Air burst systems are a standard requirement for municipalities and industries that draw water from rivers, lakes or ponds. The air burst system is designed to keep intake screens below the water level clear of debris and silt which accumulate as a result of inward water flow. The basic principle is to displace these materials with a high volume blast of compressed air in reverse flow direction through the screens. The systems are normally designed to be automatic and operate on a timed basis, calling for an air burst usually once or twice a day, or as required/established, depending upon the intake water conditions. Some basic calculations must be made to determine the volume of air required (relative to the screen size), air storage receiver size, and piping and air burst valve size based on air volume and distance from valve to screen. A good control design will allow the user to change the frequency of bursts, manually burst, provide for recharging of the air storage receiver within 15 minutes, and provide an alarm and shutdown of the compressor if it runs for an extended period of time. Such an alarm would be the result of the air burst valve failing in the open position, which is not necessarily a matter of if, but rather when the valve will fail. One of the main considerations should be the type of air compressor that will be used to charge the air receiver. Listed below are a few considerations.
Reciprocating or Rotary Screw
Lubricated air compressors are available as either reciprocating, rotary screw or rotary vane. A consideration for selection of a rotary screw or vane compressor should be the length of operating time. These compressors inject oil into the compression chamber for lubrication and cooling purposes then separate it from the discharged air for recycling through a cooler and into a collection reservoir for injection through the compression chamber and the bearings repeatedly. The compressors are designed to operate between 170º to 190º F. This temperature range allows the compressed air to remain above its pressure dew point level during compression, so water remains suspended in vapour form in the compressed air. If the operating temperature is much lower, the water vapour in the air may condense during the compression cycle, resulting in emulsification with the oil. Emulsified oil in rotary screw or vane compressors usually results in premature bearing failure. In production applications where these compressors run continuously, this is normally not a problem as normal operating temperatures will be maintained throughout the operation. However, in applications where the compressor may be called on to start and operate once or twice a day for 15 minutes, the compressor will operate below normal temperatures for most of the hours that are accumulated on the machine. Based on this condition, the rotary screw or vane compressor will likely not be the best solution in this application.
Lubricated or Oil-Less Compressors
Lubricated compressors are available as reciprocating and rotary screw. As noted above, this type of operation does not favour rotary screw or vane compressors. A reciprocating compressor is likely the better selection. However, for environmentally conscious design consideration, if a lubricated compressor is to be used, the system should employ a suitable oil coalescing filter to remove oil from the compressed air. If none is used, the air being burst into the screen will deposit oil into the water supply being drawn in. Although this may be a very small amount of oil, over time, it is accumulative. If an oil coalescing filter is used, some further complications/considerations arise as follows.
- The filter must be sized for greater flow than the capacity of the air compressor, and it must be used with regulated flow to avoid excessive velocity. The filter cannot be subjected to the high velocity air burst flow, as element damage will result.
- Oil coalescing filters become less efficient as air temperatures rise above 100ºF, therefore, an air aftercooler is required on the air compressor for proper separation of oil.
- Condensate from the compressor air receiver will contain oil, and this must be dealt with in accordance with municipal regulations. In most cases, the oil must be separated from the condensate using a water/oil separator.
- As the air compressor must be receiver-mounted for pulsation dampening and condensate/oil removal after the aftercooler, this will necessitate the requirement of a separate air storage receiver for the air burst volume.
These requirements all add extra equipment in order to deal with oil carry-over. If the system is properly designed, the issue of disposal of the separated oil and the oil from compressor oil changes must be dealt with. Most manufacturers of compressors will recommend oil changes every 500 hours or annually. If the compressor must charge the air receiver twice daily, the hours accumulated annually will only be around 180 (.5 hours x 365 days). View our new revised brochure here.
The alternative to using a lubricated compressor is to use an oil-less compressor which discharges clean oil-free air. As there is no requirement for an oil coalescing filter, the compressor air receiver would be used for the storage of burst air. It is true that the oil-less compressor is more costly however, the components that can be eliminated, and the reduced labour to separate and disposal of oil will easily offset this. The components that can be eliminated are the extra air receiver, air after cooler, oil coalescing filter, and condensate water/oil separator. Now take into consideration the duty of the air compressor of approximately 180 hours annually. Either the lubricated or the oil-less compressor require basic servicing of vee belt drives and air intake filters at various intervals, so both score the same on these issues. The lubricated compressor will require oil changing annually, and storage and/or disposal of oil recovered from the coalescing filter and condensate at specified times. The oil-less compressor does not require this. Oil-less compressors have major service requires for piston ring and wrist pin bearings replacement between 6000 to 8000 hours. Main bearings may extend to 10,000 hours. Considering the operation of the compressor is approximately 180 hours per year (do the math), the designer of the system may very likely be retired (or worse) before a major overhaul is required on the oil-less compressor.