CHAPTER -4 HEAT EXCHANGERS

CHAPTER -4 HEAT EXCHANGERS

4.1 Introduction

Heat exchangers are devices in which heat is transferred between two fluids at different temperatures without any mixing of the fluid. Specific application may be found in space heating and air conditioning, power production, waste heat recovery and chemical processing.

Heat exchangers are broadly classified into three categories:

a) Direct transfer type.

b) Storage Type.

c) Direct contact type.

4.1.1 Direct transfer type

A direct type of heat exchanger is one in which the hot and the cold fluids flows simultaneously through the device and the heat is transferred through a wall separating the fluids. E.g. Concentric tube type heat exchanger ( Tube in tube heat exchanger)

(Figure 4.1 – Parallel flow double pipe heat exchanger)

Hence heat exchange between the fluids is from the wall of the inner tube.

4.1.2 A Storage type heat exchanger

A storage type of heat exchanger is the one in which the heat transfer from the hot fluid to the cold fluid occurs through a coupling medium in the form of porous solid matrix.

The hot and cold fluids flow alternatively through the matrix. The hot fluid stores heat in coupling medium and cold fluid extracts heat from it.

(Figure 4.2 – Single matrix storage type heat exchanger)

During the operation valve A & valve B are open and valve C & valve D are closed. The hot fluid enters the heat exchanger from valve A, flows through the matrix and exit the heat exchanger from valve B. As a result the matrix extracts the heat and stores in it. Then valve A & valve B are closed and valve C & valve D are opened and cold fluid enters the heat exchanger from valve C. It flows through the matrix and extracts the heat which is stored in it. Finally it drains out of the heat exchanger from valve D.

4.1.3 Direct Contact Type

A direct contact type of heat exchanger is the one in which the two fluids are not separated. Of the heat is to be transferred between a gas and a liquid, the gas is either bubbled through the liquid or the liquid is sprayed in the form of droplets into the gas.

Working principle of Direct Contact Heat Exchanger System:

Air supplied by a blower (1) is raised in temperature by the heat source (2). 

The hot gas enters the direct contact exchanger inlet (3)and travels upward through a high efficiency packed bed (4) 

Cool water is introduced at the top of the unit through a liquid distributor (5) where it comes into intimate counter current contact with the hot gas in the packed bed as it travels downward.  Sensible heat transfer between the colder water and hot gas occurs resulting in rapid heat up of the water. 

The heated water drains into the sump section (6) and is pumped (7) through a plate and frame or shell and tube heat exchanger (8) to reheat return heat transfer liquid in a complete closed loop cycle. 

Cleaned reduced temperature gas now saturated with water vapour exits out of the top of the unit at (9)

(Figure 4.3 – Direct contact type heat exchanger)

Note – so for undergraduate course we will consider only direct transfer type of heat exchanger.

4.2 Direct Transfer Type Heat Exchanger

Direct transfer type heat exchangers are broadly classified into three categories:

a) Tubular.

b) Plate.

c) Extended surface.

4.2.1 Tubular type of Heat Exchanger

In generally there are two types for tubular type of heat exchanger:

(a) Concentric tube or double pipe or tube in tube type.

Concentric Tube (or Pipe) Heat Exchangers are used in a variety of industries for purposes such as material processing, food preparation and air-conditioning. They create a temperature driving force by passing fluid streams of different temperatures either in parallel flow or counter flow direction, separated by a physical boundary in the form of a pipe. This induces forced convection, transferring heat to/from the product.

(a)

(b)

(Figure 4.4 - Concentric tube heat exchanger (a) parallel flow (b) counter flow)

(b) Shell and tube types of heat exchanger.

It consists of cylindrical shell and bundles of tube inside the shell. Also the axis of the tube is parallel to the axis of the shell. One fluid flows through the tubes and other fluid flows on the outside of the tube i.e. shell side.

Following are the parts of shell and tube types of heat exchanger:

1. Cylindrical shell

2. Bundles of tubes

3. Tube header sheet (i.e. tubes are fixed in sheets at the ends)

4. Front and rear end heads of heat exchanger

5. Nozzle (from where fluid enters or exits the heat exchanger)

6. Baffels (there are the sheets which ensures that fluid flows in zig-zag manner, therefore they create turbulence in flow which enhances heat transfer rates)

(Figure 4.5 – Shell and tube heat exchanger)

Note - Heat transfer area available per unit volume is around 100 to 500 m2/m3

4.2.2 Plate type of heat exchanger

It is a series of large rectangular thin metal plates which are clamped together to form narrow parallel plate channel.

(Figure 4.6 - Plate type of heat exchanger)

In the plate-fin heat exchanger, b indicates the width of the passages through which the fluid travels. (b1) is the width of passage that fluid being examined passes through. (b2) is the width of the passage on the other flow side. (a) is the thickness of the plate that divides these passages.

(Figure 4.7 – Flow pattern plate type of heat exchanger)

Note - Heat transfer area available per unit volume is around 100 to 200 m2/m3

4.2.3 Extended surface type of heat exchanger

In extended surface type of heat exchanger, fins are attached on the primary heat transfer surface with the object of increasing the heat transfer area; hence it provides much more heat transfer area per unit volume than a tubular or a plate heat exchanger.

Note - Heat transfer area available per unit volume is greater than 700 m2/m3.

(Figure 4.8 - Extended surface type of heat exchanger)

4.3 Temperature Distribution in heat exchanger

During the heat transfer from hot to cold fluid, there is a change in temperature either of both the fluids or one fluid as in case of condensation and evaporation.

(a)

(b)

(c)

(d)

(Figure 4.9 – temperature distribution for various cases of heat exchanger (a) parallel flow (b) counter flow (d) condenser (d) evaporator)

Fouling factor

Over a time period of heat exchanger operation the surface of the heat exchanger may be coated by the various deposits present in the flow system. Moreover, the surfaces may become corroded or eroded over the time. Therefore, the thickness of the surface may get changed due to these deposits.

Material deposits on the surfaces of the heat exchanger tube may add further resistances to heat transfer. Such deposits are termed fouling and may significantly affect heat exchanger performance.

Scaling is the most common form of fouling and is associated with inverse solubility salts. Examples of such salts are CaCO3, CaSO4, Ca3(PO4)2, CaSiO3, Ca(OH)2, Mg(OH)2, MgSiO3, Na2SO4, LiSO4, and Li2CO3.

The heat exchanger coefficient, U may be modified to include the fouling factor Rf.

Subject - Applied thermal and hydraulic Engineering Lecture notes by Prof. D.Satish

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