The Dance of Heat Transfer: Unlocking the Secrets of Heat Exchangers
In the world of thermal engineering, heat exchangers play a vital role in transferring heat from one fluid to another. This intricate process is governed by the fundamental principle of energy balance. By applying this principle, engineers can determine the temperatures of both hot and cold fluids at the inlet and outlet of the heat exchanger.
The Energy Balance Equation
The energy balance equation is a powerful tool that helps engineers calculate the heat transfer rates, fluid temperatures, and design parameters of the heat exchanger. By solving this equation, engineers can determine:
- The temperature of the hot fluid at the inlet and outlet- The temperature of the cold fluid at the inlet and outlet- The required heat transfer area- The number of tubes- The number of shell passes
Counter-Current Shell and Tube Heat Exchangers
One of the most common types of heat exchangers is the counter-current shell and tube heat exchanger. In this design, two streams pass over each other in opposite directions:
- The hot fluid enters the heat exchanger at a high temperature and flows through the tubes.- The cold fluid enters the heat exchanger at a low temperature and flows through the shell.- As the hot fluid flows through the tubes, it transfers heat to the cold fluid, causing its temperature to rise.- The cooled hot fluid exits the heat exchanger at a lower temperature, while the heated cold fluid exits at a higher temperature.
The temperature difference between the terminal points of the heat exchanger is called an APPROACH and the temperature change obtained by an individual stream of fluid is called a 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
A required rate of heat transferred into a stream of fluid can be calculated by the above equation taking some assumption of the temperature availability of the streams.
Let W = m cp = kJ/sK = kW/K
Wh/Wc = (Tc2 – Tc1)/ (Th1 – Th2)
Information on the 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 from 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 an increase in temperature.
- The 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 the mean radius used in the heat conduction equation is given by r2 – r1/ ln(r2/r1).
- When a heat exchanger is used for condensing the vapor 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 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.