Figure 1
Much has been written about the operating and economic benefits to be gained by applying the principles of statistical process control to industrial finishing. The advent of affordable and reliable PLCs as well as the necessary pressure transducers and flowmeters suitable for the finishing environment has enabled astute finishers to instrument, monitor and control many of the process variables impacting the spray application of liquid coatings. By applying statistical process control principles to the control of the fluid, atomizing air and charging current as well as other process performance impacting variables, paint mileage has been improved and quality-related rejects and rework have been reduced. In the majority of the cases, the investment in additional control has quickly paid for itself.

However, one of finishing's more significant variables, spray booth ventilation, remains very difficult to control. While some enterprising technicians have attempted to monitor and control booth ventilation, few if any have achieved long-term success. Dry filter booth exhaust airflow varies much more than water-wash booth airflow. Resistance to airflow in filter-type booths increases as the arresting filters load with overspray. Because the tubeaxial propeller fans in widespread use are relatively inefficient air movers, the exhaust airflow steadily decreases as overspray accumulates in the booths' arresting filters. In comparison, properly maintained water-wash booths have relatively stable airflow. Traditional airflow control methods do not facilitate reliable monitoring or the effective real-time control of a dry filter or water-wash spray booth's actual exhaust airflow rate.

Compliance coatings and environmental rules have changed, making dry filter booths - once largely relegated to off-line and touch-up operations - the booth of choice for many industrial finishers. They are not plagued with nozzles and pumps that may become plugged with paint sludge nor do they contain hundreds of gallons of water laced with potentially hazardous paint-related chemicals. Unfortunately, dry filter booths do not provide a stable exhaust airflow environment. The red line on Figure 1's chart accurately depicts a typical crossdraft industrial dry filter booth's exhaust airflow variations over a complete filter loading cycle. In a typical dry filter booth it is common for the exhaust velocity to vary from 15 percent to greater than 60 percent over a single filter load cycle.

Transfer Efficiency Directly Impacted By Exhaust Ventilation

Discerning finishers know that most of the higher efficiency liquid coating application methods including air-assisted airless, HVLP and electrostatic mini-bells are especially sensitive to relative minor air movement changes in the space between the atomizer and the product being painted. Furthermore, it has been observed - all other things being equal - transfer efficiency or paint mileage increases as the average air velocity in this space decreases. The proper setup of a spray booth is a delicate balance between maintaining safe operating conditions on one hand and optimizing the system's coating performance on the other. Given the short life (often less than a full shift) of arresting filters in high production finishing operations, optimizing a production spray system's performance is somewhat akin to attempting to perform cataract surgery while riding in a school bus going down a gravel road. It isn't technically impossible, but it sure is a challenge!

Today, most industrial product finishers set up their liquid coating application lines to achieve adequate (minimum acceptable) coverage when the booth's arresting filters are clean and then tweak them to minimize appearance-related quality problems caused by the increasingly heavier film builds that usually occur as the filters load up with over-spray. The net result is that both transfer efficiency and energy conservation suffer when the filters are clean and the booth is hyperventilated. The potential under-ventilated condition that occurs when the filters approach their fully loaded condition is frequently the source for both safety and quality of finish concerns.

Typical industrial dry filter crossdraft spray booths' average airflow rates range from 80 to 150 fpm (approximately 1 to 2 mph) through the face of their arresting filter banks. Variations in airflow rates in this range are difficult to detect without the aid of sensitive instrumentation. It is almost impossible for painters to detect changes in airflow and manually compensate for them as a part of their normal production operations.

Table 1

Safety is Paramount

Failure to maintain safe operating conditions can have catastrophic consequences for the workers and the facility. NFPA 33 Paragraph 7.2 requires spray booth exhaust air movement to be "capable of confining and removing vapors and mists to a safe location" and "capable of confining and controlling combustible residues, dusts and deposits. The concentration of vapors and mists in the exhaust stream of the ventilation system shall not exceed 25 percent of the flower flammable limit." In addition, OSHA regulation 29 CFR 1910.94(c)(i) requires spray booths to be designed so that air velocity in the booth cross section is not less than that specified in Table G-10 as published in the referenced paragraph (Table 1).

Finally, OSHA 29 CFR 1910.107(b)(5) requires: "Dry type over-spray collectors - (exhaust air filters). In conventional dry type spray booths, over-spray dry filters or filter rolls, if installed, shall conform to the following:" 29 CFR 1910.107(b)(5)(i) "The spraying operations except electrostatic spray operations shall be so designed, installed and maintained that the average air velocity over the open face of the booth (or booth cross section during spraying operations) shall be not less than 100 linear feet per minute. Electrostatic spraying operations may be conducted with an air velocity over the open face of the booth of not less than 60 linear feet per minute, or more, depending on the volume of the finishing material being applied and its flammability and explosion characteristics. Visible gauges or audible alarm or pressure activated devices shall be installed to indicate or insure that the required air velocity is maintained. Filter rolls shall be inspected to insure proper replacement of filter media."

The question is: What tools are available to a prudent finisher to assist him in optimizing the performance of his coating application process and simultaneously be assured he isn't compromising the system's margin of safety? Historical attempts to control spray booth airflow have used one of two approaches - pitot tube driven variable speed air movers, or dampers actuated by pressure differentials between segmented plenums. Both approaches have serious shortcomings that have limited their viability in industrial production spray booth applications. A recent advancement in airflow sensing technology has opened the door to monitoring spray booth exhaust airflow and even more importantly controlling it in a manner that maintains safe operating conditions and optimizes the system's transfer efficiency.

This open-face cross-draft spray booth implements technology that allows spray booth ventilation to become a controllable process variable.

Pitot Tube Sensor

A pitot tube's sensing orifice is less than 0.125" dia. This orifice is positioned in the exhaust airstream with the opening facing upstream and parallel to the moving air stream. Experience has shown the small sensing orifice is can easily be blinded by over-spray penetrating or bypassing the arresting filter network. The integrity of the output signal is also dependent on proper alignment of the tube in the duct. Experience has shown the vibration in most spray booth exhaust systems will eventually cause misalignment. Finally the airflow in most booth exhaust duct is turbulent and is not uniform across the cross section of the duct. The pitot tube measures the flow velocity at the point its sensing orifice is located within the duct, not the duct's total flow.

Pressure Differential Controlled Damper

An automatic controlled damper system is programmed to maintain a constant overall resistance to airflow by adjusting the damper during operation. In other words, the damper system closes when the filters are changed and endeavors to maintain a constant load on the fan by slowly opening the damper as the filters load with over-spray. While automatic damper systems have been successfully used, they are inherently energy inefficient. The exhaust fan motor consumes needless electrical power and accelerates the wear and tear on the fan motor and its drive components.

Figure 2

Stable, Closed Loop Control

A new sensor technology has emerged to improve the operation and maintain closed-loop control during operation. The Hi-Pro (patent pending) constant airflow control technology is designed to provide the needed control, save energy and improve material efficiency (Figure 2). The sensor, installed in the exhaust duct, takes an accurate reading of the airflow and adjusts the fan volume to maintain a constant air velocity during the shift. Every element in the entire system is stable, rugged, reliable and resistant to degradation by the contaminants commonly found in spray booth exhaust streams. The Hi-Pro airflow sensor has no moving parts and needs no periodic maintenance. It provides the reliable control without the problem associated with more traditional approaches to airflow management.

The system may require an exhaust fan upgrade and it may require a marginal increase in stack diameter compared to a typical spray booth. These changes can make upgrades of existing spray booths a problem. It is frequently easier and cheaper to replace existing dry filter booths with ones specifically designed for constant airflow. It is much easier to install in a new booth.

Each of these constant airflow spray booths includes a closed-loop control system that enables the operator to select the optimum airflow for their finishing operation. The control package not only maintains the preset airflow rate, but also allows it to be remotely increased or decreased by the spray station controller in the same manner as other finishing process variables such as atomizer triggering, fluid pressure, shaping air pressure and charging current.

This system can enhance the performance of almost all types of over-spray arresting filters. Controlled laboratory tests show paint arresting filters are least efficient when they are clean. Their low initial pressure drop (typically in the 0.05 to 0.1 inch w.c.) combined with the booths fan performance curve (Figure 1) aggravates this condition. The closed-loop control in the constant airflow spray booth minimizes the clean filter inefficiency problem by limiting the airflow through clean filters to the predetermined optimal level. The airflow does not have to be higher when the filters are clean and gradually drop as the filers load. Maintenance of a constant airflow throughout the filters' entire functional life frequently increases their over-spray holding capacity.

Another feature of this system addresses the limited over-spray holding capacity of most current over-spray arresting filters, an ongoing source of frustration for many industrial finishers. A zero bypass, zero overlap, filter retaining system facilitates the quick removal of loaded filters as well as the proper installation of their replacements.

Benefits of Airflow Control

The benefits of having reliable and constant airflow control are numerous. Depending upon the process parameters associated with a finishing operation, quantifiable benefits include:

Improved paint mileage - the ability to coat from 10 percent to 35 percent more surface area per gallon of coating purchased and sprayed.

Reduced energy consumption - saving from 10 percent to 40 percent in the combined gas and electric utility cost directly associated with operating the spray booth and its air makeup unit.

Reduced total annual arrestance filter-related expense of 20 percent to 50 percent - These savings include:

  • The total annual filter purchase expense is reduced. Because the arresting filters have a significantly greater over-spray holding capacity, they last longer and fewer replacements are purchased.
  • The labor expense associated with changing filters is reduced. Because each arresting filter lasts longer the total number of changeouts is reduced by more than 50 percent.
  • Loaded filter disposal costs are correspondingly reduced.
  • Production lost to mid-shift arresting filter changeout is eliminated.

A patented and unique arresting filter retention system simplifies proper filter installation. It virtually eliminates filter bypass and therefore greatly reduces exhaust plenum and stack maintenance and housekeeping labor expense.

Quality of finish related scrap and rework expense is reduced up to 10 percent.

This constant airflow control spray booth technology provides several additional important benefits that are more difficult to quantify. The spray booth ventilation can be a controllable process variable. It can be set and maintained as the filters load with over-spray and it can be programmed up or down as required to insure safe operations and optimize transfer efficiency as part profiles and configurations change. The ability to tune the finishing process variables to improve the transfer efficiency brings with it a corresponding reduction in VOC emissions. The unique over-spray arrestance system is a substantially more efficient arrestor of the troublesome smaller (PM2.5) and mid-size (PM10) airborne particulates. This feature is of particular importance to finishers using oxidizers or systems with periodic downstream fallout problems.

This new technology solves an age-old problem that has cost companies time and money every since the first dry-filter spray booth was installed. It provides a solution that will be a great interest to knowledgeable finishing technicians and companies seeking a competitive edge.