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Secondary Air Fan in Boiler

02/27/2010 1:06 AM

WHAT IS THE ROLE AND FUNCTION OF SECONDARY AIR FAN IN A BOILER???

IS IT BEHAVES JUST LIKE A FORCED DRAFT FAN???

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#1

Re: Secondary Air Fan in Boiler

02/27/2010 11:54 PM

Hi, Riaz.

Secondary air serves to shape the flames that shoot out from burners, to control flame temperature, and to mix fuel with combustion air.

Depending on the characteristics desired for a given boiler, and depending on type of fuel, different types of burners will be used.

Single-stage burners are designed to mix fuel and air in a single pass, which results in a combustion occuring continuously with the flame produced.

Dual-stage burners are designed to burn fuel in two stages. The first part of the combustion will occur in the first stage, in which some (but not all) of the fuel will be burnt. At a certain point of the first-stage flame more fuel will be injected in, as will more air. That second shot of air is 'secondary air' serves to burn the newly-injected fuel, and it serves to shape the flame and thus teh flame's combustion zone. Sometimes, tertiary air is used, in which case the burners used are three-stage ones.

The reason for multi-staging combustion like this depends on the fuel and on the boiler behaviour that' desired. Generally speaking, burning fuel in several stages reduces the maximum temperature achieved at certain points of the flame, which reduces radiance generated from combustion. In other words, multi-staging will give you more convective heat (hot air) and less radiance (light, visible and invisible, that propagates to furnace walls). Single-staging will give you more radiance because of the flame being hotter (hotter makes higher-energy light ... blue light of a given intensity carries more energy than does red light) and less convection. Boilers are designed with this in mind, and suitable burners are chosen for each one.

The other reason for multi-staging also involves minimizing the maximum heat inside a burner flame. Nitrous oxides (NOx), pollutants formed from nitrogen and oxygen in the combustion air, are created when flame temperature is above a certain limit. NOx is a strong greenhouse gas (310 times as strong as CO2 carbon dioxide), it creates acid rain, and it creates smog.

To minimize NOx formation in the boiler, the idea is to keep flame temperature below the temperature at which NOx starts to form. This is achieved by burning fuel in stages so that we don't get too much fuel burning at a given point inside a flame, causing it to become too hot and forming NOx.

Lastly, secondary air serves to shape a flame. The flame of the first stage of combustion gets shaped by secondary air blowing around it. Secondary air can be directed to blast straight out from a furnace wall, or it can be channeled to swirl around the primary flame. In both cases, it shapes the flame and it provides air needed to burn any fuel that isn't burned in the primary stage.

As for the secondary-air fan ... it blows secondary air through openings in the burners, nothing more. The reason that primary air (and so a single fan instead of two) isn't used for this is that primary air and secondary air may need to be at different pressures and flow rates; it's simply easier to use two independent air streams (and thus, fans) to do that.

I hope this helps. Cheers! DZ

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#2
In reply to #1

Re: Secondary Air Fan in Boiler

02/28/2010 1:09 AM

hi, DreadZontar

Thank you very much for answering of my question regarding secondary air fan in boilers for capacity of 80TPH to 100TPH.

yOU ANSWERED MY QUESTION IN A VERY DETAILED MANNER AND I AM VERY MUCH THANKFUL FOR YOUR THIS KIND ACTION.

I HOPE YOU WOULD CONTINUE TO PROVIDE SUCH GOOD ANSWERS.

BEST REGARDS

RIAZ ALI

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#3
In reply to #1

Re: Secondary Air Fan in Boiler

02/28/2010 1:29 AM

This is even better than a good answer; it is excellent, understandable, and educational. Thanks for taking the time to lay it out in this way.

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#4
In reply to #3

Re: Secondary Air Fan in Boiler

02/28/2010 2:55 AM

HI TORNADO

YEAH DEFINITELY YOU ARE ABSOLUTELY RIGHT THAT THE ANSWER GIVEN BY DREAD ZONTAR IS VERY EXCELLENT AND REMARKABLE.

REGARDS

RIAZ ALI

PAKISTAN

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#5

Re: Secondary Air Fan in Boiler

02/28/2010 6:10 AM

Hi Riaz,

In a pulverised fuel coal-fired boiler (PF) the Primary Air fan transports the pulverised fuel from the pulverising mills via the PF pipes to the burners and has a substantial higher head requirement than simply providing combustion air. The Secondary Air fan (also known as Forced Draft fan) provides the balance, and majority of combustion air. Its only pressure requirement is the flow of air through the ducting and airheater. Even at low boiler loads, the head requirement of the primary air system is similar to full load with additional mills in service. Hence primary air fans in this type of boiler suit centrifugal fans. Secondary air fans can be centrifugal but are more suited to variable pitch axial flow type, providing there is no gas recirculation (with dirty gas and blade erosion) utilised. The secondary air also provides the source of overfire air for NOX control. With sub-stoichimetric combustion at the burner, for lower flame temperature, the adjustable burner vanes admit the additional air at a different point in the flame. NOX emission from a chimney stack is almost invisible but can be seen as a haze in favourable lighting conditions.

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#6
In reply to #5

Re: Secondary Air Fan in Boiler

02/28/2010 6:58 AM

hi AussieBob

Thank you very much for answering my question regarding SA Fan in boiler.

you explained in a very detailed manner and i m thankful to you very much.

best regards

Riaz Ali

Pakistan

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#7
In reply to #5

Re: Secondary Air Fan in Boiler

02/28/2010 7:17 AM

Good stuff. I've yet to have worked with coal-fired boilers, so this is good to know.

Secondary air provides most of the combustion air ... I didn't know that. And overfore air ... coal-fired use that? Circulating-fluid-bed ones will, but will ones with wall-mounted burners?

Thanks! DZ

P.S. Can someone tell me how to rate AussieBob's answer as a 'good answer', please?

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#8
In reply to #7

Re: Secondary Air Fan in Boiler

02/28/2010 7:21 AM

And Riaz ... thanks for the thanks. Much appreciated, Sir!

DZ

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#9
In reply to #7

Re: Secondary Air Fan in Boiler

02/28/2010 8:17 AM

No it is 70-30% approx in favour of PA fan usually.

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#10
In reply to #7

Re: Secondary Air Fan in Boiler

02/28/2010 8:29 AM

HI GUEST

Actually the FD fan forces air from the atmosphere to the system and usually the inlet velocity is taken as zero.Similarly the Sa fan ALSO FORCES THE air but the velocity at inlet cannot be taken as zero,it can be calculated from the flow equation as

Q=AV where Q= flow rate m3/h

A= area of duct at fan inlet m2

V= average Velocity m/s

So i may say that the FD and SA fans have same behaviour.

However the ID fan is different as it induces air from the system leaving behind zero or negative inlet pressure.

Mainly the flow rate is directly proportional to the speed ie RPM

thE STATIC PRESSURE IS DIRECTLY PROPORTION TO SQUARE OF SPEED

THE POWER IS DIRECTLY PROPORTIONAL TO CUBE OF SPEED

THESE ARE CALLED FAN AFFINITY LAWS.

BEST REGARDS

RIAZ ALI

PAKISTAN

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#11
In reply to #10

Re: Secondary Air Fan in Boiler

02/28/2010 6:25 PM

Hi Riaz,

Here are a couple of diagrams of coal fired Low NOX burners in a watertube power plant boiler showing the burner and fuel/air flow arrgt which may be helpful. Cheers, AussieBob

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#12
In reply to #11

Re: Secondary Air Fan in Boiler

02/28/2010 8:37 PM

AussieBob's provided a good example. There's even tertiary air in it.

Tertiary air does the same thing as secondary: shape the flame;swirl/mix air and fuel; and keep combustion zones from getting too hot (in order to reduce NOx).

Good work, team! DZ

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#13
In reply to #12

Re: Secondary Air Fan in Boiler

02/28/2010 10:40 PM

Question for DZ and Aussie Bob.

We have an 8MW coal fired Water Tube Boiler with C Class stoker. We too are worried about NOx emissions and my question is, is it worth monitoring the flame temp to allow SA air adjustment to avoid going over our NOx limit.

Our boiler is computer driven so it would be easy enough to install further controls to avoid bad emissions and perhaps make it more efficient.

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#15
In reply to #13

Re: Secondary Air Fan in Boiler

03/01/2010 5:45 AM

Hi whezmabeer,

Can you fill in a few gaps for background - what manufacture of boiler is it? I am guessing you might be in the canefields or an abbatoir ? I think you are going to find it is going to be too costly for the R&D part on a one-off basis and it isn't the sort of thing you could competently do as a DIY retrofit. There would need to be some substantial modelling of burner configuation, flame pattern and furnace modelling before you could contemplate an installation. There may be an Underwriter and Dept of Labour hurdle to overcome.

I am guessing an OEM retrofit, if available is going to be the most efficient path.

How do you measure NOx levels presently and what are your operating requirements?

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#21
In reply to #15

Re: Secondary Air Fan in Boiler

03/03/2010 7:23 PM

It is a John Thompson. Yes we are an abbatoir. We only have to measure it once every three years. Our release limit is 500mg/Nm3 and 60g/min

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#16
In reply to #13

Re: Secondary Air Fan in Boiler

03/01/2010 8:29 AM

Hi, Whez.

AussieBob's right, your best bet would be to call in a company that can help you control or reduce emissions.

One company, John Zink, has several ways of doing that, including replacing burners with low-NOx ones. The website: http://www.johnzink.com/

It has an affiliate office somewhere in SE Australia, though the map on its website isn't very precise and there's no address on the Contact Us page: http://www.johnzink.com/about_jz/html/about_locations.htm

Cheers! DZ

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#18
In reply to #11

Re: Secondary Air Fan in Boiler

03/02/2010 1:05 PM

hi dear AussieBob

thanks a lot for sending the diagrams.these were very useful and everyone can take advantage from these.

actually i was off from net for the last two days so i m replying you lately.Sorry for late reply.

Dear Mr.AussieBob , Would you recommend the Material "Hardox500" for the impeller blades of the fans especially for ID Fans?? when dust laiden air is to be considered.

best regards

Riaz Ali

Pakistan

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#19
In reply to #18

Re: Secondary Air Fan in Boiler

03/02/2010 1:32 PM

Hi again, Riaz.

I'm an electrical engineer, so this isn't my specialty, but I believe that what you can use is a surface-hardening products of some sort. A few years ago, I read an article that described how the maintenance staff at a paper mill welded thin strips of iron carbide on the tips of their ID fan blades to prevent erosion. Apparently, it worked quite well.

Good luck! DZ

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#20
In reply to #19

Re: Secondary Air Fan in Boiler

03/03/2010 2:13 PM

hi DreadZontar

Well i m mechanical engineer and i m also very glad by knowing that u r electrical engineer.

Well i would like to know about the "reversible motors" the motors which have both forward and reversible directions.these are required for a series of belt conveyors uesd for loading and unloading of trucks etc.

Mr.DreadZontar please tell me about these type of motors.

Best regards

Riaz Ali

Pakistan

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#22
In reply to #20

Re: Secondary Air Fan in Boiler

03/03/2010 8:27 PM

Reversible motors ... easy! Pretty much all motors are reversible; it's usually a matter of how you connect their electrical-power feeds.

The great majority of motors used in industry are 'asynchronous AC motors'. That means that they are powered by AC (alternating current) power. The great majority of those motors are three-phased ones ... there are powered by three power feeds whose electrical signals are 120 degrees out of phase with each other (0 degrees, 120 deg., 240 deg., back to 360 = 0 degrees ... three signals x 120 degrees = full circle). All power feeds have the same voltage, though, and the electrical signal on each feed is a sine wave.

Without going into the details of how these motors work inside, the three power feeds make them rotate. The direction in which a motor rotates depends on the order in which the power feeds, each with a different signal phase, are connected to the motors' electrical connections.

Let's call the power feeds A, B, and C; also, let's call the motor connections a, b, c.

If you connect power feeds to motor connections Aa, Bb, Cc, and turn on the power, the motor will turn one way. If you interchange any two connections, the motor will turn the other way; so connection the motor Aa, Bc, Cb (B and C power feeds have been interchanged to c and b motor connections), will make the motor turn the other way when you turn the power on.

In you interchange two power feeds, you'll again change the direction of the motor's rotation; so Ab, Bc, Ca (the A and B power feeds have been interchanged from the previous example) will make the motor rotate in the original direction.

To find out if a motor has been connected to turn in the correct direction, all you need to do is to start it for a short while (say a second or two) to see. If it turns in the wrong direction, just reverse two power feeds. If you want to check after that, you'll see that the motor will turn in the opposite direction now that you'll have interchanged two power feeds on the motor's electrical connections.

The above is for a motor that's meant to turn in a single direction. The way to make a motor change direction (like on a conveyor, as you say) is to use a 'reversing starter'. Motors meant to turn in a single direction are turned on and off by a mechanical or electromechanical switch called a starter; this starter is just a switch that lets power pass through to the motor if the switch is closed (if contact inside the switch os made between the switch's inlets and outlets). A reversing starter is a switch that also interchanges two power feeds within itself when a control signal tells it to; interchanging the power feeds at the switch makes the motor turn in the reverse direction.

That's all. And one word of advice: always get an electrical engineer to chooses switches, design the power-feed circuits, etc. I've just given a minimum explanation, and there's more to it than that ... but now you know how to make asynchronous AC motors reverse.

Cheers! DZ

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#14

Re: Secondary Air Fan in Boiler

03/01/2010 3:08 AM

Please turn off Caps Lock.

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#17

Re: Secondary Air Fan in Boiler

03/01/2010 1:57 PM

Are you talking about standby FD fan? or ID fan? In general only one FD fan is good enough for a boiler unless the boiler service is critical.

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