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Publication Name: AXIAL AND RADIAL TURBINES HOW DO THEY COMPARE IN THE 1-TO-3 MW POWER RANGE
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GAS TURBINES

AXIAL AND RADIAL TURBINES HOW DO THEY COMPARE IN THE 1-TO-3 MW POWER RANGE?

JAN MOWILL AND LARS-UNO AXELSSON

Axial gas turbine technology is the dom- inant and appropriate configuration for large gas turbines. Major power plants and industrial plants deploy axial gas turbines to provide power and heat for district heating, process, facilities and electricity to the grid.

Less discussed, though, are the lower power ranges where both axial and radial gas turbines are available. It is

worthwhile to compare axial

and radial turbines where the technologies overlap.

Engines with radial com- pressors and radial turbines can effectively be used in single shaft turbines in power ranges fromaslowas1kWupto approximately 2 MW. If the configuration is combined with an axial power turbine, these types of turbines would be applicable for a power range up to around 4 MW.

Consequently, industrial turbine engines below 2 MW normally use radial compressors (centrifugal), but the choice of turbine type varies. As the range lowers, radial turbines have more advantages over the axial turbines.

The chief difference between axial and radial turbines is the way the air flows through the compressor and turbine. In a radial turbine, the inlet airflow is radial to the shaft, whereas an axial turbine is a turbine in which the airflow is parallel to the shaft.

Generally, the axial turbine disc is pro- tected from the heat that the turbine blades are exposed to. Not so with the radial turbine where the hot mass-flow expands in both the impeller portion and the exducer portion of the turbine.

However, a radial turbine can accom- modate an expansion ratio of about 9 to 1 in a single stage. An axial turbine would commonly require three stages to handle such an expansion.

This difference in expansion between axial and radial turbines can be explained by the following equation:

Ws = U2*Cw2 - U3* Cw3

Where Ws is the stage work per unit mass flow, U2 is the inlet blade speed, U3 is the exit blade speed, Cw2 is the inlet tangential velocity and Cw3 is the exit tan- gential velocity.

32 Turbomachinery International • November/December 2012

In an axial turbine U2 and U3 are approx- imately equal, whereas in a radial turbine U2 is greater than U3. Looking at the above equation one can see that the stage work, for the same change in tangential velocity, is larger for a radial turbine compared to an axial turbine.

In a centrifugal compressor, the air receives greater energy as it accelerates at increasing diameters. This velocity energy is converted into pressure energy when slowed down in the static diffusor. An axial

The most convenient rotor arrangement for an all-radial configuration utilizing a cen- trifugal compressor and radial turbine would be a cantilevered arrangement with both bearings located in front of the compressor in the cold part of the turbine.

This would be difficult to achieve using axial turbines. The bearings would have to be in the hot section of axial turbines and they would have a shorter life as a result. However, if there is a requirement to combine the radial configuration with an independent power tur-

bine, bearings in the hot part of the engine would be needed.

One distinct advantage of an axial turbine is the possibili- ty of being air cooled. In this way axial turbines can be oper- ated at much higher tempera- tures than radials and achieve greater efficiencies in higher power ranges. Cooling of radial turbines have been attempted in the past, but has not been suc- cessful.

Cooling of small axial tur- bines, however, also poses problems as intricate cooling holes become smaller and more complex. The clogging of the cooling holes is a source of performance degradation during the turbine life time. Since the radial turbine normally does not include cooling holes, its performance during

the operation remains nearly the same.

It would appear, therefore, that for single- shaft turbine engines below 2 MW an all radial concept has certain advantages. Radial turbines have the capability to operate uncooled at a higher turbine inlet temperature

than uncooled axial turbines.

The radial configuration has fewer

stages, is shorter in length and is more robust than the axial configuration and can achieve longer life and less maintenance. These fea- tures are generally sufficient to give radial turbines the edge below around 2 MW.

But, if a separate power turbine is need- ed, axial turbines might be preferred. For higher power ratings, the cooled axial turbine is the logical choice. The limitation in size prevents the radial concept being used for larger sizes. TI

Authors: Jan Mowill is Founder and Chairman of OPRA Turbines. Lars-Uno Axelsson is Chief Engineer, Development, OPRA Turbines, a man- ufacturer of advanced, clean, low emission gas turbine generating sets for 1 -10 MW power generation solutions for oil and gas and industri- al customers. For more information, visit www.opraturbines.com.

An all radial 2 MW range gas turbine: The OP16

compressor flows the air parallel to the axes providing increasing lift (pressure) depend- ing on the number of stages and intermedi- ate stators.

In the combustor, heat is added causing the volume of the air to be increased. The hot gasses would enter the turbine via fixed noz- zle guide vanes directing the flow against the turbine. If the turbine is of the radial type, the peripheral speed of the turbine should be at or close to the speed of the gas stream enter- ing the turbine. In this way, the added “stag- nation” temperature, which a lower speed axial turbine would encounter would not be there.This enables the radial turbine to oper- ate uncooled at up to around 100°C above axial turbines.

Radial turbines are able to do this due to the “Eiffel Tower” cross section of the turbine with a substantial hub and thinner blades.

The radial turbine functions in the oppo- site way to a centrifugal compressor: As the flow loses speed and temperature transferring energy to the turbine, the flow enters the exhaust diffusor at lower than atmospheric pressure providing some suction before exhausting at atmospheric pressure at the end.

An axial turbine blade is similar to a small airplane wing. If too much power is extracted from it, it will stall, the equivalent of lost lift in an aircraft. That is why it takes two to three stages and intermediate stator blades to match one radial turbine stage.

www.turbomachinerymag.com

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