A heat exchanger is a component that allows the transfer of heat from one fluid (liquid or gas) to another fluid.
Reasons for heat transfer include the following:
1. To heat a cooler fluid by means of a hotter fluid
2. To reduce the temperature of a hot fluid by means of a cooler fluid
3. To boil a liquid by means of a hotter fluid
4. To condense a gaseous fluid by means of a cooler fluid
5. To boil a liquid while condensing a hotter gaseous fluid
The transfer of heat is necessary for control of:
• A fluid temperature and/or its composition and phase,
• The rate of mass transfer between phases,
• The rate of chemical reactions,
• The suitable temperature to prevent failure or reduced service life of the equipment.
TYPE
• Double Pipe HEDouble- pipe heat exchangers consist of two concentric pipes, with both inner and outer pipes having a return bend at one end.
Economically attractive for small heat transfer areas, but when an appreciable amount of heat transfer surface is required, they become too bulky and expensive.
Sometimes attractive in high-pressure service
• Plate & Frame HEThese exchangers provide a large surface heat exchange area in a small plant area. They are used to save weight and space often offshore for interchanging heat between tempered cooling water and sea water.
• Braze Plate HECorrugated Plate HE :
Made of stainless steel. Generally used in liquid and refrigerant applications, they accommodate a wide range of temperatures, from cryogenic to more than 200°C.
Because of the brazed construction, the units are not expandable, but get their reputation from their relatively tiny
size.
Plate-Fin HE :
Braze-aluminium, plate-fin exchangers are used extensively at temperatures below - 20°C, in cryogenic processing, both for gas- to-gas and chilling service. They also are used for low- temperature reboiling and condensing.
Provide approximately 25 times more surface per weight of equipment than comparable shell and tube exchangers.
Brazed aluminum heat exchanges should be used with clean fluids since they are more susceptible to plugging than other types of heat exchanger equipment; however, proper filters may prevent heat exchanger fouling.
• Air Cooled HEAir- cooled exchangers are used to cool fluids with ambient air. Processed fluid flows inside the tubes and ambient air on the outside usually moving by natural convection or blown by a fan. Because of the low heat-transfer coefficient for atmospheric air, fins are used on the outside of the pipes.
Advantages of cooling by air :
• No need for pump houses, filtering screens, pipelines, oily water separators,
• Maintenance costs on air-cooled equipment are much less than on water- cooled units,
• No danger of water contaminating the fluid to be cooled or condensed,
• In the event of an electricity failure and therefore with the fans stationary, some heat transfer still takes place owing to natural convection (up to 40%),
• The danger of water pollution is eliminated.
Disadvantages :
• Air- cooled condensers and coolers are more expensive as conventional shell-and- tube coolers for sea water service,
• Air- cooled equipment occupies more ground space, though it can be placed above other equipment,
• There is more electrical equipment,
• Leaking tubes may cause explosion hazards or fires.
• Special HESeveral common types are used for specific uses. The coil-wound, aluminium heat exchanger is very popular for LNG service.
1. Printed Circuit HE (PCHE)
2. Indirect fired heater
Heat is provided by burning gas or oil in the burner of the fire tube. The hot combustion gas and flame heat an intermediate liquid which, in turn, heats the fluid flowing in the coil or the series of tubes. The intermediate liquid must be stable at atmospheric pressure and the maximum temperature involved.
3. Direct fired heater
U- shaped fire tubeThe reboiler of the conventional glycol regenerator is
heated by U-shaped fire tubes. In this apparatus, the glycol is in direct contact with the wall of the fire tube. U-shaped fire tubes are also used in heater treaters encountered in some onshore crude oil dehydration.
Electrical heaterIn some installations, for safety reason, the heat required by the reboiler is provided by an electrical resistance instead of a fire tube. This solution deletes a fire point on the production platform.
Electric tracing
Electric tracing is used in some installations to maintain the oil or the gas at a temperature value required to prevent processing problems due to crude oil viscosity increase, gas condensation
Direct fired heaters or furnacesDirect fired heaters are the usual equipment when large amounts of heat input at high temperature are required to process the fluid. They are usually encountered in gas processing plants to run reboilers, regenerate solid desiccants.
Fired heaters come in two basic styles:
• Horizontal - tube box - type heaters (cabins, two-cell box,),
• Vertical - tube cylindrical heaters (vertical-cylindrical,).
Combustion of the gas or oil fuel occurs in burners at the bottom of the heater. Heat transfer in the main chamber is primarily by radiation. The hot flue gases travel upward through a second section in which the heat transfer is mainly by convection.
• Condenser : hydrocarbon or other vapor is condensed either partially or completely by the removal of heat.
• Cooler/Chiller : used for cooling process streams by the removal of sensible heat to temperatures usually in the neighborhood of atmosphere. The cooling medium is generally air or water.
• Reboiler : Reboilers supply the heat requirements of a distillation process in the bottom of fractionating columns.
• Heater – Super heater : A heater is any heat transfer equipment employed for heating. A "super heater" heats a vapor above the saturation
• Coil in vessel : suitable for handling small quantities of fluid. heat flux (heat transferred per unit time and per unit area) is relatively low, but this type of exchanger is inexpensive and is suitable for some limited applications.
