HEAT EXCHANGERS

MADE BY :RIMJHIM RAJ SINGH (13IME045)

SAKSHI SINGH (13IME051)

INTRODUCTION TO HEAT EXCHANGERS:

• A heat exchanger is a device in which two fluid streams, one hot and another cold, are brought into 'thermal contact' in order to effect transfer of heat from the hot fluid stream to the cold.• It provides relatively large area of heat transfer for a given volume of

the equipment.• They are widely used in space heating, refrigeration, air conditioning,

power plants, chemical plants, petrochemical plants , petrolium refineries and sewage treatments.

PRINCIPLE OF HEAT EXCHANGERS• Heat exchanger works because heat flows from higher tempreture to

lower temprature. therefore if a hot fluid and a cold fluid are seperated by a conducting surface, heat can be transferred from hot fluid to cold fluid.• two fluids of different temperature are brought into close contact but

are prevented from mixingby a physical barrier. The temprerature of the two fluids will tend to equalize. By arranginf counter current flow it is possible for the temperature at the out let of each fluid to approach the temperature at the inlet of the other. the heat contents are simply exchanged from one fluid to the other and vice versa . No energy is added or removed.

Heat transfer depends upon following factors:

• Type of the material between fluids.• thickness of materials.• surface area of materials• type of fluids.• flow rate of fluid.

Basic equation defining the heat exchanger principle:

where, Q= Rate of heat transfer between the two fluids in the heat exchanger .U=overall heat transfer coefficient.A= heat transfr surface area. = log mean temp. difference.

lmTUAQ

lmT

CLASSIFICATION OF HEAT EXCHANGERS• 1. Nature of heat exchange process• 2. Relative direction of motion of fluids• 3. Mechanical design of heat exchanger surface• 4. Physical state of heat exchanging

NATURE OF HEAT EXCHANGE PROCESS• Direct contact heat exchanger: this is done by complete physical mixing of heat and mass transfer. Examples are water cooling towers and jet condensers in steam power plants.• Regenerator: here hot and cold fluids flows alternately when hot fluid

passes, the heat is transferred to the solid matrix and then stopped the flow of hot fluid, next cold fluid is passed on the matrix which takes heat from solid matrix. Examples are Open hearth and blast furnaces.• Recuperator: the cold fluid flows simultaneously on either side of a

separating wall. Examples are super heaters, condensers, economizers and air pre-heaters in steam power plants, automobile radiators.

Relative direction of motion of fluids• Parallel flow heat exchangers: In parallel flow heat exchangers, both the tube

side fluid and the shell side fluid flow in same direction. In this case, the two fluids enter the heat exchanger from the same end with a large temperature difference.• Counter flow heat exchangers: In counter flow heat exchangers, the two

fluids flow in opposite directions. Each of the fluids enter the heat exchanger from opposite ends. Because the cooler fluid exists the counter flow heat exchanger at the end where the hot fluid enters the heat exchanger, the cooler fluid will approach the inlet temperature of the hot fluid.• Cross flow heat exchangers: In cross flow heat exchangers, one fluid flows

through tubes and second fluid passes around the tubes perpendicularly.

Mechanical design of heat exchanger surface• Shell and tube.• Double pipe heat exchanger.

Physical state of heat exchanging• Condenser• Evaporator

DOUBLE PIPE HEAT EXCHANGER • A double pipe heat exchanger, in its simplest form is just one pipe inside

another larger pipe. One fluid flows through the inside pipe and the other flows through the annulus between the two pipes. The wall of the inner pipe is the heat transfer surface. The pipes are usually doubled back multiple times as shown in the diagram at the left, in order to make the overall unit more compact.• They are one of the simplest and cheapest types of heat exchanger.• They can be used for high temperature, high pressure, and highly viscous

service.• Some designs use inner tubes/pipes fitted with longitudinal fins to increase

the heat transfer area and improve heat transfer.

Shell and tube heat exchanger

• A shell and tube heat exchanger is a class of heat exchanger designs. It is the most common type of heat exchanger in oil refineries and other large chemical processes, and is suited for higher-pressure applications. As its name implies, this type of heat exchanger consists of a shell (a large pressure vessel) with a bundle of tubes inside it. One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids. The set of tubes is called a tube bundle, and may be composed of several types of tubes: plain, longitudinally finned, etc.

ADVANTAGES OF SHELL AND TUBE •Condensation or boiling heat transfer can be accommodated in either the tubes or the shell, and the orientatioN can be horizontal or vertical. You may want to check out the orientation of the heat exchanger in our laboratory. Of course, single phases can be handled as well.• The pressures and pressure drops can be varied over a wide range.• Thermal stresses can be accommodated inexpensively.• There is substantial flexibility regarding materials of construction to accommodate

corrosion and other concerns. The shell and the tubes can be made of different materials.

• Extended heat transfer surfaces (fins) can be used to enhance heat transfer.• Cleaning and repair are relatively straightforward, because the equipment can be

dismantled for this purpose