MEC 3112 Lecture IX and X - Heat Exchangers

Course Overview

  • Course: MEC 3112 Mechanical Equipment

  • Instructor: A. Khelawan, B.A.Sc., M.A.Sc

  • University: University of Toronto

  • Professional Engineer: P.Eng, Province of Ontario, Canada

  • Lectures Covered: IX and X – Heat Exchangers

    • Introductions and Examples of Applications

    • Review of Course Content

    • Preliminary Concepts

    • Examples/Assignments

Heat Exchangers Introduction

  • Definition: A heat exchanger is a device used for transferring heat from one fluid to another through a separating wall.

  • Usage: Mainly used in process industries for heat balance calculations—addition or removal of heat via exchangers linked to hot and cold process streams.

  • Example Application: Automotive Cooling System components include:

    • Radiator

    • Cooling Fan

    • Thermostat

    • Water Pump

    • Engine Hoses

Functions of Automotive Cooling System

  1. Heat Removal: Eliminates excess heat from the engine.

  2. Temperature Maintenance: Ensures the engine operates at optimal temperatures.

  3. Rapid Heating: Brings the engine to the correct operating temperature quickly.

  • Main Components:

    • Engine

    • Radiator

    • Water Pump

    • Cooling Fan

    • Hoses

    • Thermostat

  • Energy Conversion: Up to 70% of engine energy in combustion is converted to heat.

  • Heat Transfer Process: Hot coolant circulates from the engine to the radiator, where it is cooled by air blown by the fan, returning to the water pump for recirculation.

Overview of Heat Exchangers

  • Functionality: Heat exchangers enable heat transfer without physical contact between the two fluids, primarily through conduction and convection.

  • Heat Transfer Principle: Heat transfers from a higher temperature fluid to a lower temperature fluid.

  • Common Types: Includes double-pipe, spiral, plate and frame, and shell and tube heat exchangers.

    • Shell and Tube Exchanger: Most common in process industries.

  • Operational Awareness: Process technicians should be aware of the operational aspects and any factors affecting heat exchange.

Heat Transfer Overview

  • Temperature vs. Heat:

    • Temperature: Measure of thermal energy average (hotness or coldness).

    • Heat: Transmission of energy between objects due to temperature differences.

  • Heat Transfer Methods:

    1. Conduction: Direct heat transfer through materials.

    2. Convection: Heat transfer through fluid movement.

    3. Radiation: Less common in heat exchangers.

Energy Transfer Equations

  • Fourier's Law of Heat Conduction:

    • Q_conduct = (k * A * (T2 - T1)) / thickness

  • Newton's Law of Cooling:

    • Q_conv = ha * (T1 - Tw)

  • Stephan-Boltzmann Law:

    • E_max = σ * A * T^4

Applications of Heat Exchangers

  • Industries:

    • Chemical and petrochemical

    • Food industry

    • Pharmaceutical industry

    • Fertilizer industry

    • Textile industry

    • Power production

    • Waste heat recovery

Types of Heat Exchangers

  • Basic Types: Double-pipe, spiral, cold box, air-cooled, plate and frame, and shell and tube heat exchangers.

  • Functional Classification: Includes condensers, reboilers, preheaters, vaporizers, chillers, intercoolers, and aftercoolers.

Double-Pipe Heat Exchangers

  • Description: Comprises one pipe within another; suitable for high-pressure applications but limited to low heat duty due to reduced surface area.

  • Design: Features nozzles facilitating flow through the annular and inner pipes, sometimes equipped with fins to enhance heat transfer.

Shell and Tube Heat Exchangers

  • Common Design: Consists of multiple tubes enclosed within a cylindrical shell.

  • Usage: Commonly used for chillers, condensers, coolers, and other thermal processes.

  • Heat Transfer Methods: Uses conduction (through tubes) and convection (within shell and tubes).

  • Operational Configurations: Can be single-pass or multipass depending on the design.

Kettle Reboilers and Plate Type Heat Exchangers

  • Kettle Reboilers: Often utilized in distillation processes, featuring vapor-disengaging space.

  • Plate Type Heat Exchangers: Constructed with corrugated plates, allowing for significant heat transfer due to the high surface area. Used primarily for clean and non-toxic fluids.

Maintenance and Operations

  • General Maintenance Tasks may include backwashing, water blasting, sand blasting, and acidizing to maintain exchanger efficiency.

  • Specific Cleaning Techniques:

    • Backwashing: Involves reversing flow to dislodge accumulated solids.

    • Water/Sand Blasting: Remove scale using high-pressure water or air-sand mixture.

    • Acidizing: Employs acid to dissolve deposits on accessible surfaces.