COMPRESSED AIR INTRODUCTION
The use of compressed air as a safe energy source has steadily increased ever since it was first introduced in the late 19th century. Sales of pneumatic tools and equipment have never decreased. It is now common too see complicated machinery operated solely by compressed air and no hydraulics or electricity whatsoever. This industry is expected to expand even further over the next ten years and beyond. It is unthinkable therefore of this very useful power source. Young people especially, must have some grasp of the power and the subtle dangers involved before being exposed to this vital subject.
Note: You will notice that I use metric and imperial terms. This is because industry still uses many different forms of measurement in both metric and imperial, especially CFM (cubic feet per minute), PSI (pounds per sq. inch), one bar (14.5 PSI), L/S (Litres per second) and so on. This way by referring to our “Safety Manual” (can be downloaded from our site) you may become familiar with the figure and to be able to calculate the metric/imperial equivalent without reference.
Thanks to gravity the atmosphere around us exerts a pressure on everything. At sea level the pressure is 14.7 pounds per square inch. Compressed air is in fact air that has been placed under pressure, forcing it to a higher pressure than the normal atmosphere pressure. Having been pressurized it will try to return to its normal state and if harnessed this can be used as a form of power.
An example of this is a balloon. If a balloon is blown up and then released without the neck being tied, the energy (compressed air) within the balloon forces the balloon to fly around the room.
The bicycle pump is the first step in producing compressed air mechanically. This has a simple piston that moves down the cylinder, compressing the air into a smaller space and therefore raising the pressure. Electrical compressors obviously work much faster than a hand pump and are therefore able to produce large amounts of compressed air. This air when released can be used to drive tools or spray guns. The larger the size of motor, the more air that can be produced.
At one time all piston compressors over 100 PSI were cast iron and two stage. They were however heavy, bulky and extremely expensive. The Italian compressor industry eventually designed a range of light alloy compressors which made it possible for small businesses, farmers and one-man operators to own one due to low cost. Because they were alloy, greater heat dissipation was possible and by increasing the speed, the compressors could be made smaller. Most compressors sold today are single stage up to 3 HP and all larger units are usually two stage. It is also worth noting that the cast iron cylinder has been retained in most modern compressor.
It is most important to understand the relationship of pressure and volume. A simple analogy is to look at a kitchen tap. If you have the tap on and place you finger against the flow of water you can feel the pressure. If you were to place an empty bucket under the tap what you collect in the bucket is the volume of water.
To continue the analogy if you were to use a fire hydrant the pressure of the water would be similar to that of a kitchen tap but the volume of water would be considerably more. The bucket would fill much quicker with the fire hydrant.
The measurement for pressure is PSI (pounds per square inch) or BAR pressure. The measurement for volume is CFM (cubic feet per minute). Air is also measured in litres per second (L/S) and cubic metres per hour (M3/H). The volume is extremely important. If the compressor being used has a maximum 4 CFM and the tool requires 8 CFM then obviously it is impossible to operate the tool. It is like trying to lift a two ton weight with a one ton forklift.
If a user complains that the air receiver is running out of air and the pressure is dropping quickly he is using too large a tool for the machine’s capacity.
It is important to note that there are two different volume measurements. One is displacement which is the amount of air displaced in the compressor cylinder, the other is FAD. (or free air delivered) which is the actual amount of air being produced by the compressor. The FAD is a lower measurement but is the one that is used to identify the maximum performance of the compressor. The air displacement measurement should be ignored when calculating the compressor’s capacity.
PLEASE REMEMBER your desired pressure will be achieved if you get your volume right. Once your compressor produces more volume than is required (or being used) then the air builds up to the required working pressure and stays at that pressure until the motor is switched off.
Rotary screw compressors have been with us for a long time. It is only recently however, that rotary units have begun to eat into the small piston end of the market. It is rare nowadays to find a piston compressor above 20 HP.
Most rotary compressors are lubricated. The inter-meshing screws are normally kept apart by a thin film of oil which also acts as a sealant.
This is the method normally used by various institutions to measure compressor performance. That is defined as; air at 20 Celsius, 14.7 PSI and 36 percent relative humidity. This standard can sometimes vary and should be confirmed. Metric standards do not vary;
ACFM metric = M3/Min.
SCFM metric = NM3/Min.
Please remember, when you purchase an air compressor you do so, not for its mechanical beauty but to operate the pneumatic equipment in your business. To do this the compressor must be serviced regularly.
Regular service will ensure you receive the performance from the compressor which you paid for, and ensure the safety margins of this equipment is maintained.
All compressors need parts and service so therefore make sure the people you purchase same from are reputable service agents for the brand they sell. Correct installation and good service planning will ensure your compressor works with as few interruptions as possible.
Test your supplier; ask him/her for a gasket set or safety valves! They are relatively cheap and will come in useful one day. If he/she cannot supply immediately then go else where. This seller is not interested in you once he/she parts you from your money.
Important: It is now illegal to sell a compressor without the C.E. label on the compressor. See below for correct markings of C.E. conformity.
Note: It is useful exercise to note the length of time it takes your compressor to fill the air receiver. It is important to make sure the air receiver is isolated from the rest of the system because of varying air leaks and uses. Keep a record of this time to asses if the compressor is not performing to full capacity. This will tell you if your servicing routine is adequate. Timing before and after a service will also give you an indication of how good a job your service engineer is doing!
Modular air line is a new system of compressed air delivery from the compressor to various points in the factory/small workshop. This replaces the dated method of galvanized piping which is tedious, labour intensive and gruelling to install. Modular air-line is strong yet lightweight so is safe and easy to handle. This means there is no back breaking lifting or heavy tools required. You need little more than a hack saw and an Allen key to install Teseo modular air-line!!
When you buy Teseo modular air-line you invest in new, “energy saving” technology.
By this we mean:
- Complete flexibility – You can easily relocate the pipe-work to suit your factory/workshop alterations
- You can also add as many air outlets as you require, literally within minutes
- 30% more efficient – The smooth bore combined with no air leaks (see chart below) = greater power savings
- Clean air (no corrosion or rust) means savings on maintenance because through reduced filter element and pneumatic valve changing frequencies
- Teseo looks great in any working environment, it’s totally professional and pays for itself rapidly.
- Installing cumbersome galvanized GB air-line can cost you up to 4 times the price of Teseo Aluminum Pipe-Work.
COMPRESSED AIR CAN BE PAINFULLY FATAL
What follows here are but a few examples of the injuries sustained simply by using compressed air.
1. Employees in a woodworking shop were using compressed air to clean sawdust off their clothes. One man pushed the hose between the legs of another worker.
Operations failed to save the injured man’s life. He died in agony within three hours. Investigation showed that air pressure was very low.
Some of the injuries suffered by the victim included:
• Bowel torn open in three places
• Abdominal cavity filled with fluid, blood and fecal matter
• Abdominal cavity membranes torn in several places
• Abdomen and hernia canals ballooned
• Bruises and bleeding
2. A man working on a car had cut the right side of his index finger, only about 3/8″ long (10mm), which bled very little. The mechanic was so intently occupied with his work that he continued. He went ahead washing a few small parts in some cleaning fluid, and when he had finished, he held some of them in his left hand and began blowing them dry with a compressed air gun.
Apparently the jet of air sprayed over the small cut he had received earlier. A short while later, complaining of great pain, he staggered to his shop manager and complained that his body and head felt as though they were going to explode.
He was rushed to the hospital where his condition was diagnosed as “air bubbles in his blood stream”. These and traces of cleaning fluid were in his blood. Luckily the employee recovered within for days but the doctor gave him a warning that he would not soon forget.
“You could very easily have died”, he said, “from one or both of the
dangerous elements in this situation: in using the air hose on an open wound and failing to get prompt first aid attention for a minor cut”.
3. A Vancouver garage had a mechanic working on a car, and while this man bent over the front fender, another person passed by with a air hose pointed toward the mechanic. The air was on, inflating his intestine causing several ruptures. He was rushed to the hospital and his life was saved by an immediate operations.
4. At Newport News, Virginia, a shipyard worker died after a day of intense suffering following the introduction of 90 pounds of air pressure into his body.
5. Another worker in a scrap iron company located in Waterbury, Connecticut, suffered 60 agony-filled hours after two of his friends decided to have fun with an old man and chose an air line as their toy. They same article went on to further explain that this was not the first time an air hose had caused a fatality in that state.
6. One man in a Massachusetts factory was killed in exactly the same way. Another had a narrow escape when cleaning the palm of his hand with air; the air entered his body through the pores of his skin and inflated his whole arm for nearly three days.
7. A case was cited in the Industrial commission of Ohio’s “Monitor” as a victim of air hose horseplay. For about 12 years the victim of this simple playful act was in and out of hospitals. The year of suffering which it brought to the victim are a rather heavy price to pay for gratifying an employee’s whim to have a little fun.
8. A machine operator in a woodworking plant covered with sawdust decided to clean himself off with compressed air. He held the nozzle 12″ from the palm of his left hand. When he opened the nozzle the air, under 80 pounds of pressure struck and entered his hand. Before he realized what had happened, his arm had blown up as big as a grapefruit and was shooting pain – from fingertips to shoulders. He had excruciating pain in his head and a feeling that the top of his head was about to be blown off. This feeling was so real and the pain so intense that when help arrived, he was actually trying to hold the top of his head in place.
The surgeon said it might have been worse. Had the air forced its way into the blood stream, it would have made its way to the very small blood vessels of the brain causing a clot, which would have burst the vessels and caused death.
It should be apparent by now that whenever air is used for cleaning purposes, there can be an inclination to get playful with the hose. A serious or fatal introduction of air into the body openings can be so quick and the result so damaging that we should all do everything we can to avoid even a chance occurrence.
The American Medical Association states that when air is used in this manner, there is the danger of blowing germs and harmful foreign material under the skin. There may be some materials in some operations that are not dangerous on simple contact but could be extremely fatal if they did get through skin protection. This could happen easily with the mechanical help of a high pressure stream used in blowing dust off clothing. (THIS PRACTICE IS PROHIBITED BY OSHA AND STATE REGULATIONS which also require the nozzle pressure to blow off hoses to be reduced to less than 30 psi and used only with chip guarding and personal protective equipment).
Air under pressure can be just as dangerous as high pressure steam, and when released suddenly can cause serious injury. It can maim, tear, or embed matter into the skin and bones of the human body. Air played around the face can blow out an eye, or if directed at the ear, it may puncture an ear drum and cause deafness. A person who has been painting or covered with dirt or soot can have poisonous particles blasted into the body where they immediately combine with the blood. Even air without impurities is dangerous when forced into the bloodstream through a cut or pores of the skin.
Some people believe that air in the bloodstream will cause a joint or other affected part to swell up (accompanied by severe pain) and cause small blood vessels of the brain to burst. This is only partly true. People have died because their bowel ruptured as a result of pressure as low as four pounds per square inch.
In some countries, horseplay with compressed air is a punishable offence. To point an air hose at a person is as serious an offence as to point a gun. It is worth remembering that clothing offers no more protection against compressed air than it does against a bullet.
The use of compressed air to blow away dirt, chips, iron fillings, or sharp fragments is a dangerous practice. If the pressure is strong enough to remove these particles, it will be strong enough to blow them into your eyes, ears or nose, or even the skin. It is far safer to use a brush or vacuum cleaner.
|MAXIMUM AIR FLOW THROUGH GUN BARREL AIR LINE|
|Bore size mm||12||15||20||25||32||40||50||65||80||100|
|MAXIMUM AIR FLOW THROUGH HIGH EFFICIENCY TESEO ALUMINUM AIRLINE|
|Bore size mm||12||15||20||25||32||40||50||65||80||110|
Treat air receivers as you would treat water tank in the attic. If you use more air (water) than is supplied (mains) the tank will eventually run dry. Sizing a receiver is sometimes complex in large installations but simpler in small applications.
In a 10 bar receiver with a 7 bar working pressure requirement, only approximately 25%* of the vessel capacity is useful, (allowing for pressure drop). So, a 200 litre receiver at 10 bar (gauge) will have approximately 18 CFM in reserve should the compressed air demand exceed the compressor capacity.
*10 x 200 = 2000 bar litres
25% of 2000 = 500 litres
500 divided by 28.32 (litres.) = 17.66 (18 CFM)
You can see by this example that a 3 HP compressor giving 9 CFM (free air delivery) mounted on a 200 litre receiver would not give much greater benefit if mounted on a 300 litre receiver. Using the above method* we calculate a 2.9 minute reserve instead of 2 minutes.
(300 litre receiver = 26 CFM divided by 9 = 2.9 mins)
IMPORTANT: NEVER STEP INSIDE AN AIR RECEIVER TO INSPECT IT IF IS CONNECTD TO A LIVE SYSTEM. HIGHER PRESSURE RESULTING FROM THE AIR SUPPLY BEING “ACCIDENTLY” SWICHED ON COULD RESULT IN SERIOUS INJURY (THE BENDS).
To dry compressed air we use the following (popular) methods;
Regenerative desiccant dryers.
Moisture in compressed air
Most compressed air is saturated. This is because one cubic ft of air at normal atmosphere pressure (14.7 PSI) is at least 15% saturated.
The humidity of one cubic foot of air at atmosphere pressure (14.7 PSI) is at least 15%. If you squeeze more air into this area until pressure increases to 100 PSI, you have squeezed 6.9 cubic feet of air into one cubic foot.
Therefore 6.9 x 15% = 103.5%!!
Your compressed air is now 100% saturated.
Absolute Pressure – is the existing gauge pressure plus atmospheric pressure. At sea level the gauge pressure in pounds per square inch plus 14.7 gives the absolute pressure.
After-coolers are devices for removing the heat of compression of the air or gas after compression is completed. They are one of the most effective means of removing moisture from compressed air.
Air Receivers are tanks into which the compressed air or gas is discharged from the compressor. Receivers help to eliminate pulsations in the discharge line and also act as storage capacity during intervals when demand exceeds the capacity of the compressor.
Ambient Temperature – temperature of air surrounding the equipment.
Atmospheric Pressure – The absolute pressure of the atmosphere as measures at the place under consideration.
Automatic Drain – A device that automatically discharges condensate.
Compressor Regulator – A device fitted to the compressor to control the output of the machine.
Condensate – The liquid formed from water vapour in the air because of a drop in the air temperature and/or an increase in pressure.
Condensation – The process of changing vapour or gas into liquid.
Dew Point – The temperature at which moisture begins to condense.
Note: In selecting an air dryer system only “pressure dew point” is important.
Dryers, Refrigerated – Dryers that use a refrigeration system to cool and condense moisture.
Dryers, Regenerative – Dryers that use two desiccant towers. One tower dries the incoming air while the other generates the desiccant.
Filter – A device which removes foreign matter from the working medium.
Free Air – is the air at normal atmospheric conditions. Because the altitude, barometer and temperature vary at different localities and at different times, it follows that this term does not mean air under identical conditions.
Lubricator – A device which introduces a controlled quantity of lubricant into the working medium.
Micron – One millionth of one meter or 1 meter = 1,000,000 microns.
Pressure Regulator (Pressure Reducing Valve) – A device which reduces the line pressure and maintains it relatively constant despite changes in inlet pressure and outlet flow rate.
Pressure Relief Valve (Safety Valve) – A device which limits the maximum system pressure by exhausting the compressed air to atmosphere when the required back pressure is exceeded.
Purge – Usually refers to the removal of unwanted air, water or gas.
Ring Main – An n air main which begins and ends at the compressor so that every outlet has two possible sources (routes) of supply.
Saturated Air – Air at 100% relative humidity. It takes 7. cu. ft. of free air to saturate compressed air at 100 PSIG. Therefore, as long as the atmospheric relative humidity is over 12.9% (7.8 x 12.9 = 100%), which it almost always is, the compressed air from an air compressor is saturated. (This also explains why an air dryer is a must for a compressed air system).
SCFM – Standard Cubic Feet per Minute – Standard air is defined at 68oC (20oC), 14.7 PIA (1.01 Bar) and 36% relative humidity (density, 0.0750 ibs./cu.ft).
Specific Energy Consumption – is the generally accepted method of measuring the efficiency of air compressors i.e. the amount of KW or HP required to produce 100 CFM (FAD) at full load.
Volumetric Efficiency – is the ratio of the actual capacity of the compressor to displacement and is expressed in per cent.
Compressed air runs through a series of copper or alloy tubes which are in turn submerged in a refrigerant or heat extracting liquid. This liquid is usually called Freon 22 or R134a. The heat is removed with a fan blowing across a heat exchanger. This heat is usually returned to the compressed air after it has dried by the reverse of the latter method. This is a form of energy return.
The principle is simple enough. If you chill the air down to +5ocC then you cannot extract anymore water from the compressed air unless you operate air equipment at a lower temperature.* Since most factories require at least 16oC to satisfy a good working environment then it stands to reason the compressed air will not shed any moisture. Most refrigerated dryers operate between +2 and +5oC. This is usually called “Dew Point”. It cannot drop to 0oC because the trapped moisture would freeze and block the air supply.
High Volume Low Pressure – generic term HVLP – turbine paint spray equipment for DIY d professional applications. Invented more than 30 years ago, HVLP offers a unique and cost effective way to apply coatings cleanly, efficiently and in an environmentally friendly manner.
How Does It Work?
The volume of air atomizes the material, not the pressure. Air is drawn into the turbine through a high efficiency filter, via specially designed turbine blades which trap the air within the suction housing. Theses blades rotate at between 19,ooo and 22,000 RPM creating frictional heat and raising the air temperature. Heated air is then discharged through the air hose to the spray-gun, free from oil and moisture contamination.
Depending on which model is in use, the air pressure generated from the turbine will be between 2-3 PSI.
What are the advantages?
Low pressure spray – virtually no bounceback or spraymist.
Up to 40% material savings.
Warm air spray – reduces moisture content.
No air receiver checks or maintenance.
Lightweight & easily portable.
Maintenance free turbine.
No compressor or expensive airlines.
Immediate and constant air flow.
The principle advantages of HVLP are mentioned above on this page. There is now great concern however, for the environment and by using turbines we can reduce the amount of Volatile Organic Compound (VOC) emissions into the atmosphere. This complies with new stringent Environment Protection Laws, and therefore a significant advantage.