Air Compressor Efficiency in Auto Refinishing
Written by Chris Lampe, STAR4D Program Manager at the Iowa Waste Reduction Center, University of Northern Iowa.
Compressed air is one of the most important utility requirements of an automotive shop. Compressed air is used throughout many processes and applications including pneumatic tools and spray guns. Improperly sized hoses, fittings, and quick-disconnect couplings often result in large pressure drops. These losses require higher system pressure to overcome the pressure drop, thus resulting in wasted energy. Most air tools, such as impact wrenches and sanders are rated at 80-90 pounds per square inch gauge (PSIG) inlet. Spray equipment, such as gravity feed spray guns, require much less air pressure 20-30 PSIG inlet.
More importantly for automotive refinishing, is Cubic Feet per Minute (CFM) or volume of air delivered to the spray equipment. Volume is referred to as CFM on a compressor and is a critical factor to consider when choosing the correct compressed air system. Spray guns require a large volume of air at a lower air pressure for the optimal atomization in the spray pattern, resulting in improved finish quality of the job. The CFM is what determines the atomization of the coating when spraying. Pound per square inch (PSI) refers to the pressure that an air compressor provides.
In many automotive refinishing shops, the air compressor will consume more energy than any other equipment. A system that is incorrectly sized or not operating at peak performance can not only waste energy but can lead to unsatisfactory results such as poor finish quality and lower production. Maintaining peak performance of the air compressor is an ongoing process that will require continuous attention. With proper selection, setup and maintenance of the air compressor system, savings of 50 per cent or more have been achieved.
The air compressor system should be delivering air at the lowest practical pressure needed. Operating at the minimum practical pressure at the end-user, together with a corresponding reduction in compressor discharge pressure, will reduce the consumption of compressed air, the leakage rate, and overall energy consumption.
It is important to identify leaks and understand the cost of leakage. It is common to find a leakage rate of 20 to 30 per cent in older compressed air systems that have been in use for years. Once leaks have been identified, schedule maintenance to repair them. An efficient mechanism to identify leaks in compressed air systems is the utilization of an ultrasonic leak detector. An ultrasonic leak detector identifies leaks from the noise that the compressed air generates when leaving the system. The detector will not only identify the leak but also quantify the size of the leak. Using this information combined with the energy costs paid by the facility, a calculation can be generated to determine the individual annual cost of each compressed air leak, and the amount of energy being lost.
Consumption can be minimized by turning off the compressed air supply when it is not needed. Stopping the supply of compressed air to applications not in operation will reduce the consumption of compressed air. Install a disconnection valve that is used to shut off compressed air to the system when not in use.
Anticipating future needs or expansion can avoid overloading the compressed air system. The more air that must pass through the piping, the larger the pressure drop between compressors and the points of use. Systems should allow for a minimum of 30 per cent expansion in demand for compressed air by oversizing piping, valves and controls. Compressors, however, should not be oversized but should be sized and selected with controls to allow operation at optimal efficiency at prevailing rates of flow. The savings in operating costs may greatly offset any future need to purchase another compressor.
For optimum efficiency and energy consumption, only the minimum required number and size of compressors to meet required capacity and pressure should be in service. How many spray booths will be in operation at one time? How many spray guns will be in use at any one time? Pneumatic tools such as wrenches, grinders and sanders also require air consumption. These are factors that should be considered when choosing the size and type of air compressor.
Selecting Your Compressor
Compressor selection should depend on air requirements. A two-stage rotary screw compressor may be preferred if continuous use of the system is necessary. The two-stage rotary screw compressor, with an electric motor driver, offers longer compressor life.
If use is only intermittent, an air-cooled two-stage reciprocating compressor may be more economical. Smaller reciprocating compressors are often more affordable and thus, may be preferred for smaller auto body shops. These compressors are cooled by air and do not produce as much heat. This often makes two-stage reciprocating air compressors an ideal choice. In comparison, a single-stage compressor, which you may find for many DIY users, only delivers about 1 CFM per horsepower (HP). For painting vehicles, this could be problematic. Instead, consider using a two-stage compressor with at least 5HP to provide an acceptable amount of compressed air to your spray equipment.
Air Quality and Temperature
Compressed air should be dried to a dew point that is at least 18°F / 7°C below the lowest ambient temperature to which the system or end-user is exposed to. The drier the air the higher the operating cost, so do not dry the air to a pressure dew point below what is needed for each application. A pressure dew point range of 35°F – 38°F is often adequate for most automotive applications.
Clean, dry compressed air will also result in lower operating and maintenance costs. Any dirt, water and lubricant that may enter the inner surface of pipes can result in corrosion and can cause large pressure drops in the line. Any air being supplied to respirators, hoods and helmets for breathing requires drying, filtration and treatment to meet specified safety levels. An alarm system to monitor carbon monoxide levels is mandatory. OSHA standard 1910.134 – Grade D is specified for breathing air systems.
Piping may be made from 40 steel for 100-125 PSIG service. Many systems now use stainless steel or aluminum piping to avoid any future corrosion issues. Aluminum piping provides good structural strength that is lightweight and can be easily installed. Plastic piping is typically not recommended for compressed air.
Rubber or plastic hoses are typically used and the ends should be fitted with a swivel connection fitting. Hoses that are too long or too small in diameter are often the causes of excessive pressure drops. Hoses most often used for automotive painting operations have a 3/8” inside diameter (ID). The hose length may vary but avoid excessive length hoses. For instance, a 25’ hose may be adequate enough to move around the vehicle and perform most painting operations within the spray booth. A 50’ hose may be perfectly acceptable for larger trucks and trailers but could become cumbersome and result in a greater pressure drop when painting smaller vehicles. Always use the correct length of air hose whenever possible.
Couplings and Fittings
Quick disconnect couplings should be used to ease connections to the spray gun and provide a leak-free operation. High flow quick disconnects should always be used when operating a High Volume Low Pressure (HVLP) spray gun. HVLP spray guns require a high volume of air consumption. Using smaller diameter quick disconnect couplings starves the spray gun of airflow often resulting in a high air pressure (psi) being used. Higher air pressures result in wasted coating, increased finish quality defects such as dry spray, and higher energy costs.
Each restrictive fitting, hose or bend that is added to the compressed air system results in a collective pressure drop and can have negative results on the performance of spray equipment. Identifying this and finding the right sized components will enable your compressed air system to perform better.
In conclusion, regularly check for leaks and areas of possible pressure drops. An air compressor system that produces lower CFM will have a difficult time keeping up with production demands, ultimately affecting performance and finish quality.