There is a huge importance of heat transfer in food industry and many key processes for ensuring food safety and quality such as sterilization, pasteurization and refrigeration are based on it . There are numerous number of equipments available for heat transfer, but heat exchangers are considered as the best choice among food technologist for thermal treatment of food and beverage products as it helps in achieving longer shelf life by decreasing microbial load. Heat Exchangers are considered as the best cost- effective tool for pasteurization, sterilization and refrigeration as it provides rapid heat transfer which not only minimizes the risk of food borne illness but also presents the advantage of retaining or enhancing nutritive quality, minimizing food wastage, and increasing food availability, security, and affordability.
- Definition and Working
Heat exchanger may be defined as a process equipment which works on principle of Second Law of Thermodynamics i.e.it transfers the energy from a hot fluid to cold fluid with maximum rate and minimum investment and running cost. In heat exchangers the temperature of each fluid changes as it passes through the exchangers and hence the temperature of the dividing wall between the fluids also changes along the length of the exchangers. For the heat transfer to occur two fluids must be at different temperature and thermal contact must be present between them. Heat exchangers involve convection in each fluid and conduction through the separating wall.
In order to meet widely varying applications, several types of heat exchangers have been developed which are classified on the basis of nature of heat exchange process, relative direction of fluid motion and design and constructional features.
2.1. On the basis of Nature of Heat Exchange Process
2.1.1. Direct contact Heat Exchangers
In this type of heat exchanger the exchange of heat takes place by direct mixing of hot and cold fluids and transfer of heat and mass takes place simultaneously. It is used in condition where mixing of two fluids is either harmless or desirable. e.g Direct Steam Injection
2.1.2. Indirect Contact Heat Exchangers
In this type of heat exchanger, heat transfer between fluids occurs through wall which separates the two fluids. E.g. Regenerators and Recuparators
2.2. On the basis of Heat Exchanger’s Flow Arrangement
In parallel-flow heat exchangers the hot and cold fluids enter the heat exchanger from the same end and flow parallel to each other.
2.2.2. Counter current-Flow
In countercurrent-flow heat exchangers the hot and cold fluids enter the heat exchanger from opposite sides and flow toward each other.
In cross-flow heat exchangers the hot and cold fluids enter the heat exchanger at different points and as they travel through the heat exchanger they cross paths with each other, often at right angles.
2.3. On basis of Design and Construction
2.3.1 Tubular Heat Exchanger
It is the most common type of heat exchanger used for heat transfer. It consists of series of tubes containing fluid which is required to be heated or cooled and are enclosed inside the cylindrical shell. In this one of the fluids is usually passed through pipes or tubes, and the other fluid is passed round or across these. Local temperature differences and the heat transfer coefficients are the main parameters that control the heat transfer in this heat exchanger. The fluids can be single or two phase and can flow in a parallel or a cross/counter flow arrangement. Shell and tube heat exchangers are typically used for high pressure applications (with pressure greater than 30 bar and temperature greater than 260 °Celsius) owing to its robustness.
It consists of four major parts:
Front End: this where the fluid enters the tube side of the exchanger.
Rear End: This where the tube side fluid leaves the exchanger or where it is returned to the front header in exchangers with multiple tube side passes.
Tube Bundle: this comprises of the tubes, tube sheets, baffles and tie rods etc, to hold the bundle together.
Shell: This contains the tube bundles.
The popularity of this type heat exchanger had led to the development of “Tubular Exchanger Manufactures Association” that is responsible for creating standard for its designation and use. For increasing the heat transfer efficiency the surface of the tubes are corrugated which extends the surface area and provokes dynamic turbulence in fluid. Various studies have reported that introduction of corrugation in tubes have led to 90% improvement in heat transfer. This equipments are often employed to pasteurize milk or other beverages, and the heat exchanging fluid is almost exclusively water or water steam (depending on process temperature).
2.3.2 Plate Heat Exchangers
This is the popular type of heat exchanger having widespread applications in food and beverage industry for operating low viscous food. A plate exchanger consists of a series of parallel plates that are placed one above the other so as to allow the formation of channels for allowing fluids to flow between them.The size of a plate can range from a few square centimeters (100 mm x 300 mm side) up to 2 or 3 square meters (1000 mm x 2500 mm side). The number of plates in a single exchanger ranges from just ten to several hundred.The most frequently used materials for the plates are stainless steel (AISI 304, 316), Just like Shell and Tube Heat Exchangers its plates are also corrugated for increasing surface area in order to increase the turbulence which provides mechanical rigidity. Corrugation is achieved by cold forging of sheet metal with thicknesses of 0.3 mm to 1 mm.The main advantage of this Heat Exchanger is that its dismantling is easy and quick.
2.3.3 Scraped Surface Heat Exchangers
This is the kind of heat exchanger which consists of an internal cylinder within a jacketed cylinder in an concentric manner and is fitted with scraping blades (or shafts). Continuous scraping of the surface with the help of steel blades avoids fouling, provides a longer running time and sustainable heat transfer rate during the process. This type of heat exchanger is mainly used for processes such as sterilization, freezing, gelatinization, Khoa Preparation and cooling of high viscosity products such as ice creams, chocolate, yoghurt, jams .Scraped surface exchangers can also be used to process foods sensitive to heat and/or to mechanical stress. Coefficients of heat transfer vary with speeds of rotation and with heated fluid characteristics, and are quite high, in the order of 900-4000 J m-2 s-1 °C-1.
Nature of process, process temperature, type of fluid, fluid velocity, fluid viscosity, etc are some of the performance factors which we need to keep in mind while selecting and designing of heat exchangers. Moreover, the material of construction must be corrosion resistant as well as it must prevent the accumulation of solids leading to Fouling. Fouling is the most common issue of heat exchangers which if not controlled may lead to the serious problems like failure of process operation and food safety programmes.. Therefore Heat exchangers must be selected in such a way that it provides best heat transfer efficiency by reducing problem of fouling, pressure drop and heat leakage.