5 replies to this topic

[theleftcoast]

Hello everyone. I am working on furnace sizing and have a couple questions about the equipment capability, efficiency and fuel use. This is just basic sizing to estimate purchased cost and fuel use for a process furnace as part of a senior design project. The main source is "Rules of Thumb for Chemical Engineers" by Carl Branan and the text I reference is from one of the appendices titled "Guidelines for Equipment Decisions" (quoted below). I assume these rules of thumb apply to a furnace that looks something like the sketch below.

For fired heaters: Radiant rate is 12,000 BTU/(h)(ft²); convection rate, 4000 BTU/(h)(ft²); cold oil-tube velocity, 6 ft/s; thermal efficiency, 70-75%; fluegas temperature, 250-350F above feed inlet; stack-gas temperature 650-950F. Approximately equal heat transfer occurs in both sections.

I am confused about fluid allocation because there seem to be two "sections" of a typical process furnace. Does a single process fluid enter the convection section tubes and then go through the radiant section tubes? Does one process fluid circulate in the radiant section with a completely separate fluid circulating in the convection section?

The primary purpose of this heater is to superheat 1.245*10^5 lbs/hr light naphtha vapor from 260F to 600-700F which requires a heat input of about 2.7*10^7 to 3.2*10^7 BTU/hr (duty calculated using HYSYS). A secondary need in my process is 9200 lbs/hr of 30psig steam for reboiler heating duties which will require about 9*10^6 BTU/hr to generate. Can I preheat the feed in the radiant section of the furnace and generate process steam in the convection section? Would it be better to build two furnaces to handle process heating and steam generation separately?

As for fuel use, I know the efficiency of a furnace is related to the temperature of the fluegas that escapes. Most of the plots I found to estimate thermal efficiencies were for natural gas fired heaters (HHV of 1000 for the fuel used on those plots). Since I am using a fuel gas with LHV of about 1400 BTU/scf and cant use those plots for estimation, would it be reasonable to assume a thermal efficiency of 70% for estimating purposes? I was going to calculate the SCF fuel required as follows.

SCF/hr = (.70)(Furnace Heating Duty [BTU/hr]) /(Lower Heating Value of Fuel [BTU/scf])

I know this is kind of a giant post and I am sorry for that. Thanks in advance for anybody who is kind enough to reply!

-JT

[Nikhiln]

Hi,

What you envisaged is the common setup seen in fired heaters in general. The process fluid is sent to the radiant coils, while the convection section coils can be used for any other purpose, like steam generation/superheating or for heating other fluids, preheating combustion air etc. However , it is an equally common setup to see the main process fluid being preheated in the convection coils & then sent for the main heating/reaction in the radiant section. (for instance, in reformers)
It all depends on the optimisation of fuel consumption vs temperature pinch points, like constraints on flue gas exit temperature, mimimum allowable temperature approach in certain coils etc.
You can play around with the configuration to find what is best in your case, satisfying all the constraints.

Cheers.

[theleftcoast]

Thanks for your reply Nikhiln. After switching my google search from "furnace" to "fired heater", I came across the following web page that seems very thorough.

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

This page would for sure help me with detailed design and optimization but that is a little overkill for the design work required for my project. One of my main questions was if a fired heater could handle two fluids or just one and that is now answered answered. If anybody else could still help me with a simple fuel use estimate, I would be very appreciative. Thanks!

-JT

[breizh]

Hi ,

Let you read these papers :
http://www.heatflux.com/articles.html

Breizh

[Nikhiln]

Thanks for your reply Nikhiln. After switching my google search from "furnace" to "fired heater", I came across the following web page that seems very thorough.

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

This page would for sure help me with detailed design and optimization but that is a little overkill for the design work required for my project. One of my main questions was if a fired heater could handle two fluids or just one and that is now answered answered. If anybody else could still help me with a simple fuel use estimate, I would be very appreciative. Thanks!

-JT

Hi again,

I should have written this in my first post itself.
The equation you mentioned for calculating fuel flow is correct, except for a minor flaw. You need to divide by the efficiency value (0.7), not multiply!
The equation mentioned above doesnt take into account the convection section configuration, so what it gives you is just the radiant section estimate. (ie, heat absorbed by the radiant coil only)

A typical value for furnace radiant section efficiencies is around 60%. (ie, using the above formula).But if you consider the efficiency including the convection bank, the overall efficiency could reach around 75-80%. You could try to squeeze it further by optimising the convection bank configuration, but then the limit would be basically set by the exit flue gas temperature, dictated by dewpoint considerations. As a result, the overall efficiency falls in the 70-80% bracket.

Of course there are new developments which push the envelope further using regenerative air techniques and others. But for an initial estimate, the earler values should suffice , I would think.

Hope this helps.
Cheers.

[theleftcoast]

Thank you Breizh and Nikhiln!