx) (45000+3310)x(29/22.414)x0.3x 280 = 5.2 MM kcal/h (enthalpy base 0 oC)
Enthalpy of gases (air+NG) in (45000+3310)x(29/22.414)x0.25x 20 = 0.3 MM kcal/h
Heat to process (neglecting heat losses) 28.3 + 0.3 - 5.2 = 23.4 MM kcal/h, versus 25 MM kcal/h realized.
Do I miss or oversimplify something?

[breizh]

Hi ,
Additional informations available here , look inside Publications ( left)

http://www.heaterdes...com/design0.htm

Hope this helps

Breizh

Edited by breizh, 25 June 2011 - 08:04 PM.

[sheiko]

Accounting for all the losses my Theoretical Heat Available for absorption by process side is 29.7 MM kcal/hr.
So I am not able to account for the 5 MM kcal/hr of heat.

How did you get this 29.7 MM kcal value? What value have you considered for casing losses and flue gas losses?

Edited by sheiko, 25 June 2011 - 09:05 PM.

[saurabh singh]

Accounting for all the losses my Theoretical Heat Available for absorption by process side is 29.7 MM kcal/hr.
So I am not able to account for the 5 MM kcal/hr of heat.

How did you get this 29.7 MM kcal value? What value have you considered for casing losses and flue gas losses?

Hi,

I have also considered the heat input from the flue gases which are coming from the other heater in to the convection section and the enthalpy of this flue gas is around 4.6 MMkcal/hr.
I am attaching the file for the calculation I have done.

Also I have considered standard conditions as 0 deg.C and normal conditions as 20 deg.C, thus the difference in calcuations.

Regards,

Attached Files

•  Heater efficiency.xlsx   11.14KB   54 downloads

[kkala]

Commented "XI0000012.xls" (sent by saurabh singh) is attached, indicating mainly two correction points to my understanding. Hope it is useful.

Attached Files

•  Xl0000012.xls   32KB   26 downloads