When using compressed air, there are losses in pressure throughout the delivery process, some due to leaks which should be addressed, but also every time the compressed is used. One loss that needs to be minimized is back pressure. Air mains are usually sized on air velocity to be low enough to prevent excessive pressure drop and also allow reasonable water separation from the compressed air. Tee connections should be replaced with directional angle entry connections or swept tees as turbulence caused by a 90o tee connection can cause pressure drops resulting in a back pressure sending a false “unload signal” to the compressors which can potentially cause excessive cycling of the compressor. This can lead to increased energy costs and maintenance costs.
Incorrect pipe sizing and restrictions are a major source of pressure losses in the system. Losses in the interconnecting distribution pipework between the compressor and the header distribution piping are commonplace however, the losses along these lines should be kept to a minimum.
Interconnecting piping between compressors or systems often require close attention. It is vital that they are carefully designed to avoid sending back any false signals to the compressor. Filters, regulators and other system accessories can also cause excessive pressure drops if undersized, again raising energy costs. The tendency to compensate for pressure loss is to increase the pressure at the compressor which only increases energy use.
It can also be unsafe to have high back pressure. For example, if there is inadequate pressure to a blow off nozzle or series of nozzles on a paper machine that is used to blow away trailing of a web of paper production which cut, there will be inadequate pressure to the nozzles, less force and this scrap material may not be blown away properly and could hurt nearby personnel or damage equipment. High back pressure will negatively affect the cooling rate from a vortex tube operated panel cooler or cabinet enclosure cooler. The reduced pressure will reduce the cooling effect and not provide the level of air conditioning desired.
If the supply line to an air nozzle is too small, whether it is for a flat air nozzle like the Nex Flow Air Edger, or the Air Mag air nozzle, or any type of nozzles – even noisy laval nozzles, the lower pressure will affect the force produced. When you read specifications for a nozzle, it is usually a force at a given inlet pressure, normally 80 PSIG or 5.5 bar. If the pressure is only 75 PSIG the force will be less. The 5 PSIG drop could simply be because of a supply airline that is too small thereby choking the airflow or it could be due to a fitting that is too restricted, or some other accessory such as an undersized filter, regulator or solenoid valve. All those accessories, if undersized will rapidly build up back pressure. One way to check if the supply line has too much back pressure when going into an air nozzle, air knife, or vortex tube operated device is to just install a pressure gauge a few inches upstream from the product. Then read the pressure indicated while the air is flowing. Reading the pressure when the air does not flow will give you the line pressure in a static condition which should be high. When flowing, the pressure reading should drop only slightly. That drop is the “pressure drop”.
If the pressure gauge reading drops when the air operated device is off, then you probably have a leak. That leak could be in the line somewhere, or from an accessory such as a filter (drain valve might be stuck for example), or from a bad fitting. This will not detect all leaks as they could be endemic to the system further upstream, but could assist in locating a leak nearby.
If the pressure drop is significant from the static to the active flow operation, you have an issue with either the line size itself or one or more of the accessories. Yet, you certainly need these accessories. A water removal filter (and also oil removal filter if oil is suspected in the airline) should always be used upstream from any blow off or cooling application to assure no moisture or oil or particulate enters the device or even gets onto the manufactured or processed product. Membrane filters in particular need to be inspected regularly to make sure they are still usable as after a period of time, there will be dirt build up on the membrane and pressure drop across the filters will increase. There are alternative filters now with no replaceable cartridges that are essentially maintenance free such as the Nex Flow Super Separator and the Expel filters. Sometimes these accessories might be more appropriate especially for airlines where moisture and dirt buildup is a big problem.
Back pressure is something one should always keep in mind whenever installing any compressed air operated device, whether it is a blow off product, a cooling device, a cylinder or an air tool. If the pressure drop is too high, the devices will not work to their optimum level, or maybe not at all. Even if the devices work, you would incur needless higher energy costs. Don’t forget to check out our article on 10 Ways to Immediately Save Compressed Air and Energy.