An overseas customer had a problem with their coal blasting furnace. As the workers would open a 1.2 meter by 1.2 meter door to shovel in coal, the foundry would fill with smoke. This was a hazard and a nuisance for the crew. They saw articles about how EXAIR Air Amplifiers were used in smaller ovens for exhausting hot flue gases, and they wondered if the EXAIR Air Amplifiers could be used for something much larger.
He sent me an email with some additional details about their furnace system. They had a fan that was mounted in the stack that had a capacity of 50 m^3/min. This was fed into a filtration collection system to remove the residue byproducts. The temperature inside the furnace was approximately 450 deg. C. From this information, I could calculate the required velocity to keep the smoke inside the furnace.
In sizing this application, I determined that I could use an equation from Heskestad and Spaulding. This equation was developed to find the minimum velocity required to keep smoke from egressing into corridors during fires. In this case, we were keeping the smoke from egressing into the foundry. The formula looks like this:
V = 0.64 * Sqrt(g * H * (T – To)/T) Equation 1
V – Velocity (m/s)
g – Gravitational acceleration (9.8 m/s^2)
H – Height of Opening (meters)
T – Avg. Fire Temperature (Kelvin)
To – Avg. Space Temperature (Kelvin)
In this equation, we are mainly fighting the forces of the temperature difference from inside the hot furnace area to the outside cooler area. The outside area was near 40 Deg. C, and this gave me the temperature difference. In converting these temperatures to the absolute temperature, Kelvin. I calculated the fire temperature, T, to be 450 Deg. C + 273 = 723 Kelvins; and the space temperature, To, to be 40 Deg. C + 273 = 313 Kelvin.
In placing the given information into Equation 1, the minimum velocity could be found.
V = 0.64 * Sqrt(9.8 m/s^2 * 1.2m * (723K – 313K)/ 723K)
V = 1.65 m/s
If the velocity could be maintained at this mark of 1.65 m/s, then the smoke could not egress into the plant. They had a stack fan that was flowing 50 m^3/min, or 0.83 m^3/sec. We can determine the velocity that the stack fan was producing by calculating the flow over an area:
V = Q/A Equation 2
V – Velocity (m/s)
Q – Flow (m^3/sec)
A – Area (m^2)
With a door opening of 1.2m by 1.2m, or 1.44m^2, the velocity can be calculated by placing the known values into Equation 2:
V = (0.83 m^3/s) / (1.44m^2)
V = 0.58 m/s
Now we can see why they were getting smoke pluming from the coal furnace into their facility. They required a minimum of 1.65 m/s, and the stack fan was only drawing 0.58 m/s. If we take the difference, we can determine how much additional velocity will be required to keep the smoke within the furnace: 1.65 m/s – 0.58 m/s = 1.07 m/s.
To determine how much air flow would be needed to create a velocity of 1.07 m/s through the door opening, I just had to rearrange Equation 2 to determine the flow, Q.
Q = V * A = 1.07 m/s * 1.44 m^2 = 1.54 m^3/s
To better correlate the flow data, I converted 1.54 m^3/s to 92.4 m^3/min of air flow.
EXAIR Air Amplifiers are designed to have large amplification ratios (the ratio between the amount of ambient air being moved compared to the amount of compressed air used). This makes them perfect as an efficient air mover. Being that this was a furnace application, the High Temperature Stainless Steel Adjustable Air Amplifier was required. This Air Amplifier has a temperature rating of 374 deg. C, and it can be easily mounted at a safe distance to meet this temperature requirement. The largest unit that we stock is the model 6034, a 4 inch (10cm) Stainless Steel Adjustable Air Amplifier. It has a 24:1 amplification ratio that can create an outlet flow of 34 m^3/min. (It would only need 1.42 m^3/min of compressed air at 5.5 bar to create this outlet flow). For this customer to reach the 92.4 m^3/min to keep the smoke from escaping, he would need to install three units (3 * 34 m^3/min = 102 m^3/min). He mounted the Stainless Steel Adjustable Air Amplifiers to some extraction wyes in their stack and added solenoids to them. So, when the crew opened the door to load the coal, the Air Amplifiers would operate to keep the exhaust smoke from filling the room. The company and operators were very satisfied as it made the environment clear to see and safe to work.
If you have an application where smoke and fumes are a nuisance, you can contact an Application Engineer at EXAIR to see if an Air Amplifier would work in your application.
John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb
