HEAT

Eng. Robert FairchildBEF CHaB Stove Camp

The FarmSept 2012

What is heat?

• To an engineer – Heat is the flow of energy caused by a temperature difference

• Hotter energy Colder

How does heat flow?

• Conduction – through all materials• Convection – movement of fluids ( liquids and

gasses)• Radiation -- between surfaces through

vacuum/air/any medium that transmits electromagnetic radiation (light)

Conduction - through material

•Qcond=(k/t)A(Thot-Tcold)

• Linear Function of Area, Conductivity, and Temperature difference

More heat flow with more area, higher conductance, higher temperature difference

Less heat flow with greater thickness

Thermal Conductivity of Common MetalsHeat Transfer, J. R. Holman, McGraw Hill, 1978, pp. 10.

Convection-by movement of fluids

•Qconv=hA(Thot-Tcold)• Linear function of film conductance, area, temperature

difference• Film conductance is a function of the velocity (speed)

of fluid movement• More heat flow with more area, higher film

conductance, higher temperature difference

Types of convection

• Natural – caused by changes in density (and therefore buoyancy) of fluids as they are heated

• Forced – caused by mechanical devices causing fluids to flow across the surface

( fan/blower or pump)

Heat Transfer, J. R. Holman, McGraw Hill, 1978, pp. 13.

Radiation – by “light”

Radiation is a linear function of emissivity, geometry (view factor), and area

And the difference of the fourth powers of absolute temperature

•Qrad=FεσFgA(Thot4-Tcold

4)

Emissivity depends on “color” and surface finish

Energy Transport

• Moving heated materials from the point of generation to the point of use

• Generally done with liquids (water, antifreeze, oil) in pipes with pumps, gasses (generally air) in ducts with blowers

• Can be done by moving hot solids (e.g. bricks)

Insulation• Any material that reduces the flow of heat

-Batts, sheets, blocks, cast material to reduce conduction and convection

reduced k or h-Radiative insulation

reduce Fε (change “color” or finish) of sending and receiving surfacesreduce Fg (block the view)

Heat Exchangers• Devices designed to increase heat transfer – Increase surface area– Increase fluid velocities near surfaces – Decrease conductivity • change material • reduce thickness

Heat Exchanger Types and Examples

• Gas to gas - small engine cooling fins• Gas to liquid – firetube or watertube boiler• Liquid to gas – car radiator, finned tube• Liquid to liquid – tube in tank• Radiant to air – solar air heater• Radiant to water – solar water heater

Small Engine Cooling Fins

Fire Tube Boiler(Stanley Steamer) http://en.wikipedia.org/wiki/Vertical_fire-tube_boiler

Finned Tube

Tube in Tank

Solar air heater

Solar water heater

Shell and tube heat exchangerOne fluid runs through the tubes.The other fluids runs past the tubes in the shell forced into a serpentine flow by baffles.

Barrel TLUD Heat Exchangers

• Gas water heater tank with insulating skirt• Shell and tube air heater• Counterflow tubular air heater

Shell and tube HExsecondary fire tube baffles andair manifold nozzle fire tubes

Counterflow Tubular Heat Exchanger to get hot air from flue gasses

• 30 gallon drum inside 55 gallon drum• 30 gallon drum with 4” stovepipes around conduits• Seven 2” conduit fire tubes through drums• Insulate around 4” stovepipes in 30 gallon drum• Skirt and insulate around 55 gallon drum• Blow secondary air into tubes with pipe manifold• Blow air thru duct into upper side of 55 gallon drum• Duct hot air out lower opposite side of 55 gallon drum

Counterflow HEx innerstovepipes around fire tubes

Heat Pipes

• Use sealed container of fluid to move heat by evaporation and condensation cycle

• http://www.forsteel.cz/heat-pipes.html

SOME USES FOR HEAT

• Space heating• Water heating• Drying – fuel, forage, food, lumber, …• Desiccant recharging • Evaporation/Concentration – maple syrup, sugar, sorghum, juice, milk…• Cooking• Canning• Water pasteurization• Absorption cooling/ice production• Heat engine-mechanical• Heat engine - mechanical - electrical• Thermoelectric generator (solid state generation of electricity from heat)• Distillation – alcohol, potable water from seawater, water for batteries, …• Accelerating bioprocesses: composting, biogas production,…• Accelerating chemical reactions• Melting for casting or densifying - wax, plastic, tin, zinc, lead …• Melting of phase change materials for storage and/or remote use of heat