8 replies to this topic

[Keith84]

Hi, everyone.

I'm looking for some resources that address design and behavior of forced-draft direct fired heaters. I've been poring through various books, and there's decent information on natural-draft designs, but very little that I can find on forced-draft.

In addition to just needing more design information as a while, I have two specific, related questions that I cannot seem to get answered definitively from my reading:

1) What is the pressure profile of the flue gases in the heater for forced draft? I've read that the flue gases are supposed to be at slightly negative pressure, so no hot gases are pushed out through the refractory to the outer skin. That makes perfect sense to me, since the box is not a pressure vessel or even leakproof. However, I've also read that the negative pressure profile only occurs with natural draft. At least one book says that forced draft has a positive pressure in the box, but, as usual, the information is scant and, in my opinion, sometimes contradictory between books. I don't see how you could have a positive pressure in the box without causing serious safety and refractory problems.

2) How do you design the stack for a forced-draft heater?

While I'm here, I might as well ask a couple of general fired heater questions that have been bugging me:

3) Is the stack usually insulated or not? If so, inside or outside? If not, is it still usually the same material as the rest of the box (carbon steel)? Stack gases are probably going to be around 600-800 deg F by rough estimate.

4) Is the refractory ever shielded from the flue gas? We're planning on using ceramic fiber which is fairly fragile and porous. This comes back to the pressure profile of the flue box. Suppose you had a positive pressure in the box, either continuously or transiently. What if you sandwiched the ceramic fiber between the outer shell (carbon steel) and and an inner shell of stainless sheet metal? The stainless would face the flame (possibly even helping with reflecting radiation onto the backside of the tubes), and maybe could be even be used to seal the refractory and outer shell completely away from the flue gases. Does this have any merit?

I've asked some specific questions, so if anybody has any thoughts on those, I'd be happy to hear them. And if you have any recommended sources of information for answers like these, I'd really appreciate it. I'm looking for specific info on forced-draft, but also just any good info on fired heater design, both thermal and mechanical. Thanks, everyone.

[Art Montemayor]

Keith:

Your post reads like that of a student’s. This is not a negative or demeaning observation, but rather one that gives the impression that you don’t have experience in dealing with or designing fired equipment. Some of the comments you make are wrong or down the wrong path. I have, in my younger years designed, installed, and operated both forced and induced draft combustion equipment – heaters and boilers. I believe I could still do it again if I had the need.

However, today we don’t design or contemplate fabricating such equipment since there are suppliers available who do it for a living – and do it well and it is warranted to perform as expected. You have not explained why you don’t follow this route rather than try to “invent the egg”, so to speak, so I will assume you have a need, period.

I will address your questions with the following responses:

What is the pressure profile of the flue gases in the heater for forced draft?
For any fired equipment, the pressure profile is unique and specific to that piece of equipment. The fabricator is the best one to have that information. If you want the pressure profile of a fired piece of equipment that you have not fabricated yet, you must wait until you build it and can test it.
I don’t know who you have been reading but a negative pressure profile does NOT occur only with natural draft. Additionally, contrary to your beliefs, forced draft DOES PRODUCE a positive pressure in the combustion chamber and all the way out through the flue stack. That is why it is called Forced Draft. All fluid flow requires a driving force and in order to have gases flow out into atmospheric pressure, you must create an internal pressure that is greater than atmospheric pressure – ergo, positive gauge pressure.

How do you design the stack for a forced-draft heater?
In the same manner as you do for an induced draft heater. The fluid flow is the same.

Is the stack usually insulated or not?
My stacks were not insulated and I see no reason for doing so – unless there is a specific need for it, like personnel protection. The stack temperatures of 500 -600 ^oF were within the limits of carbon steel.

Is the refractory ever shielded from the flue gas?
I have never “shielded” (protected) the ceramic fire brick in the combustion chamber – nor in my fire brick walls (where I used them as flame impingement due to the flame’s shape). Again, if you have a specific need for this then you may have to. But I have never seen, heard, or read about this need. Do not forget that the higher temperature of a stack gas is part of the driving force that makes the flue gas rise.
I see no merit is “shielding” the ceramic brick in the combustion chamber from the flame. I have operated fired boilers and heaters with positive pressures of 5 to 15” of W.C. pressure and never had any problems. I believe there are existing combustions taking place in industry at higher pressures and these have to have ceramic or other insulators to protect the outer steel casings or shells of the equipment. Pressure does not do damage to the insulating brick – at least not at the pressures we are talking about: 5 – 20” of W.C.

You have not stated the pressure values you are talking about. What are these pressures that are giving you concern?

[Bobby Strain]

Art covered the basics well. Usually in a heater of any size, forced draft is used in conjuction with induced draft, and, in many cases, an air preheater is used. So, prepare a duty spec for the heater and send it to qualified vendors for quote. Assuming that you are ready to buy.

Bobby

[SSWBoy]

Do be aware that in some places (europe) ceramic fibre is considered carinogenic, therefore its use is not allowed or at the very least should be minimised. Regardless of the health implications, some clients prefer castable refractory as ceramic fibre has a unnerving tendancy to end up in the convection section fins. Depending on the temperature of the firebox, firebrick can be used in place of ceramic fibre in the section of the fired heater which might be exposed to the flame. On low temperature fireboxs i've seen ceramic fibre throughout, on ethylene pyrolysis furnaces i've seen firebrick throughout.

Typically a pressure of 0.1" draft (i.e. negative pressure) is specified at the arch (just upstream of the convection section) of a fired heater. This is the highest pressure point in a heater, if you guarantee -ve pressure here, then everywhere else will be more negative. As you say negative pressure is required to ensure that operators opening peep (observation) doors, do so without losing any eyebrows.

[Keith84]

Thanks, Art, for your thoughtful reply.

I guess I do sound like a student, which is probably pretty typical for me, since I'm perpetually having to learn about new areas at my job. I'm a jack-of-all-trades type here, which has led to a pretty bizarre-looking learning curve. We build a bunch of small stuff at my company, so it's important for me to understand every piece of equipment as much as I can. Prices and delivery can vary pretty wildly at times for complex equipment like heaters. If we're building something small enough or if we're building a pilot plant to test a process, we will sometimes build our own equipment. This is I'm why I'm researching heaters in depth.

The books I've been reading (still very much in progress) are The John Zink Combustion Handbook, the North American Combustion Handbook, and Editions Technip: Petroleum Refining Volume 4. The latter is where I got the confusing language about the positive and negative pressures in the heater.

I don't have any positive pressures in mind that were concerning me from a safety standpoint. I read in the Technip book that positive pressure had potential to damage the insulation or the shell, but it was a vague statement, which is why I wanted to get an opinion from some actual engineers. It's heartening to know that it's not a black and white situation.

I'll report back to this thread when I have a better grasp of the situation. Thanks, again.

[kkala]

Having read the posts of the thread, following views are probably useful concerning forced draft type without induced fan.
1. Flue gases out of the stack are at pressure P= - hρg in reference to ground atmospheric pressure, where h=stack height from ground, ρ=air density at conditions, g=gravity acceleration. Pressure at stack base is -hρg+hρ'g+ΔPf = -hg(ρ-ρ')+ΔPf, where ρ'= (average) stack flue gas density (*), ΔPf=frictional pressure drop along the stack. For refinery boilers in normal operation (burning fuel oil, exit flue gases ~ 145 oC), point of 0 Barg for flue gases is a few meters upstream the stack base, according to vague information from vendors. So a small part of gas path up to the stack is at pressure slightly lower than ambient (negative), while most part of their path is at pressure higher than ambient (positive), starting from the combustion chamber. It is assumed that forced draft heaters have a similar flue gas pressure along their path.
Heated tubes are welded to make a shell, preventing flue gases going out (pressurized type boiler).
2. For stack calculations http://www.cheresources.com/invision/topic/12891-pressure-loss-and-sudden-expansion-of-gas may be useful.
3. Stacks of mentioned boilers are insulated for heat conservation. Estimated acid due point (plus a margin of 15 oC) determines exit flue gas temperature.
4. Fired heater refractory is not "sealed" from flue gas. Refractory can crack if flue gas temperature increases suddenly (e.g. at startup), especially during commissioning when refractory contains much moisture. A specific "dry out procedure" is followed at that time.
Welcomed are comments on above points for a more clear picture.

(*) ρ>ρ', since stack flue gases have higher temperature than air.

Edited by kkala, 03 January 2013 - 02:20 AM.

[Ken_der_Ami]

Keith,

You got some good information, but also some potentially very misleading information.

FYI, a very good overall reference is API Standard 560 - Fired Heaters for General Refinery Service.

Some specific comments / clarifications:

1) It is not clear when you say "forced draft" whether you mean a Fired Heater with a Forced Draft fan only, or a Fired Heater with both Forced Draft and Induced Draft fans. The proper term for a Fired Heater with Forced and Induced Draft fans is "balanced draft". In a balanced draft Fired Heater, as in a natural draft Fired Heater, the firebox should be at a slight negative pressure compared to the outside (ambient) atmospheric pressure.

2) Fired Heater stacks are not insulated for heat conservation. There is no downstream heat recovery (by definition) so therefore no reason to conserve heat. Stacks can be internally or externally insulated for personnel protection. Internal insulation is usually preferred in refinery Fired Heater applications, especially where higher-temperature flue gases are encountered, because with an internal lining a lower-cost carbon steel shell can still be used. Because it is at a lower temperature and heat conservation is not an issue, a thin lining (1-1/2" to 2" thick) of low-cost castable refractory material is typically specified. In very low-temperature climates e.g. North Slope of Alaska, external insulation is more common. Last but not least, from a structural design viewpoint, the presence (or absence) of internal insulation can also impact the susceptibility of the stack to wind-induced vibration. See API 560 and/or ASME STS-1.

3) When you ask "how is the stack...designed" I assume you mean what conditions should it be designed for. Typically, a Forced Draft or Balanced Draft heater will be specified either to shut down completely upon loss of fan, or to operate at some (possibly reduced) capacity under natural draft. In this latter case, the stack would need to be designed to accommodate the (possibly reduced capacity) natural draft mode of operation.

4) Ceramic fiber refractory linings can be "sealed" from the flue gas flow by a variety of means. Where flue gas velocities are low and there are no sharp corners or abrupt transitions, no sealing is required. In areas with slightly higher flue gas velocities, ceramic fiber blanket can still be used with application of a spray-on rigidizer, or ceramic fiber modules can be used. In very high-velocity areas, an alloy-metal "shroud" can be engineered to protect the ceramic fiber refracatory, although a lining of castable refractory or firebrick is also quite suitable and may be cheaper overall.

[kkala]

Some points arisen from post No 7 by Ken_der_Ami are further discussed below.
1. API 560 distinguishes between balanced draft and forced draft heaters in "definition of terms" (May 2001 edition). It does not seem to exclude forced draft heaters from requirement of slight negative pressure in radiant section and downstream. Can anyone advise? Forced draft water tube boiler have flue gas pressure turned negative closer to stack according to vendors' info (post No 6).
2. Stack insulation depends on economics. It may be more efficient in the future, as fuel prices increase.
Mentioned boiler (92.5% efficiency on fuel oil LHV) would keep flue gas temperature practically constant at ~145 oC along all its insulated 60 m stack (vendors' info). This results in higher overall heat recovery (due to higher upstream temperatures) and could be also applicable to fired heaters. Insulation for personnel protection (surface ~ 55 oc) may have a bit higher heat losses, but not much.
Modern fired heaters have high efficiency requirements, hence low exit stack temperatures (e.g down to 121 oC), determined by flue gas acid dew point plus some margin; http://www.cpec.nus.edu.sg/myweb/newsletter/news11/FWE.html - Process Design Considerations. High exit stack temperatures probably concern older heaters, where stacks could be even uninsulated.
3. For forced draft heaters, possibility of reduced capacity through natural draft (when fan is lost) has been heard of.
4. Post no 1 asks whether ceramic fiber could be sandwitched between furnace shell and an internal stainless steel shell, so "seal" means no contact with flue gas. I do not know ceramic fibers, since local heaters burn heavy fuel oil (7.4.19) or fuel gas. Seen refractories (apparently castable) are exposed to flue gases. Dry-out during commissioning faces the risk of cracks at sudden temperature increase, this is the most critical case for them (as understood locally).
Ceramic fibers can be selectively coated by ceramic fiber modules, protective shrouding, castable lining, etc, in areas of high flue gas velocities (7.4.8), yet these local coatings may not be characterized sealants (as above). Stainless steel vapor barrier foil (7.4.18) is not clear to me (clarification welcomed). Castable refractories (10.2.5) can be covered with impervious organic coating after curing (7.3.10).
Figures in parenthesis refer to paras of API 560, 3rd edition (May 2001). Comments on the above welcomed.

[kkala]

.... API 560 distinguishes between balanced draft and forced draft heaters in "definition of terms" (May 2001 edition). It does not seem to exclude forced draft heaters from requirement of slight negative pressure in radiant section and downstream. Can anyone advise? Forced draft water tube boiler have flue gas pressure turned negative closer to stack according to vendors' info (post No 6)....

Furnace Operations by R D Reed (Gulf, 1976) seems to concur with API 560 on the point, noting on "Forced Draft for Air Supply", Chapter 5.
Forced draft will increase the air pressure drop across the burners enough to obtain faster burning for increased heat release. But this must not be done at the expense of pressurizing the furnace. The stack breeching, convection section pressure drop relationship must be such that the stack can still hold a minimum draft of 0.03 in WC at the arch or roof of the furnace.
Above indicates no shell similar to pressurized boilers (post no 6, para 1). Clarification welcomed.