Accessories for compressed air equipment

Systems using compressed air are widespread in almost all fields of industry. Areas of application include actuating valves and operating various tools, such as impact wrenches. In addition, the airflow is used directly in paint spray guns and drying equipment.

Compressors are used to increase the pressure of air from the atmosphere. Compressors are equipped with heat exchangers to deal with the large amount of heat generated by the process. Although less efficient than hydraulic systems, this technology is more widespread as it is easier to handle. The amount of actual losses and the losses in distribution and use are defined by the three parameters used as a basis for determining the optimal system: pressure, quantity and quality. In general, useful work is determined by the quotient of input energy and transport losses or, even more specifically, the pressure loss in the network and the energy devoted to cooling the hot air generated by the compressor. This value is somewhere between 24% and 32%. Attempts should be made to decrease these losses. Consumers generally require 6.0 bars of excess pressure.
Figure: Hydraulics and pneumatics, György Veres (2012)

“The above figure shows the relationship between pressure and performance. It can be clearly seen that 6.0 bars of excess pressure is required for 100 % performance. If the pressure in the network falls, the performance decreases drastically: 1.0 bar drop in pressure results in a 20% decrease in performance. The situation is disadvantageous even if consumers are provided with more than 6.0 bars of pressure. Although this results in a digressive increase in performance, as illustrated by the above figure, it also leads to an increase in the required amount of compressed air: an increase of 0.5 bar leads to a 10% increase in air consumption.” The system needs to be designed properly, therefore, as the loss in pressure has an unfavourable effect on the efficiency of pneumatic tools, and although this may not be apparent during operation, it impacts overall productivity.

Without the components necessary for systemic operation, this may be deficient due to the above reasons. These accessories are as follows:

1. Air filters
2. Pressure sensors, indicators and regulators
3. Oil and water separators
4. Heat exchangers, air coolers and dryers

Air filters

Air filters are used to remove solid airborne particles from the atmosphere or industrial air. They may be placed on either the suction or the discharge side, so they vary in design. Intake air filters are not subject to high pressure; these are generally filter mats or framed filters used for pre-filtering. Air filters on the discharge side have been developed for different pressure intervals and generally use filter cassettes. They can withstand pressures from 5 bars all the way up to several hundred bars. Filters for lower pressures are equipped with cheaper, plastic housings, while high-pressure filters use welded aluminium, steel or stainless steel housings. The cartridges are reinforced with steel netting and enable filtration of various particle sizes from 5 µm down to 0.01 µm. They can also be supplied with activated carbon filters, which remove gaseous materials from the system.

These filters neutralise solid, liquid and gaseous materials in one pass, typically dust fractions, water, oil aerosols and hydrocarbons.

Pressure sensors, indicators, and regulators

Pressure sensors may be valves, analogue or digital devices. They are used to measure changes in pressure and indicate operational capability. Digital equipment provides an exact reading while analogue meters indicate the current state within certain intervals. They can be installed both in compressors and in the network. Remote access can be enabled via radio frequency signals or with a direct wire, which allows for immediate intervention, but on-site readings also provide comprehensive information on current statuses.

Oil and water separators

During compression, varying amounts of moisture in the air condense in the form of water as the compressed air is cooled to operating temperatures and the humidity drops below the atmospheric dew point. We should also mention pressure dew points, which also lead to condensation. This water must be removed from the system, as it cannot be present at the point of use. There are ways of doing this, including adsorption and cool drying. The former uses adsorbent materials to physically bind the moisture in the air by capillary condensation without causing any change in air temperature.

The following materials are commonly used:

♦ Silica gel: lowest pressure dew point: - 50 °C, regeneration at 120-180 °C,
♦ Activated alumina: lowest pressure dew point: - 60 °C, regeneration at 170-300 °C,
♦ Molecular sieve: lowest pressure dew point: - 90 °C, regeneration at 200-350 °C.

Cool drying is explained in point 4. Water can also be extracted with electric condenser dryers. The pump serves to drain the condensate or other non-aggressive liquid from the compressed air system in an entirely automated process. These units can be installed to provide external drainage. The condensate is collected in a tank. When the tank is full enough, enough condensate is generated without any loss of air. The water level is monitored by a capacitive level sensor. A special directly-operated self-cleaning valve ensures reliable operation.

Mineral oils end up in the airstream from work processes that require lubrication during compression. This is a result of the operation of instruments essential to compression: the main cylinders. The oils ensure the smooth operation of parts by reducing friction, providing corrosion protection and increasing working life. It should be mentioned that these oils contain various additives to enhance their beneficial properties. These include additives for improving viscosity. If the viscosity is too low, however, this increases clearance losses between moving parts, which also depends on operating pressures. The viscosity value has to be determined by taking all three of these factors into account. Lower pressures require oils with lower viscosity whereas higher pressures use those with higher viscosity in order to satisfy these competing requirements. A great advantage is that they have a flash point above 600°C, which means they are non-flammable. The liquid itself does not burn, although its fumes do. A disadvantage is that they attack organic materials, such as sealants, surface-coating paints, etc. These oils must also be removed from the airflow. The figure shows a combined filter that enables the removal of both fractions.

Heat exchangers, air coolers and dryers

“Air is cooled to + 2°C in the heat exchanger, and the water that condenses out during the cooling process is extracted by high performance droplet separators. In order to prevent compressed air at +2°C from entering the network, an air-to-air heat exchanger uses the hot compressed air exiting the compressor to warm up the cold air. Based on the above, the compressed air coming from the compressor at a pressure of 7.0 bar and a temperature of +35°C then passes through the dryer and exits at the same pressure of 7.0 bar, a temperature of +30°C and a +2°C pressure dew point. This also means that the moisture in the air will not condense out until it reaches a temperature of less than +2°C in the network. The pressure dew point of cool dryers depends on the characteristics of the air entering the dryer. Manufacturers provide the amount as calculated for the parameters on the intake side (+20°C and 1.0 bar), while calculations are made for entry into the dryer of compressed air at a temperature of +35°C and a pressure of 7.0 bar. If the actual values differ from the above, corrections must be made to the data given in the dryer catalogue. A wide range of cool dryers is available nowadays with performance ranging from 0.3 to 300 m3/minute as standard, and up to 10,000 m3/minute in case of special requirements.”