Downdraft Versus Cross-Draft Booths

Q. We are considering a new spray booth, and we’ve read a lot about the different types of airflows. What can you tell me about downdraft booths and cross-draft booths, and what should I consider in adding one to my shop?

A. Let’s break down the basics. In a true downdraft booth, airflow travels from supply filters in the ceiling to exhaust filters located in a concrete pit or raised floor basement. Airflow should be vertical and laminar throughout the booth. Booth configuration—such as hip and ceiling light panels, or narrow exhaust filter pans—determines the airflow and can be used to achieve both negative and positive effects. For instance, it’s typical to see a narrow exhaust filter pan at the center of the booth when painting vehicles. This allows the booth to be designed to a lower cubic feet per minute (cfm), and increases the velocity at and around the vehicle surface. It also draws the overspray away from freshly painted surfaces. Full floor exhaust pans are typically best in booths used for small parts or parts with large open areas.

Downdraft booth velocities range from 30 feet per minute (fpm) to well over 100 fpm, as an average when measured empty. Booths with lower velocities are designed understanding that the painted product will occupy a large area, increasing the velocity to an acceptable range. Another key factor is the large airflow volume of downdraft booths. Typically, these require 20- to 40-percent larger air volumes than a traditional cross-draft booth, resulting in a 20- to 40-percent higher operational cost and a 20- to 30-percent higher upfront capital expenditure. The performance may be worth the cost. 

A semi-downdraft booth is a common variation of the one previously described. Airflow travels from the supply filters downward until drawn to both the left and right lower wall mounted exhaust plenums. Semi-downdraft booths mimic the performance of a downdraft without the expense of a concrete pit or basement. Unfortunately, airflow is not laminar throughout the cabin, and dead spot or eddy currents can be found at the center of the booth near the floor. The dead spot is exaggerated in wide or tall booths and should be taken into consideration.

That said, the semi-downdraft booth does provide good overspray control, arguably better than a downdraft. Overspray is pulled away from the product being painted toward the wall’s plenums. In an application where dry spray contaminates adversely affect freshly painted surfaces, this can be a major advantage; however, it comes with a cost. In these applications, the painter is in the direct path of the overspray and precautions should be taken to monitor the painter’s exposure to hazardous chemicals. 

Semi-downdraft booths are commonly used to paint a variety of small parts. Parts can be arranged in front of both wall plenums, enabling multiple painters to operate with minimal overspray concerns. Semi-downdraft booth velocities are similar to that of downdraft booths but with higher costs. Upfront cost can be slightly higher than downdraft booths, excluding any concrete construction cost. Nonetheless, semi-downdraft booths can be a good compromise. 

Cross-draft booths are the most common booth style. In cross-draft booths, air travels horizontally from a wall-mounted supply plenum, filtered doors or simply a booth opening across the booth to a wall-mounted exhaust plenum. Cross-draft booths can be configured with hipped light panels without negatively affecting the airflow. When outfitted with filtered supply plenum or doors, cross-draft booths can additionally provide a clean application environment similar to that of the downdraft and semi-downdraft booths. 

The major downside of cross-draft booths is overspray control. Overspray is consistently drawn across the booth in the direction of the airflow and, in many cases, contaminates unpainted or previously painted surfaces. In applications with multiple painters, overspray exposure is expected. Velocities in cross-drafts are almost always designed to a minimum of 100 fpm, as defined by local fire and building codes. Open-face cross-draft booths are often designed to 125 fpm, to further contain overspray through the life of the exhaust filters. 

A modified cross-draft is one version of a cross-draft booth in a partial ceiling supply air plenum that is located at one end of the booth opposite exhaust. Often, the modified cross-draft is confused with a semi-downdraft booth, but performs similar to a cross-draft with the negative addition of a dead spot and eddy below the supply plenum. Another type of cross-draft booth is the cross-flow, which is the most economical and requires the least investment of the styles discussed, but provides the lowest quality and performance. 

All of the aforementioned booths meet the basic safety and containment requirements for most spray applications. These booths contain and filter overspray out of the discharge air stream and exhaust the VOCs to the building exterior. 

To make a decision on which type of booth to purchase, information is key. Read articles and discuss your process with your paint supplier and potential equipment vendor. Most importantly, involve your painters in the conversation. They will learn how the spray booth is designed to perform and will be more likely to take advantage of the new features instead of resisting change.  

Dave Rohner is president and CEO of Rohner. For more information, visit rohnerspraybooths.com. 

 

 

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Originally published in the November 2016 issue.