Q: What is an air knife?
A: An air knife is a plenum or enclosure with a means to blow air into it and a means for the air to exit. The air is usually blown in through a round tube with a hose attached. The air exits through a long thin slot, or occasionally, a row of holes.
Q: What does an air knife do?
A: An air knife creates moving air with a particular shape. The purpose of the air knife is to optimize that shape to make it as effective as possible.
Q: How are air knives used?
A: The most common use of an air knife is to dry things by blowing liquids off. Air knives can also be used to blow dry debris off of things, to heat or cool things, to control coating thickness, to dry liquid coatings, to remove static, to separate spaces that need to be maintained at different temperatures, to minimize the loss of liquids, to clean conveyors, to remove chips produced by machining, to direct debris into vacuum pickups, and to apply a hold-down pressure.
Q: What makes one air knife better than another?
A: Air knife manufacturers try to achieve two things: 1) minimum pressure loss across the air knife, and 2) laminar air flow. A tear-drop shaped air knife with the exit slot at the point will transmit 95% or more of the inlet pressure to the air exiting the slot. A round tube with holes drilled in it will typically only transmit 60% of the inlet pressure. The pressure loss represents the inefficiency of the exit design. Laminar air flow is air that is all moving in the same direction. Turbulent air flow is air that is moving in many different directions. If you want to sweep water off a flat surface, laminar air flow does that more efficiently than turbulent air flow. Air knife manufacturers achieve laminar air flow by taking advantage of the Coanda effect, which says that air blown against a flat surface tends to stick to that flat surface. This makes it possible to direct air in a more focused way than blowing it out of a round hole.
Q: Why are air knives used?
A: Almost everything that is manufactured must be cleaned before it is packaged or printed on or painted. Prior to 1987, much of that cleaning was done using solvents that contained CFC’s. These solvents would evaporate quickly and leave behind little or no residue. Therefore, there was no need to dry the product. In 1987, the world started to cut back on the use of these solvents because of their negative effects on the health of the atmosphere. Now, most industrial cleaning is done with water-based products. These do not evaporate quickly and can leave a residue behind if they are allowed to evaporate, so there is now a need to dry the products, as well as wash them.
Q: What is velocity pressure?
A: Velocity pressure is the pressure exerted by moving air. If you measure velocity pressure, you can calculate the speed at which air is moving. Air that has been pressurized, but is not moving (as in a compressed air tank) exerts pressure on the walls surrounding it. This is known as static pressure. Air that is blowing into air exerts velocity pressure. Air that is moving in a container, like the hose leading to an air knife, exerts both static and velocity pressures. The combination of these two pressures is called total pressure.
Q: Why is velocity pressure important?
A: There is an upper limit to how fast you can blow air into air. That limit is the speed of sound. At sea level and normal atmospheric pressure, that velocity is approximately 67,700 feet per minute or 770 mph. That velocity happens when the velocity pressure is approximately 10.3 PSI. Most air knife drying applications operate with velocity pressures of 2-5 PSI. One measure of the efficiency of an air knife system is how well static pressure is converted to velocity pressure. Since the velocity pressure can never be more than 10.3 PSI, the higher the static pressure you start with, the less efficient your process is. This is why air knives are typically powered using blowers rather than air compressors. It is easy to specify a blower that produces only as much pressure as you need. All the energy that an air compressor uses to go from the final velocity pressure to 100 or 150 PSI is wasted. In addition, blowers move much more air than an air compressor at the same pressure. It can easily cost four times as much to power an air knife with a compressor instead of a blower.