 |
|
An air knife is essentially a plenum or hollow vessel containing a narrow slot. Pressurised air fed into the vessel through one or more inlet ports is expelled through the slot as a high velocity sheet of air. Most air knives have straight slots, but circular slotted knives are also made. Practical slot widths for our low-pressure air knives range from approximately 0.5 to 3 mm (0.02" to 0.125").
Our air knives have fully adjustable slot widths, to allow them to be "tuned" to suit both the application and the blower used to supply air to the knife. Our knives are rugged and corrosion resistant. They are ideal for heavy-duty air knife applications, especially where hygiene is important. Examples of typical VABS air knives are pictured below.
While air knives can be powered by high-pressure compressed air, our air knives use air supplied from low-pressure blowers. As we will explain, using low-pressure air can produce well worthwhile benefits in many applications. In its simplest form, a low-pressure air knife system consists of an air knife, a length of interconnecting hose and a blower, as illustrated below.
The high velocity sheet of discharged by an air knife can be put to many uses. Some of the more common are:
Please note that we don't yet have practical experience in all of these applications. We are simply putting air knives forward as possible solutions to a wide range of process problems. Areas where we have had specific experience include:
The benefits of low-pressure air knives
Using air knives may also lead to valuable product quality improvement benefits by:
From a safety perspective, air knives may be used to remove water from products before it drips onto the floor and creates a safety hazard. Complete removal of moisture or dust not always feasible The complete removal of liquid or dust may not always be possible. This is especially so if the item to be dried or cleaned is round, contains pockets, is very fast moving or if physical constraints prevent the knife from being placed close to the surface to be acted upon. To allow the implementation of a practical and cost effective air knife solution, it pays not to over-specify the dryness/cleanliness result required. It also pays to be specific as to the parts of the product where dryness or cleanliness is critical and where it's not. For example, on a wine bottle the region of the neck to be covered by the capsule may be critical for dryness, but other parts less so. Writing a practical specification for acceptable dryness or cleanliness quality can be difficult. While a weight limit on the liquid/dust remaining on the product may appear to be a simple way of measuring the effectiveness of an air knife, the weight of the acceptable residual contamination is usually too low to be economically measurable. For example a single hemispherical drop of water measuring 0.5 mm (0.02") in diameter may be unacceptable on the neck of a premium wine bottle. That's a weight of 0.00003 gram (0.000001 oz). You won't be able to measure that fine on the kitchen scales! As another example, we have been asked to dry large plastic produce market crates after washing. Because of the large physical size, the numerous stiffening webs and the surface roughening resulting from wear and tear, these plastic crates are difficult to dry completely using reasonable energy levels and practical drying tunnel lengths. However, the bulk of the water can be removed, allowing the crates to be stacked at the exit of the drying tunnel without excessive water dripping onto the floor and causing a safety hazard. Rather than ask for complete dryness/dust removal, or set a dryness/cleanliness specification that cannot be practically measured, it may be better to simply adopt an air knife solution that has worked well in an identical application previously or that has been proven adequate through trialling. Shadow effects When using a single air knife to dry round or smoothly curved objects (such as bottles and cans), there is a tendency for a few small drops of liquid to cling to the surface and to travel round the object until they sit in the "shadow" of the air curtain. Repeated blasts from various directions with multiple knives will reduce the number of droplets remaining but it should be recognised that rounded surfaces can be difficult to dry completely. A common solution for the drying of round objects is to use pairs of opposing air knives. I.e., to mount the knives in pairs such that the air curtain from one of the knives is aimed directly at the exit slot of the opposite knife (as illustrated below). While more effective from a drying point of view, opposing air knives tend to be noisy, as discussed in the next section.
Possible noise generation effects Although the undisturbed air flow from a well-constructed air knife can be surprisingly quiet, when the high velocity air flow hits a moving object containing pockets or thin crevices, high noise levels may be generated. Knives placed in opposing pairs can also be noisy, especially when the separation distance is small, as might be the case when drying the necks of bottles. Soundproof enclosures may be used to reduce the noise problem, but these add cost. They also tend to restrict access and visibility. Although our low-pressure air knives can be relatively noisy in some situations, high-pressure compressed air knives performing the same duty may well be considerably noisier. Mist and dust discharge The liquid mist and dust driven off products by air knives has to go somewhere. Often the amount is small and of no consequence. However, if contamination of the environment is of concern, or if water on the floor might cause a slip hazard, this issue might need to be addressed. Again, a practical trial might be the best approach to see how significant the mist/dust discharge really is before investing in unnecessary shrouding and mist/dust extraction. Location constraints Low-pressure air knife systems can be relatively bulky. For fast moving objects, or difficult to dry objects, the air knives may need to be quite long to provide sufficient air contact time. Before committing to an air knife system, make sure there's sufficient space for it. The location of the blower itself also needs to be considered. The blower should not be placed where it can ingest moisture or dust driven off by the air knives. The blower should be located in a clean, dry and well-ventilated position. The points discussed above should not preclude the selection of an air knife system. It is simply a matter of being aware that air knives have certain limitations that should be considered and worked around in some cases. In cases of doubt we recommend practical testing to see how significant the problems are - which is why we have invested in the substantial test conveyor pictured below.
Air knife installation - key points The effectiveness of an air knife may be influenced by the following:
The energy discharged from an air knife For low-pressure air knives, a practical slot width generally falls in the range 0.5 mm ((0.02") to 3 mm (0.125"), with blower pressures from 100 mbar (1.5 psi) to 300 mbar (4.5 psi). Slot widths less than 0.5 mm are difficult to maintain accurately without making the knife expensive to construct. With slot widths greater than 3 mm, the problem tends to be delivering sufficient air flow to feed the knife. If the slot is too wide, the piping required to feed the knife may become impractically bulky. Our tests have also shown that unless the object to be dried or cleaned is very close to the knife, say less than 10 mm, knife pressures over 300 mbar (gauge) do not provide much extra benefit for the additional energy input required. Distance from the air knife slot and pressure effects For all of the air knife configurations we have tested to date, we have found that the impact pressure generated by the air curtain drops off rapidly as the distance from the slot increases. As illustrated in a typical test result below, the impact pressure falls from 130 mbar (1.9 psi) at the exit of the knife to just over 60 mbar (0.9 psi) at a distance of 10 mm from the slot exit. I.e., more than 50% of the impact pressure is dissipated within 10 mm and over 90% within a distance of 50 mm from the slot.
For maximum effectiveness and minimum energy usage, air knives should be placed as close as practicable to the surface to be dried or cleaned. Practical considerations often mean that clearances have to be greater than ideal to allow for product size variations, as physical impact with the air knife should, of course, be avoided. Noise generation effects may also limit the closeness of the air knife to the object and it may be necessary to use a gentler airflow and/or a greater distance to prevent excessive noise. Air knife surface impact pressures may have to be deliberately restricted when drying or cleaning objects that have a large surface area relative to their weight. Unless the object can be physically retained as it passes through the air curtain, excessive air pressure might blow it off the conveyor, or cause it to topple. In situations where an air knife might be used for processing items of different sizes and densities we recommend installing a valve that may be used to regulate the air flow to the air knife and thus optimise the impact pressure for each particular product.
Positioning the flow regulation valve to suit the chosen blower type is critical. With side channel and positive displacement blowers, opening the valve on the side arm will beneficially reduce the load on the blower, whereas stalling their air flow using the throttle valve arrangement of diagram 2 might cause these types of blowers to overheat and seize. On the other hand, allowing excessive flow from a radial or centrifugal blower using the arrangement of diagram 1 could result in an excessive motor load. For the system in diagram 1, it may be necessary to dump the waste air via an exhaust silencer or to a location where the possible exhaust noise won't cause concern.
The nature of the surface to be dried Indented surfaces and shapes are more difficult to dry. As an example, we have found that the 400 mm (16") high ribbed plastic crates used in vegetable markets are almost impossible to dry completely using air knives with viable energy inputs. Even if not deeply indented, rough and worn surfaces can be more difficult to dry. Again, as we have learnt with vegetable crates, old crates with abraded surfaces are much harder to dry than new ones. While complete dryness may not be easily achievable with awkward shapes and indented surfaces, the bulk of the water can usually be removed. This result may often meet the needs of the application. Sharp edged pockets or crevices may also cause noise problems. The speed of the items to be dried or cleaned Air knife systems are usually arranged so that the knife/knives are stationery while the objects to be dried or cleaned are moved past on a conveyor. All other things being equal, the faster the movement of the objects to be dried, the greater the amount of energy required to achieve the desired level of dryness. It may also be necessary to use longer knives to extend the contact time of the air curtain. The maximum speed of the VABS test conveyor is 100 metres/min (328 feet per minute.) Practical testing often the best method of arriving at the optimum air knife configuration Selecting an appropriate combination of air knives and blower for an air knife system requires a number of complex variables to be considered. The most critical is usually the level of dryness or cleanliness actually required. Unless a repeat of a previously successful installation, air knife systems are often best finalised after testing. In most cases, the required tests are usually not that difficult to accomplish, either on our test conveyor or on the actual installation. The VABS test conveyor can perform air knife tests on objects up to 600 mm (24") wide. Specific air knife applications Drying vacuum packed products at the exit of a hot water shrink tunnel This has been the most popular application for our low-pressure air knives in New Zealand to date. This use may be found in the meat, dairy and other industries where vacuum-packed products are passed through a hot water bath to complete the polymerisation of the plastic film. Especially with irregularly shaped meat cuts, the shrinkage of the plastic film inevitably creates pockets that trap water. It is usually necessary to remove the trapped water prior to final packaging to avoid damage to the outer packaging. Where production speeds are such that hand inversion and/or wiping of the vacuum packs isn't practical, compressed air nozzles have previously been used. Since the conveyors used in this application are typically 400 mm (16") to 600 mm (24") wide, using compressed air nozzles for this application leads to high compressed air consumption and noise. Almost always, our low-pressure air knives will be more cost-effective and quieter than compressed air nozzles in this duty. For meat and cheese processing, the usual post-shrink tunnel air knife installation consists of 2 air knives placed above the conveyor carrying the packs from the shrink tunnel. For conveyors 400 mm to 600 mm wide, a single 5.5 kW ((7 hp) side channel blower usually provides adequate drying. However, installations processing large/heavy products may benefit from using 5.5 kW of blower power per knife. Where 2 top knives are used, the first knife should ideally be placed at least 1 metre (40") downstream of the exit of the shrink tunnel. This amount of separation is desirable to avoid excessive disturbance of the steam in the shrink tunnel. This first knife should be oriented slightly off the perpendicular to drive the bulk of the water back towards the shrink tunnel. The second knife can be placed 300 mm (12") to 500 mm (20") further downstream. The air curtain from the downstream air knife should be oriented nearly perpendicular to the conveyor, so as to apply maximum force to the product. The purpose of the second knife is simply to remove more water than is possible with just one knife. Air knives will work with either solid conveyor belting or mesh/link type belting. The latter tends to result in better dryness on the underside of the pack. With mesh belting, it is possible to put an air knife under the belt to improve the dryness of the underside of the packs. Note that if a bottom knife is used, it should be placed directly below an upper knife. Having an upper knife above a bottom knife will help stop the product "floating" over the bottom knife. Conveyor belt drying The New Zealand meat industry has found that bacteria counts can be lowered by continuously washing and drying conveyor belts used for moving unpacked meat. The washing and drying can usually be accomplished on the underside of the belt on its return journey. This is a good application for a low pressure air knife as it is usually possible to get close to the belt. The close impact distance allows almost 100% dryness to be achieved with a single knife and a relatively low energy input (say less than 3 kW, or 4 hp, for a typical meat industry belt). To allow the knife to be positioned within 10 mm (1/2") of the belt, the knife should be located close to a support roller, so that the belt doesn't flap and hit the knife. Using air knives as scrapers, or to create air slides An interesting meat industry application is to use a carefully positioned air knife to slide "sticky" meat cuts off a conveyor as the belt passes around the tail roller. The air knife method may be more effective than wire scrapers, which have a tendency to jamb and cause belt damage. The air knife should be positioned so that the air curtain follows the curve of the belt as it passes around the roller. The optimum positioning is illustrated below.
For this scraper application, critical practical points are: Coating thickness control Applications of this type we have encountered are:
Note that the flour removal application needs a gentle air curtain! Bottle and can drying after filling, washing and pressure cooking Relative to compressed air nozzles and heat tunnels; low-pressure air knives have the potential to save significant amounts of energy. However, this is an application where over-specifying the level of dryness required can result in excessive energy usage and unnecessarily high noise levels. For this application it is essential to clarify the acceptable level of dryness required and the specific parts of the container where dryness is most critical. Because single air knives tend to drive drops of water around the container into the "shadow" of the air knife, the usual arrangement is to use pairs of air knives that direct their respective air curtains at each other. The resulting violent turbulence where the air streams intersect atomises the water and largely eliminates the "shadow" effect. Unfortunately, the interfering air streams also create noise, especially if the knives are close to each other. The drying of can tops is more straightforward, but multiple air knives may be required to completely dry tops with lipped edges and tear tabs. A point to watch for in this application is the temperature difference between the container and the ambient air. If the container's temperature is lower than that of the ambient air, condensation may occur. A condensation film can be difficult to remove with an air knife. Even if removed, the film may reform immediately after the air knife, as the container is unlikely to increase much in temperature as it passes through the air curtain. Where condensation is a possibility, the best solution is to increase the container temperature at least up to ambient temperature prior to drying. Dust, swarf and static removal applications There are many potential dust and static removal applications in the plastics, wood and metal working industries. If pure static removal is required (i.e., with low or negligible air velocity) it is probably better to use a purpose made de-ionising fan rather than an air knife. However, where high velocity air is necessary, air knives should be considered for removing dust and debris. One application we have sold a system into was the removal of loose metal swarf from rough-machined alloy wheels prior to X-ray. In this application, the objective was simply to dislodge the swarf that might otherwise fall off in the X-ray machine. Complete cleanliness was not a requirement. Where a high level of cleanliness is required, it may be better to: Low-pressure air knives may also be beneficially used on high production metal working machines where it is necessary to keep sliding surfaces free of metal swarf. Normally this would be done with compressed air nozzles, but low-pressure air knives can be more cost effective from an energy point of view. VABS air knife design and materials Our air knives are currently fabricated from welded stainless steel. All of the welded joints are ground smooth and the high quality of finish is such that the knives have been readily accepted in all food processing applications to date. Slot width adjustment is by means of alternating tension and jacking screws. A slot width variation along the knife length of less than + 0.1 mm can usually be achieved. While the slot width is field adjustable, we usually recommend having the slot width preset by VABS. We will adjust the slot to suit the desired operating condition of the chosen blower and this will reduce the risk of blower overloading problems during commissioning. The support stands and adjustment clamps are also made from stainless steel, but acetal adjustment clamps are available where smoother sliding and rotational adjustment is required. Information needed for selecting a low pressure air knife system In considering an air knife solution for a new application, probably the first step to take is to quantify the potential benefit of an air knife based solution. As examples;
If pre-qualification suggests that a low-pressure air knife system might be worth pursuing, please provide us with the following initial information: 1.The nature of the problem to be solved (meat packs to be dried, plastic crates to be dried, etc)? Once we have considered your preliminary information (please submit our on-line application questionairre form, we will come back to you with our suggestions for the next step. |
