5. Heat transfer (DONE)

Sensible heat

Heat you can feel

• ΔT > 0

• No phase change

• Q = McΔT

• J/kg*K

Thermal conductivity

High gamma: conductor (e.g. metals)

Low gamma: insulator

Foods can have a conductivity of 0.3-0.6 if they contain water. 

Thermal diffusivity

• α

• Combination of 3 product properties

• the measure of how quickly heat spreads through a materia

Enthalpy

• The energy content of:

  • A product with mass M

  • A flow with mass flow

• Heating/cooling → we need as much heat Q as the material gains in enthalpy

  • ± ΔQ = ± ΔH

• Enthalpy is a “state variable” → independent of history'

• T0 = reference temperature

Latent heat

Heat you cannot feel

• ΔT = 0

• Phase change

• S ⇌ L ⇌ G

• J/kg

Energy exchange in latent heat

• Heat that is supplied to evaporate water into steam

• Heat that is withdrawn to freeze a product

• Heat that is supplied to sublimate ice

Sensible vs latent heat in a graph

How to read a steam table

Different heating sources

• Hot liquids

  • Water (T<100C)

  • Oil (T>100C)

• Steam

• Electricity

Heating mechanisms

• Heat transfer by hot source

  • Hot water/hot oil

  • Steam

• Heating by electricity

  • Radiation

  • Di-electric heating

  • Ohmic heating

Indirect/direct heating

• Indirect: no physical contact of heating medium with product

  • Most common way to heat fluids, e.g. heating through a wall

• Direct: physical contact of heating medium with product

  • Less commonly used: e.g. steadm injection

External heating three methods

• Conduction

  • the process by which heat or electricity is directly transmitted through the material of a substance when there is a difference of temperature or of electrical potential between adjoining regions, without movement of the material.

• Convection

  • the transfer of heat through the movement of fluids (liquids and gases) from a warmer to a cooler area

• Radiation

What are the heating mechanisms for hot liquid, steam, radiation and di-electric heating

Cooling

• Extracts heat

• Use of refrigerant in cooling chamber

  • Transfer heat from product to outside

• Cooling media

  • Water, ice water

  • Media with low boiling point (ammonia, liquid nitrogen, isobutaan)

    • State transition (L→G) requires latent heat, extracted from product that is to be cooled

    • After evaporation: condensate/re-liquify by recompression → reuse

Steam

• Fuel burning (coal, gas, oil)

  • → convert chemical energy to heat energy

• Indirect heating of water and conversion to steam

• Steam collection in steam vessel

• Distributed through piping system

How is steam created?

• Created upon evaporation of water (as saturated steam)

  • Not diluted with air → saturated steam → use for heating

  • If diluted with air → moist air (not usable as hot source)

What is steam used for?

• Used for heating

  • Use latent heat of condensation for indirect/direct heating of product

• Industrial evaporation, re-use of vapor

  • Use released vapor (from product) as heating medium for next stage

Indirect vs direct steam heating

• Indirect: steam heating

  • Heating of wall of vessel/pan

• Direct infusion/injection of steam

  • Instant heating

  • Very efficient, very fast

  • Short heating time possible

  • Product is diluted with condensate.

Energy in steam (sensible and latent heat in graph)

Mechanism of steam heating

• Instant condensation

• High rate of heat transfer

• Fast T increase

• Product slightly diluted

• Steam temperature is regulated by pressure (more pressure = higher temperature of steam)

Flash pasteurization/sterilizaiton

Direct heating

• instant heating

• Short holding time

• Flash cooling of product by water evaporation

This provides better quality because:

• There is less denaturation

• Lower viscosity

• Less nutrient loss

Conduction

• Movement of heat: from hot to cold

• Driving force: temperature difference

• Applications: heating through a wall, heating through a semi-solid layer

• Mechanism: direct transfer of molecular energy

Heat transfer by conduction

Relation of heat transfer (Q) to ΔT, wall thickness, thermal conductivity, heat transfer area.

What does the heat transfer coefficient tell us?

The rate at which heat moves in conduction

• High HTC = heat moves very fast and very well

• Low HTC = your wall is preventing heat transfer

Convection

• Heat transfer in a fluid

• Fluid molecules heated near surface wall (pan)

• Molecules with high kinetic energy carried by the flow

• Fluid takes up the heat and disperses it

• Heat dispersed in the bulk is very fast

• Convection is very fast compared to conduction

Free and forced convection

• Free convection: due to density differences, caused by temperature differences (low h value)

• Forced convection: mechanically enhanced (stirring, pump, air blowing) (higher h value)

Convection near a wall

• Near wall: limited flow, so limited heat transfer

• Limited heat transfer in the small layer close to interface

• Close to wall: heat transfer by conduction 

What does the convective heat transfer coefficient h depend on?

Fluid properties:

• Viscosity

• Heat capacity

• Conductivity

Flow conditions:

• Flow rate

• Turbulent or laminar

• Thickness of boundary layer

• Geometry and dimensions

Nusselt number

• Shows if there is more convective heat transfer or conductive heat transfer

  • 𝑁𝑢=1: This signifies that heat transfer is dominated by conduction only, with no convection occurring.

  • 𝑁𝑢>1: This indicates that convection is playing a role in heat transfer. The larger the value of 𝑁𝑢, the more dominant convection is.

  • 𝑁𝑢>>1: A very large Nusselt number often suggests turbulent flow, which enhances heat transfer significant

Heat transfer by infrared radiation

Infrared: Electromagnetic radiatoin

• Emission of infrared radiation photons

• Infrared light: wavelength of 700nm-1mm

• Invisible to humans

• Humans feel it as heat

Infrared (halogen) radiation

Surface heating only

Electric radiation: expensive

Applications:

• Heating: grill, oven

• Drying: sun drying

Emission, absorption and reflection of radiant energy

• Emission of infrared radiation photons by any object with T>0K

• Absorption of photons by molecules of product → vibrations → increase in temperature

What does the Stefan Boltzman law show?

• The radiation intensity and how it strongly depends on the temperature of the heating element. 

• ε = emissivity (a surface property of emitting material)

  • ε = 1 blackbody

  • ε = 0 mirror

• σ = Stefan-Boltzman constant

Di-electric heating

• Electricity creates an alternating electric field

• Electromagnetic radiation by:

  • Microwaves

  • Radiowaves

Mechanism Di-electric heating

• Alternation electric field → alternating polarity

• Many materials contain dipolar molecules 

• When the electric field reverses its direction millions to billions of times per seconds, the dipoles attempt to realign with it. 

• Dipoles cannot kepp up and this causes:

  • Internal friction

  • Collisions between molecules

  • Energy loss in the form of heat

Ohmic heating

• Heating by electricity

• Electric field creates voltage gradient

• Alternating electrical current passes through the food

• Ions in product move → heat generation

• Requires electric conductivity of food