In some cities when you look along the skyline, you see flue stacks bellowing out plumes of white smoke. I never paid much attention to the structure except that they were tall and in some cases very wide. A power company contacted EXAIR about their flue stacks. They did a temperature reading, and they found a hotspot within the wall of the stack. To cool the hotspot, they contacted EXAIR for a solution.
Smokestacks are large chimneys that can be from 825 feet (250m) to 1,188 feet (360m) tall and are designed to release the smoke high in the air. As a tall structure, it is important to keep the walls stable and sound. For this customer, they were getting a hotspot reaching a temperature of 750oF (400oC). This was too hot, and it could cause premature issues to the construction of the stack. They wanted to reduce the temperature to 400oF (204oC) to keep the stack from warping and degrading. We were able to find a solution using our stainless steel Adjustable Air Amplifiers.
Adjustable Air Amplifier
The area of the hotspot in their smokestack was a section around 2 feet (0.6m). The customer fabricated a stainless-steel manifold to mount three pieces of model 6033 3” 303SS Adjustable Air Amplifiers. The model 6033 will only use 35.2 SCFM (997 SLPM) of compressed air at 80 PSIG (5.5 bar). With the high amplification ratio, the model 6033 can move 2,323 SCFM (65,780 SLPM) of air along the surface. The large volume of air created good cooling capacities to reduce the hotspot temperature. In keeping the temperature of the stack under control, they could continue operations and lessen the concern for untimely shut-downs and costly maintenance.
By using air to cool, you can do it safely and cleanly. Unlike fans which create turbulent flows, voids, and high noise levels, the EXAIR Air Amplifiers generates a large volume of laminar air to cool and clean. If you would like to speak about any cooling application, you can contact an Application Engineer; even something as large as a smokestack.
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.
Smoke starting to migrate out of the opening
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.
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’s Adjustable Air Amplifier
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.
In some cities when you look along the skyline, you see exhaust stacks bellowing out plumes of smoke. I never paid much attention to the structure except that they were tall and in some cases very wide. I received a call from a customer that had a flue stack that exhausts hot gas in their petroleum process. They had an issue with hotspots in the wall of their stack. The customer need to cool the hotspots and decided to contact EXAIR.
Flue Stack
His stack had a diameter of 6 foot (1.8 meters), and the hotspots were reaching a temperature of 750 deg. F (400 deg. C). This was too hot, and it could cause premature issues with fatiguing of the structure of the stack. He wanted to reduce the temperature to 400 deg. F (204 deg. C) to keep the stack from warping or degrading. Together, we were able to create a solution using our stainless steel Adjustable Air Amplifiers.
Adjustable Air Amplifier
The customer was able to create a stainless steel circular manifold to mount 40 pieces of the model 6032 around the circumference. This manifold ring would be mounted around the stack near the hotspot. With the high amplification ratio associated with the Air Amplifiers, it can move a large volume of cooler ambient air along the hotspot of the stack. In keeping the stack cool, he could continue operations and reduce his worry for untimely shut-downs and costly maintenance.
Whenever you have something hot and you need to cool it down, the Air Amplifiers could be the solution for you. If you would like to speak to an Application Engineer about your cooling application, you can contact us at EXAIR at 800-903-9247.
John Ball
Application Engineer
Email: johnball@exair.com Twitter: @exair_jb
