AIR KNIFE SYSTEMS
Superior Technology For Most Effective-Efficient-Energy
Saving Solutions For Drying-Cleaning-Blow
OFF -Wiping-Many more Applications.-
AIR KNIFE SYSTEMS
Superior Technology For Most Effective-Efficient-Energy
Saving Solutions For Drying-Cleaning-Blow
OFF -Wiping-Many more Applications.
With water based cleaning, drying of parts is an essential element of the process.
Level of dryness required, varies from simply removing excess water, to prevent spills and maintain a clean & safe work environment, to dry for critical processes such as thermal deburring, painting, leak testing, assembly , printing etc.
How dry is dry? This factor is easier to evaluate. Don’t dry parts any more than you need to based on what will be done next with the parts. The reason is that drying investment and costs are almost exponentially dependent on the degree of dryness needed. If you aren’t sure how dry your parts have to be assume dry to the touch is adequate. To the touch means remaining moisture is in the range of about 1 to 5%.
Parts drying choices include.
If you are going to paint your parts after cleaning match the carrier in the paint to the carrier in the cleaning agent (water or solvent).
If plating is the next step after cleaning use an aqueous cleaning agent, rinse well and don’t worry about adding water to the planting bath.
If the parts are to be stored after drying, consider letting them air dry in storage-if deposition of air borne particles can be avoided.
If a very high level of dryness is needed (<25ppm), the drying should be done in two steps: from wet down to 1% moisture and from 1% down to 25ppm. The reason is the costs of polishing drying are dependent on the amount of moisture being removed.
Predicting generalized cost of drying is an inexact science. The main cost element is energy. So, it makes sense to compare drying costs on the basis of energy equivalents.
Drying is accomplished primarily by evaporation, and physical displacement of water solution.
Evaporation rate is dependent on difference in vapour pressure between the water on the part and the air used for drying.
Increase in part and air temperature, and increasing the flow and velocity of the air all contribute to increased drying effectiveness.
Pressure is used to provide the high speed impact, to displace water from the work piece or spread it over an extended surface area to improve evaporation.
In many cases where washing temperature is high (70°C), parts will dry in ambient air within minutes of exiting the washer and no extra drying system is required.
If this is not the case, the following systems are available.
High-impact air blow-off systems such as Blower Assisted Air Knife System are designed to shear off droplets and trapped liquid from part surfaces, consequently spreading the moisture over a larger surface area and thereby accelerating the evaporation process.
When properly designed, these systems can remove over 95% of the moisture from the part surface.
Such system is suitable for high production items such as automotive components and will dry with a minimum exposure time (typically 15 to 60 seconds, many times lesser than this).
Such system requires minimal floor space, and does not increase demand on plant compressed air supply, since a blower is used to supply air for drying.
The moisture shearing effect of a high-impact air blow-off is produced with a pressure blower capable of generating 2.0-3.0 PSI pressure and a specially designed, precision air knife producing air velocity ranging from 6000 FPM TO 40,000 FPM.
When drying with high velocity air, such system would require 38 times less air, compared to low impact system, because high velocity air systems spreads moisture droplets over 15 times its original surface area.
When drying by evaporation, it would require 5 times more air velocity, if evaporation rate was required to be doubled. However, if the moisture were to spread over double the area, lower air velocity would achieve the same results.
Ring blowers are typically used in these Air knife Systems.
When air is pressurized, inherently temperature rises.
By raising air outlet temperature by 10 degree C. evaporation rate can be effectively doubled
For most difficult applications, an electric heater can be added to downstream side of the blower to increase temperature up to 120°C.
The air used for drying is not recirculated in this system, so only clean dry air (supplied by the blower from plant atmosphere) contacts the work piece.
When process enclosure is installed, air is exhausted at a rate, slightly higher than the blower capacity, to evacuate water removed from the work piece, and prevent escape of the hot humid air into plant atmosphere.
Blowers are selected depending on the application.
A number of methods are used for delivering the air to the work pieces:
a) Air Knives
These are typically used for general purpose drying, to remove excess water and in applications, where different work pieces will be processed through the same equipment.
Using multiple air knives, for prolonged exposure, can produce excellent drying results, even on complex parts. The air knives are designed to cover the entire part geometry. This system requires relative movement, between part and air knives. This is accomplished by moving the parts past the fixed air knives or by reciprocating the air knives past a fixed part.
b) Precision Nozzles
High flow nozzles are precision aimed, to dry specific areas, while the work piece is stopped in an indexing system. This is typically used for parts with complex geometry, such as automatic transmission valves
and any item with large blind holes or internal cavities.
Some parts with complex internal passages (such as fluid control valves for braking and traction control systems and distribution manifolds for diesel injection systems) are extremely difficult to dry as the internal wetted surfaces are not in line of sight of the air nozzles. In this case specially designed manifolds are clamped to the parts and air is forced through the passages for rapid drying.
For larger work pieces, such as assembled transmissions, a system using a combination of the principles used in convection ovens and high impact blow-offs is often recommended. A medium pressure (6" to 45" w.c., 0.2 to 1.6 psi) radial blower is used to recirculate heated (30 to 70°C) air for convection drying and supply high velocity air through precision aimed nozzles or air knives to physically displace water from difficult areas of the work piece. Typical exposure time required is 1 to 3 minutes.
A percentage (20 to 50%) of the main airflow is exhausted to remove the evaporated water. This system requires more floor space than a high impact air blow-off but less than a convection oven. Heating requirements are also less than a convection oven but the higher horsepower; high speed blowers generate more noise than the low pressure blowers used in convection ovens.
This system uses the impact from high pressure plant supplied compressed air (at 20 to 100 psi) to physically displace the water.
Compressed air systems are generally used to remove excess water from specific areas of a work piece and not as complete drying solutions.
Compressed air blow-offs with precision aimed nozzles are often used to compliment other drying systems to remove water from difficult areas of the work piece, such as blind holes. These nozzles can be also mounted on retractable manifolds so they can be automatically positioned to address areas of the work piece that are inaccessible to fixed nozzle manifolds or air knives due to restrictions imposed by work piece handling methods.
For clearing water from small bore holes and blind tapped holes the operation of the nozzles is rapidly pulsed for more effective operation.
The major disadvantages to compressed air blow-offs are the high demand on plant compressed air supply and generally higher noise levels than other blow-off methods. Energy costs are also very high for such system when compared with blower assisted Air Knife System.
To reduce demand a single nozzle can be mounted on a multi-axis transport system to clear multiple orifices on a work piece. Flow amplification nozzles can be used which entrain surrounding air with the compressed air jet to significantly increase flow with a minimal reduction in pressure. These nozzles also reduce noise levels. On high production equipment, part sensors are installed so that the air blow-off is only actuated when a part is present.
Compressed air blow-offs will also reduce part temperature (5 – 10°C) dependant on exposure time & flow which can be a desirable feature, if cool parts are required for downstream processes.
This process relies on evaporation only and no heat is used. The work piece is placed in a sealed vessel and a Vacuum pump is used to rapidly (10 seconds or less) evacuate the vessel to an absolute pressure of 3 to 5 Torr. The vapour pressure of water is thus reduced to a point where it will evaporate at below 40° C. This process quickly dries the work piece and since no heat is added, the heat of evaporation lost also cools the work piece. In many applications this cooling effect can eliminate the need for separate cooling tunnels required to bring work piece temperature down to acceptable levels for safe handling or processing in downstream operations such as gauging, leak testing and assembly. Typical cycle time for parts such as automotive components is between 20 and 60 seconds.
Vacuum drying is used when critical processes require absolutely dry parts and is best suited for removing thin water films that cover the entire work piece surface. Pooled water or filled blind holes will not dry quickly with the vacuum process. A blower assisted air knife blow-off for rough drying is typically installed upstream in this case.
The vacuum system is the most energy efficient drying system available, requires minimum floor space and no exhaust connections outside the plant.
This process relies mainly on heat to evaporate the water from the work piece. A low pressure (1 to 4" W.C., .04 to 0.14psi) radial blower is used to circulate a high volume of heated air past the work pieces. These systems normally operate in the range of 140°C to 350°C. To conserve energy the air is recirculated. A percentage (10 to 30%) of the main airflow is exhausted to remove the evaporated water.
Convection ovens typically require longer cycle times (1 to 10 minutes), significantly more floor space and use more energy than the other drying methods described, but the effectiveness of the process is not as dependant on work piece geometry. No special work piece fixturing or precision aimed nozzles are required; anything that will fit through the oven will dry.
For large work pieces, such as electrical control cabinets and automotive sheet metal stampings, convection ovens are normally the most practical solution.
1) Part orientation is very important; parts must be positioned so they drain effectively and areas of the part that trap water is directly
exposed as close as possible to the air knife or blow-off nozzles.
2) Threaded holes are difficult to dry. High impact air blow-offs are usually necessary.
3) Blind holes are more difficult to dry than through holes, and vertical blind threaded holes on the top horizontal surface of the part are
almost impossible to dry completely. Precision aimed, pulsed compressed air blow-offs are normally required.
4) Small bore holes (under / 6” Ø) are almost impossible to dry completely. Positive pressure drying is normally required.
5) Low density parts that do not store significant heat (e.g. aluminium sheet or plastic construction) will not dry rapidly in ambient air
when they exit the equipment.
6) Formed sheet metal parts such as pans that have seams, rolled or double wall lips are difficult to dry as they may continue to leach
water from the seams after part has past the blow-off. In extreme cases a convection oven is the only solution.
7) Continuous channels in topside horizontal surfaces such as large O ring grooves and lips of pans are difficult to blow-off since the
water tends to simply move along the channel. For these applications high impact air blow-off with multiple or reciprocating air
knives are effective.
8) Wash process has a major effect on drying capabilities.
a. Higher operating temperature will aid drying.
b. City water only does not drain off parts as effectively as R.O. or De - I water, or water (in rinse stages) with detergent residue from
previous stages or with a rinse aid.
c. When using high temperature blow-offs it is important to choose appropriate detergents and rust inhibitors that do not leave
behind excessive residue when the water is evaporated. This residue can coat the blow-off chamber and may eventually detach
and recontamination the work pieces. If this is unavoidable automatic cabinet cleaning systems must be installed.
9) In applications requiring tight cleanliness standards it is important that the drying system is constructed to prevent recontamination
of a precision cleaned work piece. Blower suction filters and stainless steel construction for piping, air knives and nozzles are
required to ensure that that only clean air contacts the Work piece.