Compressor Liners and Pistons Overview

Compressor Liners

• Liners are renewable surfaces that prevent wear on cylinder bores, saving the cost of replacing entire cylinders
• Wear is greater at the bottom of horizontal cylinders due to the weight of the piston
• Cylinders are equipped with water jackets or fins to reduce temperatures during compression
• Cast iron is used in larger, low-pressure cylinders, while steel is used in smaller, high-pressure cylinders

Compressor Pistons

• Pistons transfer energy from the crankcase to the gas in the cylinder
• Solid, single-piece pistons made of cast iron or steel are used in small bore, high-pressure applications
• Hollow pistons are used for low-pressure difference and larger diameters
• Two-piece pistons, made of aluminum or cast iron, are used for cylinder diameters of 10 inches and above
• Aluminum is preferred in two-piece designs as it reduces weight and facilitates the installation of endless rider rings
• Three-piece pistons are also used to aid the installation of endless rider rings

Compressor Piston Rings

• Piston rings are sliding seals that prevent or minimize the leakage of compressed gas between the piston and cylinder/liner

Reciprocating vs. Rotary Compressors

• Reciprocating compressors are bulky for a given discharge volume, while rotary compressors are smaller
• Reciprocating compressors have intermittent air supply, while rotary compressors have steady and continuous air supply
• Reciprocating compressors deliver dirty air, while rotary compressors deliver clean air
• Reciprocating compressors have higher compressed efficiency with a pressure ratio greater than 2, while rotary compressors have higher efficiency with a pressure ratio less than 2
• Reciprocating compressors require higher maintenance due to reciprocating parts, while rotary compressors have lower maintenance due to fewer sliding parts
• Reciprocating compressors have lower mechanical efficiency, while rotary compressors have higher efficiency
• Reciprocating compressors have complicated lubrication systems, while rotary compressors have simple systems
• Reciprocating compressors have a higher initial cost, while rotary compressors have a lower cost
• Reciprocating compressors have greater flexibility in capacity and pressure range, while rotary compressors have no flexibility
• Reciprocating compressors are suitable for low and medium gas volumes, while rotary compressors are suited for large volumes
• Reciprocating compressors are appropriate for medium and high-pressure ratios, while rotary compressors are suitable for low and medium pressures

Seal Types

• Seals separate moving and stationary parts, minimizing leakage between areas of unequal pressures
• Compressor seals prevent process gas leaks and seal fluid leaks into the process stream
• Seals can be dry or liquid lubricated, depending on the application
• Seals also depend on whether the shaft is rotating or reciprocating (piston rod)

Connecting Rod

• Connecting rods should have drilled holes for oil passage between the big end bearing and small end bearing

Crosshead

• Crossheads join the piston rod to the connecting rod and guide the piston rod in the cylinder bore
• Advantages of crossheads include the use of a narrower piston for greater valve area and efficiency, longer strokes and greater capacity, separation of the crankcase from the cylinder for oil carryover control, greater stability for the piston, and stronger piston design for higher operating pressures.
• Crossheads are equipped with shoes that slide within the crosshead guide (also called guide cylinder)
• Channels in the shoes distribute lube oil
• The connecting rod is connected to the crosshead using a crosshead pin (also called gudgeon pin)
• The piston rod is connected to the crosshead by a nut

Oil Wiper Rings

• Used to prevent crankcase oil from passing into the cylinder by scraping excess oil generated along the piston rod.
• Can prevent condensate or cylinder and packing lubricant from entering the crankcase

Distance Piece

• A distance piece separates the gas end from the power end
• It is provided with drain and vent arrangements and can be purged with buffer gas if necessary

Capacity Control in Reciprocating Compressors

• Capacity control in reciprocating compressors is used to adjust the suction flow to match process demand and to save energy
• Methods of capacity control include start-stop, suction line throttling, bypass or blow-off control, clearance pocket control, and suction valve unloading

Start-Stop Capacity Control

• The compressor delivers full output while running and nothing when stationary
• Frequently used for plants requiring low capacity, usually automatic
• Buffer tanks are installed between the compressor and equipment to accommodate pressure fluctuations
• Pressure switches turn the compressor on when pressure drops and off when pressure reaches the limit

Suction Line Throttling

• Partially closing the valve in the compressor suction line reduces volumetric efficiency, decreasing compressor capacity
• This does not guarantee a decrease in power requirement

Bypass or Blow-Off Control

• This method uses an external bypass around the compressor to recycle gas from the discharge to the inlet, or to the atmosphere
• The bypass must be downstream of a heat exchanger to ensure cooled gas is returned to the suction

Air Compressor Accessories

• Air compressor accessories include intake filters, dryers, intercoolers, condensate traps, aftercoolers, receivers, moisture separators, and air line filters

Intake Filters

• Intake filters remove solid particles (dust, dirt, and other contaminants) from the air before it enters the compressor
• They prevent premature wear on cylinders, pistons, and rings

Intercoolers

• Intercoolers are heat exchangers (air-cooled or water-cooled) located between stages of multi-stage compressors
• They cool compressed air from the first stage before it enters the second stage

Aftercoolers

• Aftercoolers are also heat exchangers that cool compressed air as it discharges from the compressor
• They cool the air before it enters the air receiver
• Single-stage compressors have aftercoolers but not intercoolers
• Multi-stage compressors have both intercoolers and aftercoolers

Moisture Separators

• Moisture separators remove water and other liquids (oil) from compressed air, as cooling the compressed air causes condensation.
• Single-stage compressors have one moisture separator, while multi-stage compressors have one after each stage

Dryers

• Dryers remove any remaining moisture in compressed air.
• They use either a refrigerated condenser or a desiccant.
• Refrigerated condensers cool the air to condense the water vapor, which is then drained from the system.