Compressor in applications

Applications

Gas compressors are used in various applications where either higher pressures or lower volumes of gas are needed:
- in pipeline transport of purified natural gas to move the gas from the production site to the consumer.
- in petroleum refineries, natural gas processing plants, petrochemical and chemical plants, and similar large industrial plants for compressing intermediate and end product gases.
- in refrigeration and air conditioner equipment to move heat from one place to another in refrigerant cycles: see Vapor-compression refrigeration.
- in gas turbine systems to compress the intake combustion air
- in storing purified or manufactured gases in a small volume, high pressure cylinders for medical, welding and other uses.
- in many various industrial, manufacturing and building processes to power all types of pneumatic tools.
as a medium for transferring energy, such as to power pneumatic equipment.
- in pressurised aircraft to provide a breathable atmosphere of higher than ambient pressure.
- in some types of jet engines (such as turbojets and turbofans) to provide the air required for combustion of the engine fuel. The power to drive the combustion air compressor comes from the jet's own turbines.
- in SCUBA diving, hyperbaric oxygen therapy and other life support devices to store breathing gas in a small volume such as in diving cylinders.
- in submarines, to store air for later use in displacing water from buoyancy chambers, for adjustment of depth.
- in turbochargers and superchargers to increase the performance of internal combustion engines by increasing mass flow.
- in rail and heavy road transport to provide compressed air for operation of rail vehicle brakes or road vehicle brakes and various other systems (doors, windscreen wipers, engine/gearbox control, etc).
- in miscellaneous uses such as providing compressed air for filling pneumatic tires.

Compressor: Gas laws

Temperature
Compression of a gas naturally increases its temperature.
In an attempt to model the compression of gas, there are two theoretical relationships between temperature and pressure in a volume of gas undergoing compression. Although neither of them model the real world exactly, each can be useful for analysis. A third method measures real-world results:

Isothermal - This model assumes that the compressed gas remains at a constant temperature throughout the compression or expansion process. In this cycle, internal energy is removed from the system as heat at the same rate that it is added by the mechanical work of compression. Isothermal compression or expansion more closely models real life when the compressor has a large heat exchanging surface, a small gas volume, or a long time scale (i.e., a small power level). Compressors that utilize inter-stage cooling between compression stages come closest to achieving perfect isothermal compression. However, with practical devices perfect isothermal compression is not attainable. For example, unless you have an infinite number of compression stages with corresponding intercoolers, you will never achieve perfect isothermal compression.

Adiabatic - This model assumes that no energy (heat) is transferred to or from the gas during the compression, and all supplied work is added to the internal energy of the gas, resulting in increases of temperature and pressure. Theoretical temperature rise is T2 = T1·Rc(k-1)/k, with T1 and T2 in degrees Rankine or kelvins, and k = ratio of specific heats (approximately 1.4 for air). R is the compression ratio; being the absolute outlet pressure divided by the absolute inlet pressure. The rise in air and temperature ratio means compression does not follow a simple pressure to volume ratio. This is less efficient, but quick. Adiabatic compression or expansion more closely model real life when a compressor has good insulation, a large gas volume, or a short time scale (i.e., a high power level). In practice there will always be a certain amount of heat flow out of the compressed gas. Thus, making a perfect adiabatic compressor would require perfect heat insulation of all parts of the machine. For example, even a bicycle tire pump's metal tube becomes hot as you compress the air to fill a tire.

Polytropic - This model takes into account both a rise in temperature in the gas as well as some loss of energy (heat) to the compressor's components. This assumes that heat may enter or leave the system, and that input shaft work can appear as both increased pressure (usually useful work) and increased temperature above adiabatic (usually losses due to cycle efficiency). Compression efficiency is then the ratio of temperature rise at theoretical 100 percent (adiabatic) vs. actual (polytropic).
In the case of the fire piston and the heat pump, people desire temperature change, and compressing gas is only a means to that end.

Types of compressors

The main types of gas compressors are illustrated and discussed below:

1) Centrifugal compressors

A single stage centrifugal compressor
Centrifugal compressors use a rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, increasing the velocity of the gas. A diffuser (divergent duct) section converts the velocity energy to pressure energy. They are primarily used for continuous, stationary service in industries such as oil refineries, chemical and petrochemical plants and natural gas processing plants. Their application can be from 100 hp (75 kW) to thousands of horsepower. With multiple staging, they can achieve extremely high output pressures greater than 10,000 psi (69 MPa).
Many large snow-making operations (like ski resorts) use this type of compressor. They are also used in internal combustion engines as superchargers and turbochargers. Centrifugal compressors are used in small gas turbine engines or as the final compression stage of medium sized gas turbines.

2) Diagonal or mixed-flow compressors

Diagonal or mixed-flow compressors are similar to centrifugal compressors, but have a radial and axial velocity component at the exit from the rotor. The diffuser is often used to turn diagonal flow to the axial direction. The diagonal compressor has a lower diameter diffuser than the equivalent centrifugal compressor.

3) Axial-flow compressors

Axial-flow compressors are dynamic rotating compressors that use arrays of fan-like aerofoils to progressively compress the working fluid. They are used where there is a requirement for a high flows or a compact design.
The arrays of aerofoils are set in rows, usually as pairs: one rotating and one stationary. The rotating aerofoils, also known as blades or rotors, accelerate the fluid. The stationary aerofoils, also known as a stators or vanes, turn and decelerate the fluid; preparing and redirecting the flow for the rotor blades of the next stage. Axial compressors are almost always multi-staged, with the cross-sectional area of the gas passage diminishing along the compressor to maintain an optimum axial Mach number. Beyond about 5 stages or a 4:1 design pressure ratio, variable geometry is normally used to improve operation.
Axial compressors can have high efficiencies; around 90% polytropic at their design conditions. However, they are relatively expensive, requiring a large number of components, tight tolerances and high quality materials. Axial-flow compressors can be found in medium to large gas engines, in natural gas pumping stations, and within certain chemical plants.

4) Reciprocating compressors

A motor-driven six-cylinder reciprocating compressor that can operate with two, four or six cylinders.
Reciprocating compressors use pistons driven by a crankshaft. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. Small reciprocating compressors from 5 to 30 horsepower (hp) are commonly seen in automotive applications and are typically for intermittent duty. Larger reciprocating compressors well over 1000 hp are still commonly found in large industrial and petroleum applications. Discharge pressures can range from low pressure to very high pressure (>5000 psi or 35 MPa). In certain applications, such as air compression, multi-stage double-acting compressors are said to be the most efficient compressors available, and are typically larger, noisier, and more costly than comparable rotary units.

5) Rotary screw compressors

Rotary screw compressors use two meshed rotating positive-displacement helical screws to force the gas into a smaller space. These are usually used for continuous operation in commercial and industrial applications and may be either stationary or portable. Their application can be from 3 hp (2.24 kW) to over 500 hp (375 kW) and from low pressure to very high pressure (>1200 psi or 8.3 MPa).

6) Rotary vane compressors

Rotary vane compressors consist of a rotor with a number of blades inserted in radial slots in the rotor. The rotor is mounted offset in a larger housing which can be circular or a more complex shape. As the rotor turns, blades slide in and out of the slots keeping contact with the outer wall of the housing. Thus, a series of decreasing volumes is created by the rotating blades. Rotary Vane compressors are, with piston compressors one of the oldest of compressor technologies.
With suitable port connections, the devices may be either a compressor or a vacuum pump. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. Dry vane machines are used at relatively low pressures (e.g., 2 bar) for bulk material movement whilst oil-injected machines have the necessary volumetric efficiency to achieve pressures up to about 13 bar in a single stage. A rotary vane compressor is well suited to electric motor drive and is significantly quieter in operation than the equivalent piston compressor.

7) Scroll compressors

A scroll compressor, also known as scroll pump and scroll vacuum pump, uses two interleaved spiral-like vanes to pump or compress fluids such as liquids and gases. The vane geometry may be involute, archimedean spiral, or hybrid curves. They operate more smoothly, quietly, and reliably than other types of compressors in the lower volume range
Often, one of the scrolls is fixed, while the other orbits eccentrically without rotating, thereby trapping and pumping or compressing pockets of fluid or gas between the scrolls.

8) Diaphragm compressors

A diaphragm compressor (also known as a membrane compressor) is a variant of the conventional reciprocating compressor. The compression of gas occurs by the movement of a flexible membrane, instead of an intake element. The back and forth movement of the membrane is driven by a rod and a crankshaft mechanism. Only the membrane and the compressor box come in touch with the gas being compressed.
Diaphragm compressors are used for hydrogen and compressed natural gas (CNG) as well as in a number of other applications.

9) A three-stage diaphragm compressor

The photograph included in this section depicts a three-stage diaphragm compressor used to compress hydrogen gas to 6,000 psi (41 MPa) for use in a prototype compressed hydrogen and compressed natural gas (CNG) fueling station built in downtown Phoenix, Arizona by the Arizona Public Service company (an electric utilities company). Reciprocating compressors were used to compress the natural gas.
The prototype alternative fueling station was built in compliance with all of the prevailing safety, environmental and building codes in Phoenix to demonstrate that such fueling stations could be built in urban areas.

Gas compressor

A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.
Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas. Liquids are relatively incompressible, so the main action of a pump is to transport liquids.

Advantage and disadvantage of gas turbine engines

Advantages of gas turbine engines

Very high power-to-weight ratio, compared to reciprocating engines;
Smaller than most reciprocating engines of the same power rating.
Moves in one direction only, with far less vibration than a reciprocating engine.
Fewer moving parts than reciprocating engines.
Low operating pressures.
High operation speeds.
Low lubricating oil cost and consumption.

Disadvantages of gas turbine engines
Cost is much greater than for a similar-sized reciprocating engine since the materials must be stronger and more heat resistant. Machining operations are also more complex;
Usually less efficient than reciprocating engines, especially at idle.
Delayed response to changes in power settings.
These disadvantages explain why road vehicles, which are smaller, cheaper and follow a less regular pattern of use than tanks, helicopters, large boats and so on, do not use gas turbine engines, regardless of the size and power advantages imminently available.

Gas turbine in commercial use

There have been a number of experiments in which gas turbines were used to power seagoing commercial vessels. The earliest of these experiments may have been the oil tanker "Aurus" (Anglo Saxon Petroleum) - circa 1949.
The United States Maritime Commission were looking for options to update WWII Liberty ships and heavy duty gas turbines were one of those selected. In 1956 The "John Sergeant" was lenghened and installed with a General Electric 6600 SHP HD gas turbine, reduction gearing and a variable pitch propeller. It operated for 9700 hours using residual fuel for 7000 hours. The success of this trial opened the way for more development by GE on the use of HD gas turbines for marine use with heavy fuels. The "John Sergeant" was scrapped in 1972 at Portsmouth PA.
Between 1970 and 1982, Seatrain Container Lines operated a scheduled container service across the North Atlantic with four 26,000 tonne dwt. container ships. Those ships were powered by twin Pratt & Whitney gas turbines of the FT 4 series. The four ships in the class were named "Euroliner", "Eurofreighter", "Asialiner" and "Asiafreighter". They operated a transatlantic container service between ports on the eastern seaboard of the United States and ports in north west Europe. Following the dramatic OPEC price increases of the mid-nineteen seventies, operations were constrained by rising fuel costs. Some modification of the engine systems on those ships was undertaken to permit the burning of a lower grade of fuel (i.e. marine diesel). The modifications were partially successful. It was proved that particular fuel could be used in a marine gas turbine but, savings made were less than anticipated due to increased maintenance requirements. After 1982 the ships were sold, then re-engined with more economical diesel engines. Because the new engines were much larger, there was a consequential loss of some cargo space.
The first passenger ferry to use a gas turbine was the GTS Finnjet, built in 1977 and powered with two Pratt & Whitney FT 4C-1 DLF turbines, generating 55000 kW and propelling the ship to a speed of 31 knots. However, the Finnjet also illustrated the shortcomings of gas turbine propulsion in commercial craft, as high fuel prices made operating her unprofitable. After just four years of service additional diesel engines were installed on the ship to allow less costly operations during off-season. Another example of commercial usage of gas turbines in a passenger ship are Stena Line's HSS class fastcraft ferries. HSS 1500-class Stena Explorer, Stena Voyager and Stena Discovery vessels use combined gas and gas (COGAG) setups of twin GE LM2500 plus GE LM1600 power for a total of 68,000 kW. The slightly smaller HSS 900-class Stena Charisma, uses twin ABB–STAL GT35 turbines rated at 34,000 kW gross. The Stena Discovery was withdrawn from service in 2007, another victim of too high fuel costs.
In July 2000, the Millennium became the first cruise ship to be propelled by gas turbines, in a Combined Gas and Steam Turbine configuration. The RMS Queen Mary 2 uses a Combined Diesel and Gas Turbine configuration.

Gas turbine in naval use

Naval use
Gas turbines are used in many naval vessels, where they are valued for their high power-to-weight ratio and their ships' resulting acceleration and ability to get underway quickly. The first gas-turbine-powered naval vessel was the Royal Navy's Motor Gun Boat MGB 2009 (formerly MGB 509) converted in 1947. The first large, gas-turbine powered ships, were the Royal Navy's Type 81 (Tribal class) frigates, the first of which (HMS Ashanti) was commissioned in 1961.
The Swedish Navy produced 6 Spica class torpedoboats between 1966 and 1967 powered by 3 Bristol Siddeley Proteus 1282, each delivering 4300 hp. They were later joined by 12 upgraded Norrköping class ships, still with the same engines. With their aft torpedo tubes replaced by antishipping missiles they served as missile boats until the last was retired in 2005.
The Finnish Navy issued two Turunmaa class corvettes, Turunmaa and Karjala, in 1968. They were equipped with one 16 000 shp Rolls-Royce Olympus TMB3 gas turbine and two Wärtsilä marine diesels for slower speeds. Before the waterjet-propulsion Helsinki class missile boats, they were the fastest vessels in the Finnish Navy; they regularly achieved 37 knot speeds, but they are known to have achieved 45 knots when the restriction mechanism of the turbine was geared off. The Turunmaas were paid off in 2002. Karjala is today a museum ship in Turku, and Turunmaa serves as a flotating machine shop and training ship for Satakunta Polytechnical College.
The next series of major naval vessels were the four Canadian Iroquois class helicopter carrying destroyers first commissioned in 1972. They used 2 ft-4 main propulsion engines, 2 ft-12 cruise engines and 3 Solar Saturn 750 kW generators.
The first U.S. gas-turbine powered ships were the U.S. Coast Guard's Hamilton-class High Endurance Cutters the first of which (USCGC Hamilton) commissioned in 1967. Since then, they have powered the U.S. Navy's Perry-class frigates, Spruance-class and Arleigh Burke-class destroyers, and Ticonderoga-class guided missile cruisers. USS Makin Island, a modified Wasp-class amphibious assault ship, is to be the Navy's first amphib powered by gas turbines. The marine gas turbine operates in a more corrosive atmosphere due to presence of sea salt in air and fuel and use of cheaper fuels.