Page 150 - Mechanical Engineer's Data Handbook

THERMODYNAMICS AND HEAT TRANSFER 139

rate and the number and dimensions of the tubes are
given.
Symbols used:
m,=cooling water mass flow (kgs-’)
m,=steam mass flow (kgs-’)
h,,=latent heat of steam (kJkg-’)
x =dryness fraction of steam
c = specific heat capacity of water
(4.183 kJkg-’K-’ for fresh water)
h,=overall heat transfer coefficient (kW m-’K-’)
t, =water inlet temperature (“C)
t, =water outlet temperature (“C) \ui’
t, = steam saturation temperature (“C) Two tube passes
C, = velocity of water in tubes (m s- I)
A,=area of tube bore (m2)
D, =outside diameter of tubes (m) 1 .25mshfp
n,=number of tubes per pass Surface area of tubes A,=
np = number of tube passes h06,
L = tube length (m)
A, = surface area of tubes (m’) (assuming 25% allowance for fouling)
p=density of water (kgm-’)
where: ern =logarithmic mean temperature difference
Cooling water flow mc=- k*hf,
-
c(t, - t,) - (4 - t, 1 - (t, - tz) (assuming no undercooling of
In condensate)
Overall heat transfer coefficient
h0=1.14(~~’5(7) ’.*’ Number of tubes per pass n,=m,/pA,C,
t+ 18
Tube length L = A$zD,n,np
where: t = (t, + t2)/2.

3.16 Combustion of fuels

3.16. I Air-fuel ratio and mixture be ‘weak’ or ‘lean’. With less air the combustion is
strength incomplete and the mixture is said to be ‘rich’ (see
table).
The following deals with the combustion of solid,
liquid and gaseous fuels with atmospheric air. The Definitions:
fuels are supposed to be composed only of carbon, Air/fuel ratio R= Amount of air
hydrogen and sulphur, with perhaps oxygen and ash. Amount of fuel
The carbon, hydrogen and sulphur combine with the (by mass for solids and liquids and by volume for
oxygen in the air; the nitrogen in the air remains gases)
unchanged. Stoichiometric air/fuel ratio R, = ratio for complete
The correct proportion of air for complete combus- combustion
tion is called the ‘stoichiometric air/fuel ratio’. Usually
the proportion of air is higher and the mixture is said to Percentage excess air x 100%
Rs