Air Intake for Boilers

[Joesteam]

There is a boiler room with three new forced draft oil burning boilers (2 ea. 300 HP and 1 ea. 100 HP). I was asked to provide the air intake fan. These boilers will be sequenced according to laod. Any experience with modulating vs. single speed fans and sizing of the air intake system will be appreciated.

 

[friartuck]

Hi Joesteam

I have a spreadsheet based on BS6644 for boilers over 60kW. Its very simple and works out mechanical and or natural ventilation but I dont know how to append the excel file.

Anyone got any ideas?



Friar Tuck of Sherwood

 

[TBP]

Most jurisdictions have a fuel code, which will usually specify combustion air intake requirements. Or, the boiler manufacturer will often specify what's required for his equipment.

 

[25362]

Joesteam,

I'd recommend a small handbook titled Fan Engineering published by the Buffalo Forge Company. It contains excellent Chemical Engineering information. Its Chapter 15 deals with the various forms of fan control: variable pitch, variable inlet vanes, dampers (inlet and outlet), variable speed and even number of fans, that may interest you.

 

[quark]

Why should you provide separate intake fan? Don't you have fans fitted to your boiler?

What is your idea? Do you want to have single variable flow fan for all the three boilers? With single fan, the excess air control may become difficult.

 

[Joesteam]

This is air for the boiler room, the boilers themselves have thier own fans.

 

[quark]

I generally prefer good natural ventilation with wide open areas. If it is not possible, and if you on/off the boilers, then a low cost option will be to go for a two speed fan. You can have variable speed fans but precise control may be a difficulty(actually, to decide what should be the control signal is an issue). You may have to calculate the speeds initially(with respect to no. of working boilers) and feed to a controller.

 

[pmover]

bottom line...

determine air requirements at 100% boiler load.
size air intake based on X velocity or Y [&Delta;]P across air intake. recommend not placing boiler room static pressure < atmospheric pressure as this can impact boiler combustion.
if installing louvers, i'd also consider placing interlock on louvers such that boilers do not operate with louvers in closed position.
if in US, contact your local boiler inspector for further direction.

good luck!
-pmover

 

[friartuck]

Joesteam

I cant send you the relevent British Standard Excel spreadsheet I have, but I can summarise the basics for you.

The BS6644 relates to boilers over 60kW.

For natural ventilation only you will need ventilation for combustion and to control boiler house temperatures. Natural openings should be at high and low level. The high level vent also allows the escapes of natural gas in the inadverdent event of a leak.

So:

HL Vent=270cm2 + 2.25cm2 per kW input (over and above 60kW)
LL Vent=540cm2 + 4.5cm2 per kW in excess of 60kW

These are free areas. Typical louvre areas are 50% free so the above results should be doubled to give overall louvre sizes.

Where you wish to use mechanical input to the plant room, you need to :

supply 1100 litres/sec per kW
extract 450l/sec per kW (input for natural draft blrs)

or for forced draft you:

supply 900l/sec per kW
extract 600l/sec per kW input

you really need the local codes but these will give you a good start.

PS it is also permissible in the UK to supply only and exhaust naturally, provided the exhaust is at high level and doesn't create a high plant room pressure.

ciao

Friar Tuck of Sherwood

 

[remp]

Those figures above seem very high. For a forced draft boiler you have a supply of 900l/s per KW. Lets say we have a 500kW boiler, these calcs say you need 450m3/s supply air. quite a big fan...!!!!!

 

[friartuck]

I wondered how long it would take for someone to spot my 'deliberate' mistake. Perhaps per MW might make more sense.

If however you decided to use per kW, I know a really good fan manufacturer who can supply 'woppers'. Well spotted that man.



Friar Tuck of Sherwood

 

[remp]

Who?, Boeing!!!!

 

[friartuck]

All right billyq, perhaps you might need 10 fans at 45m3/sec but I kinda like the idea of the jet engines. I wonder??????

Friar Tuck of Sherwood

 

[ChasBean1]

International Mechanical Code (2000) requires 1 cfm room make-up air for forced draft systems for each 2,400 BTU/h boiler rating. This would be about a 750 cfm fan for your application. I wouldn't play around with variable speeds for this small a fan. Might even make the fan 1,000 cfm as a buffer and put some gravity relief dampers in. Any time a boiler calls to fire, have the fan come on (and prove on in order for the boiler to fire).

This is a good application for an H&V unit with a steam coil, preferably with face & bypass dampers. You could use the unit also to heat the space in the winter.

Best of luck,
CB

 

[DUMechEng]

correct me if i'm wrong, i'm a recent graduate just starting in the hvac industry, but isn't a boiler hp different from a hp? i think its 1 boiler hp = 33,475 btu/hr if so then 700 hp x 33,475 btu/hr/hp = 23,432,500 btu/h. 23432500 btu/hr / 2400 btu/hr/cfm = 9764 cfm. he would need more like a 9800 cfm fan not 750. at this point would it be worth staging the fan, or mabye even two smaller fans? i would imagine that in a building with 700 hp worth of boiler this fan is a pretty small slice of the buildings total energy use.

 

[ChasBean1]

DUMechEng- you may be right about the conversion. Before I posted this I checked with my neighbor who mentioned a direct conversion, but I was skeptical as I wrote it. If that's correct, thanks for clarifying. -CB

 

[walkes]

If this is a combustion air fan you can contact an Exhausto Representative. They have systems that provide variable combustion air to the boiler room based on the boiler requirements. The chimney is also fitted with a variable speed fan to maintain negative stack pressure to ensure that with any or all of the boilers operating the stack pressure is appropriate.

Check out

http://us.exhausto.com/