The starting point in any combustion system is the supply of fresh air. To avoid serious problems, the boiler must have an adequate supply of fresh air and a supply system that does not affect the boiler operation. An adequate, proper and consistent supply of fresh air must be supplied into the boiler room for burner combustion air. A rule of thumb is to provide four to six square inches of open, unrestricted area for every boiler horsepower. Example: For a 75 HP boiler, 300 to 450 square inches of open unrestricted area are recommended.
How much air is required?
In general, the following formulas have been developed to determine the minimum amount of combustion air required for any boiler room with a package firetube boiler firing gas or oil fuels:
NOTE: We recommend at least (2) Louvers for a single boiler application. However, for more than one boiler installation we recommend at least one louver per boiler. The end-user may like the louvers tied in to each of the boilers control scheme.
| CFM |
|
| Combustion: |
Air = BHP x 8 CFM |
| Ventilation: |
Air = BHP x 2 CFM |
| Total Required: |
Air = BHP x 10 CFM |
BHP refers to the total maximum boiler horsepower in the boiler room.
| FPM |
|
| 0-7 Feet Above The Floor: |
250 FPM |
| Above 7 Feet Above The Floor: |
500 FPM |
| *Duct From Air Supply To Boiler: |
1,000 FPM (*See Air Ducting Below) |
These calculations are adequate for installations up to 1000 feet above sea level. For any installation above 1000 fasl, add 3% air for each 1000 fasl. What size of opening to the outside is required?
The size of the fresh air inlet openings and their location are very important. There should be a minimum of two permanent air supply openings in the walls of the boiler room. Whenever possible, they should be at opposite sides of the boiler room and no higher than 7 feet above the floor. This will promote mixing with the air already in the boiler room, proper cooling of the boilers, and tempering of potentially colder outside air prior to its entering the burner for combustion.
The air inlets should be provided with some type of weather protection, but should never be covered with a fine mesh wire screen. This type of covering results in poor air flow and is subject to clogging by dust, dirt, paper, and other small items.
*Air Ducting
In some applications, the boiler room is located in a building such that it has no outside walls. Many of these applications do not have sufficient excess makeup air in the factory to allow for combustion air requirements. In these cases, there are two possible solutions:
1. Ducting fresh air into the boiler room. Where this is required, the general rules for the size of the wall opening can be used to determine the size of the ducting and wall opening in the boiler room. In addition, the pressure drop through the duct must never exceed 0.05" w.c.
2. Ducting fresh air directly into the boiler. This method of air supply should be avoided whenever possible. The disadvantages of this type of system far exceed any perceived advantages. If used, the ducting becomes a part of the boiler system and can affect the stability of combustion due to varying weather conditions, wind direction and velocity, humidity, and temperature. Changes in temperature can lead to massive CO production, soot formation, and unstable, unsafe combustion.
*If direct ducting is to be used, we suggest the following steps be followed:
1. Each boiler has its own, completely separate, fresh air ducting and exhaust stack to minimize combustion problems.
2. Boiler directly connected to fresh outside air ducts must be checked for proper combustion adjustment and operation every three months by a certified package firetube boiler specialist.
3. The duct work supplying the fresh air to the boiler must be sized so that it has a maximum pressure drop of 0.05" w.c.at maximum flow.
4. The fresh air supply should have an electric, hot water, or steam heater to temper cold outside air to at least 50 deg F.
5. If the application is utilizing a low emission with flue gas recirculation, do not use direct ducted outside air. The potential problems associated with a standard burner are intensified with a low emission burner.
A Boiler Room Example
| Boiler Horsepower: |
1,000BHP Each |
| Number of Boilers: |
Three (3) |
| Total Boiler Horsepower: |
3,000BHP |
How much air is required?
| A; CFM |
|
|
| Combustion: |
Air = 3,000BHP x 8 CFM |
24,000CFM |
| Ventilation: |
Air = BHP x 2 CFM |
6,000CFM |
| Total Required: |
Air = BHP x 10 CFM |
30,000CFM |
BHP refers to the total maximum boiler horsepower in the boiler room.
| B; FPM |
|
| 0-7 Feet Above The Floor: |
250 FPM |
| Above 7 Feet Above The Floor: |
500 FPM |
| *Duct from Air Supply to Boiler: |
1,000 FPM (*See Air Ducting Below) |
| TOTAL AREA REQUIRED: |
|
| |
| Area (Ft2) = |
30,000 CFM |
| |
250 FPM |
| |
|
|
| |
|
| LOUVER SIZE & QUANTITY: |
| Area (Ft2) = |
120/3-boilers |
| |
|
|
| |
| Area (Ft2) = |
40Ft2 x 3 each minimum |
| |
|
|
Therefore; The minimum quantity and size of the combustion/Ventilation Louvers is three (3) units at 40FT2 each. The top of the louvers to be located no higher than 7-feet above the floor of the boiler room. No additional required to compensate for the elevation of the boiler room. We recommend the combustion air louvers be motorized and wired in to the boiler combustion system to open when the boilers are fired.