When we design an air knife system for an application, the first thing we do is to estimate what air velocity will be needed to do the job. This estimate is based on more than twenty years of experience with many different applications. It takes into account many things: the shape of the part, how fast the part is moving, how the part is being held, how close the air knife can be to the part, etc. Once we have settled on a velocity, we calculate the pressure needed to achieve that velocity. This is a straightforward calculation. Knowing the velocity and the size of the air knife opening, we can also calculate the amount of air we will need. Now we have the information we need to specify a blower that will produce that amount of air at that pressure. (Please see also Article #80 “Blower basics”.) There is one more thing we need to account for. As the air travels from the blower to the air knife through tube or hose, some pressure will be lost. If we don’t account for that lost pressure, our air knife system will not give us the velocity we designed for.
Many people seem to think that air is so thin that it can be forced through any size hole or pipe with no losses. Unfortunately, that is not the case. Improper sizing or shape of hoses can cause surprising pressure losses. If the air were routed from the blower to the air knife using smooth-walled metal tubing with few bends, and if the distance between blower and air knife is small, and if the tubing is big enough, then pressure losses would be very small. That, however, is not the way most air knife systems are set up. It is cheaper and easier to use a flexible hose to route air to the air knives. Since the inside of a flexible rubber or thermoplastic hose is not nearly as smooth as rigid metal tubing, the pressure losses per foot of length in the flexible hose are many times the losses in the metal tubing. For this reason, we try to keep the flexible hoses as short as possible.
We do not try to calculate the pressure losses for every stage of a system we engineer. We simply specify a blower that can give us more pressure than our chosen velocity would require. How much more pressure depends on the system configuration. What size hose is being used? How long is it? Are there wyes or plenums between the blower and the air knives? In order to achieve the velocity we want at the air knife, we specify a blower that will provide that pressure plus the pressure we expect to lose in piping, at the flow rate the air knife requires. (Please see also Article 80 “Blower Basics”.)
Most air knife systems have the blower very close to the air knives, usually right beside the line. If the hose length is less than ten feet, then there is not much pressure lost between the blower and the air knife. Occasionally, we run into situations where the blower must be mounted some distance away from the air knives. In these cases, we do two things. We expect to lose more pressure, and we try to minimize that extra pressure loss. What this means in a practical sense is that the air should be routed through smooth-walled, rigid metal tubing from the blower to a spot near the air knives. Then the last few feet can be done with flexible hose. The metal tubing should have as few bends as possible and those bends that are necessary should have as long a radius as possible. The bigger the diameter of the tubing, the lower the losses are. By limiting the velocity of the air in the tubing, it is possible to calculate a minimum size.
There is another situation in which pressure losses must be considered. The air flow coming out of an air knife slot should be even across the full length of the slot. The pressure at one end of the air knife should be the same as the pressure at the other end. For an air knife with one inlet in the end, there is a rule of thumb that says the area of the inlet must be twice the area of the slot. The cross-sectional area of the air knife body must be at least the size of the inlet and, of course, is always bigger. The end result of all this is that, given a certain inlet size and slot opening, there will be a maximum air knife length possible. For example, with one ¾” diameter inlet and a 0.035” slot, the maximum air knife length possible is only 4.3 inches. (That’s “four point three”, not “forty three”.) With a 1-1/2” inlet and a 0.035” slot, the maximum length is 21”. Those lengths can be doubled by putting an inlet on each end. But if you tell me you need an air knife 60” long, but you only have room for a body with 1-1/2” inlets, I am going to tell you that we need to find another place to put that air knife, or we have to create more room. Long, skinny air knives do not work.
There is one more source of pressure losses in an air knife system. All the air knife systems we sell have filters. The filter is there to protect the blower and to prevent dust and dirt from being blown on the product. Over time, the filter clogs up. The blower has to work harder to suck air in. The blower is only capable of producing a certain amount of pressure differential between its inlet and outlet. If more of the differential is needed on the inlet side, then less of it is available on the outlet side. As the filter gets dirty, the velocity at the air knife drops. If the filter gets dirty enough, the air knife does not do the job any more. Filters tend to be neglected. We have seen filters that were so dirty, they had collapsed because the vacuum pressure had gone up so much. It is important to clean or change filters on a regular basis. (Please see also Article #47 “Mainte