A double pipe concentric heat exchanger operating in counter-current mode is constructed in 5 m lengths fitted in series. Each section consists of an outer pipe of 100 mm inside diameter, and an inner pipe of 55 mm outside diameter and wall thickness of 3mm. Both pipes are made of steel with a thermal conductivity k of 16 W/mK. Steam is supplied to the shell side at 393 K and the condensate is not sub-cooled (discharged at 393 K). The duty of the exchanger is to heat 12,000 kg/hour of process fluid from an inlet temperature of 283 K to an outlet temperature of 353 K. The heat transfer coefficient of condensing steam is 8000 W/m2K.
Physical properties of the process fluid at the mean bulk temperature of 318K are as follows:
Thermal conductivity k = 0.17 W/mK
Density ? = 800 kg/m3
Viscosity ? = 0.3 mNs/m2
The specific heat capacity Cp of the process fluid is not known at the mean bulk fluid temperature but is a linear function of temperature, and must be calculated using the following information, Cp at 300K is 1.98 kJ/kgK, and Cp at 400K is 2.65 kJ/kgK
To estimate the film coefficient for the inner pipe, use the Nusselt correlation as given below:
Hence or otherwise determine
(i) The flow velocity of the process fluid
(ii) The inside heat transfer coefficient
(iii) The overall heat transfer coefficient U
(iv) The heat transferred based on the heating of the process fluid
(v) The number of heat exchanger sections required to meet the duty