Wet compressed air is not a good combination for any dry abrasive blasting machine. Wet air is caused by a number of things and there are a number of solutions for each…listed below are the most common reasons for having wet air when the application is for automotive type machinery.
WHAT CAUSES WET AIR?
- Too small or UNDERSIZED air compressors
- Long part processing times with or without marginal air supply volume
- Closely located air compressor and blast machine
- Small air compressor storage tank
- Non existing air dryer or cooling equipment
Items listed above can all cause abrasive flow problems by introducing visible and non-visible moisture into any dry abrasive blasting cabinet. It is important to remember that moisture does not need to be visible to be harmful. The smaller the abrasive the more critical dry air becomes. Any abrasive size smaller than 100 mesh can be affected by moisture.
All compressed air is manufactured hot. Frictional heat is generated during piston compression and storage of compressed air. When air is cooled, vapor is no longer able to remain as vapor and appears as a liquid. Simply put, remember what happens when your automobile air conditioner cools the air inside your car. If your not sure look below any automobile that has the air conditioner running and you will see water, lots of water, on the ground. Hot air is also cooled by a cold automobile tail pipe and that’s why you will see water dripping out of a newly started car on a cold day until the exhaust system is heated.
Cooling of compressed air does happen and not paying attention to this simple fact can result in warm compressed air entering a cool steel blast cabinet. Blast cabinets are subject to the wind chill factor created when the exhaust blower replaces dusty air for clean outside air. It is almost impossible for any blast cabinet to remain ambient with the removal of cabinet air created during the dust collector operating cycle, just think about blowing on a bowl of soup!
Too small an air compressor: Using a marginally sized air compressor really means the compressor is pumping or making air as fast as you are using the air. This means hot air is entering the compressor tank and leaving the tank very quickly. All compressor tanks are basically air-chilling tanks in addition to the collection tank. Chilling the compressed air makes water and that’s why you regularly drain water from any air compressor tank. If you feel the tank and it feels warm, air will be leaving the tank and entering the colder air supply line and finally the blast cabinet. This really means the air gets cooled again inside the airline and the blast machine. If you throw in a little potting soil you might be able to germinate seeds. Having a simple air filter will not eliminate all of this condition. Water can still remain as vapor when it passes through the air filter, the air filter will not remove the moisture unless it is already a liquid. Air filters only remove already existing water that occurred at some previous location. The first cooling location is at the compressor tank. The airlines traveling to the blast machine become the second cooling location acting as a heat exchanger but in a straight line. How often have you started the day only to find water in the airlines? You can cure this condition if you use an air cooler. The most easy to install device is called an Ambient Dryer or often miss-termed a cooler. Other items include air dryer…refrigerant dryer…desiccant dryer and more. Finally you found a reason you kept that old racing oil cooler. Run compressed air through the inner coils and circulate cold water around the outside and you just made a water cooled air cooler.
Another often-overlooked item is the location of the airlines. Often companies install the compressor air dryer at the compressor to make sure all the air is dry and moisture free. Then they run air lines into the ceilings to permit distribution throughout the building. The ceiling is the hottest part of any building, hot air raises….Air in the lines in the ceiling is again heated allowing the air to enter the point of use on the ground heated. This is often the case and very often an overlooked condition.
Long processing times with marginal compressed air supply: This is also related to having a compressor that is too small for the demand of the application. Any compressor that does not have adequate off time will eventually heat the receiver tank and lines sending hot air to the blast chamber if no dryer device has been installed. If the compressor used is just large enough to supply the blast nozzle with just enough air for processing at the required blasting pressure, it is important to make sure you have cool, moisture free compressed air enterring the cabinet. The smaller the air compressor and the more you use any air compressor, the more critical the proper cooling of the compressed air becomes.
Closely located air compressors: This is not a negative condition but it can often cause a secondary negative result. Often the longer length or distance between compressor and blast machine can be helpful, if not for flow at least for the cooling aspect. Taking into consideration all the above information allows a longer distance between compressor and blast to act as a secondary cooling line. The air is often cooled during the transfer to the point of use resulting in the vapor turning into a liquid. This allows any simple air-filter water-trap to remove the troublesome liquid from the compressed air before it enters the blast cabinet. Almost all larger air tools are equipped with air-filter water traps used to remove the already existing liquid. Do not consider this an item capable of removing moisture stored as a vapor. Think of the gas filter in your automobile. Something solid will be filtered out of the gas, something dissolved in the gas will simply pass through the filter. If this something reacts negatively with your gas tank, you might then damage the tank.
The longer the distance between air source and blast machine, as long as the flow is not restricted, the more time you have to lower the temperature of the air and liquefy troublesome moisture for removal. Consider this, you might say the shorter the distance the more undesirable the layout.
Smaller than normal compressor storage tanks: Tanks are used for chillers or moisture collectors as well as air storage. This was described above but the simple fact is, “The smaller a tank size the less time it takes to start warming the tank from piston heat generated during compressed air manufacturing by the compressor pump.. Warm tanks transfer warm air to the point of use or the blasting machine. The smaller the tank the warmer the air leaves the tank because it leaves the tank sooner.
No existing air-drying or moisture removing equipment: Any air compressor and blast machine combination can be operated for short periods of time without any moisture removal equipment. At some time you will reach the point when the size of the compressor, the size of the holding tank and the distance between the two items becomes critical. If you start feeling a warm tank, you have reached that point. You can also feel the airline going into your blast machine. If you feel any temperature you need some form of cooler or dryer.
Ambient dryers are the simplest to install because you install them at the point of use or the blast machine... This means you only need enough dryer to dry the air used by the machine it is attached to as opposed to drying all the air coming from the compressor. If you are in a very high-humidity area then a refrigerant or desiccant dryer is best.
Consult Media Blast and or any authorized Media Blast dealer or compressor dealer for real solutions to that troublesome problem of wet air! This is really only a problem if you allow it to become a problem!
Always check the air demand of any blast machine and remember more is always better.