MODES OF HEAT TRANSFER

Heat is transferred by conduction, convection and thermal radiation. In practice, heat is usually transmitted by two or all the three modes of heat transfer concurrently.

CONDUCTION

Heat conduction of simply conduction is the transfer of heat by a direct contact between the elementary particles of a body, Viz molecules, atoms, free electrons, when the bodies involved are at rest. Pure conduction takes place in opaque or non transpired solid.

In goses, conduction occurs due to random motion of the molecules (the diffuse from high concentration region to lower region. In this made of heat transfer, it is very common with metals and thus call for high thermal conductivity. Also the heat transfer per unit area is proportional to normal temperature gradient given as:

                   Q       =       dt                =       (1)

                   A                dx

Fourier postulated an expression for heat transferred by conduction called fouriers law gives by:

                   Q       =       KA dt         =       (2)

                                           Dx

Where Q rate of flow of heat J/S or welts

K       =       Fourier’s constant

A       =       Area of heat transfer perpendicular to the direction of heat flow (M2).

dt/dx temperature gradient (0C or K  ).

CONVECTION

This involves the transfer of heat from one body to another by the mobile particles of liquid, gas or coarse solids during their relative motion in space. Convection heat transfer can be illustrated by the transfer of heat by heated air from a stove to the upper layers of the room air. Convection consist of forced and natural convection. Forced convection is widely used in chemical processing industries. The expression below shows heat transfer by convection.

          Q       =       KA (Ti – To)        =       (3)

                   =       hA (Ti – To)         =       (4)

          where h k/x

Q       =       heat transfer rate J/S or welts

K       =       Proportionality constant

X       =       Distance over which heat is transferred (m)

A       =       Area (M2)

Ti and To    =       Temperature at different point (0C or K)

h        =       Convection heat transfer co-efficient (w/m2k)

RADIATION

This is a process of heat transferred by electromagnetic waves through a machines which is transparent to thermal radiation. During this process, a fraction of the thermal energy of a hot body is converted into radiant energy which, when encountering an opaque body again turns partly into heat.

From the second law of thermodynamics, STEFANBOLTZ MANU prop Proposed that heat is directly proportional to the fourth power of the temperature and the surface area.

Thus given as:

          Q       =       såAT – 4      =       (5)

Where  s     =       Stefan – Boltman constant

å       =       Emissivity surface

T       =       Absolute temperature (0C or K)

A       =       Area of heat transfer (M2)

Q       =       Heat transfer rate J/S or welt

HEAT TRANSFER EQUIPMENT

There are various types of heat exchange equipment generally defined by the function it performs in a chemical industry. Generally, heat exchange is the equipment whose primary objective is to transfer heat energy between two fluids. These equipment are classified into three categories mainly:-

(a)     Regenerators

(b)     Open type heat exchangers and

(c)      Closed type bread exchanger or recuperations

a.       REGENERATORS

These are heat exchangers in which the hot and cold fluid flow through the same space alternatively with a little physical mixing as possible occurring between the two streams. The amount of heat transferred depends on the fluid and flow properties of the fluid streams as well as the geometry and thermal properties of the surface.

b.      OPEN TYPE HEAT EXCHANGERS

These are devices where by fluid stream flow into an open chamber and there the complete mixing occurs. Hot and cold fluid enter the exchanger separately and will at the other end leaves as single fluid stream.

c.       RECUPERATORS OR CLOSED TYPE HEAT EXCHANGERS

They are those in which the heat transfer occurs between the two fluid stream that do not physical contact each other. The fluid streams involved are separated from one another by a tube wall or a pipe. Heat transfer occurs by convection from the hot fluid to the solid surface, by conduction through the solid surface and then by convection through solid surface to the cooler fluid.

Another classification is based on relative flow direction of the two fluid streams. They include:-

i.  Parallel Flow: When the fluid stream flow in the same direction.

ii.  Counter Current Flow: The fluid streams flow in opposite direction.

iii.  Cross Flow: If the fluid stream flow at right angle to one another.

The other classification is based on tube construction:-

i.  Double – pipe heat exchanger

ii.  Shell and tube heat exchanger

iii.  Extended – surface exchangers

iv.  Spiral plate exchanger

v.  Graphic block heat exchangers

vi.  Scrap surface exchanger.

Tags:   CONSTRUCTION OF SHEEL AND TUBE HEAT EXCHANGER   CONSTRUCTION OF SHEEL   CONSTRUCTION OF TUBE HEAT EXCHANGER