HEAT EXCHANGERS ( Heat Transfer – 3 )

By MacanyTech Editorials

Through this article, you are able to take a wide knowledge about the followings.

  • What is known as a Heat Exchanger, Fouling, Copper in Heat Exchangers
  • Types of Heat Exchangers
  • Uses and applications of Heat Exchangers
  • Mechanism of a Heat Exchanger

Introduction

Heat Exchanger is a system used to transfer heat between two or more fluids. They are used in both cooling and heating processes. The fluids are not in direct contact. They are separated by a solid wall to prevent mixing.

When designing heat exchangers, they are designed to maximize the surface area of the wall between the fluids and to minimize the resistance to fluid flow through the exchanger for the efficiency of the exchanger.

Fouling:

When impurities deposit on the surface of heat exchangers, fouling occurs. Due to this, the effectiveness of heat transfer decreases. It is caused by:

  • Low wall shear stress
  • Low fluid velocities
  • High fluid velocities
  • Reaction product solid precipitation

Copper in Heat Exchangers:

When designing a heat exchanger, it is important to select material which can conduct and transfer heat fast and efficiently.

Copper has such properties. Copper is an excellent conductor of heat. It has a high thermal conductivity. So that, heat passes through it quickly.

Some other properties of copper in heat exchangers as follows.

  • Corrosion resistance
  • Bio fouling resistance
  • Stress
  • Internal pressure
  • Creep rupture strength
  • Fatigue strength
  • Hardness
  • Thermal expansion
  • Specific heat
  • Anti-microbial properties
  • Tensile strength
  • Yield strength
  • High melting point
  • Alloy ability
  • Ease of fabrication
  • Ease of joining

 Non-copper heat exchangers are also available. They are included some alternative materials: aluminium, carbon steel, stainless steel, nickel alloys and titanium.

Some applications for copper heat exchangers:

  • Electric power plants
  • Chemical and petrochemical plants
  • Solar thermal water system
  • HVAC system (Air conditioning systems, motor vehicles, gas water heaters)
  • Electric systems (in heat sink, heat pipes)

Types of Heat Exchangers

Heat Exchangers are classified on basis of nature of:

  • Nature of heat exchange process
  • Relative direction of fluid motion
  • Design and constructional features
  • Physical state of fluids

01. Nature of heat exchange process

On the basis of nature of heat exchange process, we can classify heat exchangers as follows.

  • Direct contact (or open) heat exchangers

Examples: Cooling towers, Jet condensers, Direct contact feed heaters

  • Indirect contact heat exchangers

-Regenerators

-Recuperators or surface exchangers

Regenerators:

The performance of the regenerators is affected by the following parameters:

  • Heat capacity of regenerating material
  • The rate of absorption
  • The release of heat

Advantages of regenerators:

  • Higher heat transfer co-efficient
  • Less weight per kW of the plant
  • Minimum pressure loss
  • Quick response to load variation
  • Small bulk weight
  • Efficiency quite high.

Disadvantages of regenerators:

  • Costlier compared to recuperative heat exchangers
  • Leakage is the main trouble, therefore, perfect sealing is required

Examples: IC engines and gas turbines, Open hearth and glass melting furnaces, Air heaters of blast furnaces.

Recuperators:

Examples: Automobile radiators, Oil coolers, intercoolers, air preheaters, economizers, super heaters, condensers and surface feed heaters of a steam power plant, Milk chiller of pasteurizing plant, Evaporator of an ice plant

Types of recuperators

Advantages of recuperators:

  • Easy construction
  • More economical
  • More surface area for heat transfer
  • Much suitable for stationary plants

Disadvantages of recuperators:

  • Less heat transfer coefficient
  • Less generating capacity
  • Heavy and sooting problems

The flow through direct heat exchangers and recuperators may be treated as steady state while through regenerators the flow is essentially transient.

02. Relative direction of fluid motion

Heat Exchangers can be divide according to their flow arrangements.

  • Parallel – flow heat exchanger:

The two fluids enter the exchanger at the same end and travel in parallel to one another to the other side.

Example: Oil coolers, Oil heaters, Water heaters

  • Counter – flow heat exchanger:

The fluids enter the exchanger from opposite sides. The counter-current design is the most efficient one. This can achieve a greater amount of heat or mass transfer than others.

  • Cross – flow heat exchanger:

The fluids travel roughly perpendicular to one another through the exchanger.

Example: Automobile radiator

03. Design and constructional features

By considering the design and constructional features of the heat exchangers, they can be classified as follows.

  • Concentric tubes:

Two concentric tubes are used, each carrying one of the fluids. The effectiveness of the heat exchanger is increased by using swirling flow.

  • Shell and tube heat exchanger:

They consist of a series of tubes. Those tubes contain fluids that must be either cooled or heated. A second fluid which can provide the heat or absorb the heat required runs over the tube. They are used in high-pressure applications. The tubes may be straight or bent in the shape of a “U”.

Followings are some thermal design features which must be considered when designing the tubes in these heat exchangers.

  • Tube diameter
  • Tube thickness
  • Tube length
  • Tube pitch
  • Tube corrugation
  • Tube layout
  • Baffle design
  • Multiple shell and tube passes:

They are used for enhancing the overall heat transfer. They are those who which re-route the fluid through tubes in the opposite direction.

  • Compact heat exchangers:

These are special-purpose heat exchangers. They have a very large transfer surface area per unit volume of the exchanger.

Examples: Plate – fin, Flattened fin tube exchanger

04. Physical state of the fluids

On the basis of the physical state of the fluids, heat exchangers are classified as follows.

  • Condensers
  • Evaporators

Some other types of heat exchangers are as follows.

  • Double pipe heat exchangers are the simplest heat exchangers used in industries. This type of heat exchangers are cheap for both designing and maintenance. They suitable for small industries. Their efficiency is low.
  • Fixed tube liquid-cooled heat exchangers are used for marine and harsh applications.
  • Marine heat exchangers are also similar to non-marine heat exchangers. They are found in aboard ships. Many systems use heat exchangers onboard vessels. Regular maintenance of these heat exchangers is important. There are two primary types of marine heat exchangers. They are:
  • Plate type marine heat exchangers
  • Shell and tube marine heat exchangers
  • Plate heat exchangers
  • Plate and shell heat exchangers
  • Adiabatic wheel heat exchangers
  • Pillow plate heat exchangers
  • Fluid heat exchangers
  • Waste heat recovery units
  • Dynamic scraped surface heat exchangers
  • Phase – change heat exchangers
  • Direct contact heat exchangers
  • Microchannel heat exchangers
  • Spiral heat exchangers

Uses and applications of Heat Exchangers

They are widely used in space heating, refrigeration, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural gas processing and sewage treatment.

The classic example of a heat exchanger is found in an internal combustion engine.

Another example is the heat sink.

Mechanism of Heat Exchangers

Heat exchangers work as a result of heat naturally flows from higher temperature to lower temperature. Thus if a hot fluid and a chilly fluid area unit separated by heat conducting surface, heat is transferred from the new fluid to the cold fluid.

Two fluids of various temperatures area unit brought in to shut contact however area unit prevented from compounding by a physical barrier.

The temperature of the two fluids can tend to equalize.  By composing counter-current flow, it’s attainable for the temperature at the outlet of every fluid to approach the temperature at the water of the opposite.

The warmth contents area unit merely change from one fluid to the opposite and the other way around. No energy is supplemental or removed.

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Here we come to the end of part 1. Stay tuned for the next part. We will publish it here on our website www.macanytech.com. For email notification please subscribe our news alert service.

CONDUCTION ( Heat Transfer – 1 ) – Click here

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