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Blowers for air knives? Actually, it’s a very good idea.

By: Stephen Horne

Industrial air knives direct air flow toward a product and use the air’s velocity for product drying, cleaning, coating and cooling in many manufacturing and packaging plants. In their most simplistic form, holes are drilled into pipes. Involutes or air amplifier designs are used in more advanced systems. In either case, these applications require high air velocities but not high pressure.

While compressors are commonly used for air knife process applications, they are not the most energy efficient means when compared with rotary lobe blowers. A significant amount of energy is required to compress air to pressures of 80-110 psig before it is stored, dried and filtered. However, a blower is only pressurizing the air to 3-6 psig – mainly to overcome friction losses in the supply piping and through the air knife orifice – and this air does not require storage or further drying because of the low compression ratios.

Substantial energy savings can be achieved by replacing air knife processes currently using compressed air with blower-supplied air. For a given process, the flow rate will be constant, though a centralized low pressure air system may need to be designed to produce varying flows to accommodate production lines going on and off line.

Q: Don’t I need pressure to remove water or particles from my product?
A: Pressure in a tank is static energy. What removes water or particles from a product is kinetic energy. Once pressure leaves the pipe, it rapidly dissipates into the atmosphere. The dissipation rate gives you velocity. If your part is in range of the expanding air, the energy can be used to remove water or material from a part. A blower generates high volumes of air flow at low pressures. When it exits the pipe or nozzle, the same condition occurs, but the velocity already exists in the air flow which can be generated at a lower cost.

Q: Aren’t blowers loud?
A: Blowers can be noisy, but with properly designed air knives and a well-designed blower, noise levels in the plant can be reduced. Furthermore, a positive displacement blower can be located away from the product floor in a compressor or utility room. This can drastically reduce plant noise and free up plant floor space.

Q: When do I use blowers and when do I use compressed air?
A: This will be a function of current compressed air usage and how much low pressure air you require. If the low pressure usage is very small, it may be difficult to justify the investment, but if the low pressure demand is drawing away from the effectiveness of the compressed air system, a low pressure investment can pay for itself in just a few months.

Consider blowers for air knife applications including:

Update: September 13, 2013. In response to a comment on this post, here is a table showing a comparison between different blowoff devices used with compressors and blowers. Please see the explanation included in the comments of this post for more information.

Blowoff Device Comparison: 2 x 12″ Air Knives for Water Stripping
Blowoff Device Type Operating Pressure
(psig)
Air Consumption
(cfm)
Power
(hp)
Maintenance
Costs
Annual
Electrical Cost*
Flat Nozzles 60-80 psig 257 cfm 60 hp Same as compressor $9,742.07
Drilled Pipe 60-80 psig 175 cfm 40 hp Same as compressor $6,387.68
Venturi Air Knife 60-80 psig 72 cfm 20 hp Same as compressor $3,280.16
Low Pressure Air Knife 2-4 psig 345 cfm 15 hp Same as blower $1,165.32
*Assumes 110 psig with 30% idle power. Duty cycle determined by required cfm/unit cfm. kW calculation assumes 0.83 power factor, 8.3 cents/kWh, 40 hours/week, 52 weeks/year.

 

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