Heating the Paint Booth — How it Works
A curing paint booth provides four successive operating cycles:
- Spray mode
- Bake mode
- Cool down
Phase 1: Spray Mode
The spray mode is the period of time in which the paint material is being sprayed onto the vehicle. 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 to the paint booth, then sets the appropriate switch on the control panel to “spray.” The spray cycle is as follows:
- The damper positions itself automatically to allow the intake blower assembly to draw in only outside fresh air
- All of the air passes through the pre-filter, then through the burner or around the heat exchanger
- The outside air is heated to the preset temperature on the control panel and enters the plenum of the paint booth
- Here the air passes through the ceiling filters, enters the paint booth and is evenly distributed throughout the paint booth cabin
- The air is then exhausted beneath the floor through the paint arrestor filters, where most of the overspray is removed
- It then enters the exhaust side of the mechanical unit, where it is expelled through the duct exhaust to the outside.
Phase 2: 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 before the final cure.
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 the paint product recommendations.
Phase 3: Bake Mode
The bake mode is the period of time required for the curing of the paint applied to the vehicle. In this phase, the control unit maintains the operator’s pre-selected temperature (up to 200 degrees Fahrenheit) for proper results. For code compliance, no one should enter the spray booth during bake mode.
The operator switches 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 starts when the spray booth reaches the preset temperature for this phase.
The operating cycle is as follows:
- The damper automatically positions itself to permit the intake blower assembly to draw a portion of the air from the outside (10 to 15 percent) and re-circulate the remaining air (85 to 90 percent)
- All of the air then passes through the pre-filter and around the burner or heat exchanger
- It is heated to the preset temperature on the control panel and enters into the plenum of the paint booth
- Here the air passes through the ceiling filters, then enters the paint booth and is evenly distributed throughout the booth cabin
- The air is exhausted beneath the floor through the paint arrestor filters, then it enters the exhaust chamber, where 10 to 15 percent of the air is expelled outside and the remaining 85 to 90 percent is re-circulated
Phase 4: Cool Down
The cooling phase is the period of time required to cool down the heating unit and the interior of the paint booth. 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 the paint booth 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 percent fresh air from the outside, like in the paint cycle. Never turn off power to the paint 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 overheat and be damaged. If it is absolutely necessary to interrupt the cooling cycle due to an emergency, turn off the main power switch.
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 unit per minute
T = Temperature of air leaving the unit (should be the 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 minutes/hour
H = Total hours of operation during costing period
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 hours per week at 65 degrees Fahrenheit. It is fueled by natural gas at $0.40/ft3.
To determine the annual operating hours:
7 months of operation x 52 weeks x 60 hours/week = 1,820 hours/F x E
Translate heating value into BTU/100 ft3
1,021 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
This sum represents the greatest cost to operate the air make-up unit. Actual cost could be less.