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The heart of any body shop is the compressed air system. Often mistreated and misunderstood, the air compressor is usually taken for granted – until it ceases to function. The cost of metal men not being able to sand and painters not being able to spray paint mounts really quickly when no one has any compressed air.
Most compressor problems are caused by dirt pulled into the system. Shop dust can nick seals, abrade smooth surfaces, block orifices and jamb valves open or closed. One manufacturer estimates that 35 percent of all compressed air generated is wasted to leaks. A 1/4-inch leak in the system will cost $10,000 per year. This alone is reason enough to understand and maintain your compressed air system.
Pump It Up
It’s a system because several components are involved besides the compressor pump. A compressor pump increases the air pressure by reducing the air volume. Using either reciprocating pistons or meshing rotors, incoming air is compressed in volume to raise the pressure. Atmospheric pressure (which surrounds us all) is 14.7 pounds per square inch (PSI).
A single-stage piston compressor pulls in the air at 14.7 PSI and, in one cylinder, reduces the volume to reach 125 PSI. A two-stage piston compressor pulls in air at 14.7 PSI and compresses the volume to reach 40 PSI in the first cylinder. The second stage pulls in the air at 40 PSI and compresses the volume to 175 PSI in the second cylinder. A quick test to identify single- or multi-stage compressors is to check the maximum PSI output. Only multi-stage piston compressors can reach 175 PSI.
Rotary screw compressors trap the incoming 14.7-PSI air between two spinning rotors. The air volume is compressed, and the air pressure raised as the rotors “screw” the air into a smaller space. They have many fewer moving parts and don’t create pulses of air like reciprocating compressors do when intake and exhaust valves open and close. They run at a constant speed and deliver a very smooth flow of compressed air. Often seen in larger units than body shops employ, they’ve recently become more popular in the 10hp to 25hp range. Pressure can reach 175 PSI with no spikes or dips along the way. Be sure to have the compressor vendor explain the advantages of either type pump; they both have several.
Air Volume & Efficiency
Compressor pumps measure their output in air volume as well as air pressure. At a given air pressure, say 100 PSI, the pump can produce a certain air volume. Air volume is measured in cubic feet per minute (CFM) in two different ways: displaced air and delivered air.
Displaced air volume is a theoretical calculation. Multiply the bore of the cylinder times the length of the piston stroke times the RPM of the pump, and ÒXÓ CFM of air is displaced by the piston. Delivered, effective air volume is lower. Air is lost to friction as it works its way through the pump, the intercooler and various fittings.
Calculations for rotary screw compressors are similar. Multiply the swept area of the two rotors times the length of their mesh times the RPM they spin to calculate volume.
A general rule of thumb for either style of pump is to figure approximately 4 CFM for each one horsepower on the electric drive motor. So a 5-hp compressor should displace about 20 CFM (5 X 4 = 20).
The compressor manufacturer must provide delivered air volume amounts. While displaced air volume is a math calculation, delivered volume depends on what size pipe, how many turns it takes and how many fittings are between the pump and the outlet on the tank. (One 90-degree elbow causes the same friction loss as 4 feet of pipe.)
An excellent way to compare one compressor to another is to measure their efficiency. If the 5-hp compressor from Brand A displaces 20 CFM and delivers 14 CFM at 90 PSI, it’d be 70 percent efficient (14 divided by 20 = 70 percent). The compressor from Brand B displaces 20 CFM but delivers 15 CFM at 90 PSI (PSI must be the same to compare pumps). It’s 75 percent efficient. In this example, the compressor from Brand B does a better job of moving the compressed air all the way though the various components and out the receiver tank.
Waterlogged: Moisture in the Air
Many of the air volume losses are the result of trying to manage the heat and moisture created when air is compressed. The action of compressing the air and raising the pressure generates a lot of heat. Discharge air temperature on a screw compressor is approximately 175 degrees F, while a two-stage piston pumps out air at 250 degrees F and a single-stage running full tilt will expel air at 350 degrees F.
The warmer the air, the more moisture it’ll hold. For example, at 40 degrees F, 1 cubic-foot of air will hold 3 grams of moisture. The same cubic foot of air at 80 degrees will hold 10 grams of moisture, and at a toasty 180 degrees, the same air holds 180 grams of moisture.
The measure of how much moisture air can and will hold is known as relative humidity. During the summer when the air feels so thick you could cut it with a knife, the air is holding a lot of water and the relative humidity is high. For example, at 80 degrees, the air can hold 10 grams of moisture. And let’s say on that day it does hold 8 grams – so the relative humidity is 80 percent. When the air holds 100 percent of the possible moisture, it’s raining!
At any given temperature, air can hold a certain amount of moisture. And once it exceeds that amount, the water vapor in the air will condense back to liquid. This is the dew point.
Dew forms on the flowers (or the cold beer can) when the temperature cools off and the air can no longer hold as much moisture. The air is saturated and condensation occurs. The beer can had no moisture on it when it was inside the refrigerator. But take the can out into the hot, humid air in the summertime kitchen, and the kitchen air nearest the beer can cools off. So the now cooler air right next to the can will no longer hold as many grams of moisture, and the water condenses on the outside of the cold can.
Once moisture turns from a vapor to a liquid, it’s easy to trap out of the compressed air stream. Much of an air compressor’s design and piping is designed solely to cool off the hot compressed air.
The first place to lose some heat is between the cylinders on a two-stage piston compressor. This finned aluminum or copper pipe is called the intercooler and serves to dissipate some heat as the hot air moves from the low-pressure cylinder to the high-pressure cylinder. (By the way, you can tell the low from the high side on the compressor by the diameter of the cylinders. The high side is roughly half as big as the low side.)
The primary cooling is done after the pump but before the tank. This aftercooler pipe is often finned and twisted to run in the moving air stream produced by the belt-driven flywheel. The moving air created by the blades on the flywheel cools both the pump itself and the compressed air inside the aftercooler.
Once inside the air tank, also called a receiver, the air is further cooled by contact with the sides of the big metal tank. In each case, the goal is to cool the air, lower the dew point and condense the moisture to liquid.
Methods for Trapping Moisture
Once the air leaves the tank, the secondary air treatment begins. The goal is dry, clean, oil-free compressed air at the point of use. And this is all about removing the moisture in the compressed air.
Temperature isn’t much of a problem. Your sander will run just fine on 150-degree air. Moisture is the problem – it’ll rust the sander and ruin the paint job with water blisters trapped in the paint film or a white haze in the finish.
There are four ways to remove the moisture:
- A separator trap.
- A membrane filter.
- A refrigerated dryer.
- A desiccant dryer.
- Water separators are also called impact traps and are the most common solution to catching moisture in a body shop. The air undergoes a sudden reversal of direction (smashes into a baffle), and the heavier water particles are flung out of the air stream. The inside of the trap is designed to swirl the air and facilitate extracting the water. This style of trap is incredibly efficient. They typically trap out 99 percent of the moisture that’s turned liquid (i.e. condensed). They trap 0 percent of the moisture that’s still hot and in vapor form. This is why it’s so important to always be cooling the compressed air. For every 20 degrees F you lower the air temperature, one-half the moisture will turn to water. And once it’s liquid, simple impact-type traps will knock it out of the air stream. Draining the trap often is the key. In a typical body shop, the 10-hp compressor creates 3 quarts of water every hour.
- Membrane filters pass the compressed air through a bundle of hollow membranes. The water is trapped out by the porosity of the special fibers used to make the hollow tubes. Water permeates the membrane toward the outside of the filter, and dry air passes through the middle. While effective on moisture, some compressed air volume is lost through the membrane.
- Refrigerated dryers can lower the dew point to 30 to 35 degrees F – dry enough for most body shop work. They chill the air by using a condenser much like your air conditioning unit at home. By lowering the average tank temperature of 100 to 110 degrees F down to 35 degrees, virtually all the moisture in the air will condense to liquid. The liquid is collected using the exact same style of impact trap described above. It’s 99 percent efficient if the moisture is liquid. The refrigeration unit cools the air and condenses the water; the attached separator traps the water out of the air stream. Again, the trap must be drained frequently, or the water is picked back up by the swirling air and carried out of the trap.
- Desiccant dryers dry the air by trapping the moisture vapor. They provide the driest air with dew points as low as minus 150 degrees F. Rather than cool the air with refrigeration, these dryers remove the moisture in vapor form. All desiccant dryer chambers are preceded by an ordinary impact trap to collect any water that’s already cooled and condensed; the desiccant stops the remaining vapor. There are two basic styles of desiccant dryer, and the difference is in the chemical media they use to trap the moisture vapor.
Absorption desiccant dryers use media that absorbs the water vapor. The media is exhausted over time and must be replaced or regenerated. Eventually, the media slowly dissolves from repeated contact with water. Some types of bead media can be cooked dry again once they’re removed from the desiccant canister. An example of the simplest absorption desiccant dryer is the style that uses a roll of toilet paper to trap the moisture. When the toilet paper has absorbed all it can, the roll must be replaced, or the next water entering the chamber will push an equal amount out the exit end. The plastic ball filter many painters screw onto their spray gun is an absorption desiccant dryer. The desiccant material is either absorbent beads, leaves of paper or cotton wadding – depending on brand. In the case of the toilet paper filter or the plastic ball filter, they’ll fill with water in mere minutes if there’s much moisture still in the compressed air.
The other style of desiccant dryer uses an adsorption media. Rather than collect the moisture vapor by soaking it up like a sponge (or toilet paper), this style of desiccant collects the moisture on the outside of the media. The moisture vapor is collected on the sharp edges of granule material. Silicone dioxide or spherical alumna are used to make a desiccant that can be dried and regenerated in place. Once the desiccant has collected the maximum amount of moisture on each granule or bead, heat is used to release the moisture back into the air. By blowing a brisk flow of air in reverse over the media, the water is swept away from the media and out of the canister. It is de-sorbed by heat-and-purge air flow. At issue with either style of desiccant dryer is regular replacement or regeneration of the desiccant. They provide the driest possible compressed air but absolutely require conscientious maintenance to function.
A Draining Topic
No matter which method you use to collect the water, it must be drained from the system frequently. There are four types of drains: manual, timed, float and demand.
Manual drains are nothing more than the screw-out petcock on the bottom of the receiver, tank or filter canister. While the most economical, they’re the worst choice. Sure, you’ll drain each unit 10 times an hour. This tends to make it difficult to fix cars at the same time.
Timed drains open and drop the water at set intervals. While this is certainly better than sporadically opening the petcocks manually, they also open the same number of times while everyone is at lunch and no air is being used.
Float drains work like a toilet tank in reverse. When you flush the toilet, the water runs in the tank until it floats the on/off arm up until it shuts off the water. In a float drain filter, the water collects until the float reaches a level that triggers the bottom valve to open and dump the water. And it won’t drop water again until the float rises. One problem, however, is the rust, scale and gunk that collects in the canister, which can clog the float’s movement or the valve’s function.
Demand drains are electronic or magnetic and open and close based on usage. More water collected means more drain dumps. No air used at lunchtime? No cycle for the demand-type drain.
Any style automatic drain is better than manual – largely because no one actually drains the traps often enough. And as water accumulates inside the canister, it’s swept back up into the air stream to cause problems further down the line.
Not the Place to Be a Tightwad
Don’t just whine about your air problems. Take steps to prevent them. You’ll get more work done!
Remember, the air compressor is the beating heart of your shop – and a poor place to try to save money. Re-plumb, re-pipe or re-engineer to get the driest air possible from your system.
Writer Mark Clark, owner of Professional PBE Systems in Waterloo, Iowa, is a well-known industry speaker and consultant. He’s been a contributing editor to BodyShop Business since 1988.Special thanks to John Werner – assistant damage analysis manager at Sterling Autobody in Cuyahoga Falls, Ohio – for not only being brave enough to hug an air compressor, but for letting us take a picture of him doing it! Thanks John!