Exhaust Flue Gas - Duct Material

Hi capri,

You are basically talking about introducing an 'economiser' into the flue ducting to improve the boiler efficiency. As the 230C inlet temperature is well within the capabilities of MS ducting I would certainly not be specifying anything as exotic as SS unless the fuel being burnt has an extremely high sulphur content with the possibility of the acid dew point being exceeded within this section. If this was likely to occur then I doubt that you would be contemplating such a heat recovery device any way.

You may be a little more cautious of the down stream conditions ,post HE, as there may be similar problems depending on the exit temperature. Do you know what the exit conditions will be?

If you are still above the condensation temperature the the MS ducting could still be the best option. You may need to lag it to keep the surface temperatures above the critical dew pt. I would not chose galvanised steel under any circumstances as the flue gases will be guaranteed to remove it quite quickly and therefore irs a waste of money. It is generally better to specify heavier gauge MS plate rather than anything more exotic.

Hope this helps,

Massey.

How long do you expect this recovery system to last?

It only a casing. You should have some numbers on the savings this should help your decision, unless your design only consists of mechanical fabrication drawings and no efficency numbers.

p911

There is an assortment of ceramic insulating sprays for hot, corrosive flue ducts. Very effective. The ducts can be made from regular mild steel. Consult Delta Thermal Services.

Thanks Massey, P911 and the guest. You guys are very helpful.

P911, I would like that to be in service for atleast five years.

Capri

Hi capri,

The mild steel ductwork is more durable than most people admit. In situations where you think there might be an issue with corrosion it might be prudent to use 6mm plate this is also appropriate for rectangular X-sections. Otherwise spiral or rolled and seamed sheet of 3mm thickness works well.

Coatings can be a mixed blessing because unless they are absolutely perfect (and what is?) then you can easily produce the very condition you are trying to avoid, viz an area under the coating at sub-dewpoint where the liquid can work away at the underlying steel. Not long before a hole appears! I've seen that happen on a municipal incinerator after the heat recoverey section.

Your mild steel will gain a rusty appearance but this layer of oxide will actually reduce the rate of corrosion provided the place is not running in condensate.

The comment about using a spray recuperator and dropping the flue gas to 10C is a very specific application and whilst they use the effluent in a cooling tower it can have other problems associated with the long-term operation of the kit.

Good luck,

Massey.

I agree. Since you are based in India I suggest go through http://www.jyoticeramic.com/download/zircoat.pdf for application of Zirconium coating. Local Jyoti company representative should be able to help you. Good luck.

We have had good success using an insulating coating in heat retention on boilers and other heat producing industrial units such as incinerators and industrial air pollution control equipment.

You can find several of the results and some statements and pictures at http://www.ct-texas.com

We have some of the results from a test conducted in Canada at a steam power plant. Here are a few of the results;

Sootblower pipe - Bare 470F Coated 290F

Fly Ash Pipe - Bare 138F Coated 109F

Hot Air Duct - Bare 148F Coated 122F

Boiler - Bare 620F Coated 370F

On an old hospital boiler in California; Bare metal 267F Coated 155F

Hal

ceramatech_engineering@yahoo.com

Along the same issues regarding expected performance- Do you anticipate that you will drop exhaust gas temp any where close to dew point? If you think that will happen- check the metalurgy of your coils- you may be condensing on them without condensing the total gas mix and will likely need corrosion protection from likely acid produced. As indicated below- have the exhaust go horizontal with a drain if condensation on the tubes is possible to avoid any upstream damage.

Note- normal natural gas exhaust from a boiler condenses at just under 60C. If your feed water is below that temperature (likely) localized condensing on your heat pipe tubes could occur even if you are not below 60C with your total exhaust.

Second- If you think that you will be condensing exhaust gas- allow for the latent heat that will be collected by your heat pipe (12-15% of total thermal input) in your design water flows AND use StStl for the exhaust beyond the heat pipe. Also- have the exhaust go horizontal and provide a drain for the condensate before ultimately exhausting (which can still be vertical).

A system that we use for heat reclaim intentionally reduces exhaust gas temperatures from those similar to yours to as low as 10C. The exhaust from a 4.2 MMBTU (sorry about the "English") input produces 1,400,000 liters of water per year- which we use for cooling tower make-up. The water is slightly acidic (pH = 6.5 to 6.7).

These systems recover about 50-60% of the heat that means the outlet temp of the flue gases will hover around 100-110DegC. The inlet temp of the feed water is at 50DegC (warm enough). So, in this condition do you still suspect that condensation on the tubes would take place? moreover, the heat pipes attains equilibrium temp very fast.

The system is designed with a horizontal passage to the flue gas and with a facility for draining condensate at the bottom inspection cover. The copper heat pipes are also coated for protection.

I am trying to attach an image of the proposed system. I have to go back to the forum where 'Sparkstation' had detailed the procedure. Hope to get back soon.

Capri

50C inlet water will likely NOT cause condensation if you anticipate 50-60% heat reclaim effectiveness but your plan for coated tubes is justified.

Since you are building this product for broad distribution, it is still possible that SOMEONE will install it SOMEWHERE that will have colder inlet water (5C is very possible) SO- with 60% heat reclaim (likely with the lower inlet water) you would see (230-5)x0.6=135C temp drop or 95C leaving air (OK) but the heat pipe tube surface could likely fall below the 60C dew point.

If that happens, you will see more overall heat reclaimed (no noticeable change in air temp) so you will need to specify at least 10% higher minimum water flow to assure that all the reclaimed heat is recovered and the water is not heated above desired temperature.

Thanks energygod, You have been quite specific and to the point.

I understand your concern on condensation over the tubes. You are also quite right in pointing out that temp of inlet water and its flow rate will decide the overall quantity of heat removed for a constant flue gas temp of 230DegC.

That being agreed, the system with a fixed Nos. of Heat pipe can always be specified in relation to these factors - 1. Flue gas inlet temp. 2. Flue gas mass or flow rate (Kg/hr 'or' CuMtr/sec), 3. Inlet water temp and 4. Flow rate of the water. Right?

Thanks once again.

Capri

Whew!! Got it for you all to see.