In a counter flow double pipe heat exchanger water flows through a copper tube (19 mm O.D. and 16mm ID), at a flow rate of 1.48 m/s. The oil flows through the annulus formed by inner copper tube and outer steel tube (30 mm O.D. and 26 mm I.D.). The steel tube is insulated from outside. The oil enters at 0.4 kg/s and is cooled from 65°C to 50°C whereas water enters at 32°C. Neglecting the resistance of the copper tube, calculate the length of the tube required.
Data given :
Nu = 0.023 (Re)0.8 (Pr)0.3
Fouling factor, water side = 0.0005 m2 /W
Fouling factor, oil side = 0.0008 m2K/W
Water and oil properties:
|
Property
|
Oil
|
Water
|
|
r (kg/m3)
|
850
|
995
|
|
cp (kJ/kg K)
|
1.89
|
4.187
|
|
K (W/mK)
|
0.138
|
0.615
|
|
n(m2/s)
|
7.44 ´10-6
|
4.18 ´ 10–7
|