Unit 14 -Air Preparation

1. Unit 14 -Air Preparation In this unit, we will discuss the preparation of air for use in a pneumatic system. Raw compressed air should not be used by pneumatic components until it has been cleaned, had its pressure adjusted, dehumidified, and in some cases lubricated. No matter how sophisticated a pneumatic system may be, if the air in the system is contaminated, the system will not operate properly.

2. Dirt and its Effect in a Pneumatic System Dirt, simply put, is any “free floating” solid or semi-solid particle in the stream of air flow. Besides the intake of dust and moisture vapor, compressed air usually produces many unwanted compounds which are carried into the receiver tank. While most will remain in the tank, some will inevitably make their way into the system where, if not removed, will cause problems ranging from sticking valves to total component failure.

3. Dirt is Pollution All compressors should have an intake filter. The intake filter will remove a great deal of contamination but cannot remove fine dusts or water vapor. An intake of 8 cubic feet of air will be reduced about 8 times so the humidity left in the compressed air is multiplied by as much. The combination of fine dirt particles, moisture, and compressor discharge temperatures of about 450 degrees will produce a variety of contaminant types in such a degree as to necessitate the cleaning of air at every work point or piece of equipment.

4. Contaminant Type and Source Contaminants are produced from any one of 3 sources. First, as the system is assembled, they may built in as would happen in a dirty environment. Second, dirt can be generated when moving parts begin to wear. Third, dirt can be ingested by filthy reassembly practices or through a seal in an actuator working in a dirty environment. There are 3 basic types of contamination. Abrasive Dirt: Dust ,Grinding,Compounds, Sand Soft Dirt: Plastics, Pipe Compounds, Pipe Tape, Paint Chips Entrained Liquids: From water vapor

5. Micrometer Scale and Limits of Visibility As discussed in an earlier lesson, the micrometer scale is based on units used in the SI system of measurement. As it relates to contamination, the human eye can only see a particle if it is larger than 40 microns. This lower limit of human visibility would suggest that a lot of contamination cannot be seen by the naked human eye.

6. Industrial Air Air is basically a mixture of gases, 78% nitrogen and 21% oxygen. Besides moisture, air carries a lot of other things that are not desirable to have in a pneumatic system. The primary contaminants are dust particles and water vapor. In most cases it is best to remove as much of the dust and moisture as possible before the air enters the system and this is accomplished through various devices but mainly by filters and dryers. The first place to start the removal of dirt is at the compressor intake.

7. Air Intake Filter The intake filter of a pneumatic system is the primary defense against contamination. As is with an internal combustion engine, the intake filter is critical in relation to proper system operation and if ignored can reek havoc upon the system components.

8. Compressor Air Air drawn into the compressor from the outside is squeezed into a smaller volume which, according to Boyles’ Law, will increase its absolute pressure. This compressed air is under pressure, hot, and contains a significant amount of water vapor as well as other contamination and some lube oil which passes around the piston from the crankcase . First we will discuss some methods of moisture removal starting with the air that comes directly out of the compressor. This moisture laden air is sent to a device called an after cooler which is usually mounted somewhere between the compressor and receiver tank.

9. After-Cooler and its Operation As we have discussed in past lessons, the after cooler removes moisture simply by cooling the hot air. As hot air cools it loses its ability to hold moisture and condensation occurs. The moisture collects in the after cooler and is then drained.

10. Effects of Moisture in a Pneumatic System Moisture in a pneumatic system causes a wide range of problems. In larger quantities, moisture will wash away lubricants and cause excessive wear on moving parts. Also, moisture reacts chemically with heated air and deteriorate seals. In addition to these problems, water vapor is a nuisance to the operator and a health hazard. Consider that a 100 SCFM system can produce as much as 50 gallons of water in 24 hours and one can appreciate the need to control moisture and limit its access to the system.

11. Moisture Removal A primary consideration for the removal of moisture is in design the air delivery system. When a loop system is not practical, air piping systems should incorporate a drop in the piping and take off connections that encourage water to stay on the bottom while moisture free air is allowed to continue on to the components it feeds.

12. Moisture Removal Refrigeration Air Drier There are two main types of moisture removal. One method is by refrigeration and the other is by adsorption. Adsorption Air Drying Unit

13. Refrigeration Dryer Operating Principle To appreciate the operation of the refrigeration type dryer, recall that heat transfers to any surface with less heat. Once inside the dryer, compressor air freely gives up its heat as it comes into contact with the much cooler walls of the dryer which are being cooled by refrigerant gas. As the hot air cools, it loses its ability to retain moisture and condensation takes place. The moisture then collects in the bottom of the dryer unit where it is drained from the system. The much lighter dry air moves out into the system.

14. Air Line Filters Controlling pressure and filtering air start at the compressor. Air also has to be filtered , pressure adjusted, and in some cases lubricated at the beginning of branch line circuits. A combination of the filter, regulator, and lubricator is commonly referred to as a FRL.

15. Air Line Filters Inlet Outlet Filter Baffle Deflector Plate Air entering a filter is forced to take a path down across the deflector plate which causes a swirling action. This swirling action, aided by inertia, throws out dirt and at the same time causes water vapor to collect against the walls of the filter bowl where gravity pulls it down to the lower level. In the lower level where the air flow is low, the water and dirt collect where they can be removed from the system by draining.

16. Filter Elements As with hydraulics, the purpose of a pneumatic filter is to remove contamination by the action of retention. There are two basic design types; depth and edge. The depth type filter element uses a “thickness” of material to knock contamination out of the air. Porous bronze and plastic are common materials used for this type of filter. Edge type filter elements trap dirt by concentrating the flow of air into a tiny area which causes debris to collect while allowing air to keep moving. A paper, impregnated with resin, is most common.

17. Air Line Lubricator There are many different types of air line lubricators. The one shown uses vacuum to draw oil into the air stream where it is “busted up” or atomized and suspended in the air to be carried downstream where it can lubricate tools and equipment.

18. FRL An FRL is a unit consisting of a filter, regulator, and lubricator. FRLs should be placed at the delivery of all branch circuits in a pneumatic system to give the greatest range of protection and pressure adjustment.

19. Maintaining Filters and Lubricators Filters and lubricators must be checked periodically or their presence will not make a difference. A general rule on filters is to simply replace them after a certain number of hours in service. Lubricators require constant inspection to ensure they don’t run low on oil. The best way to keep track of maintenance is to keep records and schedule maintenance.