Fans Blowers and Compressors P M V Subbarao

Fans, Blowers and Compressors P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Machines to Generate Lively Gases….

First Law Analysis & ASME Classification SSSF : Hardware

Nearly Incompressible Flow machines : Fans

Fluid Dynamics of Centrifugal Fan

Centrifugal Fan Operation • Fans cause a pressure increase through two methods • Centrifugal force is created by the rotation of the column of air trapped between two blades. • Kinetic energy is supplied to the air through the impeller • Total pressure = static pressure + velocity head • Blades are backward-curved, forward curved, or radial (straight) Airfoil-types are complex and expensive but very efficient; they’re used for large systems where the cost is justified.

Axial Flow Fans • Common types: propeller, tubeaxial, vaneaxial • Tubeaxial: impeller is inside a tube to guide airflow and improve performance. • Vane axial: like a tube axial except vanes downstream of the impeller are used to reduce swirl and improve performance • Used to deliver large flow rates but small increase in pressure • Examples include fans used for ventilation without ductwork, mobile room fans, and fans used to cool computers. • Tube-axial fan for computer cooling • Tube-axial fan for ventilation • Vane-axial fan for high air resistance electronics cooling • Straightening vanes are located inside tube.

Types of Axial Flow Fans

Anatomy of Large Axial Flow Fan

Fan Laws Power Pressure rise Operational and performance data of different but geometrically similar fans can be cast onto a single universal curve using dimensionless numbers

Actual Fan Power

Fan Flow Control • Fans rarely are operated continuously at the same pressure and volume discharge rates. • There exists a need for a system which can vary the vane output according to requirements. • Common methods of controlling fan output are: • Damper control • Inlet vane control • Variable speed control. • Axial fans can control flow using variable pitch blades.

Characteristic Curves: Radial Vane Centrifugal Fan Static Pressure Efficiency r e Pow

Characteristic Curves: Forward Vane Centrifugal Fan Noise Level Static Pressure E cy n e i ffic r e w Po

Characteristic Curves: Backward Vane Centrifugal Fan Noise Level Static Pressure Efficiency Power

Comparison of Fan Characteristic Curves : Static Pressure straight Backward Forward

Characteristic Curves: Axial Fan Noise Level Power E cy n e i ffic Static Pressu re

ASME Classification

Classification of Compressors Rotodynamic Compressors Centrifugal Compressors Mixed Compressors Positive Displacement of Compressors Axial Compressors Reciprocating Compressors Rotory Compressors

Duty of Compressors • A two step life infusion into gas. • Rotor : Mostly Energy Transfer • The centrifugal compressor consists essentially of a stationary casing containing a rotating impeller which imparts a high velocity to the air. • Only Energy Conversion: • A number of diverging passages (Diffuser) in which the air is decelerated with a consequent rise in static pressure.

Centrifugal Compressors • Air is sucked into the impeller eye and whirled round at high speed by the vanes on the impeller disc. • Centripetal acceleration leads to increase in rise of some amount of static pressure. • The remaining increase in rise of pressure occurs in diffuser.

Impeller – Diffuser : Rotor - Stator

Encapsulated Diffuser

Anatomy of A Centrifugal Compressors

Performance of centrifugal compressors • Total Pressure Ratio Design clue for Blades:

Centrifugal Compressors for High Pressure Ratio

Performance of Radial Vane Compressor

Pressure Ratio Vane Geometry : Compressor Characteristics Mass Flow

b 2 =9 0 Performance of Backward Vane Compressor b 2 5 =7 b 2 0 =6

Remarks on Centrifugal Compressors • • • High pressure ratio per stage (r 0 p= 3 to 4). Less flow area per unit mass flow rate. A multistage centrifugal compressors have shorter lengths. They are best suited for low mass flow rates. As the flow is turned by 900, the efficiency of centrifugal compressor is relatively low. The impeller is an integral unit of blades and a disk. Even if one blade is damaged the entire unit is to be replaced. Look for other fluid dynamic phenomenon for ingestion of life into fluids. Aerofoil Theory.

Evolution of Compression Phenomenon Centrifugal Compact & Less Efficient Aerofoil Bulky & More Efficient

GE T 700

AXIAL FLOW COMPRESSORS An Efficient Way to Ingest Life in Gases !!!

A Compressor Stage

Axial Flow Compressors

Axial Flow Fluid Machines The power, P of a fluid Machine

Similarity Analysis • Experimental performance curves or maps provide a versatile data for compressor engineers. • The data is translated into most general form. • Data in general form is minimally effected by air speed, altitude etc. • The total pressure ratio of a compressor is: Using dimensional analysis & Buckingham Pi Theorem:

Similarity Analysis

Test Rig For Centrifugal Compressor

Compressor Characteristics Maximum Efficiency point Zero Efficiency Points Supply pipe Compressor exit

Centrifugal Compressor Map

Small Centrifugal Compressor Maps

Axial Compressor Maps

Variable Speed Operation

Axial Flow Compressors: Current Design Practice Fan or low pressure Compressor Parameter Pressure ratio for single stage Pressure ratio for two stages Pressure ratio for three stages Inlet mass flow rates Tip speed Range 1. 5 – 2. 0 – 3. 5 – 4. 5 195 – 205 kg/m 2. s 427 – 457 m/s

Axial Flow Compressors: Current Design Practice High pressure Compressor Parameter Stage loading coefficient Flow coefficient Hub/tip ratio Inlet mass flow rates Tip speed Range 0. 3 – 0. 35 0. 45 – 0. 55 0. 6 – 0. 75 175 – 185 kg/m 2. s 386 – 457 m/s

Multi Stage Hybrid Compressor

GE 9 X : The worlds Next Great Jet Engine : 2020 6 -stage turbine 14 -stage compressor contributing to an overall engine pressure ratio of 60: 1.