Axial Reaction Turbines
Barber-Nichols Inc. ( BNI ) is a designer and manufacturer of both single stage and multiple stage
Axial Flow Reaction Turbines. Reaction turbines can provide higher efficiencies than impulse turbines;
however, the head or enthalpy drop across a stage is more limited than that of impulse turbines.
Axial flow reaction turbines are typically used for expanding hydrocarbon gases or fluorinated refrigerants where a high molecular weights results in a low enthalpy drop across the turbine. When air or steam is used the head drop across a single turbine is usually too high; therefore, multi-stage turbines can be used to reduce the head across each stage. Additionally, the power and resulting flow rates should be high enough to result in specific speed values of 60 or higher. If specific speeds are low, then impulse turbines may be better suited for an application.
The term "axial
flow" indicates that the primary flow path through the turbine is parallel to the turbine
shaft with a small change in radius due to increasing blade lengths along the flow path. The
reaction in a turbine stage refers to the amount of enthalpy or head drop across the rotor expressed
as a percentage of the total head drop across the stage. An impulse turbine has zero reaction
because the entire head drop is across the nozzle. A 50% reaction turbine drops half its head
across the rotor and the remaining half across the nozzles. This is a common reaction turbine
design because it provides high efficiencies with manageable rotor tip speeds. When the reaction
degree is 50% the relative velocity vector the rotor is axial. The blade turning angle in a reaction
turbine is then somewhat less than 90 degrees, typically around 70 to 75 degrees. The low turning
angle combined with fluid acceleration due to head drop across the rotor results in extremely
high rotor velocity coefficients and good energy transfer. This is one reason why reaction turbines
can achieve higher efficiencies than the impulse turbines.
Reaction turbines do however have potential thrust problems due to the pressure drop across the rotor. The exit side pressure is lower than the inlet side and some means of thrust balance may be required. Additionally, tip leakage over the top of the blades represents a loss because this fluid does not produce any work.
Turbines can be designed with lower amounts of reaction if high energy ie. large enthalpy drops are required that would result in rotor tip speeds that are too high for mechanical integrity. The 50% reaction turbine reaches its highest efficiency at Turbine Velocity Ratio (U/Co) values of 0.7. Alternatively, the impulse turbine reaches its highest efficiency at U/Co values that are less than 0.5. Designing turbines for reaction percentages that are less than 50% provide improved efficiency over the pure impulse design along with an increase in the optimum U/Co value.
Single stage turbines are the best choice when the enthalpy drop across the turbine can be accommodated and when the rotor tip velocity is within sound mechanical limits. However, when the total enthalpy drop is such that the required U/Co can not be attained due to mechanical limitations, the enthalpy drop can then be divided between two or more stages. Interstage leakage and bearing thrust loads are variables that must be considered when deciding whether a single or multistage turbine will be most appropriate.
Barber-Nichols can assist you in the selection of the best turbine concept for your application.