The heat exchanger is a system that is used to transfer heat between two or more fluids. Heat exchangers are applicable in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or could even be in direct contact. Heat exchangers have extensive use in chemical, petrochemical, oil & gas, power generation, refrigeration, pharmaceuticals, HVAC, food & beverage processing, and pulp & paper industries. The variants depend on the type of heat exchanger employed and the heat transferring process can be gas to gas, liquid to gas, or liquid to liquid.
The Functioning principle of the Heat Exchanger– This system functions on the principle of thermodynamics which is a science that deals in heat energy flow, temperature, and the relationships to other forms of energy. Heat is transferred typically in three ways- conduction, convection, and radiation.
Conduction is the passing of thermal energy between materials that are in contact with each other. Temperature increases the average kinetic energy of molecules in a material and the warmer object transmits energy to a cooler object and this process continues till thermal equilibrium has been achieved.
Convection is the transfer of thermal energy from a surface by way of a medium such as a fluid or air or water that has been heated.
Radiation is a mechanism of heat energy transfer that involves the emission of electromagnetic waves from a heated surface or object and no intermediate medium is required in this kind of transmission.
Heat exchangers are of various types but the tube and shell exchangers are the most commonly used heat exchange equipment. A shell and tube exchanger is made up of a lot of tubes which is placed inside a shell mostly cylindrical in nature and is widely used by designers of such equipment because of their capacity to sustain a wide range of temperature and pressure. It is of such a nature that two or more liquids can exchange heat and is devised in such a manner that one of the fluids flow inside the tube, while the other flows outside the tube. The fluids can be single or double-phased and the flow can be in a parallel manner or in a counter flow manner. This system comprises of four main parts:
Front head/Stationary header– This header is an inlet from where the fluid enters the side of the exchanger where tube is available.
Rear header– This header is an outlet from where the fluid exits the exchanger or the place from where the fluid is returned back to the front header where there is a need for multiple tube passes.
Tube bundle– This Bundle holds all the tubes, baffles, tube sheets and the rods that tie and hold the entire bundle together.
Shell- The importance of the Shell to hold the maximum number of tubes is the ideal condition for heat transfer as it increases turbulence within. The Shell holds the entire tube bundle.
The tube thickness should be such that it withstands the internal pressure along with the required corrosion allowance. The tube thickness is expressed in terms of BWG (Birmingham Wire Gauge) and true outside diameter (OD). The most common materials used for the tube are Stainless steel, admiralty brass, copper, bronze, and alloys of copper-nickel. Each tube contains a turbulator which helps in the turbulent flow and prevents any sediment deposit or foulness. Turbulence in shells can also be increased with the help of baffles which maximize the amount of thermal mixing that occurs between the shell-side fluid and the coolant pipes. The principle of working of a shell and tube heat exchanger is to pass a hot fluid through a cold fluid without them getting mixed so that only their heat is transferred without any interflow among them. There are inlets and outlets for the entry and exit of the fluid. The tube bundle is secured by metal plates which are also called tube sheets or tube plates. The side flow of the tube passes through this bundle and then exits the tube outlet. In a similar manner, the shell side flow also begins at an inlet and after passing through these tubes exits at the shell outlet. There are headers made on both sides of the tube bundle to create reservoirs for the tube-side flow.
These devices are among the most effective means of exchanging heat, as they are easily built, require low maintenance, are compact, and provide efficient heat transfer. They are widely distributed in industry, being most useful for condensers, turbine coolers, evaporators, feed water preheating, and much more.