HEAT EXCHANGER - DEFINATION, TYPES, FLOW OF ARRANGEMENT AND USES

 WHAT IS Heat Exchanger?

• Heat exchangers are equipment that transfer heat from one medium to another.

• A heat exchanger is a component that allows the transfer of heat from one fluid (liquid or gas) to another fluid.

Example:

• From hot water to cold water,

• From hot steam to cold water,

• From hot gas to cold water,

• From hot water or unsaturated steam to cool air.

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

Types Of Heat Exchangers:

1. Tube And Shell Heat Exchangers
2. Plate Heat Exchangers
3. Regenerative Heat Exchangers

TUBE AND SHELL HEAT EXCHANGERS:

• This type of heat exchanger consists of a set of tubes in a container called a shell. The fluid flowing inside the tubes is called the tube side fluid and the fluid flowing on the outside of the tubes is the shell side fluid.

• At the ends of the tubes, the tube side fluid is separated from the shell side fluid by the tube sheet(s). The tubes are rolled and press-fitted or welded into the tube sheet to provide a leak tight seal.

• In systems where the two fluids are at vastly different pressures, the higher pressure fluid is typically directed through the tubes and the lower pressure fluid is circulated on the shell side.

• This is due to economy, because the heat exchanger tubes can be made to withstand higher pressures than the shell of the heat exchanger for a much lower cost.

• The support plates shown on Figure also act as baffles to direct the flow of fluid within the shell back and forth across the tubes.

PLATE HEAT EXCHANGERS:

• The hot and cold fluids alternate between each of the plates. Baffles direct the flow of fluid between plates.

• Because each of the plates has a very large surface area, the plates provide each of the fluids with an extremely large heat transfer area.

• Therefore a plate type heat exchanger, as compared to a similarly sized tube and shell heat exchanger, is capable of transferring much more heat.

• This is due to the larger area the plates provide over tubes.

FLOW OF ARRANGEMENT:

1. Parallel Flow
2. Counter Flow
3. Cross Flow

• 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.

• In a cross-flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger.

REGENERATIVE HEAT EXCHANGER:

• A third type of heat exchanger is the regenerative heat Exchanger.

• In this, the heat from a process is used to warm the fluids to be used in the process, and the same type of fluid is used either side of the heat exchanger (these heat exchangers can be either plate-and-frame or shell-and-tube construction).

• These exchangers are used only for gases and not for liquids. The major factor for this is the heat capacity of the heat transfer matrix.

Uses of a heat exchanger:

• Raise the temperature of a fluid, using a warmer one.
• Cool a fluid using another that is at a lower temperature.
• Condense gases.
• Evaporate liquids.