Heat Exchangers

1. Introduction

A heat exchanger is a device used to transfer heat between two or more fluids at different temperatures, without mixing them. Heat exchangers are widely used in power plants, refrigeration and air-conditioning systems, chemical industries, automobiles, and process plants.
The fluids may be liquids, gases, or a combination of both, and heat transfer occurs mainly by convection and conduction, and sometimes radiation.

2. Purpose of a Heat Exchanger

• To heat or cool fluids
• To recover waste heat
• To maintain required process temperatures
• To increase thermal efficiency of systems

3. Principle of Heat Exchanger

Heat flows from a hot fluid to a cold fluid due to the temperature difference between them.
The rate of heat transfer depends on:

• Temperature difference
• Surface area available for heat transfer
• Heat transfer coefficient
• Flow arrangement

4. Classification of Heat Exchangers

4.1 Based on Contact Between Fluids

a) Direct Contact Heat Exchanger

• Hot and cold fluids come into direct physical contact
• Heat transfer occurs by mixing
• Example: Cooling towers

Advantages:

• High heat transfer rate
• Simple construction

Disadvantages:

• Fluids mix, so limited applications

b) Indirect Contact Heat Exchanger

• Fluids are separated by a solid wall
• No mixing of fluids
• Most commonly used type

4.2 Based on Flow Arrangement

a) Parallel Flow Heat Exchanger

• Both fluids flow in the same direction
• Temperature difference is maximum at inlet and decreases along the length

Characteristics:

• Simple design
• Lower effectiveness compared to counter flow

b) Counter Flow Heat Exchanger

• Fluids flow in opposite directions
• Maintains a more uniform temperature difference

Advantages:

• Higher heat transfer efficiency
• Better temperature control

c) Cross Flow Heat Exchanger

• Fluids flow perpendicular to each other
• Used when one fluid is gas and the other is liquid

Example:

• Car radiators
• Air coolers

4.3 Based on Construction

a) Shell and Tube Heat Exchanger

• One fluid flows through tubes, other flows through shell
• Most widely used in industries

Advantages:

• Suitable for high pressure and temperature
• Easy maintenance

b) Plate Heat Exchanger

• Consists of corrugated metal plates
• Large heat transfer area in compact size

Advantages:

• High efficiency
• Easy to clean

c) Finned Tube Heat Exchanger

• Fins increase surface area
• Used when one fluid has a low heat transfer coefficient (air)

5. Heat Exchanger Components

• Tubes
• Shell
• Baffles
• Tube sheets
• Inlet and outlet nozzles

6. Heat Transfer Rate in Heat Exchanger

The amount of heat transferred is given by:$$Q = m c_p (T_{in} – T_{out})$$

Where:

• $Q$Q = heat transfer rate (W)
• $m$m = mass flow rate (kg/s)
• $c_p$cp​ = specific heat (J/kg·K)
• $T$T = temperature (K)

7. Log Mean Temperature Difference (LMTD)

Since temperature difference varies along the length, the log mean temperature difference is used:$$\Delta T_{lm} = \frac{\Delta T_1 – \Delta T_2}{\ln\left(\frac{\Delta T_1}{\Delta T_2}\right)}$$

Where:

• $\Delta T_1$ΔT1​ = temperature difference at one end
• $\Delta T_2$ΔT2​ = temperature difference at the other end

8. Overall Heat Transfer Equation

$$Q = U A \Delta T_{lm}$$Q=UAΔTlm​

Where:

• $U$U = overall heat transfer coefficient (W/m²·K)
• $A$A = heat transfer area (m²)

9. Effectiveness of Heat Exchanger

Effectiveness (ε) is defined as:$$\varepsilon = \frac{\text{Actual heat transfer}}{\text{Maximum possible heat transfer}}$$ε=Maximum possible heat transferActual heat transfer​

It indicates how efficiently the heat exchanger performs.

10. Fouling in Heat Exchangers

Fouling is the deposition of unwanted materials on heat transfer surfaces.

Effects:

• Reduces heat transfer
• Increases pressure drop
• Increases operating cost

Prevention:

• Regular cleaning
• Proper material selection
• Filtration of fluids

11. Advantages of Heat Exchangers

• Efficient energy utilization
• Compact design
• Wide range of applications
• Improves system performance

12. Limitations

• High initial cost
• Fouling reduces efficiency
• Design complexity for high-performance units

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