Introduction to Heat Exchanger

A heat exchanger is a device, which transfers thermal energy between two fluids at different temperatures. In most of the thermal engineering applications, both of the fluids are in motion and the main mode of heat transfer is convection. Examples are automobile radiators, condenser coil in the refrigerator, air conditioner, solar water heater, chemical industries, domestic boilers, oil coolers in a heat engine, milk chillers in pasteurizing plant.

Classification of heat exchangers:

1. Classification according to construction:

• Tubular heat exchangers.
• Plate heat exchangers.
• Extended surface heat exchangers.
• Regenerative heat exchangers.

2. Classification according to the transfer process:

• Indirect contact heat exchangers.
• Direct contact heat exchangers.

3. Classification according to flow arrangement:

• Parallel flow exchangers.
• Counter flow heat exchangers.
• Cross flow heat exchangers.

4. Classification according to pass arrangement:

• Single pass arrangement.
• Multi-pass arrangement.

5. Classification according to surface compactness:

• Gas to liquid
• compact (β ≥ 700 m2/m3)
• non compact (β ≤ 700 m2/m3)
• Liquid to liquid and phase change
• compact (β ≥ 400 m2/m3)
• non compact (β ≤ 400 m2/m3)

6. Classification according to a number of fluids:

• Two fluids.
• Three fluids.
• N-fluids.

7. Classification according to the phase of the fluid.

• Gas- liquid.
• Liquid-liquid.
• Gas-gas.

Materials for heat exchanger:

1. Aluminum
2. Stainless steel
3. Copper

Pipe in pipe heat exchangers:

Hairpin heat exchangers (often also referred to as “double pipes”) are characterized by a construction form which imparts a U shaped appearance to the heat exchanger. In its classical sense, the term double pipe refers to a heat exchanger consisting of a pipe within a pipe, usually of a straight-leg construction with no bends. However, due to the need for removable bundle construction and the ability to handle differential thermal expansion while avoiding the use of expansion joints (often the weak point of the exchanger), the current U-shaped configuration has become the standard in the industry.

Working: 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.
The term ‘hairpin heat exchanger’ is also used for a heat exchanger of the configuration in the diagram. A hairpin heat exchanger may have only one inside the pipe, or it may have multiple inside tubes, but it will always have the doubling back feature. Some heat exchanger advertises the availability of finned tubes in a hairpin or double pipe heat exchanger. These would always be longitudinal fins, rather than the more common radial fins used in a cross-flow finned tube heat exchanger. In a double pipe heat exchanger design, an important factor is the type of flow pattern in the heat exchanger. A double pipe heat exchanger will typically be either counterflow or parallel flow. Crossflow just doesn’t work for a double pipe heat exchanger. The flow pattern and the required heat exchange duty allow calculation of the log mean temperature difference. That together with an estimated overall heat transfer coefficient allows calculation of the required heat transfer surface area. Then pipe sizes, pipe lengths, and a number of bends can be determined.

Plate type heat exchanger

Construction and working of Plate type heat exchanger.

It consists of a series of closely spaced parallel plates with fins held in between. The plates separate the two fluids which flow through passages alternately formed between the plates. It also has fins attached over the primary heat transfer surface so as to increase the heat transfer area. This improves the effectiveness of the heat exchanger. Fins from the individual flow passages for single fluid. A typical cross-flow, both fluids unmixed arranged is shown below, in which heat is transferred between fluid “A” and fluid “B”.
The counter flow or parallel flow arrangement can also be possible. The fins may be plain fin (Straight or corrugated) or interrupted and are attached to the plate by brazing or soldering. They are more suitable for gas to gas applications. Plate fin type heat exchanger is as shown in figure

Applications of plate type heat exchanger:
a. Milk chilling plants
b. Radiator in automobile
c. Air conditioning
d. Food industries

Shell and tube type heat exchanger

Shell and tube heat exchanger consists of a bundle of round tubes placed inside the cylindrical shell. The tube axis parallels to that of the shell. One fluid inside the tubes while the other over the tubes.

The main components of this type of heat exchanger are:
i. Shell
ii. Tube bundle
iii. Front and rear headers of shell
iv. baffles

The baffles provide the support to tubes and also deflect the fluid flow approximately normal to tubes. This increases the turbulence of shell-side fluid and improves heat transfer. The various types of baffles are existing and their type, spacing, shape, will depend on the flow rate, shell side pressure drop, required tube support, flow vibrations, etc.

The fluid combination may be :
1 Liquid to liquid
2 Liquid to gas
3 Gas to gas

Advantages of shell and tube type heat exchanger : 

• Less expensive as compared to Plate type coolers
• Can be used in systems with higher operating temperatures and pressures
• Pressure drop across a tube cooler is less
• Tube leaks are easily located and plugged since pressure test is comparatively easy
• Tubular coolers in the refrigeration system can act as a receiver also.
• Using sacrificial anodes protects the whole cooling system against corrosion
• Tube coolers may be preferred for lubricating oil cooling because of the pressure differential

Disadvantages of shell and tube heat exchanger : 

• Heat transfer efficiency is less compared to plate type cooler
• Cleaning and maintenance is difficult since a tube cooler requires enough clearance at one end to remove the tube nest
• Capacity of tube cooler cannot be increased.
• Requires more space in comparison to plate coolers

Shell and Coil type of heat exchanger

Construction of Shell and Coil Heat Exchanger
1. Shell and coil type heat exchanger consists of a helical coil.
2. The helical coil is compact in the shell.
3. The cooling medium is passed through the coil and liquid to be cooled is passed from the top of the shell.
4. The cooled liquid is taken out from the bottom of the shell.
5. Generally in this type of heat exchanger, the counter-flow arrangement is used.

Spiral Plate Heat Exchanger

It is a form of a plate heat exchanger usually made of stainless steel. It is often used in cellulose industries where the heat exchanger is subjected to severe fouling and corrosion. The plates of this type of heat exchanger are very long and thickness of passage between the plates must be rather small so that after the sheets forming the upper and lower surfaces are welded together, the unit can be wrapped into a spiral form. That’s why it is called as a spiral heat exchanger.

The technical features of this type of heat exchanger are :
(a) Flow rates are relatively low
(b) Pure counterflow heat exchanger
(c) Highly compact (more than 700 m2/ m3)
(d) Can withstand pressure up to 10 bar only

Regenerator

Heat exchangers in which there is an intermittent flow of heat from hot to cold fluid via heat storage and heat rejection through the exchanger surface or matrix are referred to as indirect or storage type heat exchanger or regenerator. The regenerative type heat exchangers are either static or dynamic.

Static Type Regenerator
(a) No moving parts.
(b) Consists of a porous medium (balls, pebbles, powders, etc.) through which hot and cold fluid pass alternatively.
(c) A flow switching device regulates the periodic flow of the two fluids.
(d) Compact for use in refrigeration and Stirling Engines.
(e) Non-compact in high temperature (900 – 1500oC) applications.
(f) Low cost and ruggedness are essential for the stationary type.

Storage Type or Regenerative Heat exchanger

The storage type or regenerative heat exchanger is shown in Figure 14.6. In this heat exchanger energy is stored periodically. Medium is heated or cooled alternatively. The heating period and cooling period constitute 1 (one) cycle.

Features
(a) Periodic heat transfer-conduction.
(b) Heat transfer fluid can be a liquid, phase changing, non-phase changing.
(c) Solid storage medium is called matrix.
(d) Matrix may be stationary or rotating

Classical Applications of Regenerator : 
(a) Gas turbine regenerators: Heating the compressed air by the gas turbine exhaust before the air goes to the combustor.
(b) Reversed Stirling engine for liquefaction of air-Philips refrigeration machine.

Applications of heat exchangers.

1. Food and beverages.
2. Petroleum/chemical processing.
3. Hydrocarbon processing.
4. Polymers.
5. Pharmaceuticals.
6. Industrial.
7. Energy and power.

Some Questions and Answers : 

1. Suggest the type of heat exchangers for following applications –

(i) A dairy plant (Milk Chilling Plant)
(ii) The condenser of the refrigeration system. (Household system) Justify your answers

Answers: Types of Heat Exchanger Used for

1) Dairy Plant (Milk Chilling Plant)- Plate Type Heat Exchanger
Because It is made up of an aluminum alloy which provides a higher rate of heat transfer.
Due to larger surface area, It has more heat transfer as compared to other heat exchangers which
is useful for dairy plants.
It is lighter in weight.

2) The condenser of Refrigeration System:- Counter Flow tube type heat exchanger

Because High performance due to large surface area
Compact and light in weight
In tubes generally, turbulent flow is developed which reduces scale deposition.
Less installation and maintenance cost.

2. Suggest the type of heat exchangers for following applications –

Answers :

1) Mills Chiller Plant:- Plate type heat exchanger
Justification:-
a) Non-reactive material.
b) Leakproof joints
c) No mixing of two fluids
d) Non-toxic material
e) Non- corrosive material

2) The radiator of an Automobile:- Plate and tube type Heat Exchanger (Air Water Convective Radiator)

Justification:-
a) it is used to cool the engine of an automobile
b) Water flows through jacket along with engine and carried away heat by convection
c) This water enter into the radiator where it gives its heat to the air of the atmosphere which is passed over water tubes. Though plates.

3. Classification of the heat exchanger and their applications

1. Nature of heat exchange process Heat exchangers, on the basis of nature of the heat exchange process, are classified as follows :
(a) (i) Direct contact (or open) heat exchangers:- Examples : (i) Cooling towers ; (ii) Jet condensers ; (iii) Direct contact
(ii) Indirect contact heat exchangers.
(a) Regenerators:- Examples : (i) I.C. engines and gas turbines ; (ii) Open hearth and glass melting furnaces ; (iii) Air heaters of blast furnaces.
(b) Recuperators Examples : (i) Automobile radiators, (ii) Oil coolers, intercoolers, air preheaters, economizers, superheaters, condensers and surface feed heaters of a steam power plant, (iii) Milk chiller of pasteurizing plant, (iv) Evaporator of an ice plant

2. The relative direction of fluid motion

According to the relative directions of two fluid streams the heat exchangers are classified into the following three categories
(i) Parallel-flow or unidirectional flow :-: Examples: Oil coolers, oil heaters, water heaters etc.
(ii) Counter-flow:- Examples: The cooling unit of refrigeration system etc.
(iii) Cross-flow. Examples:- Automobile radiator etc.

3. Design and constructional features

On the basis of design and constructional features, the heat exchangers are classified as under :
(i) Concentric tubes.
(ii) Shell and tube
(iii) Multiple shell and tube passes.
(iv) Compact heat exchangers:- Example: Plate-fin, flattened fin tube exchangers, etc.

4. The physical state of fluids
Depending upon the physical state of fluids the heat exchangers are classified as follows: Condensers (ii) Evaporators

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