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• 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
• 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.
• 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.
• Heat can travel through a medium and also
• There are three modes of heat transfer,
• 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
• 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
• In gases and liquids, it is due to the collisions of molecules
during their random motion
• 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.
• 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
– 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
• It is the measure of a material’s ability to resist
• Thermal resistance to heat transfer offered by
the wall, Rw = B/kA
– B = thickness of wall
– K = thermal conductivities
– A = surface area
• 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
• 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
• 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
– 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
• Heat transfer coefficient has SI units in watts per meter square kelvin (W/m2K).
• It is the inverse of thermal insulance.
• Energy is rediated from all materials in the form of electromagnetic
• 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
• Heat transfer through vacuum is called thermal radiation.
• Radiative heat transfer can be mathematically expressed with Stefan-
• Q = σAT4
– Q = heat transfer per unit time
– σ = stefan Boltzmann constant
– A = Area of the emitting body
– T = absolute temperature
• 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
• A heat exchanger is required to maintained the bioprocess at a
• 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
– Internally located helical or baffle coil
– External heat exchangerss