Page 148 - Mechanical Engineer's Data Handbook

THERMODYNAMICS AND HEAT TRANSFER 137
Emissivity of surfaces (&WC except where stated)

Aluminium : oxidized 0.11, 0.12 (250°C) Tile 0.97
polished 0.04, 0.05 (250°C) Water 0.95
anodized 0.72, 0.79 (250°C) Wood 0.90
Aluminium-coated paper, Paint: white 0.95, 0.91 (250°C)
polished 0.20 black gloss 0.96, 0.94 (250°C)
Aluminium, dull 0.20 Paper 0.93
Aluminium foil 0.05 (average) Plastics 0.91 (average)
Asbestos board 0.94 Rubber: natural, hard 0.91
Black body (matt black) 1 .oo natural, soft 0.86
Brass: dull 0.22, 0.24 (250°C) Steel: oxidized 0.79, 0.79 (250°C)
polished 0.03, 0.04 (250°C) polished 0.07, 0.11 (250°C)
Brick, dark 0.90 Steel: stainless
Concrete 0.85 weathered 0.85, 0.85 (250°C)
Copper: oxidized 0.87, 0.83 (250°C) polished 0.15, 0.18 (250°C)
polished 0.04, 0.05 (250°C) Steel: galvanized
Glass 0.92 weathered 0.88, 0.90 (250°C)
Marble, polished 0.93 new 0.23, 0.42 (250°C)

3. I5 Heat exchangers

In a heat exchanger, heat is transferred from one fluid ‘recuperator’, in which the fluids exchange heat
to another either by direct contact or through an through a wall; the ‘regenerative’, in which the hot and
intervening wall. Heat exchangers are used extensively cold fluids pass alternately through a space containing
in engineering and include air coolers and heaters, oil a porous solid acting as a heat sink; and ‘evaporative’,
coolers, boilers and condensers in steam plant, con- in which a liquid is cooled evaporatively and continu-
densers and evaporators in refrigeration units, and ously, e.g. as in a cooling tower. The following deals
many other industrial processes. with the recuperative type.
There are three main types of heat exchanger: the

3. IS. I Shell and tube heat exchangers Symbols used:
U = overall heat transfer coefficient
One fluid flows through a series of pipes and the other A =surface area of tubes (mean)
through a shell surrounding them. Flow may be either ha = heat transfer coefficient for hot side
‘parallel’ (both fluids moving in the same direction) or h, = heat transfer coefficient for cold side
‘counter flow’ (fluids moving in opposite directions). 0 =temperature difference (“C)
Another possibility is the ‘cross-flow’ arrangement in t =Temperature (“C)
which the flows are at right angles. Other types have el=lt,-lt,; e,=,t,-,t,
more complex flows, e.g. the ‘multi-pass’ and ‘mixed-
flow’ types. The following formulae give the heat Parallel flow
transferred, the logarithmic mean temperature differ-
ence and the ‘effectiveness’. 61 -02
Logarithmic mean temperature difference Om = -
0
In 2
02