Terms

Module 5: Heat Exchangers

  • Heat Exchanger: A device facilitating effective heat energy transfer between two mediums without mixing.

  • Heat Transfer Mechanism: Heats or cools by transferring heat energy through conduction.

Overall Heat Transfer Coefficient (U)

  • General equation in heat exchanger design involves the inside or outside surface areas with corresponding heat-transfer coefficients.

Resistances in Heat Transfer

  1. Convection Resistance (Inner Surface): Heat flow resistance from fluid to the wall.

  2. Inside Fouling Resistance: Resistance due to internal deposits during operation.

  3. Heat Conduction Resistance: Resistance through the solid wall separating fluids.

  4. Outside Fouling Resistance: Resistance due to external deposits during operation.

  5. Convection Resistance (Outer Surface): Heat flow resistance from wall to fluid.

  • Note: Conduction resistances can often be negligible for first estimates.

Fouling Factors

  • Over time, heat-transfer surfaces can become coated or corroded, leading to increased resistance.

  • Represented by a fouling factor (R_f), included with other thermal resistances for overall heat-transfer coefficient calculations.

  • Experimental determination required for clean vs. dirty conditions.

Types of Heat Exchangers

  1. Recuperative Type: Heat exchanged between fluids separated by a barrier.

  2. Regenerative or Storage Type: Material is heated by a hot fluid, followed by a cold fluid.

  3. Direct Mixing Type: Fluids mix and reach a common temperature; rarely used.

Heat Exchanger Arrangements

  1. Single Tube Arrangement (Double-Pipe): One fluid flows inside the tube, the other outside.

    • Flow Directions:

      • Parallel Flow: Fluids flow in the same direction.

      • Counter Flow: Fluids flow in opposite directions.

      • Cross Flow: Outside fluid flows perpendicular to pipe axis.

  2. Shell and Tube Arrangement: Suitable for larger capacities:

    • Small-bore pipes inside a shell, one fluid through tubes and other through the shell.

    • Subtypes: Various shell and tube pass configurations.

  3. Cross-Flow Heat Exchanger: Popular for air or gas applications.

    • Subtypes: Unmixed or partially mixed fluids.

Page 2: Log-Mean Temperature Difference & Heat Exchange Components

  • Log-Mean Temperature Difference (LMTD):

    • Temperature difference varies spatially.

    • Total heat flow calculated either by summing heat flow over elemental areas or calculating average temperature difference.

Condenser

  • Used to condense a gaseous substance into a liquid state through cooling, releasing latent heat.

  • Applications: Air conditioning, distillation, steam power plants.

Boiler

  • A closed vessel for heating fluid (typically water) without necessarily boiling it.

Module 6: Evaporation

  • Definition: Process of removing vapor from boiling liquid to concentrate solution.

    • Examples: Concentration of solutions (e.g., sugar, milk).

Distinction from Other Processes

  1. Distillation: Vapor usually a single component.

  2. Drying: Residue remains liquid, often viscous.

  3. Crystallization: Focus on concentration rather than crystal formation.

Processing Factors

  • Concentration: Low viscosity aids heat-transfer coefficients.

  • Solubility: Exceeding solubility limits leads to crystal formation.

  • Temperature Sensitivity: Heat-sensitive materials may degrade.

  • Foaming: Solutions like skim milk may foam during boiling.

  • Pressure and Temperature: Boiling point elevation occurs with dissolved solids.

  • Scale Deposition: Reduced solubility/decomposition leads to scale and reduced heat transfer.

General Types of Evaporators

  • Heat Exchanger: Condenses steam and adds latent heat to liquid feed.

  • Vapor Space or Head: Large chamber for separating liquid from vapor, often with impinging steam.

  1. Open Kettle: Boils liquid in an open pan (steam or direct heat).

  2. Horizontal-Tube Circulation: Steam condenses in tubes surrounded by boiling liquid.

  3. Vertical-Tube Circulation: Creates natural circulation.

  4. Falling-Film Evaporator: Liquid flows down tube walls.

  5. Forced-Circulation: Pump-driven circulation enhances heat transfer.

  6. Agitated-Film: Improves heat transfer for viscous materials.

  7. Open-Pan Solar Evaporator: Relies on solar energy for evaporation.

Methods of Operation

  • Capacity: Weight of water evaporated per hour.

  • Steam Economy: Water evaporated to steam ratio.

Boiling-Point Elevation

  • Increase in boiling point due to dissolved solids.

    • Factors: Minimal in dilute solutions, significant in concentrated salt solutions.

Page 3: Single-Effect & Multiple-Effect Evaporators

Single-Effect Evaporators

  • Process Description: Steam condenses to heat the solution, producing vapor.

  • Material and Energy Balance: Involves latent heat calculations and boiling-point rise.

Multiple-Effect Evaporators

  • Utilizes vapor from one effect as steam for the next, enhancing steam economy.

    • Types:

      • Forward Feed: Feeds flow same direction as vapor.

      • Backward Feed: Feeds flow opposite vapor, requires pumps.

  • Energy Balance: Considers latent heat transfer and temperature differences.

Steam Economy

  • Defined as vapor weight released versus steam weight used, improves with multi-effect systems.

Module 7: Crystallization

  • Definition: Process forming solid particles from a homogeneous phase, focusing on yield, purity, and uniform crystal size.

Crystal Geometry

  • Defined as orderly arrangement of atoms, appearing as polyhedrons.

Classes of Crystals

  1. Cubic System: Equal axes at right angles.

  2. Tetragonal System: One axis longer in right angles.

  3. Orthorhombic System: Unequal axes at right angles.

  4. Hexagonal System: Equal axes in a plane at 60°.

  5. Monoclinic System: Three unequal axes with two at right angles.

  6. Triclinic System: Three unequal axes at non-standard angles.

  7. Trigonal System: Three equal axes, equally inclined.

Equipment for Crystallization

  • Classified by methods used for supersaturation:

  1. Cooling with Negligible Evaporation: Tank and batch-type crystallizers.

  2. Evaporation with Little Cooling: Evaporator-crystallizers.

  3. Combined Cooling and Evaporation: Adiabatic evaporators.

    • Specific Equipment Types:

      1. Tank Crystallizers: Open tanks for cooling saturated solutions.

      2. Scraped Surface Crystallizers: Semicircular cooling jackets.

      3. Double-Pipe Scraped Surface: Internal agitators for heat transfer.

      4. Crystallizing Evaporators: Generate supersaturation and recycle.

      5. Vacuum Crystallizers: Circulate magma for crystal growth.

Crystallization Theory

A. Nucleation Theories

  1. Solubility and Crystal Size: Surface energy varies with particle size.

  2. Homogeneous Nucleation: Molecules cluster from fluctuations.

  3. Contact Nucleation: Caused by contacts with walls or agitated crystals.

  4. Commercial Nucleation: Low supersaturation and optimal agitation for growth.

B. Rate of Crystal Growth

  1. Crystal Growth Mechanism: Layered growth on outer faces.

  2. ∆L Law: Size changes are consistent regardless of crystal size.

C. Particle-Size Distribution

  • Defined by the coefficient of variation (CV).

Module 8: Mass Transfer

  • Definition: Molecule flow from one body to another, driven by concentration differences.

  • Key Concept: Rate of mass transfer is proportional to concentration gradient; inversely related to medium resistance.

Industrial Mass Transfer Operations

  • Tranfers components between phases, based on transfer within phases.

  • Categories:

    1. Direct contact of immiscible/partially miscible phases.

    2. Phases separated by membranes.

    3. Direct contact of miscible phases.

Diffusion Mass Transfer

  • Occurs without mixing; governs by Fick’s Law.

  • Governing Equations include mole diffusion details.

Equimolar Counter Diffusion

  • Constant total pressure; differences in partial pressures drive diffusion.

Convective Mass Transfer

  • Occurs through mass concentration interchange. Resistance causes concentration gradients.

Transient and Interphase Mass Transfer

  • Resistance in phases leads to varying concentration gradients.

Dilute Systems

  • Mass transfer equations for gas and liquid phases.

Concentrated Systems

  • Different equations apply at high solute concentrations.

Module 9: Simultaneous Heat And Mass Transfer

  • Definition: Coexists concentration and temperature gradients between two or more species.

  • Applications: Humidification involves simultaneous heat and mass transfer during liquid contact with gas.

Psychrometry

  • Involves gas-vapor mixture properties including humidification/dehumidification.

Major Areas of Air-Water Contact

  1. Humidification: Control specific moisture content in drying processes.

  2. Dehumidification: Common in air conditioning systems.

  3. Water Cooling: Used for coolers and condensers.

  4. Gas Cooling: Cooling air streams via water contact.

Humidification Terminology

  • Absolute Humidity (Y): Vapor mass per dry air mass.

  • Specific Humidity (YW): Vapor mass per gas-vapor mixture mass.

  • Mole Ratio (z): Moles of vapor per mole of gas.

  • Mole Fraction (y): Moles of vapor per gas-vapor mixture.

  • Volumetric Humidity (Yv): Vapor mass per volume of gas-vapor mixture.

  • Vapor Pressure (p): Partial pressure in gas-vapor mixture.

  • Saturation Vapor Pressure (ps): Pure vapor at a temperature.

  • Relative Humidity (RH): Vapor pressure to saturation ratio.

  • Dew Point (Tdew): Temperature for gas-vapor mixture saturation upon cooling.

Psychrometric Charts

  • Graphical representation of parameters for gas-vapor mixtures.

Operations Involving Gas-Liquid Contact

  1. Adiabatic Operations: Direct cooling or humidifying operations.

  2. Non-Adiabatic Operations: Like evaporative cooling and refrigeration dehumidification.

Drying Terminology

  1. Bone-Dry Solid: Solid without moisture.

  2. Bound Moisture: Liquid with vapor pressure lower than pure liquid.

  3. Constant-Rate Period: Water removal maintains constant rate.

  4. Dry Basis: Moisture content per bone-dry solid weight.

  5. Falling-Rate Period: Decreasing drying rate as surface dries.

Drying Equipment

  1. Tray Dryer: Trays in a heated chamber for batches.

  2. Rotary Dryer: Continuous drying using a revolving cylinder.

  3. Spray Dryer: Produces fine dry particles from atomized liquids.

  4. Fluid Bed Dryer: Fluidizes solid with gas for rapid drying.

  5. Drum Dryer: Evaporates liquid from thin layers on heated rolls.

Drying Rate Curve

  • Induction Period: Initial heating phase prior to drying.

  • Constant-Rate Period: Maintained vapor pressure keeps surface wet.

  • Falling-Rate Period: Drying rate declines as surface dries.