How to Prevent or Minimize AirEntrapment in Industrial Fluids
Air entrapment is one of the most common causes of poor fluid performance and parts failure in fluid management. Loris Medart, a fluiddispensing engineer, discusses the eight most common reasons for gas entrapment and how they can be avoided.
by Loris Medart
Air bubbles can have a devastating impact on pro- duction. Medium viscosity adhesives, silicone, UV, epoxies, latex, and oil are just some of the fluids thatare particularly affected. Trapped gasses can ruin the fluidor the part it is dispensed onto costing tens of thousands ofdollars in lost production time. In the electronics industry,where epoxy resin encapsulation or coating is often an essential process, air bubbles lead to catastrophic parts failure.
There are, however, some simple steps that can be taken toavoid or greatly minimize the chances ofgas entrapment in industrial fluids.
1. Avoid Excess Fluid
Fluids change over time and the shiftingchemical composition can itself cause airbubbles to form. Production managers using too much fluid will often find that itsproperties have changed significantly afterjust a few short days, with production processes having to be altered to suit and thenre-altered to accommodate a fresh batchof fluid—whereupon the process starts allover again. The best practice is to use justthe amount of fluid needed for a singleproduction day. This de-risks the likelihood of air bubbles and also reduces lostproduction time by preventing the need tocontinually adapt production processes tothe changing properties of the fluid.
2. Size Matters
One of the issues many production managers encounter, particularly those using legacy equipment, is the sheer size and capacityof vessels. This can often leave operatives having to manage fouror even nine times the amount of air to fluid needed in a one(five) or 3-gallon (10-liter) tank. A smaller one or half-gallon(2-liter) vessel adapted to the daily volume usage minimizes thelikelihood of air bubble contamination inside the fluid.
3. Under Pressure
Pressure requirements change over time in line with the
amount of fluid used. The pressure set by the operator at the
start of production with a full tank to deliver a certain size
deposit will have changed significantly halfway through the
production period. As the volume of fluid goes down, the
volume of air grows with operators increasing the air pressure
inside the vessel to compensate for the flow rate and achieve
the desired deposit size. A sudden burst of air entering the
pressurized tank at force, however, can penetrate the liquid
causing air bubbles. The absolute best way of addressing this
is to make use of an automated system that gradually adjusts
the pressure in line with the volume of fluid used—ensuring
consistent flow rates and deposits throughout production. If
for any reason this cannot be done, however, a simple and
effective way of reducing (but not eliminating) the formation
of air bubbles is to install an elbow fitting to deflect the air jet
onto the side of the fluid container.
4. Managing the Levels
Another common cause of air bubbles is too little fluid inthe pressure tank. On automated or fast-moving productionlines, the level of fluid can inadvertently drop below the diptube. Air then enters the dip tube traveling through the feedline to the dispensing/spraying head. Using a level sensorwill prevent the system from getting to this point avoiding afull production reset. Other methods include assessing levelsby eye, something that has been made a lot easier followingthe commercial availability of transparent pressure vesselsor weight monitoring.
5. From R&D to Full Production—Is This Fluid Rightfor Me?
Often in applications requiring adhesives such as cyanoacry-late, the wrong adhesive is selected for the application. Whilstperfectly capable of doing the job it was selected for at theresearch and development stage, multiplying that many timesover to get to full capacity (e.g 1 part/day produced vs 100
parts/day produced) can often lead to a number of problemsincluding the production of air bubbles. In some cases, thismay mean going back to the drawing board and identifyinga new adhesive or other industrial fluid. If this isn’t possible,then having complete control of the environment the fluidis in during production using some of the steps mentionedabove can lead to a custom solution that enables the fluid tomeet application and production requirements.
6. Chemical Compatibility
Often overlooked, but something to consider when other lines of inquiry have beenpursued is whether fluids are reacting withany wetted parts to create gas bubbles?Sterile production systems with limitedcomponents are the best practice for anyprocess involving fluids. A chemical reaction can cause chemicals and air to leachin and out of fluids.
7. System Setup
There are a number of practical steps thatcan be undertaken to minimize the likelihood of the formation of air bubbles atthe system setup stage. Minimizing thedistance between the fluid container andthe dispensing/spraying head is one, alongwith considering pressure and flow rateand how everything is connected to thecompressed air. One simple area that is often overlooked however is the fluid tubing itself. If too long, with an oversizeddiameter, there is a risk for loops, high points, and low points,that can cause air bubbles to be formed and dragged throughthe system by the fluid.
8. Fluid Transfer
The movement of fluid from one container to another canabsolutely cause air bubble entrapment. Depending on thefluid viscosity trapped gas may raise to the surface over time,but this is not guaranteed. Attention should always be givento the manufacturer’s advice on the best process and systems for storage and handling. However, as a rule of thumb,dispensing directly into the pressure vessel is often the bestapproach. In some cases, the production chamber can also beplaced into a vacuum or centrifuge to remove bubbles aheadof production.
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