Article # 77- Adjustable Air Knives

Some of our competitors offer adjustable air knives in which the exit opening, or gap, can be made wider or narrower.  By opening the gap, you can lower the system pressure, making the air knife blow gentler.  By closing the gap, you can raise the pressure, making the air knife blow harder.  We think adjustable air knives are not a good idea.  There are two reasons for this. 

The first has to do with what it takes, in terms of pressure and flow, to do the job.  Pressure has to do with how fast the air is moving.  Flow has to do with how much air is being applied.  Air knives do their work by transferring momentum.  Momentum is a product of mass and velocity.  A certain amount of air, moving at a certain speed will remove water from a surface.  If there is more water to be removed, it may take more air, or a higher velocity, or both.  Obviously, blowing sawdust off a wood panel moving 60 feet per minute is easier than blowing water off steel plate moving at 2500 feet per minute.  Because of the way blowers work, if you close an air knife gap to boost the velocity, you usually get less air.  If you open the gap to get more air, you usually get less velocity.  If the air knife does not work at one setting, it probably will not work at another setting.  The root problem is not the air knife setting.  The problem is that the wrong velocity and amount of air were chosen to begin with.  Our years of experience allow us to predict what it will take to do most jobs.  If we cannot predict, then we will do testing at our facility or send the prospective customer an adjustable demo system to determine what the application requires.

The second reason is that, by adjusting the air knife, you risk damaging the blower.  With some kinds of blowers, narrowing the air knife gap will raise the pressure, which will raise the current going through the motor.  At some point, that current will exceed the motor’s safety factor and you will be overheating the motor windings and breaking down the insulation.  Ultimately, the motor will fail.  With other blowers, the same effect can happen by widening the air knife gap.  With these, the increased flow causes the increase in motor current.  And, with some blowers, narrowing the gap can cause the blower to surge, which can do physical damage to the blower.  It seems to be human nature that if you give someone something that can be adjusted, they will adjust it.  If the person doing the adjusting is not aware of the dangers, they can easily destroy a blower.

For most applications, adjustable air knives are not a good idea.  But that does not mean adjustable air knife systems are a bad idea.  Some applications require adjustability.  One example is using air knives to control the thickness of a coating.   In this case, we are not trying to remove all the coating, just some of it.  If we hit it too hard, it is a problem.  And it is impossible to know what the right combination of pressure and flow will be.  Here, an adjustable system is a necessity.  As we pointed out above, when you change the air knife opening, if the pressure goes up, the flow goes down and vice-versa.  The two tend to cancel each other out.  This does not give you the kind of adjustability you need to control coating thickness.  However, there are three ways to achieve adjustability without risking your blower.  Please note that none of these three ways involves changing the air knife opening.

The first way is sometimes too crude to give good results, but it can work.  The system can be adjusted simply by moving the air knife closer to, or farther away from, the product.

The second way is simple, inexpensive and much better.  A wye is placed in the blower outlet.  One outlet goes to the air knife.  The other goes to a valve.  If the valve is closed, all the air goes to the air knife and it operates at the highest pressure and flow that it can.  If you open the valve part way, some of the air is bled off and the air knife now operates at a lower pressure and flow.  This method can give you good adjustability over a fairly wide range.  And the valves can be cinfigured so that the blower will not be damaged no matter where the valve is set.  One drawback is that, if you want to use your system with different coatings, or you want the same coating, but a different thickness, you have to adjust the system by hand.

The third way is to power the blower with a variable frequency drive (VFD).  This is more expensive, but it has two big advantages.  One is that when you turn the blower down, it uses less power.  The other is that the drive can be integrated into a control system, for instance, a PLC.  Now, you can have a device that measures the coating thickness.  If it is too thick, that device sends a message to the PLC, which adjusts the VFD to make the blower turn faster, making the coating thinner.  And, of course, your control system can have different programs for different coatings or thicknesses.

In summary, there are places for adjustable air knife systems.  The way to achieve that adjustability is not by making the air knife adjustable.  Not only does that not work well, it can also damage the blower.  That adjustability is best achieved using other methods.