AIR COMPRESOR Chapter-13

AIR COMPRESSOR INTRODUCTION - Compressed air is costly and effective use in industries. It provides fast and accurate control to automatic control devices. Compressor supplies instrument air and service air. DESCRIPTION OF COMPRESSOR: The compressor consists of two independent banks, each bank having a set of suction filter cum silencer, low pressure cylinder, low pressure discharge vessel, inter cooler, high pressure suction vessel and high pressure cylinder. After the H.P cylinder, there is a common discharge header, an after cooler and moisture separator. The compressor is capable of delivering air at a pressure of 8.1 kg/cm² at the rate of 1000 Nm³/hr (1243.2 m³/hr).

Compressor House

SUCTION FILTER CUM SILENCER : Atmospheric air is sucked in to L P cylinders through suction filters. It is dry type, filters particles more than 5 microns. It prevents foreign particles, dust . LOW PRESSURE CYLINDER : It is a horizontal cast iron cylinder in which the aluminum alloy-casting piston with carbon filled Teflon piston rings reciprocates. Due to the reciprocating action of piston, the air enters into the cylinder through six suction valves and after compression, is delivered through six discharge valves . Each low-pressure cylinder is capable of delivering the air at the rate of 621.6-m3/hr at a pressure of 2.1 ksc and a temperature of 155 c. water-cooling arrangement provided for cylinder jacket takes out the heat due to compression of air.

LOW PRESSURE DISCHARGE VESSEL : Due to suction of HP cylinders there will be pressure fluctuation. To minimize such variations air is admitted into vessel which acts as damping vessel. INTER COOLER: Due to the compression in L.P cylinder the temperature of air gets increased from ambient temperature to 155 c. To reduce the air temperature before admission to the H.P cylinder, an inter cooler is introduced in between L.P &H.P cylinder. Inter cooler consists a carbon steel shell in which 70 numbers brass tubes supported by carbon steel and tube sheets are placed . Auxiliary cooling water at a pressure of 3-4 KSC and 40 degree temperature passes out side of the tubes and LP air flows through the tubes of inter cooler. Air temperature is reduced from 155 degree to 45 degree and water temperature rises to 44 degree

HIGH PRESSURE SUCTION VESSEL: The low-pressure cold air before entering the high-pressure cylinder passes through a Pressure damping vessel in between intercooler and HP cylinder. This is called High-pressure suction vessel. HIGH PRESSURE CYLINDER: The material of HP cylinder and its components are similar to that of LP cylinder. HP cylinder delivers the air at a rate of 621.6 m3/hr at a pressure of 8.1 ksc and at a temperature of 160 C respectively.

COMMON DISCHARGE HEADER AND AFTERCOOLER: The air from HP cylinders enters a common discharge header. At HP cylinders temperature of air is 160 to reduce temperature it is taken to after coolers. The after cooler consists of carbon steel shell in which 148 numbers of brass tubes supported at their ends by mild steel tube sheets are provided. The high pressure (8.1 ksc) hot air (160C) passes through shell outside tubes and its pressure and temperature reduces to 8 ksc and 45C respectively due to flow of auxiliary cooling water (pressure 3 to 4 ksc, temperature 40C).

SAFETY VALVES AND AUTO TRAP DRAIN : MOISTURE SEPARATOR: In the process of cooling in after cooler, the water vapour present in the air may condense. Hence in order to remove water particles from air, a moisture separator is provided before the air receivers. SAFETY VALVES AND AUTO TRAP DRAIN : At the outlet of each intercooler a safety valve is present is any pressure increase due to failure in suction of HP suction valve. A auto trap drain is provided at inter cooler and moisture separator to drain out water.

COOLING WATER ARRANGEMENT : The rate of flow of auxiliary water is 3-4 ksc at 12.84 m3/hr. Auxiliary water is supplied to HP, LP cylinder jacket, inter cooler, lube oil cooler Auxiliary line after supplying return to return bus. AIR RECEIVERS : There are two air receivers for each set of three compressors. The air receiver is provided with a safety valve at its top and an auto trap drain at its bottom. Out let of air receiver service is connected to common main bus and for instrument air it goes to drier.

AUXILIARY COOLING WATER

UNLOADING DEVICES OF THE COMPRESSOR: Capacity of compressor will be normally higher than regular consumption taking into account. If air consumption is less than compressor output then pressure will rise inside. To avoid such rise compressor is provided with unloading device. It consists of plunger with finger arrangement which will cause permanent opening of suction valves whenever delivery pressure reaches a preset value and keeps the compressor on NO LOAD condition.

STARTING OF A COMPRESSOR : AUTOMATIC OPERATION: While the compressor is in service, 3 way starting lever is at horizontal position and the solenoid valves will be in energized condition. When air pressure exceeds 7.5 kg/cm², then the 50% solenoid gets de-energized and the auto air goes and keeps open outer end suction valves of LP & HP cylinders. Now 50% load of compressor is reduced. Similarly when the pressure exceeds 8.1 kg/cm²and the 100% solenoid get de-energized and supply auto air to keep open inner end suction valves of LP & HP cylinders and the compressor runs on 100% no load. STARTING OF A COMPRESSOR : The following are to be checked before starting the compressor 1) Availability of 415V A.C for compressor 110 V A.C for annunciation and for main control. 2) Proper coupling of motor and V belt pulley shaft. 3) Proper guarding and coupling. 4) Condition of V belts and their tension. 5) Oil level in the cylinder lubricator and crank case.

A- Input side B- Diaphragm C- Pressure chamber D- Pressure relief passage E- Electro Mechanical Solenoid F- Output side

STOPPING: 7) Position of 3 way valve lever-vertical position. 6) Availability and sufficient flow of Auxiliary cooling water to LP/HP cylinder jacket cooling, inter cooler, oil cooler and after cooler. 7) Position of 3 way valve lever-vertical position. 8) Availability of impulse air upto 3 way valve and solenoid valves. 9) Availability of pressure gauges and temperatures indicators at appropriate positions. 10) Open position of discharge valves in the lines to Air receivers. 11) Open position of discharge valves in the line coming from Air receiver either to Air dryer or compressed air bus. STOPPING: 1) The compressor should be put on unloading by bringing the 3 way-starting lever to the vertical position and then the compressor is to be stopped. 2) Cooling water supply should be continued till the temperature of cylinder, intercooler etc comes down sufficiently. 3) The locked up cooling water can be drained off.

USES OF INSTRUMENT AIR: The instrument air compressors supply cleaned and dried air for the following 1) Temperature control valve for fuel oil tank floor coil heater and fuel oil suction heater. 2) Temperature control valve for secondary heaters. 3) Fuel oil pressure control valve in oil re circulation line. 4) Fuel oil flow control for oil burners. 5) Pressure control valve in atomizing system. 6) Temperature control valve in SCAPH. 7) SCAPH drain tank level control valves. 8) Mill hot air control dampers. 9) Secondary air control dampers. 10) Hopper air control dampers. 11) Oil burner air dampers.

12) Smoke density analyzers. 13) O2 probes. 14) Auxiliary steam pressure control valves. 15) Auxiliary steam temperature control valves. 16) Feed water pressure control valve for auxiliary steam temperature control. 17) Hot well make up control valve. 18) Condenser re circulation valve. 19) Excess condensate return to condensate storage tank. 20) Level control valves of low-pressure heaters.

USES OF SERVICE AIR: Service air compressors deliver air without drying at various places under. 1) Boiler local areas at various elevations. 2) Turbine hall at various points. 3) Diesel generator house. 4) Fuel oil pump house. 5) Workshop house. 6) Hydrogen plant. 7) Water treatment plant. 8) Crusher house. 9) Lignite handling control room. 10) E.P control room. 11) Ash slurry pump house.