Optimizing Blow-off Systems for Manufacturers

3/14/2019

Air knives are used in many industries to remove unwanted materials from processes. This can range from blowing water off of bottles in a bottling facility to removing cooling fluid in a metal rolling plant to removing crumbs from a bakery conveyor system. While the use of air knives for blow-off is widespread, so are the errors in setup that cause ineffective or inefficient operation. In this article we will review some of the fundamentals of implementing an effective blow-off system.

Choosing between Air Nozzles and Air Knifes

There are two ways to deliver the air in a blow-off system – an air knife or an air nozzle:

An air knife produces a flat curtain of fast moving air that acts to strip liquid, debris, or residue off a surface. Because of the flat, straight shape of the air curtain, air knives tend to work best on flat objects such as conveyor belts, sheet metal, etc.
Air nozzles produce a cone of fast moving air. This type of air flow works best if the object being dried is not a constant distance from the nozzle or if the surface in question is contoured. Applications such as bottle or can blow-off are well suited to the use of air nozzles. In many cases, a combination of knives and nozzles are used to get the best results.

Positioning Air Knives and Air Nozzles

There are several dimensions to air knife positioning. Firstly, the air knife or nozzle should be placed as close as possible to the target object. Efficiency decreases sharply as distance increases between the air knife and the object; the same is true with air nozzles, although to a lesser extent.

Another aspect of positioning relates to two angles, the angle of attack and the sweep angle.

Angle of attack: If the air knife or nozzle is positioned normal to the target surface there is risk of is residue being blown equally in all directions, contaminating already cleaned surfaces. To more effectively remove residue, the knife or nozzle should be positioned at approximately 15 degrees from normal, against the direction of travel. This causes the residue to be swept off in a direction opposite of the travel direction of the target, avoiding contamination of cleaned areas. See figure 1.

Figure 1. Angle of Attack when drying a flat surface.

Sweep Angle: The knife or nozzles need to be angled in reference to the plane of the object (see Figure 2). For example, when drying a can, rather than the knife being aligned with the length of the can, the air knife is positioned between 30 and 60 degrees from vertical so that the top of the can is dried first and as the can moves along the conveyor the water is swept down the length of the can. Another example is when drying a sheet of metal the knife will be positioned so that the liquid is swept from one side to the other or from the centre to the two edges. The sweep angle is typically at least 30 degrees.

Figure 2. Top view of the sweep angle of air knives drying a sheet.

Sizing an Air Knife

When sizing an air knife or considering the number of nozzles it is important to take the sweep angle into consideration. The sweep angle means that the air knife’s actual length could be much longer than the height or width of the object being processed. Correctly sizing the air knives is important because the length of air knife or number of nozzles is a determining factor for the size of blower required

Choosing the Right Blower

Once you have determined the total length of air knife and, if necessary, the number of nozzles required the blower can be sized. Choosing a blower that is undersized is a common mistake. A good rule of thumb to follow is 10 CFM for every inch of air knife and 30 CFM for every nozzle to ensure effective blow-off. These values can change with the width of the air knife outlet gap or the diameter of the nozzle.

Although the concept of blow-off is a simple one, in practice blow-off systems are commonly designed and implemented without using best practices. STANMECH Technologies would be happy to discuss your blow-off challenges and make sure you get the best system for your application and budget.