1.1 HEAT  EXCHANGER

A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air.

There are two primary classifications of heat exchangers 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 from the heat (transfer) medium. See counter current exchange. In a cross-flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger.

For efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing resistance to fluid flow through the exchanger. The exchanger's performance can also be affected by the addition of fins or corrugations in one or both directions, which increase surface area and may channel fluid flow or induce turbulence.

The driving temperature across the heat transfer surface varies with position, but an appropriate mean temperature can be defined. In most simple systems this is the "log mean temperature difference" (LMTD). Sometimes direct knowledge of the LMTD is not available and the NTU method is used.

The transfer of thermal energy between fluids is one of the most important and frequently used processes in engineering.   The  transfer  of  heat  is  usually  accomplished  by  means  of  a  device known as a heat exchanger.  Common applications of heat exchangers in the nuclear field include boilers, fan coolers, cooling water heat exchangers, and condensers. The basic design of a heat exchanger normally has two fluids of different temperatures separated by some conducting medium.   The most common design has one fluid flowing through metal tubes and the other fluid flowing around the tubes.  On either side of the tube, heat is transferred by convection.   Heat is transferred through the tube wall by conduction. 

Heat exchangers may be divided into several categories or classifications.  In the most commonly used type of heat exchanger, two fluids of different temperature flow in spaces separated by a tube wall.   They transfer heat by convection and by conduction through the wall.   This type is referred  to  as  an  "ordinary  heat  exchanger,"  as  compared  to  the  other  two  types  classified  as “regenerators" and "cooling towers."An ordinary heat exchanger is single-phase or two-phase.  In a single-phase heat exchanger, both of the fluids (cooled and heated) remain in their initial gaseous or liquid states.   In two-phase exchangers,  either  of  the  fluids  may  change  its  phase  during  the  heat  exchange  process.   The steam  generator  and  main  condenser  of  nuclear  facilities  are  of  the  two-phase,  ordinary  heat exchanger classification

1.1.1 REQUIREMENTS OF HEAT EXCHANGERS

1. High thermal effectiveness

2. Pressure drop as low as possible

3. Reliability and life expectancy

4. High-quality product and safe operation

5. Material compatibility with the process fluids

6. Convenient size, easy for installation, reliable in use

7. Easy for maintenance and servicing

8. Light in weight but strong in construction to withstand the operational pressures

9. Simplicity of manufacture

10. Low cost

11. Possibility of effecting repair to maintenance problems The heat exchanger must meet normal process requirements specified through problem specification and service conditions for combinations of the clean and fouled conditions, and un corroded and corroded conditions.

 The exchanger must bemaintainable, which usually means choosing a configuration that permits cleaning as required and replacement of tubes, gaskets, and any other components that are damaged by corrosion, erosion, vibration, or aging. This requirement may also place limitations on space for tube bundle pulling, to carry out maintenance around it, lifting requirements for heat exchanger components, and adaptability for in-service inspection and monitorin