Paint Booth Pressure Levels Richard Thelen, PE, Global Finishing Solutions Much thought has been given to paint booth pressures.  Usually this discussion centers on whether the booth is under a negative or positive pressure.  Some discussions insist that the booth be held at a positive pressure relative to the outside so that dirt and debris cannot enter the painting chamber and soil the object being painted.  This is a serious consideration in booth design and is generally solved by having very good seals on the doors and completely caulking the panels.  Others insist the booth be at a negative pressure relative to the outside.  This prevents emissions and VOCs from entering the room adjacent to the spraying chamber.  Once again, having very good seals on the doors and caulking the panels solve this problem. The aim of the booth designer is to achieve laminar airflow at all relevant points in the paint booth.  This can be accomplished by managing the airflow.  Using a sheet metal shroud to manage the airflow is an effective means and is approved by NFPA-33.  Laminar airflow does not necessarily mean the airflow is constant.  In fact, if left at a constant level, the airstream has a tendency to degrade and develop vortices (room effects), which are damaging to the painter’s environment.  A slight acceleration of air speed from front to back is helpful in controlling the vortices. Room effects are hard to see and to understand.  It is better to follow simple empirical practices to negate the effects of the room.  Acceleration of the airflow is one such way to manage the problem.  The other is by understanding booth pressure. This negative/positive pressure discussion generally assumes that the paint booth is a place that is at a uniform pressure level.  This is not true and the science of balancing paint booths, especially so-called “big rooms” is easy to understand.

Figure 1 - Pressure Difference In order for air to move a pressure difference must exist from one end of the booth to the other.  In a draw-thru booth (Figure 1) the pressure at the entrance will be near zero, and it will get progressively more negative as it approaches the filter bank and exhaust fan.  One may think of the spraying chamber as a section of ductwork, since it is completely enclosed.

Figure 2 - Draw-Thru Balance Notice the gradation of pressure as the air moves from one end to the other.  If the room pressure were to be measured relative to the outside area at the center of the booth, you would find that the pressure would be different at the end of the booth nearest the exhaust filters. Notice that in the draw-thru system, the air is drawn through a filter door or wall.  This is usually the product entry door because it has sufficient area to allow for good filtration.  It is an important addition to a booth to add a filter door (as opposed to using an open front booth) because air carrying dust and dirt particles is filtered prior to entering the painting chamber removing most of the cause of imperfections.  Filters will also smooth out the airflow by diffusing the air.

Pressurized Systems In pressurized systems an air replacement system consisting of a fan and maybe a heater system adds replacement air directly to the booth in lieu of drawing the air thru a filter door.  These systems are difficult to balance, but once the fundamentals are understood the balancing is very simple.  Let us look at a system that is incorrectly balanced (Figure 3). Firgure 3 - Pressurized with turbulence In this system, more air enters the booth than is allowed to leave.  When the air systems are first turned on, the suction from the exhaust tries to evacuate the room, but in time the surplus air builds up pressure in the middle of the booth causing a cloud to form.  Air is a compressible fluid and will just “bunch up” at the point of interference. This cloud is invisible until the painter starts his painting operation.  Then the air is colored with paint and the cloud is very visible (See Figure 3). Turbulence is also obvious when using a small handheld velometer.  Since the meter is a vane type device, air entering from one side causes the vane and its needle to move to the appropriate place on the scale.  When air enters from both sides such as at a location of turbulence, the needle and vane oscillate and it is difficult to get a reading on the meter that is stable. In any case, turbulence is one of the worst things that can happen to a painter.  It causes all manner of blemishes in the paint from thinly applied paint to overspray and dirt inclusions, to low visibility.  Turbulence will show up as paint overspray on the walls and lamps of the paint booth, on the floor, and on the painter himself.  It makes the working conditions very difficult and the chances of customer dissatisfaction very likely.

Figure 4 - Pressurized and Balanced Looking at a well-balanced system (Figure 4), the booth is at a negative pressure (relative to the outside) from the time the fresh filtered air enters the booth until it leaves through the exhaust filters.  This is the same pressure profile as the draw-thru system.  It is as if the exhaust fan were reaching out to the zero point of pressure in the booth and gathering the air, while the supply fan is pushing the air just enough to move it to the zero point of pressure in the booth. It is best to design a booth to move the zero point of pressure in the painting chamber to the intake filter section of the booth.  In this manner the fan reaches out to the filters and pulls the air through them into the booth.  This is the same principle used in the draw-thru system.

So much for theory! What about actual practice?  When the “perfectly balanced” environment of a spray booth is disturbed by the object to be painted as well as by a painter and spray apparatus, the perfect booth also changes.  This is natural, obeying physical laws. The most noticeable thing that happens is the filters become soiled with paint and particulates.  This fact of life is a sign that the paint booth is functioning correctly.  If the opposite were true, the paint booth would be greatly out of balance and paint and particulate would soil the object and the floor or walls.  As the filters become clogged, the exhaust fan sees a greater resistance to flow and moves less air.  The intake fan is less likely to see a change to its resistance and continues to supply the same volume of air. The effect is the zero point of pressure in the booth starts to migrate toward the center of the booth.  Wherever this zero point is located a cloud of paint will be seen when the painter is working at that location.  A velometer will also be helpful in demonstrating this effect.  To move the zero point back to the intake of the booth, the exhaust fan must run at a faster rate to compensate for the increased static pressure. The practical way of doing this is to install a variable speed drive system (VFD) on the fan allow the fan speed to increase as the filters load with overspray.  There are different ways to control airflow in draw-thru and pressurized systems. With a draw-thru system, a partial perforated plate is installed in the ductwork after the exhaust fan.  This acts as an orifice plate of sorts.  The differential pressure across this plate is directly proportional to the airflow within the range we wish to control the speed of the fan.  As the differential pressure builds the pressure sensor and pressure controller increases the speed of the fan using the VFD. With a pressurized system, a sensor and a controller measure the room static pressure relative to the outside pressure and adjusts the fan speed by using the VFD.  It is not important where the sensor is located since the gradation of pressure is essentially linear. If a pressure of -.03” is held at the center of the booth, holding the pressure to -.05” nearer the exhaust filters can have the same good results. The object (to be painted) introduces challenges that can frustrate a painter at times.  But painters are clever creatures and by experimenting with their stance and the location of the spray they can overcome most of the negative effects of the object interferences.  Mostly, these effects show up in turbulence and the overspray causes a soiled area on the object to be painted.  By choosing the method of applying paint, much of these effects can be mitigated.  The painter may find that by spraying from back to front works better than the opposite way.  Perhaps re-orienting the object will modify the turbulence to the point that it is no longer a problem. Balancing a booth can be a difficult job unless the technician understands the principles of air movement and pressure gradients.  Once armed with these principles, the job is less an art than a science.

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