X-Y-Z: Clearing the Air About FRLs
For compressed air to be safely and adequately delivered to pneumatic equipment, it must pass through a filter, a pressure regulator, and in some cases, a lubricator. Often packaged together, these devices are collectively known as FRLs.
Passing the air through a filter prevents dirt, moisture and compressor oil from damaging pneumatic devices. Air enters the filter and passes through louvers that direct it into a swirling pattern. Centrifugal force throws liquid droplets and dirt against the side of the filter bowl. "It's like a tornado inside that bowl," says John Lokken, technical support engineer with the Pneumatics Div. of Bosch Rexroth Corp. (Lexington, KY).
The dirty liquid runs to the bottom of the bowl, where it can be drained away. The air then exits through a filter element, which removes any remaining particulates.
The bowl is usually made from clear polycarbonate, says Jeff Carter, leader of the air preparation group at SMC Corp. of America (Indianapolis). However, solvents, alcohol and compressor oils can attack the plastic. If these substances are present, a metal bowl with a glass window should be used. A metal bowl is also needed if the filter is exposed to temperatures above 125 F or pressures above 150 psig.
The bowl can be drained manually or automatically, says Clayton Fryer, product champion at IMI Norgren Inc. (Littleton, CO). An automatic drain closes when the system pressurizes. A float inside the bowl rises as fluid accumulates. When the float reaches the maximum level, the drain opens. The drain also opens when the system depressurizes. A tube carries the fluid to a receptacle, so it can be disposed safely.
Automatic drains should be used if the equipment operates continually, or if maintenance is difficult or unreliable. "With a manual drain, the only way to get the water out is to open it," warns Fryer. "If you don't drain the bowl, water will get into the lubricator and air lines, and that's a bad deal."
Various materials are used for the filter element, including sintered bronze, plastic, paper and felt. Elements are rated according to the size, in microns, of the particles they're designed to catch. For most assembly applications, a 40-micron filter is sufficient. Finer filtration-5 microns or less-is required for sensitive equipment, such as gauges, air bearings and high-speed air motors.
Choosing a filter size often requires trade-offs, warns Lokken. Air exits a filter at a lower pressure than when it entered. The smaller the filter's micron rating, the higher the pressure drop, and that translates into higher operating costs. Conversely, a filter that is too large won't clean the air effectively.
"Sometimes, people choose a larger filter size than they need, so it will last longer between cleanings," says Lokken. "But, you can't go too far overboard, because the airflow gets so low inside the filter that you won't get enough spinning action to remove the particulates."
Regulators reduce inlet pressure to the level required by downstream equipment. They minimize pressure variation at the point of use, and they limit the force of cylinders. The most common type of regulator has a control spring that acts on a diaphragm to regulate the air pressure. "When downstream pressure increases, it pushes against the diaphragm, which decreases airflow," explains Carter. "When downstream pressure decreases, the spring pushes against the diaphragm, which increases airflow."
Lubricators send a fine mist of oil into the air line. Air flowing through the lubricator creates a pressure differential between the air line and the oil reservoir. Oil rises from the reservoir into a dome, where it drips onto an atomizer and becomes a mist. This mist then condenses on the moving parts of downstream devices. The amount of oil sent downstream is varied by adjusting the drip rate.
When selecting FRLs, engineers should specify the pipe size, inlet pressure, airflow demand, and range of operating pressures. Airflow demand can be estimated by adding the air usage of all downstream machinery. The airstream temperature and ambient temperature are also important.