mp&t07 heat transfer

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




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.




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)




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




Identify the types of heat transfer occurring in this picture




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.




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




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




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.




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




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.




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.




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.




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

mp&t07 heat transfer

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