mp&t07 heat transfer
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8/6/2019 MP&T07 Heat Transfer
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Manufacturing and Processing Technology
Heat Transfer
At the end of this topic you should be able to:
Recognise the different heat exchange mechanisms
Identify the options for heat transfer in chemicalreactors
Distinguish between the different types of externalheat exchangers
Name common heat exchange media
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Manufacturing and Processing Technology
Heat Transfer
Energy transfer in the form of heat is part of
almost all chemical processes.
Estimating and controlling the rate of heatflow in chemical reactors, separation
processes, furnaces and boilers represent
important factors in plant design and
operation.
Efficient energy utilisation is also important.
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Manufacturing and Processing Technology
Driving Force
Temperature difference Heat only flows from a
hotter to a colder part of a system.
Because of relationshipbetween heat transfer rate and area, heattransfer equipment is
often designed to havelarge heat transfer surface areas
T UAQ (!
Q = heat transfer rate
U = heat transfer coefficient
A = heat transfer area
T = Temperature difference (oC)
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Manufacturing and Processing Technology
Mechanisms
Conduction Transfer of heat energy
between molecules in afluid or a solid (e.g. heatingmetal bar from one end).
Convection This is the transfer of heat
through the movement of warmed matter i.e. throughcurrents.
Radiation Electromagnetic waves
transfer heat through space
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Manufacturing and Processing Technology
Identify the types of heat transfer occurring in this picture
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Manufacturing and Processing Technology
Heat transfer in chemical
reactors
Most chemical reactions need an initial
supply of heat
Most chemical reactions are exothermic. Therefore during chemical processing the
reactor design must include provision for heat
transfer.
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Manufacturing and Processing Technology
Heat transfer in chemical
reactors
The options are:
(a) A heat transfer jacket on
the reactor
(b) Internal coils in the reactor (c) Limpet coils external to the
vessel
(d) An external heat exchanger
through which the reactor
contents are recirculated.
(a) Jacket
(c) Limpet coils
(b) Internal coil
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Manufacturing and Processing Technology
Continuous Flow Heat
Exchangers
Very convenient when both materials are
fluids
Two fluids, primary and secondary, flowcontinuously through the heat exchanger
Heat exchange is controlled by the local
temperature differences and heat transfer
coefficients at any point in the equipment
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Manufacturing and Processing Technology
Continuous Flow Heat
Exchangers
Co-current (parallel flow): Fluids flow in the samedirection.
Counter current flow: Fluids flow in opposite
directions. Cross Flow: Fluids flow at right angles to each other.
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Manufacturing and Processing Technology
Shell and Tube Heat
Exchangers
One fluid passes through a bank of tubes
The other fluid passes outside the tubes
The efficiency of the transfer is increased bymaximising the contact between the hot and
cold fluids.
Shell and Tube Heat
Exchangers
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Manufacturing and Processing Technology
Plate Exchangers
Formed by bolting together
a series of corrugated
plates separated by
synthetic rubber or plastic
gaskets
Bolted together in frame
The fluids pass through
alternate spaces between
the plates and in oppositedirections.
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Manufacturing and Processing Technology
Graphite Block/Leaf Heat
Exchangers
Constructed from blocks of graphitewith passages bored through or leaves of graphite
Fluids flow in opposite directions
through these passages Can to handle corrosive liquids
and vapours
Thermal conductivity very similar tomost metals
Brittle - must be handled with careduring installation and maintenance
Used in conjunction with glass-lined and hastelloy reactors.
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Manufacturing and Processing Technology
Spiral Flow Heat Exchanger
The hot fluid enters at the
centre of the unit and flows
from the inside outward.
The cold fluids enter at theperiphery and flows towards
the centre.
Circular motion makes it
easier to achieve turbulent
conditions. Good for cooling/heating slurries/
viscous liquids etc.
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Manufacturing and Processing Technology
Heat Transfer Media
Cold Water (for heating and cooling)
Hot Water (for heating and cooling)
Steam (for heating)
Glycol (used mostly for cooling) Brine (for cooling)
Methanol/water mixtures (used for cooling)
Liquid nitrogen (for cooling)
Dowtherm/paratherm etc. (for both heating andcooling)
Use of tempered loop
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