Heat exchanger is equipment that transfers energy from a warmer fluid to cooler fluid through a solid wall separating the two fluids; with maximum rate and minimum investment cost. The transferred heat maybe sourced from a condensation or vaporization process. This is majorly used and designed by chemical Engineers. Therefore, it is common practice in chemical engineering.
Examples includes: milk chillers, automobile radiators and intercooler.
Types of heat exchanger
For the benefit and effective heat transfer process, it is necessary to classify heat exchangers according to:
- Nature of heat transfer process: this deals with the mixing or contact between the fluid system. It could be direct or indirect contact. In direct contact, the hot and cold fluid mixes together in order to transfer heat due to temperature difference. The fluid are must be harmless to the system. For indirect contact, heat is transferred through the walls separating the fluids. The advantages of both classes are; easy construction, higher heat transfer coefficient (regenerators), and more surfaces are for heat transfer (recuperator).
- Relative motion of the fluid: the movement of the fluid could be in the following ways;
- Parallel flow
2 Counter current flow
- Mixed flow: the fluid crosses each other in the space. The fluid does not mix together but the tube through which the pass intersects each other vertically and horizontally. Example is the cooling fins of a refrigerator and Automobile radiators.
- Design and constructional features: this is classified as follows
- Shell and tube: this is recommended when the heat transfer surface is large. Practically, these tubes are constructed and placed in a bundle and it’s end is mounted in a tube sheet.
- Concentric tube: the concentric tube maybe two or more, each carrying different fluid. The flow can be parallel or counter current.
- Double pipe heat exchanger: this is the most simple in construction and used. It’s operation is similar to concentric tubes, but the fluid is re-routed to flow out it enhances the overall heat transfer.
- Compact heat exchanger: they are employed in convective heat transfer coefficients. They have high transfer surface are per volume of the exchange.
- Physical state of the fluid: this deals with the state of fluid in motion. These machines are placed under consideration for operations are condensers and evaporators.
Heat transfer coefficient analysis
In heat exchangers, shaft work, potential and kinetic energy are negligible. The heat transfer equation is given as:
U = overall heat transfer coefficient
θm = logarithmic temperature mean difference.
Where h is the heat transfer coefficient
The overall heat transfer coefficient depends on:
- Fluid properties
- Surface area
- Thickness of material
- Flow rate of fluid
- Heat exchange geometrical configuration.
R.K.Rajput, heat and mass transfer. S.Chand &company LTD 2008