1.3.1.a System Design
The needs of the application — what is to be painted in the booth — determines everything about the booth system.
Objects from small to gigantic are painted in spray booths. Coatings are applied by humans and robots alike. Perhaps the application is refinishing and the object is a car or other vehicle. Or maybe a complicated coating is applied, under the most stringent temperature, humity and airflow requirements, to super-secret jet fighters. Or perhaps parts travel on a conveyor system and pass through the booth system to be sprayed.
Regardless of the application, careful matching of the system and equipment to the needs of the coating operation is critical to the successful performance of the finishers and the spray finishing equipment.
Selecting the booth and sizing it for an application takes review of several areas. Knowledge about the facility and production process is important in choosing the right equipment. It is important to take time to understand the application and any future plans that may influence the choice of spray booth design. The following are some general guidelines for selection and sizing.
All booths require regular maintenance for optimum performance. As a first step, evaluate the capability of the maintenance department or maintenance contractor. This will influence the sophistication level of the equipment required.
Balancing the application requirements and available funds will help identify the most effective exhaust chamber, air flow and booth options for the job.
3. Selecting the Booth Design
The first step in selecting an appropriate booth design for an application begins with an investigation of the production requirements, which helps determine the direction of air flow through the booth, as well as the appropriate filtration method, either dry filter or water wash.
The spray booth is an investment that pays many dividends by providing a cleaner painting environment for a better quality finish, a means of increasing productivity, and a superior working environment for the finisher.
1.3.1.b System Design
Part Size and Configuration
The size and style of vehicle/part, the carrier that conveys it into/through the booth, and the relationship of the spray gun to the vehicle/part, all play a role in determining the direction of airflow as well as the velocity or speed of air through the booth. Airflow and velocity are needed to transport paint and coatings overspray into the filter assembly.
There are three types of airflow through a booth: crossdraft, semi-downdraft and downdraft
Production Rate and Transfer Efficiency
Production rate is a measure of the number of vehicles/parts that can be finished within a certain time frame, usually per hour, per shift or per day. Production rate includes the entire production process, including any time the vehicle/part spends drying or curing.
Transfer efficiency is the percentage of material being sprayed that adheres to the part; the remainder is overspray. The type of application equipment — conventional, electrostatic or HVLP (high volume low pressure) — largely determines how efficiently paint is transferred from the gun to the part. The recent development of a laser-guided sight for spray guns and the resulting precision spraying has increased transfer efficiency an additional 25-30% in some instances, proving that the technique of the spray operator also impacts transfer efficiency.
Together, production rate and transfer efficiency influence the choice of air flow.
Material Being Sprayed
The type of material being sprayed affects the choice of filtration or exhaust method to remove overspray from the booth, and determines whether or not heat or other curing means is also required to bring the finish to a final state.
A dry-filter or paint-arrestor booth traps airborne paint particles (overspray) in disposable filters. A dry filter can be used where the material usage does not exceed 2 gallons per hour with minimal overspray. This represents the majority of applications. Depending on the material being sprayed, removal efficiency ranges from 95% to 99%.
If more than one type of material is being sprayed, it's important to ensure the compatibility of the materials. Combinations of incompatible materials in the dry filter can be a cause of spontaneous combustion.
In a water wash booth, air washing action traps the paint solids from overspray. Removal efficiency for a water-wash booth can be as high as 98-99%, depending on the type of material being sprayed.
If the coating material requires a period of heat or air movement to finish it, then heaters and other curing mechanisms will be part of the final configuration as well.
The quality of the finish on the completed part has become more critical as customers' expectation levels have increased. The total process must now be considered in order to achieve first-time-through quality levels.
The spray booth design is a key element. Airflow, direction, filtration, air velocity and balance are critical to accomplishing the various desired quality levels. Unpressurized crossflow designs are at the low end and pressurized downdrafts are at the high end of quality potential.
But it's important to also realize that the spray booth is only one part of the process. Many other elements of the process must be designed and controlled to achieve the desired quality level. These elements include the preparation and cleanliness of the object going into the booth, the maintenance of the booth and surrounding processes, the quality of compressed air to the tools (including spray gun), the quality of clothing and equipment the painter uses, and the quality of the paint or coating preparation activities. The finish quality can only be as good as the design and control of the process.
1.3.1.c System Design
Sizing the Booth System
Determining booth size is an important step in developing the system design. It will influence the booth location and be influenced by the type of operation (manual or automatic). Reviewing the facility layout and proposed booth location will help determine whether the allotted space is adequate for the size and style of booth.
A properly-sized booth for manual spray operations will give the operator and the finishing equipment adequate room in which to work. Adequate means enough space for the operator to move around, stoop down and bend over, and allows an even, fluid arm motion.
For an automated application, the correct size booth will provide enough space for automatic equipment to operate effectively. This includes allowing for the operation of side-to-side and overhead reciprocators, and providing the necessary clearances for electrostatic equipment. During finishing, there should be sufficient velocity through the booth and past the equipment to keep it in clean operating condition. When conveyors are transporting parts through the booth, the booth size is directly related to conveyor speed.
Minimum and maximum part dimensions determine the booth width, height and depth. Conveyor openings are required when a conveyor moves parts through the spray booth. Conveyor openings should allow 6-in. minimum clearance around the part. In this graphic, the booth design on the left is poor. The conveyor openings are too large and too close to the exhaust chamber resulting in less air flow past the painter. The better design on the right makes use of entrance and exit vestibules.
A vestibule is a protected entryway into the booth. It provides better airflow control through the booth by effectively blocking the tunnel leading into and out of the booth with product. The vestibule length should be a minimum of the gap between parts so the vestibule always contains a part.
Measure the width dimension of the largest article, including fixture or pallet, and add two feet minimum clearance on each end.
In multiple-operator booths allow a minimum of 6 to 8 feet for each finisher.
In conveyorized processes the width must be sufficient to allow finishers to complete the finishing operation within the allotted time, and spraying should not be closer than 2 ft. from the conveyor opening.
This space permits the part to be turned if necessary and enables the operator to work comfortably.
The height of the booth is determined by the overall height of the largest item, plus the height of its holding fixture — plus 2 - 3 ft. clearance.
This measurement gives the operator sufficient room to coat the top of the part without coating the booth ceiling. The part should also be high enough above the floor to allow the operator room to spray the lower edges and the underside easily
Sufficient working depth will allow at least 3 ft. between the rear of the part and the filter pads or water wash tank, at least 3 ft. between the front of the part and the booth face or intake filters, and allow for automatic machines, such as reciprocators, in conveyorized applications.
Working depth should be sufficient for the part, including the fixture or pallet, to be entirely within the booth enclosure during finishing, plus allow for clearance at the rear.
The finisher should work within front line of booth, except on bench or leg type booths.
1.3.1.d System Design
Heating The Booth
OSHA requires the work compartment of a spray booth be maintained at a minimum temperature of 65° F. To meet this regulation, it is mandatory that heated air make-up be used during the winter months in most areas.
In addition, many coatings require a heat-enhanced curing period after application to reach their final finished state, and this heat is applied through a heater or burner unit
- Types of Heaters
- How it Works
- Calculating Costs
Types of Heaters
The world of process heaters has been divided into two categories, indirect-fired and direct-fired, defined as follows:
"Indirect-fired process heater" means any process heater in which the combustion gases do not mix with, or exhaust to the atmosphere from the same stack(s), vent(s), etc. with any gases emanating from the process or material being processed.
"Direct-fired process heater" means any process heater in which the combustion gases mix with and exhaust to the atmosphere from the same stack(s), vent(s), etc. with gases originating with the process or material being processed.
Thus, indirect-fired units are used in situations where direct flame contact with the process material is not wanted because, among other reasons, of the problems of contamination and ignition of the material. Direct-fired units are used where such problems are not a factor. Emissions from indirect-fired units consist entirely of products of combustion (including those of incomplete combustion). Emissions from direct-fired units, on the other hand, consist both of products of combustion along with emissions of the process material. Thus, emissions to the atmosphere from indirect-fired process heaters are generic to the fuel in use and are common across a wide range of industrial sources while those from direct-fired units are unique to the given process and may vary widely both within a given industrial process (if the process material is changed) and between industrial sources (where widely varying process materials may be handled).
Many imported spray booths use indirect-fired burners. This is because the booth and all its components are being manufactured in an area where there is no vast natural gas network. Instead, the primary fuel is oil. With oil, the only choice is indirect-fired. A direct-fired oil burner, if ever developed, would have to overcome the many issues associated with combustion by-products of oil. It is more economical for importers to offer the same equipment in North America that they make for their other global customers.
The subject of combustion by-products leads us to another question - don't direct-fired burners pollute the atmosphere or cause health issues for the workers? While it is true that a direct-fired burner comes in direct contact with the outside air being heated to replace the air exhausted from a facility or a paint booth, many years of research and development have gone into these burners to assure that they burn as clean as possible and the products of combustion are be well below any governmental limits for all of these products. In addition, burners are tested by an independent testing laboratory to assure that they are in compliance with the American National Standards Institute (ANSI)
For the paint booth application, the ideal unit should be able to heat the air to the desired temperature quickly, maintain that temperature closely, be able to modulate down to only raise the temperature a few degrees on those mild days, and be as safe, reliable and economical as possible. The direct-fired system meets all of those requirements, while the indirect-fired system only meets some of them.
How a Heater Typically Works
A curing paint booth provides basically two successive operating cycles:
-1st Phase - Spray Mode and Flash-Off
-2nd Phase - Bake Mode and Cool Down
Phase 1- Spray Mode
The Spray Mode is the period of time during which the paint material is being sprayed onto the vehicle.
During this phase, the operating cycle ensures the correct air pressure and temperature for the painter, as well as excellent air filtration for proper results of the paint application.
The operator turns on the power and sets the appropriate switch on the control panel to "spray".
The spray cycle is as follows: The damper (1) positions itself automatically to allow the intake blower assembly (2) to only draw in outside fresh air (3).
All the air then passes through the pre-filter (4) then through the burner or around the heat exchanger (5). The outside air is heated to the preset temperature on the control panel and enters into the plenum (6) of the booth. Here, the air passes through the ceiling filters (7), enters the booth (8) and is evenly distributed throughout the booth cabin. The air is then exhausted beneath the floor (9) through the paint arrestor filters (10), where most of the overspray is removed. Then it enters the exhaust side of the mechanical unit (11) where it is expelled through the duct exhaust to the outside (12).
Phase 1- Flash-OFF
The flash-off phase is the period of time between two applications of paint or between the last application and the bake cycle. This time is necessary to allow the paint to flow out and release solvents.
This is an extremely variable phase, which may or may not be necessary, depending upon the type of paint and application method used. The time setting will be determined in each case by the painter and paint supplier
The flash-off phase is identical to the spray mode, except for the possible change in air temperature supplied to the booth, therefore:
During the spraying and flash-off phases, the booth should always be operating in the spray mode, with 100% fresh air. Do not turn off booth. This is to avoid any possible build-up of solvents in the booth, which could reach levels of flammability and/or explosion.
Phase 2- Bake Mode
The bake mode is the period of time required for the curing of the paint applied to the vehicle.
During this phase, the control unit maintains the operator's pre-selected temperature (up to 176 degrees F) and excellent filtration for proper results.
No one should enter the booth during the bake mode.
The operator sets the switch on the control console to "bake." This automatically activates the bake timer which should have been set in advance with the correct cure time. The bake time counter will start as soon as the booth reaches the preset temperature for this phase.
The operating cycle is as follows: The damper (1) automatically positions itself to permit the intake blower assembly (2) to draw a portion (10 - 15%) of the air from the outside (3) and re-circulate the remaining (85-90%). All the air then passes through the pre-filter (4) and around the burner or heat exchanger (5). It is heated to the preset temperature on the control panel and enters into the plenum (6) of the booth. Here the air passes through the ceiling filters (7), and then enters the booth (8) and is evenly distributed throughout the booth cabin.
The air is then exhausted beneath the floor (9) through the paint arrestor filters (10), then it enters the exhaust chamber (11) where 10-15% of the air is expelled outside (12) and the remaining 85-90% is re-circulated.
Phase 2- Cooling
The cooling phase is the period of time required to cool down the heating unit and the vehicle.
This phase starts automatically upon completion of the bake period. The length of this phase is preset and controllable via a thermostat. A sensor is located above the burner or heat exchanger and close to the connecting duct between the spray booth and the monoblock. If the thermostat temperature setting is too low, making it impossible for the outside air to cool it to the preset temperature, a preset timer will interrupt the cooling even though the preset temperature has not been reached.
The operating cycle is similar to the spray mode, in that the dampers automatically position themselves to draw 100% fresh air from the outside, like in the paint cycle.
Never turn off the power to the booth when it is operating in the cooling cycle. Doing so will stop the blower assembly, thus preventing the proper cooling of the combustion chamber, which could then overheat and be damaged.
The red emergency button is not operational during this phase.
Power to the unit should be turned off only when the blower assembly is not in operation or when absolutely necessary.
If it is absolutely necessary to interrupt the cooling cycle due to an emergency, turn off the main power switch.
Calculating Heating Costs
The formula for calculating heating costs is as follows:
Annual fuel cost = cfm x (T - To) x 1.08 x H x C / F x E
cfm = Actual cubic feet of air delivered by the air make-up per minute
T = Temperature of air leaving unit (should be same as space temperature)
To = Average outside air temperature during heating season
1.08 = Constant arrived at by multiplying 0.075 (air density) by 0.24 (specific heat) by 60 min/hr
H = Total hours of operation from October through April inclusive
F = BTU value of one unit of fuel (generally, 1,021 for natural gas per cubic foot)
E = Efficiency of unit (0.92 for direct-fired air make-up unit)
C = cost of one unit of fuel (must be expressed in the same units as those used for F)
A 10,000-cfm air make-up unit in a building in St. Louis operates 60 hrs per week at 65°F space temperature. It is fueled by natural gas at $0.40/ft3.
We find the annual operating hours by
7 months of operation x 52 weeks x 60 hrs/wk = 1820 hours /F x E
Translate heating value into BTU/100 ft3
1021 BTU/ ft3 x 100 ft3. = 102,100 BTU/100 ft3
Figure annual costs
10,000 x (65 - 43.1) x 1.08 x 1,820 x 0.40 = $1833.10
102,100 x 0.92
Remember that this sum represents the greatest cost to operate the air make-up unit. Actual cost could be less.
1.3.1.e System Design
Evaluating System Functioning
How do you know your booth is running efficiently and correctly? What instruments are there to help gather data about booth functioning? Why would anyone want to evaluate system functioning in the first place?
In our own technical department, we evaluate system functioning each time a customer feels his/her booth is not performing as expected. Almost all problems will evidence themselves in the finish, but not all problems stem from the same source.
Depending upon the exact symptoms, the field representative will take measurements and examine booth functioning in a variety of ways.
Used to measure temperature within the cabin or within duct. Used during spray and/or bake. Think of the probe as a thermometer placed in the airstream.
LASER PARTICLE COUNTER
Measures the size and quantity of dirt particles passing through the instrument. The instrument can be set to measure any particle size, typically 1 - 10 microns. It is designed for measuring particles in the spray booth around the vehicle or in the spray area to confirm that the ceiling filters are collecting particles of 10 microns and larger. It can also be used for determining sources of dirt, for example, around door seals or other openings to the exterior of the cabin.
DIGITAL LIGHT METER
Used to measure footcandles to determine light distribution across the vehicle's surface.
SOUND LEVEL METER
An instrument that measures sound outside or inside the cabin. Uses the dBa scale to quantify the degree of sound.
A portable close-up viewing device used to examine suspected contamination on the vehicle's surface.
A meter designed to measure incoming power source voltage (3 phase or single phase). It measures the actual voltage at each electrical device (motor, light fixture, contact, etc.).
A meter designed to measure the amperage usage at each motor to confirm that it is operating within the specified limits.
Measures actual RPMs (revolutions per minute) of a motor.
REFINISH MIL THICKNESS GAUGE
Measures the thickness of paint after it has been applied to a metal surface.
AIR FLOW METER or VELOMETER
This instrument measures the speed of air. Air passes into the tube and registers the results in feet per minute on the instrument face. It can be used inside ductwork, or inside the booth to measure airflow through the ceiling filters or exhaust filters, or the airflow around the vehicle.
This hand-held device is used to introduce visible non-hazardous chemical white smoke into the airstream to view its flow patterns.
NON-CONTACT TEMPERATURE PROBE
This pistol-like instrument's infrared beam reads the painted surface temperature.
Used for documenting site conditions.
1.3.1.f System Design
By design, a spray booth collects solids known as particulate emissions. Filtration media, which can be filter pads or water, and moving air are the primary tools in this collection. A spray gun or similar device, with either human or robotic assistance, applies the coating material. The airstream moving through the booth gathers the solids and transports them to the filtration medium. The force and direction of the air, the efficiency of the filtration, and the characteristics of the coating equipment — these three elements — determine not only the overall efficiency of the coating operation, but also the quality of the finish.
Prep workstations and mix room systems work in similar fashion. Moving air transports solids to filtration. In a mix room, the moving air stream also transports harmful and dangerous evaporants out of the working area, minimizing worker injury and hazard of explosion.
Altering Booth Efficiency
The relative efficiency of a booth system therefore can be altered by making changes in:
-coating equipment (transfer efficiency);
-coating material (percent of solids in paint), and/or
-the air flow (cfm),
rather than changes only in booth design.
For example, if a painter switched from conventional air spray equipment to HVLP equipment, the higher transfer efficiency possible with HVLP would increase the relative efficiency of the system.
Measuring Booth Efficiency
An efficiency factor, called "grain count", measures how effectively a spray booth and filter system will be in trapping particulate emissions. The following formula is used to determine the relative efficiency of a specific system.
Grains/ft3= solids by weight in lbs. x transfer inefficiency x collection inefficiency x 7000 / 60 minutes x cfm
Grain count is normally expressed in grains / 1000 ft3, therefore:
Grain count= grains/ft3 x 1000
Efficiency and the EPA
Because of its ability to trap particulate matter, a spray booth can help the end user meet EPA requirements.
Unfortunately, efficiency factors have at times been misrepresented as providing an assurance that a spray booth will meet EPA requirements. While some spray booth designs are more efficient than others at preventing material from entering the environment, high efficiency factor ratings do not automatically ensure EPA compliance.
1.3.1.g System Design
Maintaining the system and its parts and their functioning is an ongoing need.
Preventative System Maintenance
As stated earlier, the primary function of a spray booth is to reduce the likelihood of fires and explosions.
Regular maintenance of the booth system and maintaining its systems in good order contribute to the fire prevention effort by removing flammable accumulations and dust, as well as make a clean finish possible.
Use of the booth requires a regular schedule of filter replacement. Codes require that filters be inspected after each period of use and that clogged filters be discarded and replaced immediately, another practice that contributes to fire prevention as well as a clean finish.
Click on Image to learn more about Preventative Maintenance