THE BUSINESS OF VISCOSITY BLOG



    What Does Temperature Have to do with “Spitters” Anyway?

    Posted by Mike Bonner

    May 13, 2020 9:53:00 AM

    We recently did a survey of Industrial Coaters on LinkedIn to determine the pressures they were experiencing on their business. The first question dealt with the common defects they experience. We got plenty of the “usual suspects” like orange peel and run & sag, but amongst the most interesting and unexpected responses were:

    “…water dripping from the fixture[s]…”

    And,

    “Spit from the paint heads happens quite a bit.”

    In the industry, these phenomenon are referred to as “spitters”.

    What is a “Spitter”?

    No alt text provided for this imageThe Saint Clair Systems Paint Defects Troubleshooting Guide defines “Spitters” as:

    …the result of condensation which forms when the compressed air used for turbine drive and shaping air decompresses, falling below the dew point temperature of the booth…

    In more simple terms, this is water, condensed from the air in the paint booth, dripping into the process and contaminating the parts. And whether you’re painting with solventborne or waterborne paint, it’s a problem.

    Where Does the Water Come From?

    Water is an important part of every paint operation. Booth humidity is usually tightly controlled and often at the higher end of the recommended 40% - 80% range. Frequently, a waterfall is used to catch and remove overspray, dust and dirt – especially in high-volume operations. The booth is also kept warm – usually in the 70°F – 80°F range, which assists in managing the humidity.

    In short, water in the air is a part of our normal process.

    How Does It Come Out of the Air?

    This is where things can get a little technical…

    Compressed air is used to power bell atomizers, and often to generate or assist with atomization in guns. It is also used for “shaping” the atomized cloud and directing the paint particles to the part.

    Greatly simplified, the air applies its force by decompressing. As it gives up its energy to perform the work, the decrease in energy causes the air to get cold. In turn it cools everything it touches along the exhaust path. In fact, ice formation is a common failure mode for air-driven devices!

    Think of what happens when you pull a cold beer out of the fridge. A haze of water droplets (dew) almost immediately forms on the outside of the can or bottle. This is the humidity (water) condensing from the air onto the cold surface. This condensation occurs because the surface is below the “dew point” of the surrounding air.

    If you would like a more detailed explanation of this process, please refer to our white paper: “Eliminating Condensation in Pneumatic Drive Systems in Modern Paint Operations”.

    Why Does It Affect My Paint Job?

    Just like on your beer, this condensation will continue to build until gravity causes it to form drops that run down the side of the container and pool at the bottom. When you pick it up to drink, it drips all over the table, your pants, your shirt, whatever. 

    It is no different in your paint process…

    As you paint, condensation will continue to build on your bell or gun, or parts in the exhaust stream, until it forms drops that gravity causes to fall into your process. It may drip onto your parts. It may fall into your atomized cloud. It may be blown around by your shaping air. Regardless, the result is defects on the surface of your painted parts, and that means rework, or worst case, scrap. Rework and scrap represent excess cost which, put another way, means reduced margins. And if you are painting in the modern world, your margins are already probably thinner than you’d like.

    How Do I Stop It?

    The answer probably seems obvious: Heat the air. 

    But it’s not that simple. It’s not just heating the air that matters – it’s where and how much you heat it. Heat it too far from the gun or bell and it just cools again along the way to the applicator – increased energy costs, but no gain. Heat it too much and you run the risk of trading spitters for dry spray – and rework is rework.

    This is where temperature control comes into play.

    “How” Matters…

    The most common solution is an in-line heater outside of the booth. This would seem to make sense in that most booths are intrinsically safe environments that require careful steps to assure that volatiles in the air don’t ignite. The problem with placing the heater this far from the point of application is that the heat is gone by the time the air reaches the gun or bell. The heating must take place directly adjacent to the point of use via a means that is both naturally intrinsically safe and controls the air fed to the process at a temperature above the dew point but below the dry spray threshold.

    Reach out to us to learn how our patented process solves all of these problems at once and allows you to remove a couple more defects from your “daily headache” list.

    Topics: Temperature control, Industrial finishing, automotive painting

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