MODULE 4

HEAT EXCHANGERS ANALYSIS

• Heat Exchangers are the devices that transfer heat from one fluid to another.

• They are used in the industry for three reasons:1. They are necessary to the function of some

plant systems.2. They are used to make certain systems operate

more efficiently and3. They help to ensure safety and protect

equipment and personnel

Factors that affect heat transfer

The four factors are as follows:1. The type and thickness of the material through which

heat is transferred2. The kinds and amounts of fluids flowing through the heat

exchanger3. The difference in temperature between the two fluids in

the heat exchanger4. The presence of any contamination in the fluids or in the

surfaces that transfer heat in the heat exchanger• The direct transfer of heat is affected by factor 2 and 3 The

indirect transfer is affected by all the four factors

• The first factor affecting the heat transfer is the materials used. The heat moves faster through thinner tube wall, the tube must also be able to resist erosion.

• The second factor affecting the heat exchanger is the type and amount of fluids flowing through it. The greater the capacity in both volumes and flow of a heat exchanger, the more heat can be transferred from one fluid to another fluid.

• The third factor is the temperature difference. The greater the difference in temperature between one substance and

another, the faster the heat will flow between them.• The fourth factor is contamination. Contaminant can also reduce the

efficiency of heat exchanger, oxygen and other gases present in steam can block the flow of heat from the water.

TYPES OF HEAT EXCHANGERS

• Double Pipe• Shell Tube• Kettle• Air coolers• Plate type• Calandria type• Condensers

FEATURES OF DOUBLE PIPE HEAT EXCHANGERS

• The outer pipe is called the shell• The inner pipe is called the tube• The space between the shell and the tube is

called the annulus• Double pipe exchangers are usually used for

low flow rates and high temperatures. Double pipe H.E is also called as a pipe within a pipe exchanger.

http://en.wikipedia.org/wiki/File:U-tube_heat_exchanger.PNG

http://en.wikipedia.org/wiki/File:Straight-tube_heat_exchanger_1-pass.PNG

http://en.wikipedia.org/wiki/File:Straight-tube_heat_exchanger_2-pass.PNG

• The temperature variations of the fluids in parallel and counter flow are shown in fig

• Temperatures are plotted against length or area of heat exchanger surface.

• The inlet end, where length or area is zero is regarded as being the end where the hotter of the two fluids enters.

• The fluids are regarded as being hot or cold for convenience, and th is a temperature of the hot fluid, tc a temperature of cold fluid. Suffixes 1 and 2 are used for inlet and outlet of individual streams, and Ɵi is the temperature difference between fluids at the inlet end and Ɵo the difference at the outlet end of the exchanger.

• The primary purpose of a heat exchanger is to achieve the required transfer rate using the smallest possible transfer area and fluid pressure drop.

• The heat transfer requirement ,Q, can be expressed in three ways.

• Ɵm is a mean temperature difference between the fluids and UA and UL are mean co-efficients , in kW/m2K and kW/(mK) or equivalent units, applicable over the entire area A or length L of the exchanger.

• The mean temperature difference Ɵm for both parallel flow and counter flow given by.

Ɵo and Ɵi are defined in fig2

Problems:

THE FEATURES OF SHELL AND TUBE HEAT EXCHANGERS

• It has an outer casing called the shell which contains one fluid .This is called the shell side fluid.

• The fluid flowing through the tubes is termed as the tube side fluid.

• The amount of heat exchange can be increased by increasing the velocity of the shell –side fluid.This is done by means of baffles,which are metal discs installed inside the shell.

• The efficieny of a shell and the tube exchanger can be increased by increasing the velocity of the tube-side fluid. This is done by changing the direction of the flow, thus increasing the flow path inside the exchanger.

• Exchangers in which the tube-side flow is more than one direction are called multipass exchangers

• The type of exchanger can be identified by the position of the inlet and outlet pipes.In a two-pass exchanger which will cause the tubes to expand.

• As the temperature increases, the force will also increase until either of the following occur:

1. The tube bends2. The tube breaks free of its fixing

• To prevent this happening in a heat exchanger which is designed for large temperature differences a floating head can be used.

• One end of tube is fixed,the other end is free to move or float.The floasting head can be removed to enable the tubes to be cleaned

TUBE BUNDLE ARRANGEMENTS IN SHELL-AND –TUBE HEAT EXCHANGERS

• Tube Bundles-Consist of three main parts:tubes,tube sheets, and baffles.

• Triangular Pattern-This gives the most tubes for a given size but cleaning the outside of the tubes can only be done by chemical means

• Square Pitch- This type is the easiest to clean the outside of the tubes.

• Diagonal Pitch-is a similar to square pitch but being rotated through 45o. This layout contains the least number of tubes.

• Increasing the number of passes on the tube or shell side increases the velocity or flow of the fluid which in turn increases the heat transfer rate.

BAFFLE ARRANGEMENTS

• The three types of transverse baffles used to increase velocity in the shell side are:

1. Orifice baffles2. Segmental baffles3. Disk and Doughnut bafflesWith a bored shell clearance and consequent

leakage increases because of corrosion.With unbored shell clearance may be considerably

large because of greater tolerances.

KETTLE TYPE HEAT EXCHANGER

• Kettle type boiler or reboiler as it is called when connected to a distillation column .

• A horizontal shell contains a relatively small tube bundle, two pass on the tube side, with u-tube and tube sheet.

• The tube bundle is submerged in a pool of boiling liquid, the depth of which is set by the height of an overflow weir.

• Feed is admitted to the liquid pool from the bottom. Vapor escapes from the top of shell ,any unvaporized liquid spills over the weir and is withdrawn from the bottom of the shell.

AIR COOLERS

• Air coolers are usually used when cooling water is expensive or where the temperature to which the fluid is to be cooled is high and air can provide adequate cooling.The purpose of the fin is to increase the surface area of the tubes pulls air across the tubes and is known as an induced draft fan, and a fan mounted below the tubes pushes air across the tubes and is known as a forced draft fan.

• To control the outlet temperature, the air flow through the finned elements is varied, either by varying the pitch of the fan blade or by opening and closing louvers.

• The fan speed is set and determined by the tip speed .all fans have vibration switches and in case of severe vibration, the motor is stopped automatically .during normal operation regularly check the header boxes for leaking plugs.check the fan blades for rotation and completeness.

PLATE TYPE HEAT EXCHANGER

• Can be used for multiple duties,several different fluids can flow through different parts of the exchanger and can be kept separate from one another.

• Are very effective with viscous fluids.

CALANDRIAS

• Vertical shell and tube units are also known as THERMOSIPHON REBOILER.

• Are used for distillation and evaporation operations.

A portable boiler

Diagram of a fire-tube boiler

Diagram of a water-tube boiler