Heat Processes

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Lecturer:

Tutor:

Supervisor:

Department of Process Engineering

Synopsis:

Fundamentals of thermodynamics and heat transfer: internal energy, enthalpy and entropy (diagrams T-s, h-s). Enthalpy balancing or how to calculate energy demands for example in a pasteurization line for thermal treatment of milk.

Mechanisms of heat transfer: conduction (solids), convection (fluids), radiation (empty space). How to calculate heat transfer rate: thermal resistance and engineering correlations for heat transfer at different geometries (pipes, flow around sphere,...), effects of flow regimes (laminar/turbulent) and phase changes (evaporation, condenstation). Dimensionless criteria: Nusselt, Prandtl, Reynolds and Rayleigh numbers. Tricks as how to enhance the heat transfer (induced secondary flows,...) and possible problems with fouling.

Heat exchangers: functional description and mechanical design (powerpoint presentation]. Hydraulic and thermal design (NTU - epsilon method, graphical design). Different flow arrangements (parallel, counter and cross-flow), networks of heat exchangers. Specific features of shell and tube and plate HE design. Optimisation of heat exchanger networks - pinch analysis and targeting.

Evaporators: functional description (evaporators with natural and forced convection, falling, climbing and wiped film evaporators). Mass and enthalpy balancing. Multistage evaporators and evaporators with mechanical and thermal recompression of vapours (and how to design a Laval nozzle).

Drying and dryers: functional description (contact, convective, radiation and spray dryers). Properties of dried materials and drying media, humid air. Static and kinetics of drying (mass and enthalpic balancing, sorption isotherms drying rate controlled by heat transfer and by diffusion of water). Spray drying, droplets, evaporation of a liquid droplet.

Combustion. Combustors and furnaces. Fuels, composition and reaction enthalpy. Enthalpy balancing (how to calculate flame temperature etc.). Radiation absorption and emission by flue gases. Application of CFD for prediction of temperature and concentration distribution.

Electric heat. Microwave and direct ohmic heating.

Requirements:

Fundamentals of thermodynamics and hydromechanics. Elementary mathematics (derivatives, differential equations). Most of important basic notions are repeated during lectures, depending upon the students' background and reaction.

Syllabus of lectures:

1. Introduction and rules (form of lectures, presentations of papers by students, requirements for exam). Preacquaintance (photography). Schedule of student presentations. Working with databases and primary sources.

2. Thermodynamics fundamentals. State variables, internal energy (example: car driven by a compressed air). First law of thermodynamics, enthalpy (example: heating water at elevated pressure with phase changes). Entropy and the second law of thermodynamics (example: entropy change of heated water). Ts and hs diagrams (example: Ts diagrams for air). Thermodynamic cycles Clausius Rankine, Carnot factor (examples: nuclear power plants with light water moderator, another examples heat pump, thermal compressors, if there will be enough time).

3. Isobaric and isoenthalpic processes, choking and Joule Thomson effect in real gases (derivation of JT coefficient). Application of JT effect for liquefaction of gases in Linde process (kryogenics). Enthalpic balances (example: two stage compressor cooling, ph diagrams). Entropy and exergy balances. Exergetic losses: choking and heat exchangers. Heat processes design based upon entropy generation minimization EGM (derivation ds/dt).

4. Mechanisms of heat transfer. Conduction, convection (heat transfer coefficients), radiation (example: cooling cabinet). Fourier?s law of conduction, thermal resistance (composed wall, cylinder). Unsteady heat transfer, penetration depth (derivation, small experiment with gas lighter and copper wire). Biot number (example: boiling potatoes). Convective heat transfer, heat transfer coefficient and thickness of thermal boundary layer. Heat transfer in a circular pipe at laminar flow (derivation Leveque). Criteria: Nu, Re, Pr, Pe, Gz.

5. Heat transfer in turbulent flow, Moody?s diagram. Effects of variable properties (Sieder Tate correction for temperature dependent viscosity, mixed and natural convection). Noncircular profiles and equivalent diameter of pipe. Compact and plate heat exchangers. Hydraulic and thermal analysis of chevron type heat exchanger (H.Martin). Heat transfer enhancement (static mixers, centrifugal forces, Deans? vortices). Flow invertors. Performance criteria (PEC). Fouling (example: crude oil fouling - Polley model and diagrams).

6. Heat transfer at outer flows around sphere, cylinder and pipe bundle (derived asymptotic formula Nu=2 for sphere, paradox of cylinder). Experiment: hot air blown from hair drier to metallic cylinder with thermocouple; air flowrate calculated from temperature differences. Correlations VDI Warmeatlas. Heat exchangers: powerpoint presentation of HE design.

7. Shell and tube HE. Comparison 1-1 and 1-2 arrangements from point of view of pressure drops and heat transfer. Enthalpy balance of HE, temperature profiles, effectiveness, LMTD, sizing and rating design methods. NTU-epsilon method for parallel flows (eigenvalue problem, derived temperature profiles and eps), counter current and cross flow arrangement of streams (sheet of selected NTU-eps correlations from Rohsenow). Asymptotical properties. Zonal method. Graphical design (Roetzel Spang diagrams from VDI).

8. Heat transfer at phase changes (boiling and condensation). Evaporation and evaporators. Powerpoint presentation of evaporators (falling, climbing film, multiple effects, vapour recompression). Mass and enthalpy balances. Boiling point temperature and its elevation. Design of thermal vapour recompression (Laval nozzle and Ts diagram).

9. Drying and dryers. Powerpoint presentation (convective and contact dryers, lyofylization, fluidized bed and spray dryers). Properties of drying air (Mollier diagram, dew point, wet bulb temperatures) and dried material (moisture, sorption isotherms). Mass and enthalpy balance. Kinetics of drying. Drying experiments. First and second stage of drying. Moisture diffusion (example: coffee beans dying). Spray drying, evaporation of flying droplet.

10. Combustion and burners. Powerpoint presentation (pulverized coal, biofuels, oil and gas burners, NOx reduction, CFD analysis of gas burner). Properties of fuels, reaction enthalpy, combustion heat. Enthalpy balances, adiabatic flame temperature. Heat transfer by radiation, emissivity and absorptivity of flue gases. Hottel?s diagram.

11. CFD analysis of combustion. Transport equations and modeling of chemical reactions. Non-premix combustion and mixture fraction methods. Lagrangian and Eulerian methods.

12. Electroheat. Direct ohmic heating, radiofrequency heating and microwave heating.

Syllabus of tutorials:

Consult dr.Dostal

Study Objective:

Generally speaking the goal of this course is Practicing Engineering approach to heat transfer unit operations design. Do you know that for example a Heat Exchanger is most frequently acountered apparatus in mechanical engineering? A more specific goal is „learning to read engineering journals“. A part of lectures (approximately 15 minutes) is devoted to a presentation of papers selected from the Elsevier database on topics related to the currently delivered lectures. Students select these papers according to keywords (e.g. „mitigation“ and „fouling“).

Study materials:

There exist many excellent engineering handbooks on heat transfer, but I believe, that the lecturenotes would be sufficient.

McCabe W.L., Smith J.C., Harriot P.: Unit Operations of Chemical Engineering. Fifth Edition, McGraw Hill, Inc., N.Y.

Note:

Further information:

No time-table has been prepared for this course

The course is a part of the following study plans: