Gas Compressors: Positive Displacement and Dynamic

• Gas compression serves as a major tool in the gas industry, enabling the transportation of natural gas.
• The flow of gas through pipelines requires energy to maintain movement, which manifests as pressure loss.
• Compression restores energy to the gas, offsetting energy depletion from the reservoir and preventing premature abandonment of formations.

Compressor Classifications

• There are two main classifications of compressors: positive displacement and dynamic.

Positive Displacement Compressors

• Positive displacement compressors work by compressing gas through the pushing action of pistons, vanes, or lobes.
• They compress a definite amount of gas per stroke or revolution.
• Two general types of positive displacement compressors: reciprocating and rotary.
• Reciprocating compressors employ pistons, while rotary compressors use vanes or lobes.

Dynamic Compressors

• Dynamic compressors accelerate gas using impellers or blades, converting gas velocity to pressure in a diffusing section.
• Two general types of dynamic compressors: centrifugal flow and axial flow.

Reciprocating Compressors

• A piston is drawn back in a cylinder, filling it with gas, then moved forward to force the gas out.
• The cylinder requires intake and discharge valves to manage gas flow.
• Reciprocating compressors vary in designs and arrangements.
• They're classified by single or double-acting mechanisms, cylinder arrangement, compression method, and drive method.

Single vs Double-Acting Compressors

• Single-acting compressors compress gas at one end of the cylinder, with one compression stroke per crankshaft or flywheel revolution.
• Double-acting compressors compress gas alternately at both cylinder ends, with two compression strokes per revolution.

Single-Acting Compressor Operation

• In a single-acting compressor, gas compresses during the piston's upward stroke and is discharged.
• Components a single-acting compressor: plate type valve, cylinder, cylinder water jacket, automotive type piston, connecting rod, crankcase, crankcase door, crankshaft counterweight, oilscreen, and low oil pressure alarm.

Double-Acting Compressor Operation

• When the piston moves left, a vacuum forms in the right cylinder end, drawing gas in through a suction valve. Simultaneously, gas compresses and exits the left end through a discharge valve.
• When the piston moves right, it compresses gas on the right and forces it out through a discharge valve, while drawing gas through the left suction valve.

Reciprocating Compressor Applications

• Reciprocating compressors are suitable for wide compression ratios, typically operating from 1.1:1 to 5:1 in gas work.
• They offer relatively high efficiency, with major loss due to valve pressure loss, which decreases in effect as ratios increase.
• A major disadvantage is the high maintenance costs due to numerous moving parts.
• Gas engine driven compressors are rated at 1.5 MW and upwards.
• They are used for gas transmission pipelines, gas boosting, pressure maintenance, repressuring, recycling, petrochemical and chemical plants, natural gasoline plants, gas storage, etc.

Rotary Compressors

• Similar to reciprocating types, rotary compressors are positive displacement types.
• Common designs: sliding vane type, lobe type, and screw type.

Sliding Vane Compressors

• Sliding vane compressors are in portable air compressors for pneumatic tool service.
• These are self-contained, with controls, cooling, and lubrication, driven by gasoline or diesel engines.
• They use a cylindrical rotor with radial slots for sliding vanes, housed in a water-jacketed cylinder or casing.
• As the rotor rotates, vanes extend against the casing wall due to centrifugal force.
• Air or gas pockets are trapped between the vanes and casing, decreasing in volume due to eccentricity as vanes move, compressing the gas.
• Sliding vane compressors are less efficient than reciprocating ones, but offer lower initial costs and steadier gas flow.
• They have low starting torque needs, since compression starts after sufficient speed is reached for vanes to move outward.
• Limited pressure is due to the strength of the vanes.
• Valving is absent; vanes trap and compress gas as they slide past the inlet, discharging it upon passing the discharge port.
• These compressors work only at a predesigned discharge pressure.
• Leakage is a major factor.
• Increase pressure ratios can result in increased leakage losses.
• Rotary sliding vane is used for portable low-capacity (25 m³/min) compressors for pneumatic tools.

Rotary Lobe Compressors

• The rotary lobe compressor has two figure-eight impellers revolving in opposite directions inside a casing.
• Impellers are kept in relative positions using timing gears, where one impeller drives directly and the other via timing gears.
• As each lobe sweeps past the inlet, a pocket of air/gas is trapped between the lobe and casing wall, carried around to discharge.
• The impellers do not contact each other or with the casing, this eliminates the need for cylinder lubricant.
• The lobe type runs at up to 1750 r/min, direct driven by an induction or internal combustion engine or gear steam turbine.
• Single-stage produces very low pressure at 200 kPa; two-stage operation produces 300 kPa.
• They are compact, requires to inlet/discharge valves, and produces an even flow of oil-free air or gas.

Rotary Screw Compressors

• This rotary screw compressor has two intermeshing rotors in a close fitting casing.
• The male rotor has four convex lobes; the female rotor has six concave flutes.
• As both rotors turn and intermesh, gas compresses and forced out the discharge.
• Internal lubrication is not required, the rotors do not contact each other or the casing.
• The male rotor typically drives the female rotor through timing gears.
• Rotary screw compressors are suitable for high speeds (3000-12,000 r/min) and direct driven by steam or gas turbines.
• A speed increasing gear may be required if induction motor is used,.
• Single stage develops discharge pressure to 800 kPa; multistage designs are used for higher pressures.
• Rotary screw compressor advantages are compactness, vibration-free operation, smooth flow, and oil-free gas with requiring suction or discharge valves.
• Efficiency is less than reciprocating types.

Centrifugal Compressors

• Centrifugal compressors have existed since the turn of the century for low compression, large volume applications like mine ventilation and steel furnaces.
• They expanded to higher compression ratio uses in air compression and the petrochemical field.
• Turbo compressors became large scale in the 1950s thanks to increased volume needs, better prime movers, higher efficiency, and simplicity.
• The centrifugal versions rapidly rotate impeller vanes to accelerate gas, converting velocity to pressure in the volute casing and/or diffuser vanes.
• Impellers rotating at high speed are surrounded by a volute shaped casing.
• The impeller in centrifugal compressors draws in gas and discharges it at periphery to the volute casing or diffuser vanes at high velocity.
• Gases pass through diffuser vanes and partially convert velocity to pressure before reaching the volute casing for final conversion.
• Diffuser passage design greatly impacts velocity conversion to pressure.
• Seven or eight stages (impellers) can be built, elevating gas pressures from 0-7000 kPa at flows ranging to 7000 m³/min or 120 m³/sec.
• They have simple, rugged construction and low maintenance needs for large volume, low-pressure uses.
• Supplying oil free gas, centrifugal compressors don't need internal lubrication.
• They are less efficient than positive displacement, and are unsuited for low capacity.

Axial Flow Compressors

• Axial flow compressors are a more recent invention than the centrifugal type.
• Axial types developed to handle demand for large volume compressors.
• Typical units handle quantities that range up to approximately 200 m³/min.
• Single units can reach discharge pressures up to 800 kPa.
• By "pushing" gas in an axial motion like a fan, axial compressors accelerate gas.
• Raise the pressure and lower velocity, using fixed stator blades and a discharge volute .
• In its basic design, axial compressors resemble reaction turbines, use moving blades attached to the rotor alternating with fixed blades attached to the casing.
• As the rotor spins, gas velocity and pressure rise in moving blades.
• The pressure of gas increases further through it passes through the fixed blades as its kinetic energy converts.
• Every pair of moving and fixed blades creates a stage.
• Due to low pressure rise per stage, a large quantities of stages must be used for higher pressures.
• Axial flow compressors advantages are also present centrifugal compressors.
• Higher efficiency (10% greater) than centrifugal designs is an added benefit.
• The axial flow compressor blades corrode and erode easier than the impellers of the centrifugal type.
• Higher operating speeds, steam or gas turbines, electric motors and internal combustion engines with speed increasing gears are often required.
• Pressure rises per stage are decreased and therefore require more stages for desired pressure differentials.