Jet engine compressors
The turbo compressor ( also turbo compressor ) belongs to the thermal turbo machinery. He works in the reversal of the principle of the turbine, similar in components and construction also promotes the centrifugal pump and the respective medium also by transfer of kinetic energy in the form of a spin pulse. In contrast to the pump, the gas is compressed in the compressor, that is, at the same mass flow is the volume flow at the exit of less. The expended working volume change does not have to be applied by a pump, and therefore at the same pressure, the pump assembly work is substantially less than the compressor resp. Compressor work. For this reason, working steam power plants more efficient than comparable power plants, which are operated with air / gas.
Construction
Have all turbo-compressors
- A housing with appropriate guidance devices
- A shaft having at least one
- Impeller with blades or a directly fitted to the shaft rotor blade row.
Turbo compressors are divided into the main types radial and axial compressor. The axial compressor, the gas to be compressed flows in a direction parallel to the shaft of the compressor. The centrifugal compressor, the gas flows axially into the impeller of the compressor stage, and is then deflected outwardly ( radially ). In multistage centrifugal compressors so that a flow deflection is necessary after each stage. General support axial compressors higher flow rates produce higher pressures during radial compressor. Combined with their designs suck Axialstufengruppe at high flow rates, which are compressed in the radial subsequent stages to high pressures. The diagonal compressors are a combination of two principles.
In the often -built single-shaft machines that may also be driven by a separate transmission to different compressor stages are arranged one after another on the shaft. The arrangement is often the case that there is a main flow direction ( from the suction to the discharge nozzle ), in part, a "back to back " is also used for compensation of the axial thrust of the wave - arrangement chosen by individual compressor stages or stage groups, so that more than one suction - results or discharge nozzle. Another type are the transmission compressors in which the individual radial compressor stages are grouped directly to a transmission case around. This transmission has a central large ( dental ) wheel, which several parallel pinion shafts, each carrying one or two wheels drives. The large wheel is either directly coupled to the prime mover ( motor drive) or its part by a mostly lying down drive pinion ( turbine drive ) driven. In this case, each stage group run at their optimum, depending on the diameter speed. Geared compressors achieve very high compression ratios up to about 70
Application
Turbo compressors are used if high gas flow rates are to be compacted, for example,
- In gas turbines and jet engines
- As blast furnace and steel mill fan
- In plants to air or gas liquefaction
- As air or nitrous gas compressor in nitric acid plants
- In petrochemical plants and refineries
- To increase the pressure in gas pipelines
- A vacuum fan in the paper industry
Operation
At the intersection of the pressure drop characteristics of the downstream plant components with the respective compressor characteristic raises an operating point is. Here are the special properties of the medium to be observed gas: When pressurizing the volume decreases, at the same time, the temperature rises.
Driving machine
The performance of large centrifugal compressors is above one megawatt to about 60 MW. The choice of the engine is significantly influenced by the application. If - as in chemical plants, steel mills or foundries - and sure enough steam available, it is preferred to use a steam turbine. For pipeline and offshore applications, gas turbines offer. In addition, often the drive by an electric motor which can be operated via a frequency converter with variable speed, if necessary. To take advantage of " superfluous " pressure at the end hot- gas expander chemical processes are used, which can be regarded as a special form of the turbine.
Power control
Adjusting the compressor capacity, either the speed of the engine or the volume flow of the compressor can be controlled. Speed control is usually implemented with electric motors via a frequency converter. Within certain limits, the speed can be varied during the turbine engine. For flow regulation is usually adjustable vanes ( axial flow ) or adjustable inlet guide vanes (radial compressor ) may be used. The rules on Saugdrosselklappen applies because of excessive energy losses as obsolete. Combined Reglungen (speed and flow control) are possible, but are avoided if possible.
Compressor pumps
One of the characteristics of each turbo- compressor is a tendency in adverse operating conditions to "pump " (English Surge) - a phenomenon which occurs when at a given pressure, a minimum flow rate is exceeded. From this, the flow to the compressor blades depends. Too small mass flow of the angle of attack is so great that it stalls. Thereby is transferred to the medium, so that the mass flow rate further decreases and the flow in other tearing blades or compressor stages less pulse. The pressure difference between inlet and outlet can not be maintained, there will be a reverse flow through the compressor. In this case, the pressure ratio decreases ( the suction pressure increases) and the compressor delivers back to the surge point ( less than the minimum delivery flow ) is reached again. This process runs cyclically with a frequency of about 0.3-5 Hz, and is connected to a typical noise. Pumping occurs depending on the strength of the mismatch of the compressor in different intensities. With a slight mismatch there is a "rotating stall", where only single blades of the compressor experienced a stall. This rotates the stall opposite the compressor rotation. This is associated with a growling or humming noise and leads to a slight loss of performance of the compressor. The compressor blades are already excited to vibrations, which can cause blade breakage. If the mismatch is larger, it can come to a complete stall, the power totally collapses and creates a loud bang. When the compressor is part of a modern aircraft engine, a destruction of the engine is thus usually associated.
Therefore, virtually all turbo-compressors have a surge control, which is intended to prevent a certain minimum flow rate is exceeded. This minimum flow is to be understood as the respective pressure-dependent setpoint of the pump limit regulation and, in turn, forms the control line of the anti-surge, which runs about 10% of the surge limit in the map to the right. When passing over the control line by opening a relief valve (VA air compressors) or Umblaseventils is ( other gases), the volume flow increased again, so that the operating point is held at least on the control line. If a longer Umblasebetrieb is expected a Umblasekühler is required, otherwise the sucked gas would be increasingly warmer, which in turn facilitates the pumping of the compressor, due to the thereby decreasing density.
In particular, axial compressors are very sensitive to surges and must therefore be protected, particularly against this operating condition. They often have other, independent means for pump protection, protecting the machine from destruction if the surge limit control fails (failure of measuring or control devices, altered gas composition).
The decrease or Umblasen degrades the efficiency of the compressor dramatically as a partial flow of the gas is useless promoted back to the suction port or into the atmosphere. Therefore, this method is not used for power control, but is used mainly to protect the engine. If a machine can be operated at normal process conditions only partially open -surge control valve, the machine is not optimally adapted to the process or must be overhauled (dirt / wear of the vanes and diffusers ).
In Linz ( Austria ) were measured around 1980 ground vibrations at about 1 Hz, and the cause is found in compressors of industrial ammonia synthesis pressure and created there a remedy.
Intercooling
The compression in the compressor runs in the real case, polytropic, and adds to the pressure ratio to temperature increase; the cooling of the compressor is low because the expended compressor work decreases. The cooling can also possibly via the isentropic curve out and theoretically cool the flow so strong that the compression is nearly isothermal. This, however many intermediate cooling operations are required.
Technically, this is often achieved by an external intermediate cooler, in particular for gear compressors which have already connecting pipes between the individual stages. In addition, compact designs are common, where the coolers are integrated in pairs between the stages in the compressor housing.