Variation Of Fluid Temperatures In Heat Exchangers

CAD drawing of a heat exchanger
•In a heat exchanger, heat flows from hot fluid to cold fluid. By energy balance over the heat exchanger, all the temperature of the hot and cold fluid at inlet and outlet can be determined. Even the area required number tubes and a number of a pass on shell can also be calculated. For simple counter current shell and tube heat exchanger, two streams pass over each other as;

  • Hot fluid inlet and outlet
  • Cooled fluid inlet and outlet 

The temperature difference between the terminal points of the heat exchanger is called as APPROACH and the temperature change obtained by an individual stream of fluid is called as RANGE. A point temperature difference is a local difference between the hot and cold fluids at a specified position of the heat exchanger which is Th - Tc. 

Q = mh cph (Th1 – Th2) = mc cpc (Tc2 – Tc1) 

Neglecting any loss of heat, the overall heat balance can be written from the law of conservation of energy, where heat supplied by the hot fluid is equal to heat observed by the cold fluid.
Subscripts h & c refer to hot and cold fluids;
1 & 2 are inlet and outlet respectively
m = mass flow rate, kg/s 
cp = specific heat, kJ/kg K

For a required rate of heat transferred into a stream of fluid can be calculated by the above equation taking some assumption of temperatures availability of the streams.

Let W = m cp = kJ/sK = kW/K
Wh/Wc = (Tc2 – Tc1)/ (Th1 – Th2)
Information on heat conduction process:
  • The flow of heat by conduction is the result of the transfer of vibrational energy from one molecule to another, the transfer of kinetic energy, and the movement of free energy.
  • Transfer of heat by conduction is generally formed one part of a body to another part of the same body and between two bodies in physical contact.
  • When compared with non-metallic solids the metallic solids usually have much higher thermal conductivity.
  • Among the liquids, water has a comparatively high value of thermal conductivity which is due to partial ionization.
  • For gases, thermal conductivity value increases with increase in temperature.
  • Prandtl number for water varies from 5 to 10.
  • With the increase in concentration, the thermal conductivity of liquid generally decreases.
  • For conduction through a thicker walled tube the value of mean radius used in the heat conduction equation is given by r2 – r1/ ln(r2/r1).
  • When a heat exchanger used for a condensing the vapour of the distillation column or any equipment, to maintain the constant temperature on the shell side of the heat exchanger only single component vapor should be allowed and that to its temperature should be below superheated temperature. At this condition for a constant pressure on the shell side, the constant temperature will be maintained. Consistent condensation of vapors is also achieved.