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Heat Transfer

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Heat transfer in bioreactors

Veröffentlicht in: Wissenschaft
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Heat Transfer

  1. 1. • Heat is a form of energy which passes from a body at higher temperature to a body at a lower temperature. • Heat is defined in physics as the transfer of thermal energy across a well-defined boundary around a thermodynamic system. • The SI unit of heat is the joule (J). • The flow of heat is all pervasive. • The overall driving force for this heat transfer is the temperature difference. • When the driving force becomes negligible, then the transfer will cease to occur, and the system will reach equilibrium. • Heat cannot be measured directly by an instrument as temperature is by a thermometer.
  2. 2. • The exchange of kinetic energy of particles through the boundary between two systems which are at different temperatures from each other or from their surroundings. • Heat transfer always occurs from a region of high temperature to another region of lower temperature. • Heat transfer changes the internal energy of both systems involved according to the First Law of Thermodynamics. • The Second Law of Thermodynamics defines the concept of thermodynamic entropy, by measurable heat transfer. • Thermal equilibrium is reached when all involved bodies and the surroundings reach the same temperature. • Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.
  3. 3. • Heat can travel through a medium and also through vacuum. • There are three modes of heat transfer, a) Conduction b) Convection c) Radiation
  4. 4. • Transfer of thermal energy between neighboring molecules in a substance due to a temperature gradient. • It always take place from a region of higher temperature to a region of lower temperature, and acts to equalize temperature difference. • It can take place in all forms of matter i.e. solids liquids and gases, but does not require any bulk motion of matter. • In solids it is due to transfer of vibrational energy between molecules. • In gases and liquids, it is due to the collisions of molecules during their random motion
  5. 5. • Also called fourier’s law • It states that the time rate of heat transfer through the material is proportional to the negative gradient in the temperature and to the area, at the right angles to that gradient, through which the heat is flowing.
  6. 6. • If we consider a case of heat transfer through the wall by the process of conduction, then the rate of heat conduction is given by • ΔQ = -kA.dT/dX – Where, – ΔQ = rate of heat transfer – K = Thermal conductivity of the wall – A = surface area perpendicular to the direction of heat flow – dT/dX = Temperature gradient • The negative sign indicates that heat always flows from hot to cold areas
  7. 7. • It is the measure of a material’s ability to resist heat transfer. • Thermal resistance to heat transfer offered by the wall, Rw = B/kA – Where, – B = thickness of wall – K = thermal conductivities – A = surface area
  8. 8. • Movement of molecules within fluids • In reality, this is a combination of diffusion and bulk motion of molecules. • Because it occurs at macroscopic levels, it is therfore, confined to gases and liquids. • Molecules in fluids are further apart and have negligible cohesive force. • Convection currents are set up much faster in gases than in liquids because of the extremely low cohesive forces existing between the molecules of the gases. • Convection can be – Natural convection – Forced convection
  9. 9. • Natural convection occurs when temperature gradients in the system generate localized density differences which result in flow currents. • In forced convections, flow currents are set in motion by an external agent such as a stirrer or pump. • The heat transfer per unit surface through convection was first described by Isaac Newton and the relation is known as Newtons law of Cooling • The equation for convection can be expresssed as • ΔQ = h. A. ΔT – Where, – ΔQ = heat transferred per unit time – ΔT = temperature difference between the surface and the bulk fluid, – A = heat transfer surface area – h = convective heat transfer coefficient • The convective heat transfer coefficient is dependent on the type of media, gas or liquid, the flow properties such as velocity, viscosity, and other flow and temperature dependent properties • Heat transfer coefficient has SI units in watts per meter square kelvin (W/m2K). • It is the inverse of thermal insulance.
  10. 10. • Energy is rediated from all materials in the form of electromagnetic radiations. • Radiation is also described as the flow of heat from one place to another by means of electromagnetic waves. • All bodies absorb and emit radiation. • No medium is required between two bodies for heat transfer to take place. • Heat transfer through vacuum is called thermal radiation. • Radiative heat transfer can be mathematically expressed with Stefan- Boltzsman law; • Q = σAT4 – Where, – Q = heat transfer per unit time – σ = stefan Boltzmann constant – A = Area of the emitting body – T = absolute temperature
  11. 11. • An electric bulb in a room produces both light and radiant heat. • The radiant heat is absorbed by the materials in the room, which in turn give out radiant heat of lower energy. • Because of the nature of production, radiant heat is an electromagnetic wave that causes heating effect in objects that absorb it.
  12. 12. • A heat exchanger is required to maintained the bioprocess at a constant temperature • Biological fermentation is major source of heat, therefore in most cases bioreactors need cooling. • The exchange of heat in case of bioprocess is always between fluids that should not mix with each other. So fluids are separated using a highly conducting medium. • Such system are called heat exchanger. • Various designs of heat reactors used for heat exchange in bioreactors may include – An external jacket or coil through which steam or cooling water is circulated – Internally located helical or baffle coil – External heat exchangerss