heat flux and maximum film temperature for fired heaters - cheresources
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HEAT FLUX AND MAX FILM TEMEPERATURES IN HEATERSTRANSCRIPT
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Heat Flux And Maximum Film Temperature For Fired Heaters18 December 2012
Posted by ankur2061 in Technical
Today I was going through some writeup related to fired heaters from old manuals of a worldclass operations company and found some very goodinformation related to "Heat Flux" and "Maximum Film Temperature" for fired heaters used in various services. Although process design engineersworking for general engineering consulting firms could never think of designing a fired heater on their own, one of the most common activity related tofired heaters would be the preparation of a process data sheet for a fired heater by a process engineer. Data such as average and peak heat flux andmaximum film temperature would be required to be filled in the datasheet for a fired heater.
Before going into the figures for heat flux and maximum film temperature it is important to understand what these terms mean in context to a fired heater.
In general heat flux is the main determinant for the tube area of a fired heater. It has the units kW / m2 in SI units and Btu / hr*ft2 in English units. It is the rate of heattransmission through the tubes into the process fluid. The area used in the specification of heat flux is usually the outside tube area in the radiant section. Heat flux canbe classified as the average heat flux and the peak heat flux for a fired heater.
Average Heat FluxThe average flux is obtained by dividing the duty of the radiant section by the total exposed outside tube area. This approach is valid for "wellmixed" fireboxes wherethere is little variation of flue gas temperature throughout the box. (Steam reformer heaters in hydrogen, ammonia and methanol plants are an important exception).
If the flux is too high in the radiant section of an existing heater, the only way to decrease it is by reducing the firing rate.
Peak or Maximum Heat FluxSuppose a bank of tubes, with a centertocenter spacing of two tube diameters, is placed one or two tube diameters away from a refractory wall and is uniformly firedfrom the opposite side of the wall then the side of the tube facing the flame will receive direct radiation and the opposite side will receive radiation reflected from thewall. The hottest point on the tube will be on the side facing the flame where the surface is at a right angle to the flux. This is the peak or maximum heat flux and it willbe about 1.8 times the average value.
When the bank of tubes is placed in the center of the firebox and is fired uniformly from two sides, the ratio of peak to average flux will be 1.2. This illustrates theadvantage of twosided or double firing. Thus for a given peak flux, the average flux will be 50% greater than in the case of single or onesided firing.
For most fired heater applications the peak flux values are in the range of 47 to 69 kW/m2 (15,000 to 22,000 Btu/hr*ft2) and average flux values are in the range of 25 to
47 kW/m2 (8,000 to 15,000 Btu/hr*ft2). Refer the attached table for average heat flux values.
Maximum Inside Film TemperatureFlux, either peak or average, is a useful parameter to fix when specifying a fired heater for design. Excessively high flux will cause tube overheating and even lead todamage of the heater refractory walls. But what is really of concern from the process view point is the maximum inside fluid film temperature. This is the maximumtemperature that the fluid sees.
The maximum temperature difference across the inside film can be calculated as the peak flux (adjusted to the basis of the inside tube area) divided by the inside(convective) heat transfer coefficient. This difference when added to the maximum process bulk temperature provides the maximum film temperature. Recommended
maximum film temperatures for some applications are provided in the attached table. Inside heat transfer coefficients range from a minimum of 0.11 kW/m2*K (20
Btu/ft2*h*degF) for gases up to approximately 5.7 kW/m2*K (1000 Btu/ft2*h*degF) for boiling water (steam generation).
FoulingFor heaters with with tubes known to be subject to inside fouling, the process engineer may specify a temperature allowance for fouling. This temperature differenceis added to the maximum inside film temperature to get the inside tube metal temperature. It will affect the mechanical design of the tubes. Temperature allowances arein the range of 10 to 38 deg C (50 to 100 deg F). Refer the attached table.
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chem101 Dec 29 2012 05:22 PM
sajju Apr 08 2013 08:19 AM
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kem23 Oct 11 2013 09:26 AM
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sharp fire Oct 14 2013 11:53 PM
Robert74 Nov 02 2013 03:25 AM
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pratyayan Aug 11 2014 12:06 AM
So that is about it for heat flux and maximum film temperature for fired heaters. The table attached for recommended values is the recommendation of a particularcompany. These values may be different based on design and operating experience of another company and hence I advise the engineers to verify the figures forsuitability to the fired heater design objectives of any other company for which they are providing the data.
Hoping to have some comments on this entry from the readers of my blog.
Regards,Ankur.
Ankur,
I really appreciate that you take time and write these beneficial article for young engineer like us.
thank u for ur valuable info. kindly reply me back with the process design calculation for indirect water bath heater if possible.
sir, how can i size the shell length and diameter of water bath indirect line heater
Thanks a lot sir for sharing this valuable info with us. Could you please post How to calculate Heater Efficiency/duty and Burner efficiency as well. Thanks in advance
Dear Sir,Many thanks for the great information. If you have a basic procedure for the mechanical and process design ofthe fired heater for the crude distillation please upload it. ThanksRobert
Where do I find the values of film coefficients for typical process heaters ? I was trying to calculated the peak film temp for a Visbreakerfurnace as per API 530. It needs the heat tranfer coeff for both liquid & vapor phase at the bulk temp. However, I am unable to get that. If you can point me to any source, that will be wonderful regards Chakraborty
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Guidelines / Rules for PostingCheresources.com Community → Community Blog → Ankur's_Chemical_Engg_Blog → Heat Flux And Maximum Film Temperature For Fired Heaters
ankur2061 Aug 11 2014 01:29 AM
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Akalabeth Dec 18 2014 11:12 AM
pratyayan, on 11 Aug 2014 10:46 AM, said:
Where do I find the values of film coefficients for typical process heaters ? I was trying to calculated the peak film temp for a Visbreaker furnace asper API 530. It needs the heat tranfer coeff for both liquid & vapor phase at the bulk temp. However, I am unable to get that. If you can point me to any source, that will be wonderful regards Chakraborty
Chakraborty, Have a look at the link below: http://www.ijetae.co...TAE_0313_09.pdf Regards,Ankur.
Hi Ankur, thanks for share your knowledge. You know, where i can find information for calculate heat transfer coefficient in fired heater, specifically, convection section. Furnace type: box, with top vertical burners.Application: Steam reforming (methanol plant)Fuel: Natural Gas Coils in convection section: Somes are finned, other not. Mi intention, not are design. Thanks for all.
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