Spray Booth Types & Airflow

 

Paint spray booths can be separated into many different type classifications based on the method in which the air flow through the booth is handled, filtered and directed.  All booths can be classified as either open type, non-pressurized, or pressurized. 

Open Type Spray Booths

An open type (or open face) spray booth consists of two side walls, a ceiling, and a rear filtered exhaust plenum. The air flows through the booth from the opening in the booth, through toward the exhaust plenum and is exhausted out of the building by the exhaust fan. In cold climates it may be a code requirement that the air exhausted by the booth is replaced into the building with heated “make-up air”.

Non-Pressurized Booths

A non-pressurized spray booth is an enclosed booth that typically will draw air through an inlet area in the spray booth that has been designed to allow air into the spray area.  This inlet area usually consists of a set of filters mounted in the booth doors or sometimes in a plenum. The air that flows through the booth is exhausted from the building by and exhaust fan. In cold climates it may be a code requirement that the air exhausted by the booth is replaced into the building with heated “make-up air”.

Pressurized Booths

A pressurized spray booth is an enclosed booth that is exhausted to the outdoors by an exhaust fan but is supplied with a similar volume of air that is ducted directly into the booth. In cold climates the air is supplied to the booth by a make up air system or a process air heater.  This creates a situation where the booth system can more effectively control the temperature and cleanliness of the air flowing through the booth cabin. Some form of pressure control is recommended to ensure that the same volume of air is exhausted from the booth as is being supplied to the booth.

Non-pressurized and pressurized types of booths can be found in a number of different configurations.  These configurations include crossflow, semi-downdraft, side-downdraft, and downdraft.

Crossflow Booths

A crossflow booth is one that is typically designed such that the air moving through the booth travels across the area from one end (or side) of the booth to the other. 

Semi-Downdraft Booths

A semi-downdraft booth is one that is typically designed such that the air moving through the booth travels from an input area in the booth ceiling to an exhaust area at one end of the booth structure. 

Side-Downdraft Booths

A side-downdraft booth is one that is typically designed such that the air moving through the booth travels from an input area in the booth ceiling to an exhaust area along both sides of the booth structure. 

Downdraft Booths

A downdraft booth is one that is typically designed such that the air enters the booth through the ceiling of the booth and exits through the floor.  There are a few styles of downdraft booths commonly available on the market.  Most common are pit style booths that utilize an excavated pit and tunnel system to draw air from the centre portion of the booth floor.  Also quite common are raised floor or “basement” style booths that draw air through grating in the booth floor into a fabricated box that supports the entire booth structure.  A number of variations integrating these two styles of booths can also be found in use.

Each type and style of spray booth has applications for which it is ideally suited.  Quite often a certain style or configuration of spray booth may not suit a particular application due to factors independent of the actual booth.  Such cases include the inability to install a downdraft booth in a shop that has insufficient ceiling height to accommodate typical booth structure.  Each shop has its own specific requirements which the equipment must meet.  In most cases, however, a shop should be equipped with the spray booth that will most effectively suits the needs of the shop production schedule and required quality level.

Pros and Cons

Although many shops can produce high quality work at relatively high production levels with comparatively low end booth equipment, it is widely accepted that today’s technology allows a properly trained staff to produce showroom quality finishes at extremely high production levels with minimal effort.  With the ongoing improvement in coatings and the rapid development of paint curing systems, the auto body shop industry has advanced in recent years to a level ten years ago most thought was not possible.

It should be noted that it is still quite common to find new spray booth products that have not integrated recent technology into their systems.  These systems are usually less expensive to purchase than newer booth systems but come to the new owner with the same limitations one might find in a much older spray booth system.  An example of this situation may be found in some new hot air baking booth systems that over pressurize when changing between spray mode and bake mode resulting in booth doors blowing open and potential dust contamination of freshly applied clear coat.  While most shops can deal with this slight inconvenience, the new booth buyer should be aware that systems are available that do not possess such air volume characteristics and their inherent problems.

Air Flow Concepts

Cross flow booths have been in use since the invention of wind.  The concept behind a crossflow spray booth is to move air past the object being sprayed in order to remove the solvent from the area and to keep the resultant overspray from being deposited on the newly coated substrate. The pressurized crossflow booth was designed in an effort to better control the quality of the air moving across the booth.  A non-pressurized spray booth gives the operator limited or sometimes no control over the air temperature in the booth. The temperature in a well designed pressurized spray booth may be controlled with very good accuracy.  The other key limitation found in a non-pressurized booth is the inability to control the air cleanliness.

Typical exhaust fans in paint spray booths have the ability to move a specified amount of air with a certain amount of restriction in the system.  As the system restriction (called static pressure) increases, the air volume moved by these fans decreases greatly.  Since the exhaust fans in non-pressurized spray booths are responsible for all of the air flow, these booths must be designed such that limited static pressure is created in the system.  As a result, the filters used at the inlet end of the booths are usually of a very low grade of filtration.  These low grade filters allow for reasonable air movement (due to low static pressure imparted on the system) but also allow visible dust and dirt particles to pass through into the spray area.

Pressurized crossflow booths are typically designed with inlet filters that provide a higher grade of filtration.  Air flow is not limited by the restriction caused by these filters because the make up air unit feeding the paint booth creates a “pressure” behind the filters.  This pressure allows air to pass through the filter media without requiring the exhaust fan to actually suck the air through. The input filters utilized in pressurized spray booths are often made of “diffusion media”. This type of media is designed to create a very even flow of air through all areas of the input filter.

It is apparent then, that pressurization of a paint spray booth allows the operator to work in an area that has a greater possibility of evenly moving clean, temperature controlled air through the spray area.  This allows the painter to concentrate all of his/her efforts on the application of paint product to the substrate.

The principal disadvantages found in any crossflow booth are due inherently to the direction of the air flow.  Because the air move from one end of the booth to the other, it is unavoidable that some jobs will cause dry overspray to be carried over a part that will be painted momentarily (or worse, has already been painted).  While an experienced painter may still be able to create a flawless finish in this situation, it is not without concern and extra effort that this is accomplished.  It is also common to notice areas of extremely turbulent airflow in crossflow paint booths.  This is especially apparent in drive through style pressurized crossflow booths.  Drive through crossflow booths require that the booth air is input and exhausted through yoke or leg style plenums.  This design creates turbulence where two banks of air must turn and impact each other on the path toward the other end of the booth.  This turbulence is mimicked at the exhaust end of the booth where a relatively laminar air flow must split into two banks to be exhausted out of either side of the doorway.

Similarly, semi-down draft and side-down draft booths also have inherent air flow disadvantages due to the required motion of the air passing through the booth.  The air in these styles of spray booths is introduced into the booth through one surface and must turn prior to leaving the booth.  This necessity to turn creates an area where particulate may circulate.  This turbulence can cause difficulty for the painter when applying product.  The circulating air flow pattern can also be responsible for depositing dry overspray back onto freshly coated areas.  It should be noted that while these air flow characteristics do not assist the operator in production efforts, a painter with the proper training and experience can produce extremely high quality work in these booths.

Downdraft paint spray booths have become the accepted standard in the automotive refinishing industry and are becoming increasingly popular in industrial applications as well.  It is due to the air flow characteristics found in these booths that this standard has been reached.  Well designed downdraft paint spray booths typically are designed such that the air is introduced into the booth through a large area of ceiling filter and is exhausted through some type of pit system in the floor.  This area in the floor is generally designed to such that when a part is placed or vehicle is parked in the booth, the pit draws air evenly from all areas around the part or vehicle.

There are many operational advantages that are created in downdraft spray booths due to this top to bottom directional air flow.