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A heat exchanger is a equipment for efficient heat transfer from one medium to another, whether the media are separated by a solid wall so that they never mix, or the media are in direct contact.They are widely used in power plants, chemical plants, petrochemical plants, petroleum refineries, and natural gas processing.
Countercurrent (A) and parallel (B) flows
Heat exchangers may be classified according to their flow arrangement. In parallel-flow heat exchangers, the two fluids enter the exchanger at the same end, and travel in parallel to one another to the other side. In counter-flow heat exchangers the fluids enter the exchanger from opposite ends.
The counter current design is most efficient, in that it can transfer the most heat. See countercurrent exchange. In a cross-flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger.
|Fig. 1: Shell and tube heat exchanger, single pass (1-1 parallel flow)||Fig. 2: Shell and tube heat exchanger, 2-pass tube side (1-2 crossflow)||Fig. 3: Shell and tube heat exchanger, 2-pass shell side, 2-pass tube side (2-2 countercurrent)|
SHELL & TUBE TYPE HEAT EXCHNGERS
TEMA-style shell-and-tube-type exchangers constitute the bulk of the unfired heat-transfer equipment in chemical-process plants, although increasing emphasis has been developing in other designs. These exchangers are illustrated in Fig. 11-35, and their features are summarized in Table 11-11.
Cools a fluid to a temperature below that obtainable if water only were used as a coolant. It uses a refrigerant such as ammonia or Freon.
Condenses a vapor or mixture of vapors, either alone or in the presence of a noncondensable gas.
Condenses vapors at a point high enough to provide a temperature difference sufficient to preheat a cold stream of process fluid. This saves heat and eliminates the need for providing a separate preheater (using flame or steam).
Condenses the vapors to a final storage temperature of approximately 37.8░C (100░F). It uses water cooling, which means that the transferred heat is lost to the process.
Cools liquids or gases by means of water.
Performs a double function: (1) heats a cold fluid by (2) using a hot fluid which it cools. None of the transferred heat is lost.
Imparts sensible heat to a liquid or a gas by means of condensing steam or Dowtherm.
Connected to the bottom of a fractionating tower, it provides the reboil heat necessary for distillation. The heating medium may be either steam or a hot-process fluid. Thermosiphon Natural reboiler circulation of the boiling medium is reboiler obtained by maintaining sufficient liquid head to provide for circulation. Forced-circulation A pump is used to force liquid through the reboiler.
Distillation set-ups typically use condensers to condense distillate vapors back into liquid.
Power plants which have steam-driven turbines commonly use heat exchangers to boil water into steam. Heat exchangers or similar units for producing steam from water are often called boilers or steam generators.
In the nuclear power plants called pressurized water reactors, special large heat exchangers which pass heat from the primary (reactor plant) system to the secondary (steam plant) system, producing steam from water in the process, are called steam generators. All fossil-fueled and nuclear power plants using steam-driven turbines have surface condensers to convert the exhaust steam from the turbines into condensate (water) for re-use
In order to conserve energy and cooling capacity in chemical and other plants, regenerative heat exchangers can be used to transfer heat from one stream that needs to be cooled to another stream that needs to be heated, such as distillate cooling and reboiler feed pre-heating.
This term can also refer to heat exchangers that contain a material within their structure that has a change of phase. This is usually a solid to liquid phase due to the small volume difference between these states. This change of phase effectively acts as a buffer because it occurs at a constant temperature but still allows for the heat exchanger to accept additional heat. One example where this has been investigated is for use in high power aircraft electronics.
Heat exchangers in industry
Heat exchangers are widely used in industry both for cooling and heating large scale industrial processes.
Heat exchangers are used in many industries, some of which include:
- Waste water treatment
- Refrigeration systems
- Wine-brewery industry
- Petroleum industry