HEAT TRANSFER 147
PROBLEM 9.16
A heat exchanger is to be mounted at the top of a fractionating column about 15 m high to
condense 4 kg/s ofn-pentane at 205 kN/m^2 , corresponding to a condensing temperature
of 333 K. Give an outline of the calculations you would make to obtain an approximate
idea of the size and construction of the exchanger required.
For purposes of standardisation, 19 mm outside diameter tubes of 1.65 mm wall thick-
ness will be used and these may be 2.5, 3.6, or 5 m in length. The film coefficient for
condensing pentane on the outside of a horizontal tube bundle may be taken as 1.1kW/m^2
K. The condensation is effected by pumping water through the tubes, the initial water
temperature being 288 K. The latent heat of condensation of pentane is 335 kJ/kg.
For these 19 mm tubes, a water velocity of 1 m/s corresponds to a flowrate of 0.2kg/s
of water.
Solution
The calculations follow the sequence of earlier problems in that heat load, temperature
driving force, and overall coefficient are obtained and hence the area evaluated. It then
remains to consider the geometry of the unit bearing in mind the need to maintain a
reasonable cooling water velocity.
As in the previous example, then-pentane will be passed through the shell and cooling
water through the tubes.
Heat load
QD
4. 0 ð 335 D1340 kW assuming there is no sub-cooling of the condensate.
As in Problem 9.15, the outlet temperature of the cooling water will be taken as 310 K,
and for a flow ofGkg/s:
1340 DGð 4. 18
310 288 orGD 14 .57 kg/sTemperature driving force
1 D
333 288 D45 deg K, 2 D
333 310 D23 deg Kand: mD
45 23 /ln
45 / 23 D 32 .8deg K
Overall coefficient
Inside:
For forced convection to water in tubes:
hiD 4280
0. 00488 T 1 u^0.^8 /d^0 i.^2 W/m^2 K (equation 9.221)whereT, the mean water temperatureD 0. 5
310 C 288 D299 K;u, the water velocity
will be taken as 1 m/s — a realistic optimum value, bearing in mind the need to limit the