Duct Design Upstream of HRSG Help 1

[ctmecheng]

Hello,

I am a young mechanical engineer that is starting a new project to design a new section of duct that will be bypassing and existing stack and rejoining an incinerator outlet just before a new HRSG unit. The duct is 12'-6" in diameter and the air in the duct at the tie-in is 1000ºF. The purpose of this design is to maintain the temperature as it flows into the HRSG, the additional duct from the tie-in to the HRSG will be approximately 300'. From some research it seems that most materials experience embrittlement or precipitation at long term exposure to elevated temperatures. From that I believe it would not be wise to have the duct insulated externally. I think it would be best if the duct was lined with insulation to minimize heat loss and have the duct material exposed to a much lower temperature. I have ordered "The Structural Design of Air and Gas Ducts for Power Stations and Industrial Boiler Applications" and started reading through it to try and expand my understanding of this topic. I am having trouble finding a good starting point and would like to get some advice from some more experienced professionals.

I know this is a broad request but any help is appreciated.

Thank you.

 

[racookpe1978]

300 feet! How will you keep the inside exhaust air temperature hot that far?

Draw your problem: Plan AND Elevation - to scale as best possible. Too many other issus going on to think abou a single best answer. Yet.

 

[MJCronin]

racookpe is right ..... 300 feet is a lot of duct, but not as long as the following project:

https://www1.nyc.gov/html/dcp/pdf/env_review/riverside/appendixf_fseis.pdf

(You should notice that due to the expense, this duct project report was performed by Senior engineers at a Nationally recognized consulting firm, not by an inexperienced young mechanical engineer starting a new project ..... I can only conclude that your boss has an MBA)

Longer hot duct produces an expensive system pressure drop that must be considered in an economic evaluation

Was the newly revised plant arranged by a new CAD designer ???? Usually, revised plant arrangements are made to minimize this expensive duct length even to the point where buildings must be demoed to facilitate the function of a new system.

With this long high-temperature duct run, you can expect to spend a lot of money on expansion joints.

http://www.effoxflextor.com/pdf/Effox_Flue_Duct_Expansion_Joints.pdf

MJCronin
Sr. Process Engineer

 

[ctmecheng]

Racookpe1978, That is one of the items we are trying to determine. I am thinking a refractory lining for duct, but more work will need to go into determining the best form of insulation.

MJCronin, I agree with you 100% that this should be done by senior engineers, but this was a project I was given and I am trying to do this at the best of my ability. I believe we have contracted another firm to help verify the validity of the design. However, I am trying to gain some ground by finding a good starting point for my design.

The arrangement is driven by the current configuration of the plant. The new duct will be bypassing the existing stack and then rejoining the incinerator exhaust, which then will enter the HRSG. The duct length is already close to a minimum.

MJcronin, if the duct is lined with refractory and the duct material experiences a much lower temperature will that not lower the amount of thermal expansion the duct experiences?

Thank you for the help.

The attached image is just a proposed routing with dimensions.

 

[ctmecheng]

other image

 

[georgeverghese]

For pressure drop calcs, use an isothermal compressible flow expression

For heat loss to ambient through this duct at winter conditions with normal max wind speed, you can read up DQ Kern's Process Heat Transfer - also account for the external htc at the air - insulation jacket interface.

For duct pressure drop, would suggest we allocate an amount no more than say 10% of the dp through the HRSG. But you also have to see what the effect of total dp is on the upstream.

For heat loss, you could allocate an amount equivalent to say no more than 5% of the heat recovered at the HRSG.

These would give you approx sizes to work with. For better results, an economics analysis would be required as suggested.

The ducting should be free draining to avoid internal corrosion when the duct cools down on shutdown. Do you have SO2 in this flue gas ? At these temperatures, mineral wool insulation may be appropriate. In the long term, pipe would be preferable to flimsy thin wall ducting, but pipe wont be possible at 12.5ft ID. Find a way to avoid corrosion under insulation if this is at a coastal location.

 

[curtis2004]

Just a few thoughts:
1. Material for the duct is probably 1/4" thick (as minimum) ASTM A36 with rolled angle (or C-Channel) reinforcement "skeleton".
2. You have to design structural supports also. Because of size of the duct, you need to consider dead weight, seismic, wind loads on your duct and also structural supports.
Structural Engineer need to review your design.
3. Internal refractory may seem smart idea, however, it will be the way expensive than mineral wool insulation with corrugated AL cladding outside of the duct. For 1000 F mineral wool is OK.
4. You will need expansion joints. Once you figure out structural support locations, and have an idea how much your duct is growing, you can spec out quantity and quality of your expansion joints.
5. When calculating the coast of your duct, do not forget man doors, dampers ( maybe gulliotine dampers), instrumentation and controls.

Regards,
Curtis