Compressor Liners and Pistons Overview

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

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

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

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

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

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

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

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 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

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

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

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 include intake filters, dryers, intercoolers, condensate traps, aftercoolers, receivers, moisture separators, and air line 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 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 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 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 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.