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Join Date: Mar 2017
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Insulation Between ID FAN and Chimney

03/26/2017 3:58 AM

Dear All
I want to know why there is insulation required between ID fan and chimney. In my plant there is insulation between ID fan and chimney. There is no chance of acid dew point corrosion because we use natural gas as fuel which contains no sulphur so there is no chance of foramation of sulphuric acid. Considering this point is insulation required? If yes then why? Is it anything to do with draft or something?
Regards
Pradeep A

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Guru
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#1

Re: insulation between ID FAN and chimney

03/26/2017 4:38 AM

Can you please provide some additional detail?

Are you talking about electrical insulation or thermal?

What is the wiring code expectation where you are?

Is it possible that the airflow could include solvent?

Is it possible that what you see as insulation is just an expansion joint of "insulating" material?

Fans, conductive ducting/chimneys, electricity with ot without solvents create multiple failure modes.

Fan short to "chimney" and someone on the roof gets zapped.

Air flow through chimney creates static potential, either with sparks or equipment shutdowns.

And so on.

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#2
In reply to #1

Re: insulation between ID FAN and chimney

03/26/2017 7:25 AM

I am talking about thermal insulation not electrical insulation

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Guru
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#3

Re: insulation between ID FAN and chimney

03/26/2017 7:30 AM

Natural gas exhaust contains a lot of moisture which will condense in cold ductwork.

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#4

Re: Insulation Between ID FAN and Chimney

03/26/2017 8:15 PM

pradeep,

A hotter flue gas stream means a deeper chimney draft, which is desireable.

There may be insulation in this area for personnel protection also.

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Guru
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#5

Re: Insulation Between ID FAN and Chimney

03/27/2017 3:21 AM

<...is insulation required? If yes then why?...> The insulation is there primarily to maintain the temperature of the flue gases so that, by the time the gases arrive at the chimney top, they remain above the dew point of sulphuric acid, and the top of the chimney is thereby protected from rotting. Secondarily it is there for personnel protection, partly to provide better conditions in that location and partly as protection from inadvertent contact. Thirdly, see #4⇑.

<...natural gas as fuel which contains no sulphur...> Nonsense. What happens if the source varies (rhetorical question - NNTR)?

Oxides of nitrogen can also be present, which will contribute to equipment rotting.

Discuss the matter with the boiler maintainers and the reason for the insulation will become evident. Any proposal to remove it may well be blocked as nonsensical.

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#7
In reply to #5

Re: Insulation Between ID FAN and Chimney

03/27/2017 1:51 PM

No question in science and technology is nonsense which you are saying every body has different questions in mind and they are free to ask. What is nonsense about it?

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Guru
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#8
In reply to #7

Re: Insulation Between ID FAN and Chimney

03/29/2017 4:44 AM

The statement <...natural gas as fuel which contains no sulphur...> is nonsense. Natural gas has mercaptans added to it, which contain sulphur.

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#6

Re: Insulation Between ID FAN and Chimney

03/27/2017 9:04 AM

Dear Mr. pradeep4u,

The insulation between ID Fan exit and Chimney is very much required - to avoid Temp. Drop in the flue gas for two reasons.

1. To maintain the DENSITY DIFFERENTIAL which is directly linked to draught.

2. Personnel Safety to prevent burn injury.

The Draft created by a chimney depends upon the difference between the flue gas exit gas temp, the ambient temp. and the height of the chimney.

The higher the difference of the flue gas exit gas temp, and the ambient temp - the higher the NATURAL DRAFT created.

Higher the chimney height, higher the difference in ability to create the natural draft.

The equation governing the Temp. of of flue gas and height of the chimney is given in Standard Text Books of Thermo Dynamics, Book on "COMBUSTION FOSSIL POWER SYSTEM" EDITED by Mr. JOSEPH G. SINGER, published by COMBUSTION ENGINEERING Inc. USA.

I have the formula and retrieve from my archives and post here later.

Pressure differences resulting from various temperature differences and various stack heights, in pascals

Notes:

1. The red figures in the body of the table are the number of pascals (Pa) of pressure difference that result from the intersecting temperature difference and stack heights. One Pa is equal to 0.004 inches of water column ("wc).

2. The lower two rows of the table body can be used to roughly estimate total stack effect (- & + values) in houses during cold weather.

3. A single point flue gas temperature measurement, usually at the flue pipe, will give a higher temperature than the average for the total system because of heat loss through the chimney, and therefore will inflate predicted draft levels unless a correction is applied.

4. Combustion and venting system height measurements should be taken from the base of the firebox; i.e. from where it gets hot.

The chimney's function is to produce the draft that draws combustion air into the appliance and safely exhaust the combustion gases to the outside. But a good chimney must do more than that. It must:

  • protect the house structure from the hot gases passing through it;
  • tolerate the high gas temperatures that can result from chimney fire;
  • conserve flue gas heat to produce strong draft;
  • resist corrosion on the inside and weather effects on the outside; and
  • be sealed to prevent leakage.

DHAYANANDHAN.S

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dhayanandhan (1); Just an Engineer (1); MJCronin (1); pradeep4u (2); PWSlack (2); TonyS (1)

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