ENEM20003: Thermofluids Engineering ApplicationsFluid Machinery

Fluid Machinery• Source Text: Chapter 12 and Reference materials

• Fluid machinery can be divided into two basic groups• Machines that convert mechanical energy into fluid

energy. Examples: pumps, compressors, propellers, fans.

• Machines that convert fluid energy into mechanical energy. Examples: turbines, motors.

Pump• Positive displacement pump

– It takes clearly identified volumes of fluid from inlet and transfers them to the delivery side.

– There are seals to prevent the fluid returning to the inlet side when the pump is stopped.

• Rotodynamic pump– It relies on the hydrodynamic action of suitably shaped

impellers to induce the fluid from the inlet side to the delivery side.

A rotodynamic pump has no ability to sustain pressure when at rest & is not able to generate such high pressures as can a positive displacement pump.

Advantages– It can handle large volume flow rates– It can handle slurries and contaminated liquids– Low care and maintenance requirements– Comparatively low cost

The difference between centrifugal, mixed and axial flow pumps lies in the direction the fluid takes through the impeller.

• Compressor– Is used to primarily increase the pressure of a gas.

Can be positive displacement and rotodyrotonamic types.

• Fans– Deliver large volumes of gas without a significant

increase of pressure. Mostly axial flow type (rotodynamic).

• Blowers– Also deliver large volumes but with a greater pressure

increase. They are mostly positive displacement type.

Pump performance:The shaft power required to drive the pump is

Pump Efficiency:

30 is rad/sec, N is rpm

TNP T πω

ω

= =

is the fluid power. Other efficiencies contribute to the overall efficiency.

gHQP

gHQ

ρη

ρ

=

• Mechanical efficiency, , indicates a mechanical frictional loss in bearings, seals

• Hydraulic efficiency, , indicates fluid friction losses

• Volumetric efficiency, , indicates a slippage or leakage of fluid within the pump.

mη

hη

vη

Fluid Energy Equation

Where Hp is the pump head (note that your textbook uses ha)Pump & System relationship•The inter-relationship between the capabilities of the pump & the requirements of the system is important in pump operation.•The pump must supply what the system requires. There must be an equilibrium between the head & discharge by the pump & that demanded by the system

System head equation• It is the sum of two components:

– A static component, Hstatic consisting of pressure head plus the potential head. This component does not change with the flow rate (or velocity).

– A dynamic component, Hdyn consisting of the head loss plus the change of velocity head. If the friction factor is constant, this component varies as the square of the flow rate.

Where A is a constant = HstaticB is a constant, if friction factor does not vary with flow rate.

For a system, application of Bernoulli equation shows that

A system head equation is a parabolic curve as seen

The pump will operate where the head required equals the head available at the same flow rate. This is indicated by the point of intersection of the two curves. This equilibrium point is often termed as the ‘duty point’

Characteristic Curve • The fluid quantities involved in all

hydraulic machines are Q, Head (pump head), where as mechanical quantities are power P, speed N, size D and η

• The characteristic of most general interest is the H vs Q for a given speed, H=A-BQ. Where A and B are constants and the theoretical H vs Q is a straight line; however real one is a curve due to losses.

• To achieve this performance a power input is required – which involves η.

• For turbines, the fundamental characteristics consist of a plot of P against N at constant head H.

• When the pump performance curve and the system curve equation are superimposed – the intersecting point gives the duty point.

• Mixed flow fan impeller Centrifugal pump impeller (unshrouded)

• Centrifugal pump impeller (shrouded)

Energy TransferThe transfer of energy, forces or torque between a moving stream of fluid and solid elements of machine is governed by the principles of momentum equation.

Pump (centrifugal)• The performance of a centrifugal pump is essentially

dependent on the action of the impeller blades. • The action of the vanes is revealed through the study of

the fluid velocity into & out of the impeller. • The directions of the velocities relative to the impeller,

are tangential to the inlet and outlet edges of the vanes.• Tangential velocity component (whirl velocity) relates to

the pump head.• Flow velocity (radial component) relates to pump

delivery.

Rotodynamic Pump• If a rotodynamic pump is the best type of pump for the

system under consideration, it is necessary to make a selection from those commercially available.

• To select a stock pump, it is necessary to refer to pump manufacturer catalogues.

• These outline the dimensions and materials used in the range of pumps they produce

• The performance curves for each type and size of pump are usually given.

• Rotodynamic pumps are often denoted by their inlet and outlet diameters and the nominal impeller diameters.

• For example 100 X 80 – 160 pump would have a 100 mm inlet diameter an 80 mm outlet diameter and a nominal impeller diameter of 160 mm.

• Initial selection of appropriate pump is often assisted by use of a range chart

• When using the chart, decide on approximate flow rate and head needed in the system

• Suppose, for a system head of 14 m and the required flow rate is 20 L/s, it can be seen that 125 X 80 - 250 centrifugal pump running at four-pole speed is suitable for this application.

• The pump is manufactured with three standard impeller diameters: 260, 230 and 204 mm. It is now necessary to select the best based on the impeller that gives the lowest energy cost.

Questions?

ENEM14014 : Capstone Thermofluid EngineeringFluid MachinerySlide Number 3PumpSlide Number 5Slide Number 6Slide Number 7Slide Number 8Slide Number 9System head equationSlide Number 11A system head equation is a parabolic curve as seenCharacteristic Curve Slide Number 14Slide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 20Energy TransferPump (centrifugal)Slide Number 23Slide Number 24Slide Number 25Slide Number 26Slide Number 27Slide Number 28Rotodynamic PumpSlide Number 30Slide Number 31Questions?