10.Turbomachinery

7/25/2019 10.Turbomachinery

1/21

CHAPTER 4INTRODUCTION TOTURBOMACHINERY


2/21

CHAPTER 4 : INTRODUCTION TO TURBOMACHINERY

Introduction

Turbomachinery is one of the most widely used applications in theindustry.

Machines that add energy to fluid flow are called power producingmachines. Example: pumps.

Machines that draw energy from fluid are called power absorbingmachines. Example: turbines

These machines are often connected to a rotating shaft. Thats why wecall it turbomachinery. Turbo means spin in Latin.

Pumps

Machines that deliver fluidspumps

Machines that deliver gassesfan (small pressure rise)blower (up to 1 atm)


3/21

compressor (above 1 atm)

Pumps increase the work in a system by increasing pressure, producinga flow or increasing flow rate.

An external power supply is used to rotate the impeller

Two types of pumps- positive displacement pumps: force fluid by volume changes- dynamic pumps: fast moving blades or vanes add momentum

to the fluid

Positive-displacement pumps operate by a change in volume; energyconversion is intermittent. Examples in the human body include theheart (diaphragm pump) and the intestines (peristaltic pump). In areciprocating pump (e.g. a bicycle pump) fluid is sucked in on one part ofthe cycle and expelled (at higher pressure) in another.

In dynamic pumps there is no change in volume and energyconversion is continuous. Most pumps are rotodynamic devices where

fluid energy is exchanged with the mechanical energy of a rotatingelement (called a runner in turbines and an impeller in pumps), with afurther conversion to or from electrical energy.

Further classifications of pumps


4/21

Pump Performance Criteria & Curve

Free delivery : The maximum volume flow rate through a pumpoccurs when its net head is zero, H = 0; this flowrate is called the pumps free delivery.

Shutoff head : The net head that occurs when the volume flow rateis zero, and is achieved when the outlet port of thepump is blocked off. Under these conditions, H islarge but V is zero; the pumps efficiency is againzero, because the pump is doing no useful work.

Best Efficiency Point (BEP) : The pumps efficiency reaches itsmaximum value somewhere between

the shutoff condition and the freedelivery condition. It is notated by anasterisk (H*, bhp*, etc.).


5/21

It is common practice in the pump industry to offer several choices ofimpeller diameter for a single pump casing. Reason for this

- save manufacturing cost- to enable capacity increase by simple impeller replacement

-

standardize installation mountings- re-use the equipment for a different application

When plotting the performance of such a family of pumps, pumpmanufacture do not plot separate curves for each impeller. They preferto combine the performance curves of an entire family of pumps ontoa single plot.

Flowrate (m3/h)

The pump operates at a duty pointwhere the head supplied by the pumpprecisely matches the head requirements of the system at the samedischarge; i.e. where the pump and system characteristics intersect.

Head

(m)


6/21

Centrifugal Pump

Centrifugal pumps and blowers can be easily identified by their snail-shaped casing, called the scroll.

They are found all around your home; in dishwashers, hot tubs, clotheswashers and dryers, hairdryers, vacuum cleaners, kitchen exhausthoods, bathroom exhaust fans, leaf blowers,furnaces, etc.

They are used in cars; the water pump in the engine, the air blower inthe heater/air conditioner unit, etc.

Centrifugal pumps are ubiquitous in industry as well; they are used inbuilding ventilation systems, washing operations, cooling ponds andcooling towers.

A centrifugal pump is shown below.

The centrifugal pump consist of a series of blade attach to an impellerconnected to a shaft.


7/21

The impeller is placed inside a casing which has inlet and outlet.

Fluid enters the pump through inlet and exits the pump through outlet.

The inlet and outlet are connected to pipes.

When fluid enters the inlet (called the eye) the pressure is low.

The fluid then receives energy (pressure and velocity) from the rotatingimpeller.

The fluid leaves the impeller radially into the scroll.

The cross section area of the scroll increases (like a diffuser) to reducevelocity and further increase the pressure until the fluid leaves throughthe exit.

Centrifugal Pump Classification

Forward-InclinedBlades

Radial Blades Backward-InclinedBlades

The basic performance parameters of a pump among others are head(H), power (P) and efficiency ()


8/21


9/21


10/21


11/21


12/21


13/21

Cavitation

What is CAVITATIONS?- Cavitations only happen in liquid.

-

It is possible for the local pressure inside the pump to fallbelow vapor pressure of the liquid, Pv

- At these locations, the liquid will boils locally and creates thecavitations bubble will appear

- These bubbles will be transported and will collapse once itreach the region with higher pressure, Plocal> Pv

- The collapse of the bubble is undesirable, particularly at theregion near to blade or chasing surface

Appearance / Formation of Cavitations Bubble

Cavitation bubbles forming and collapsing on the suction side of animpeller blade.


14/21

Damage caused by cavitations


15/21


16/21

NPSHA is the NPSH available at the pump suction nozzle anddepends on the suction system design. It must always be equal to orgreater than the NPSHR. NPSHA can be determined by direct fieldmeasurement if the vapor pressure is known.

NPSHR is the NPSH required by the pump for stable operation. It isdetermined by the pump manufacturer and is dependent on manyfactors including the type of impeller inlet, impeller design, pump flow,rotational speed, nature of the liquid, etc. It is usually plotted on thecharacteristic pump performance curved supplied by the pumpmanufacturer.

Example NPSHA calculation

Hydraulic Scaling

Provided that the mechanical efficiency is the same, the performanceof a particular geometrically-similar family of pumps or turbines(homologous series) may be expected todepend on:

discharge,Q [L3T1]pressure change,gH [ML1T2]power, P [ML2T3]rotor diameter, D [L]rotation rate,N [T1]


17/21

fluid density [ML3]fluid viscosity [ML1T1]

(Rotation rate is typically expressed in either rad s1 or rpm.)

Since there are 7 variables and 3 independent dimensions,Buckinghams Pi Theorem yields arelationship between 4 independentgroups, which may be taken as :

whereHead Coefficient

CH= gH/N2D2= gH/w2D2

Capacity CoefficientCQ= Q/ND

3= Q/wD3Power Coefficient

CP= P/N3D5= P/w3D5

Suction Head CoefficientCNPSH= gNPSH REQUIRED/N

2D2= gNPSH REQUIRED/w2D2

For fully-turbulent flow the dependence on molecular viscosity andhence the Reynolds number (4) vanishes. Then, for geometrically-similar pumps with different sizes (D) and rotation rates (N):

Change of Speed - For the same pump (i.e. same D) operating atdifferent speedsN1 andN2:

Thus,

Q N , H N 2, P N 3


18/21

Example


19/21


20/21


21/21

Documents

10.Turbomachinery