You can see the blower, filter, wye, hose, air knives, mounts, conveyor and bottles.  In this illustration, the bottles would be moving from the right to the left.  This would cause the air from the air knives to hit the tops of the bottles first and gradually push the water down the bottles as they pass between the air knives.

If you are drying cans, or bottles with wide-mouth lids, we add an air knife above to dry the tops, shown below.  Notice that the upper air knife is positioned to hit the top before the side air knives.  If you are drying the typical can with a raised lip around the top, or if your cans or bottles are moving at high speed, it may take more than one pass to dry.

The bottoms of cans and bottles are often difficult, because usually bottling/canning conveyors are mostly closed; there is no way to blow air up from underneath.  If it is important to dry the undersides of your cans or bottles, you may have to provide a section of conveyor that is more open underneath.  Then we can position an air knife to blow off the bottoms.

There is another consideration for bottling lines.  There are usually rails on either side of the conveyor that guide the bottles or cans.  These rails will block the air being blown by the air knives.  They need to be cut to allow the bottle to be properly blown off.  The lips of the air knives will act as guide rails in the cut section.

We have used glass as an example, but the same general principle applies to many differently shaped parts.  This kind of air knife setup is often used to dry parts that have gone through a washer on an overhead conveyor.  In that situation, it is common to have to dry many different sizes and shapes.  The air knives have to be placed to allow the widest parts to pass.  This means the air knives can be some distance away from the narrowest parts.   For this reason, these systems usually have to operate at higher velocities and may require more passes, four or six or eight air knives instead of just two.  Complex shapes may require the addition of nozzles to treat difficult areas such as blind holes where water can be trapped.

Portable systems can also be provided on four-wheeled carts or on stands with feet that can be moved with a forklift.  The stand on which the air knives are mounted can be removable or permanently attached to the cart or stand.

Of course, air knives are only one part of a complete air knife system.  That system will include hose, hose clamps, blower, filter, mounting clamps and other fittings, at a minimum.  Let’s look at these other components.

The hose and hose clamps are easy.  Hose clamps can be had in stainless steel.  There is also hose available that is certified by the USFDA to be used in food applications.

The blower is usually the biggest problem.  Most blowers and motors are made of painted steel or aluminum.  There are blower manufacturers that will fabricate a blower for you out of stainless steel.  Those blowers are much more expensive.  Most customers prefer to deal with the blower in one of two ways.  The blower can be put in a different room and the air piped into the processing room.  The blower can also be put in a stainless steel washdown enclosure.

There is another question that we get asked.  Customers wonder what happens if a bearing seal fails.  Does this allow oil or grease to get into the air stream?  The simple answer is no.  When the seal fails, the higher pressure inside the blower housing pushes the lubricant away from the blower and not into the air stream.

The filter can be purchased with a stainless steel housing and connected to the blower with stainless steel fittings.  This allows the filter to remain outside the blower enclosure, making it easier to service.  Alternatively, the enclosure can be made bigger so the filter can be place inside it.

Our mounting accessories usually utilize stainless steel tubing and aluminum clamps.  In some applications, it is acceptable to anodize the aluminum clamps.  We can also provide clamps in stainless steel or UHMW plastic.

It is possible to make every part of a Turbotech Air Knife system acceptable for use in your food processing application.

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.

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

I do not want to make this sound to simple there is a lot to consider including losses due to configuration, space available and ambient conditions of location.  This is just a few major items.  Here at T.J. Bell we are also glad to provide just the air knives or a complete turnkey system and any stage between.  Additionally we can provide heat to 800° F, anti-static bars, articulated systems, self regulating systems, aluminum or stainless steel and sound enclosures just to name a few things the other guys can’t offer.

Some of the competition brags about the adjustable lips as a feature that adds versatility and adjustability to their air knives. We have and can make adjustable exit lips but frankly discourage that choice because it creates more problems than it solves.
First, if the blower and air knife system are sized correctly in the first place there is no need for adjustability.
Secondly, when they do make adjustable lips the springs and screws interrupt the air flow causing gaps and streaks on the target product that are not dried or wiped because of the missing air.  The client is generally not equipped to do that kind of fine adjustment and in many cases causes damage to the blower with too little or too much system pressure.  The bolts or screws on the exit lips are there to prevent the opening from getting larger when air pressure is applied.
Thirdly, another mistake they all make is putting the retainer screws too close to the exit again blocking exit air needed on the product.  We move our retainer back far enough that the air has time to reform after passing the screw so we provide a continuous solid air curtain.

We make both Stainless Steel and Aluminum air knives.  We sell more stainless steel air knives because they are tougher, will take a factory environment better and we can customize them easier than our Aluminum designs.  The Aluminum air knives are extruded which helps to make them less expensive and they are a good choice if the environment is not too harsh.  One flaw in our competitors Aluminum air knives is the end caps where you need to attach an air supply hose.  The competitors’ attachment tube is too short and is tapered again because it is cheaper to build that way.  Their hoses continually fall off and can only take a limited amount of pressure.  Our Aluminum end caps are longer and machined straight so they do not have a taper.  Our hoses do not blow off when powered up.

Turbotech HVLP Aluminum air knives have built in mounting bosses for alignment and attachment and the unique Turbotech tear drop high efficiency design.  The can be anodized and treated in a number of ways if necessary to protect them from some chemical environments.  That decision is made when we discuss the application and as compared with the cost of Stainless Steel.

Most air knife companies can supply air knives from eighteen inches to sixty inches in length and have only one body style and size.  Turbotech HVLP air knives are available from two inches to the longest one so far of one hundred ninety two inches (16 feet) long.  There are three basic body shapes and two basic body sizes.  We have also built a variety of nozzles, circular air knives, as well as custom designs for special applications.  We also offer a wide range of accessories including blowers, filters, ionization bars, adjustable mounts, hoses, air flow controls, and heaters to name just a few.

Many factories have one or more large compressors supplying air to the entire plant from a central system.  This makes a lot of sense from a maintenance and noise point of view.  It also provides for staged usage to meet current demands for air during peak and slow work hours.  Compressed air is easy to plumb to a new location.

Over years of growth companies have added compressors as the demand has increased and to provide a back-up in case a compressor fails.  I don’t think anyone would disagree that the reason air is compressed to 125 psi or more is because it is needed at that pressure to operate pneumatic equipment.  Typically it is also conditioned by filtering out condensed water and by adding oil for the tools it is used to operate.  One of the problems with compressed air systems is that it is so easy to add another blowoff.  For example, some parts that were dipped or washed needed blown off so a hand nozzle was added.  In another location they needed to remove the excess coolant from a machined part so another blow off was added.  Over time these kinds of air stations increased with production and more blow-offs were added as needed because the air was easily accessible and thought to be free.  Unfortunately, not only is that air not free, it is horrendously expensive.  We have worked with people who do energy audits at manufacturing plants.  They all agree that compressed air is the most expensive utility in any plant.  We are told that one 1/8” diameter nozzle blowing at 100 PSI 24/7 costs approximately $20,000 a year to operate.   With that kind of cost, they are as concerned with tracking down and fixing the many small leaks in the system as anything else.  They also all agree that the worst possible thing you can do with compressed air is to use it to blow something off.  The reason is that compressed air is simply the wrong tool for the job.  It’s like trying to drive nails with a pipe wrench.  You can do it, but you’re going to hurt yourself in the process.

There are several reasons for this.  The first reason has to do with pressure.  An air compressor is designed to provide air at pressures of 100 PSI and greater.  Most blowoff applications can be done at 2 or 3 PSI.  That means that all the energy used to compress the air from 3 PSI to 100 PSI is wasted.  That means that something like 97% of the energy used is wasted.  The higher the pressure the compressor produces, the worse the situation becomes.  Some of them operate at 250 PSI.  Another aspect of this waste is that a lot of compressors are reciprocating; pistons moving back and forth in cylinders.  This means that half the time the compressor is operating it is pushing that piston without producing any compressed air.  Blowers are rotary.  They are producing pressurized air all the time.

The second reason has to do with flow, how much air the compressor puts out.  A typical 15 horsepower, single-stage air compressor can put out 113 cfm at 100 PSI.  (Taken from a particular manufacturer’s published data)  A 15 horsepower blower can put out 1,000 cfm at 3 PSI (while operating at 80% efficiency).  Let’s return to that 1/8” nozzle we mentioned earlier.  At 3 PSI, one of those nozzles use 3.17 cfm.  The 15 hp compressor, with proper piping and regulators could feed a maximum of 35 of those.  If you added more, the pressure would start to drop and you would not get the same performance.  The 15 hp blower could feed 315 such devices without losing pressure.  The blower is capable of doing 9 times as much as the air compressor.   The blower fits in a space 30” x 16” x 27” and weighs 280 lbs.  The air compressor requires a space 84” x 67” x 72” and weighs 3,440 lbs.

If the comparison stopped here, it would be enough to convince you not to use compressed air for blowoff.  But, this is not the end of the expense for compressed air.  As mentioned above, compressed air is almost always treated to remove condensed water and to add oil.  Both of these are done to protect air tools powered by a compressor.  That treatment is expensive, and since blowoff devices use much more air than the typical air tool, the bulk of that treatment expense should be assigned to the blowoff.

In addition to all of this, now you are blowing off your product with air that has had oil deliberately added to it.  If your product is unpainted steel, this might be an advantage.  For everything else, it’s a problem.

Above, we looked at a 1/8” nozzle and discovered that a blower could do nine times as much as a similar sized compressor.  We can’t always provide that big an advantage, so let’s work through a more realistic comparison.  A typical air knife system might consist of two 12” air knives with 0.045” gaps.  Most drying applications can be done at 28,000 fpm air velocity.  If I were trying to set up such a system, a 7-1/2 hp blower would not be big enough.  I would have to specify a 10-hp blower, and instead of 28,000 fpm, I would be getting more like 34,000 fpm and using 255 cfm. The motor would be pulling about 7.2 kW.  To provide the same cfm using an air compressor operating at 100 PSI would require (acc. to Machinery’s Handbook) approximately 40 hp, 29.8 kW.  The blower-powered system would cost $6200.  If you are paying $0.06/kWh for electricity, you would save about $1.356/hr.  Operating two shifts a day, 40 hours a week, the electricity cost savings would pay for the system in a little over thirteen months.   After that, you would be saving about $6,000 a year.  This does not include the cost of treating the air, which is entirely absent for the blower-powered system.  The blower-powered system has a reasonable life expectancy of more than five years and there are similar systems out there that have been running for ten years.  The blower I chose is a relatively inefficient blower.  With a more efficient blower, more constant operation, or a higher electricity cost, the situation is even better.

In addition to just being expensive, using a compressed air nozzle can actually create problems.  Because of the high pressure, the air is moving very fast.  And the air is dense because it has been compressed to six or ten times its normal density.  When this high-energy blast of air is targeted at a small area, a lot of momentum is transferred.  Some of the liquid you are trying to remove may be atomized, that is, it may be blown completely off of the surface and into the air.  If you blow off water or oil with a high pressure nozzle it may remove the oil or water; it may also make a terrific fog that spreads water or oil over a wide area.  We think it makes more sense to use a controlled air curtain at 2 or 3 psi to remove the liquid like a squeegee on a window.  More air at a lower velocity gives you the same transfer of momentum, but in a much more controlled fashion. At the same time it reduces the work load on the plant compressor and uses a fraction of the power to do the same job and do it better.

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.