Don't know if anyone has really addressed this yet but here's my take. We opened a school here in Rochester, NY in January 2012 and had to get approval from our town. This is an excerpt from the report we submitted for approval, keep in mind the actual numbers used are specific to our shop and NY but it shows the concepts:
Recommended Rate of make-up air for Smithies:
The recommended rate of make-up air for smithies is given as (6-7 liters/sec per square meter) 1.18-1.38 CFM per sq ft[1]. This recommends the shop have 2950 to 3450 CFM for the space. The supply of outside air for combustion alone provides 400 CFM. When the combustion intake air is combined with the natural ventilation provided by the two doors there is sufficient air exchange. For an example of natural ventilation consider a 15 mph wind[2] blowing directly through a window with an open area of 36 sq ft can move 25,000 CFM or more through the building if the air can escape through a second doorway or other large opening.
Qwind = K x A x V, where
Qwind = volume of airflow (m³/hr)
A = area of smaller opening (m²)
V = outdoor wind speed (m/hr)
K = coefficient of effectiveness
If it is assumed in the winter the man door is fully closed and the overhead door is open between 6” to 8” (5 to 6.67 sq ft) the natural air flow will be between 2,461 to 6,577 CFM.
Q = 0.4 (45 degree incidence) x 0.464 (0.620) sq m x 22,530.8 m/hr
Q = 4,181 (5,588) meters cubed per hour = 2,461 (3,289) CFM
Q = 0.8 (90 degree incidence) x 0.464 (0.620) sq m x 22,530.8 m/hr
Q = 8,363 (11,175) meters cubed per hour = 4,922 (6,577) CFM
This gives the shop a total airflow of between 2,861 to 6,977 CFM in the winter.
Propane combustion air:
For proper combustion and safety precautions it is necessary to provide the smithy with appropriate openings for fresh air supply while using the propane forges. If the combustion air supply is limited incomplete combustion can occur, producing carbon monoxide.
NFPA 54 section 5.3: Air for Combustion Required by Gas fuel burning devices:
Minimum No. openings required 2
The facility has two exterior doors and an air intake fan to supplement the indoor air for combustion during use of the propane forges.
Outdoor opening (sq. in./Btu h) 1/4000
To verify that enough air is being provided with proper openings the amount of Btu’s generated needs to be calculated. Since it is known that the proposed burners will use about 1 pound per hour during typical use and that propane holds 91,547 Btu’s/gallon and weighs 4.24 pounds/gallon usage can be determined.
91,547 Btu’s per gallon/4.24 pounds per gallon = 21,591 Btu’s/hr per burner
With 2 burners per forge and 5 forges there will be 21,591 Btu’s/hr/burner * 10 burners = 215,910 Btu’s/hr
An outdoor opening of 1 sq inch per 4,000 Btu’s/ hr is required:
215,910 Btu’s/hr/4,000 Btu’s/hr/sq in = 54 sq inches
Using the 120 sq ft from the facility doors plus 28.274 sq inches for the 6” air intake for a total of 17,308 sq inches
There is not any permanent duct work proposed for the propane forges since they are portable appliances. To ensure that the forges are not overburdening the space or depleting the environment the volume of air used for combustion needs to be calculated.
It is known that propane needs 1 CF of room air per 1 CF of propane burned for ideal combustion, a simple 1 to 1 ratio. Propane creates 2,504 Btu’s per CF burned.
Having already calculated that the forges could burn 215,910 Btu’s/hr it would follow that 215,910 Btu’s/hr/2,504 Btu’s/CF = 86.23 CF/hr of propane is needed for all 5 forges.
We have established a 1 to 1 ratio so 86.23 CF of combustion air will be required every hour.
Over the course of the 8 hour day the forges will utilize approximately 800 CF of combustion air, <0.02% of the shop volume. This translates to about 2 CFM, the 6” air intake fan that was over sized for the coal forges can provide up to an additional 200 CFM of fresh air to the room. To additionally ensure that the environment stays safe the amount of Carbon Dioxide, CO2, created during the ideal combustion process needs to be determined so the amount of Carbon Monoxide, CO, produced can be estimated. CO2 is the final product of all hydrocarbon combustion. With ideal propane combustion, nearly all carbon in the fuel is emitted as CO2, minor amounts, typically 0.01%, are emitted as CO. So during inefficient combustion we can estimate up to 10% of the anticipated CO2 becoming CO.
Carbon Dioxide, CO2, emission from propane is 3 pounds of CO2 to 1 to pound of propane. The forges are burning about 10 lbs/hr resulting in about 30 lbs of CO2 each hour.
100 CF of CO2 weighs 12.36 pounds,
So 30 pounds of CO2 amounts to (100/12.37)*30 = 242.5 CF of CO2
With propane combustion, nearly all carbon in the fuel is emitted as CO2. Of the remainder minor amounts, typically 0.01%, are emitted as Carbon Monoxide, CO. This means that in an ideal system the created CO2 concentration would be less than 0.006% of the shop area volume each hour and less than 0.05% of the shop area for the day with the amount of CO created up to 0.00007%.
The burning of propane is:
C3H8 + 5O2 ---> 3CO2 + 4H2O
The incomplete combustion of propane is:
2C3H8 + 7O2 ---> 2C + 2CO + 8H2O + 2CO2
Most people will not experience any symptoms from prolonged exposure to CO levels of approximately 1 to 70 ppm, 0.007%. At sustained CO concentrations above 150 to 200 ppm, 0.015% disorientation, unconsciousness, and death are possible. To doubly ensure the safety of those within the space there will be 2 CO detectors placed within the forging area.
[1] Guideline from The Engineering Toolbox at www.engineeringtoolbox.com/ventailation-air-flow-rate-d_115.html
[2] Average Rochester, NY wind speed in the winter is 14 mph and the summer is 10 mph.
So once you calculate numbers for your usage you should be able to design/size an exchange/exhaust system to accommodate your specific usage. Typically if the forges are to be stationary an overhead fume hood for exhaust is utilized and is designed to draw air based on the calculated rates. In conjunction with the hood you need to supply combustion air as well as room replacement air with some sort of intake vent/system. If your forges are portable you need to ensure that the intake system and the exhaust system adequately cross the space to ensure the air is being moved and mixed through the room.
-c
