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Compressed Air

Compressed air is mainly used to operate machinery, processes or equipment. Compressed air is air, which has been put under greater pressure than the air in the general environment. It is one of the most expensive of all services to be provided, but in contrary is regarded as free and often ignored when it comes to conservation. Significant energy and eventually cost can be saved by efficient and sensible generation & utilization of compressed air.

The compressed air system normally consists of various subsystems and equipment such as:

  • Air compressor with air filters, inter coolers, after coolers, dryers, oil and moisture separators
  • Lubrication and cooling water system
  • Air receivers with safety valve, pressure gauge and drain valve
  • Motor and drive transmissions
  • Distribution system comprising piping, valves, non-return valves, pressure regulators, lubricators, flow meters, couplings and bends
  • End use equipment such as pneumatic drills & presses and spray guns.

2 Types of Compressors

Broadly air compressors are mainly classified into two types. This classification is based on their construction and operation. Types that are commonly used are described below:

Figure 1: Types of compressors
Source: Training Manual on Energy Efficiency for Small and Medium Enterprises, APO.

Positive displacement compressors mechanically displace a fixed volume of air into a reduced volume. They will deliver a nearly constant volume, when operated at a fixed speed; while the discharge pressure is determined by the system load conditions.

Dynamic compressors mechanically impart a velocity to the air, through the use of impellers rotating at high speed, in an enclosed housing. The air is forced into a progressively reduced volume. The volumetric flow will vary inversely with the differential pressure across the compressor.

 

Adiabatic and isothermal efficiencies are computed as the isothermal or adiabatic power divided by the actual power consumption. The figure obtained indicates the overall efficiency of compressor and drive motor. Isothermal compression is close to ideal, and with inter and after coolers, the work of compression is optimized with resulting power economy.

Isothermal efficiency

Isothermal Efficiency = Isothermal power/Actual measured input power

Isothermal power (kW) = P1 x Q1 x loger/36.7

Where,
P1 = Absolute intake pressure kg/ cm2
P2 = Absolute delivery pressure kg/ cm2
Q1 = Free air delivered m3/hr.
r = Pressure ratio P2/P1

Isothermal efficiency is normally lower than adiabatic efficiency as it does not consider power needed to overcome friction. Reported value of efficiency is normally the isothermal efficiency.

As an easy reference, indicative specific power consumption values for various compressor types are presented below:

At 7 kg/cm2g

Figure 2: compressor type and SPC values

Source: Training Manual on Energy Efficiency for Small and Medium Enterprises, APO.

Payback of energy saving options

Experience from the past has shown that implementing energy saving options in compressed air system is highly profitable with payback of investment of less than 3 years.

Table 1: Payback of investment of energy saving options for compressed air system

Options Estimated payback period
Replace reciprocating air compressors by screw compressors 2 years
Optimize compressed air pressure to process needs < 1 year
Improve performance of inter cooler and after cooler < 1 year
Arrest leakages < 1 year
Install VFDs for capacity control About 2 years
Adopt heat less dryers for compressor air < 1 year
Adopt transvector nozzles for low pressure needs < 1 year
Improve performance of filters, regulator, lubricator in compressed air use < 1 year

Source: NEEP 2012-2016, IGEA

 

Potential areas of Energy Efficiency improvements in Compressed air system

  • Ensure that air intake to compressor is not warm and humid by locating compressors in a well-ventilated area or by drawing cold air from outside.
  • (Every 4ºC rise in air inlet temperature will increase power consumption by 1 %)
  • Clean air-inlet filters regularly. Compressor efficiency will be reduced by 2% for every 250 mm WC pressure drop increase across the filter.
  • Keep compressor valves in good condition by removing and inspecting them once every six months. Worn-out valves can reduce compressor efficiency by as much as 50%.
  • Install manometers across the filter and monitor the pressure drop as a guide to scheduled maintenance and replacement of elements.
  • Consider the use of regenerative air dryers, which use the heat of compressed air to remove moisture.
  • Fouled inter-coolers reduce compressor efficiency and cause more water condensation in air receivers and distribution lines, resulting in increased corrosion. Periodic cleaning of inter-coolers must be ensured.
  • Compressor free air delivery test (FAD) must be done periodically to check the present operating capacity against its design capacity, and corrective steps must be taken if required.
  • If more than one compressor is feeding to a common header, compressors must be operated in such a way that only one small compressor handles the load variations, while other compressors operate at full load.
  • If pressure requirements for processes are widely different (e.g. 3 bar to 7 bar), it is advisable to have two separate compressed air systems.
  • Provide extra air receivers at points of high cyclic-air demand, which permits operation without extra compressor capacity.
  • Retrofit with variable speed drives in big compressors (say over 100 kW) to eliminate the 'unloaded' running condition altogether.
  • Maintain the minimum possible range between load and unload pressure settings.
  • Automatic timer-controlled drain traps waste compressed air every time the valve opens. So frequency of drainage should be optimized.
  • Trans-vector nozzles are to be used for low and intermediate pressure air requirements at user points. These nozzles draw a small quantity of motive air at higher pressure and mix it with ambient air, to produce compressed air at intermediate pressure, in an efficient manner, as against routine valve control method.
  • Check air compressor logs regularly for abnormal readings, especially motor current cooling water flow and temperature, inter-stage and discharge pressures, and temperatures and compressor load-cycles.
  • Install equipment interlocked solenoid cut-off valves in the air system so that air supply to a machine can be switched off when it is not in use.
  • Pneumatic transport can be replaced by a mechanical system as the former consumes about 8 times more energy than the latter

References

Bureau of Energy Efficiency, 2010 Guidebook for National Certification Examination for Energy Managers and Energy Auditors: Book 3

Asian Productivity Organization, 2010 Training Manual on Energy Efficiency for Small and Medium Enterprises

Nepal Energy Efficiency Programme (NEEP), 2012-2016, Investment Grade Energy Audits