Keep your compressors running smoothly

August 19, 2009
Plant Magazine

Compressors supply air, provide an energy source for chemical processes or compress gas in petroleum and natural gas, chemical and plastics operations; or they compress gases for refrigeration and air conditioning systems. Understanding the systems, maintenance issues and their lubrication needs will keep them running efficiently.

There are three types of compressors: positive displacement (reciprocating) units; rotary (vane and screw) compressors; and aerodynamic (centrifugal and axial flow) machines.

Centrifugal compressors (sometimes referred to as radial compressors) are a special class of radial-flow work-absorbing turbomachinery that have density increases greater than 5 per cent. Axial compressors are rotating, aerofoil-based and the working fluid principally flows parallel to the axis of rotation.

An impeller, the heart of a huge industrial compressor manufactured by Siemens.

Reciprocating air compressors are the oldest type, delivering a wide range of pressures and flow rates up to 150 psi and 3,500 cubic feet per minute. They comprise less than 15% of the market.

Rotary air compressors cover the other 85% of the market. They, too, deliver up to 150 psi of pressure, but are more compact, quieter and less expensive to run.

Reciprocating units generate pressure using a series of pistons and valves. The machines may be either single or multistage.

Problem areas are high-temperature operation; corrosion-resistance properties; deposit formation on valves and air lines; oil churning in crankcase; and the requirement of separate cylinder lubrication.

Rotary units generate pressure through a rotating action, where swept volume decreases, resulting in an increase in delivered air pressure. These machines may contain one or two rotors, making them either sliding vane or rotary screw compressors. Problem areas are rapid oxidation of the lubricant; oil separator plugging; limited roller bearing life; and corrosion resistance. Vanes, rings and bearings require proper lubrication, and these units need a coalescence to separate oil and air mixture.

Appropriate lubrication for any compressor is critical. Lubricants seal the compression mechanism, remove heat from the compression area, lubricate moving parts, and protect the machine. The lubricants are exposed to high temperatures, compression, dilution, condensation and abrasive contamination. Conventional oils comprise hydraulic oils, turbine oils, auto transmission oils or synthetics. Oil properties should include good oxidation resistance, corrosion protection, low pour point, high flash point, high viscosity index and foam resistance.