Heat Exchanger

Heat Exchanger

• device for transferring heat from one fluid to another

• fluids are separated by a solid wall so that they never mix

Examples of where Heat Exchangers are used:

• Refrigeration

• Air conditioning

• Petroleum refineries

• Power stations

• Chemical processing

How does a Car Radiator work?

hot water is cooled by flow of air over the surface of the radiator

Principle (Concept) of Heat Transfer

Heat transfer involves the movement of heat from one location to another for heating and cooling

Heat (Thermal Energy)

• form of energy that is present in all substances

• It exists because of the vibration of the molecules within the substance

  • the greater the vibration of the molecules, the greater the energy.

Important measurements of Heat Transfer

In a heat transfer process the most important measurements are those relating to:

• the temperature of a substance, and

• to the heat energy of a substance

Temperature

• result of the amount of vibration of the molecules within a substance

  • (the kinetic energy of the molecules)

• Celsius (°C) or Kelvin (°K)

True

(T/F) Heat energy and Temperature are not the same thing

False

(T/F) a 15 degree rise in the temperature of the cooling water does means a 15 degree drop in the temperature of the liquid we are cooling

Joule (J)

Unit of energy (SI) used for heat energy (and all forms of energy)

Heat Transfer

• movement of heat from one substance to another

• from one place within a substance to another place within the same substance.

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What happens when the heat transfer process is done?

final temperature of each substance will depend on the duration and rate of heat transfer

Unit of Rate of Heat Transfer

Joules/second → Watt (W)

Three

1. Conduction (solid)

2. Convection (liquids)

3. Radiation (gas)

Two ways of heat transfer in heat exchangers

Conduction

Convection

Conduction

• mainly to solids

• transfer of heat from one part to another part of the same substance

• from one substance to another substance that is in direct contact with it

What are factors affecting the rate of conduction?

• Temperature difference

• Area of contact

• Thermal conductivity

• Thickness

Temperature Difference

• Heat flows as a result of (_______) (∆T)

True

(T/F) The greater the temperature difference the greater the conduction

Area of Contact

heat transfer rate increases with an increase in (_____)

similar to increasing number of traffic lines on road, increasing the car carrying capacity.

Thermal Conductivity

The greater the (_____) of a substance the greater the rate of heat conduction.

Thickness

• this adds a resistance to the transfer of heat

• The temperature difference across the material may remain the same but the increasing the (_____) means that the temperature difference between adjacent molecules is less and the rate of heat conduction is therefore lower.

Convection

• mainly fluids

• This involves the transfer of heat by mixing the warmer portions of the fluid with the cooler portions

2 types of Convection

• Natural Convection

• Forced Convection

Natural Convection

• this occurs as a result of temperature differences within the fluid

  • the temperature differences cause differing fluid densities — mixing as less dense fluid rises

Forced Convection

• occurs when mechanical agitation increases mixing of fluid

• results in a much higher heat transfer rate because of the higher fluid velocities

Radiation

• where heat rays transfer the heat.

• These rays do not need a substance to pass through

• easily passes through a vacuum

False

(T/F) A smaller temperature difference increases the rate of heat transfer

thinner, thick

(fill in the blanks) Heat travels faster through _____ materials. However, _____ walled pipes may be needed to withstand high pressures.

allows more of the hot fluid to be close to the cold fluid for heat transfer to take place at a higher rate

What is the purpose of a larger surface area?

As the rate of flow of the fluid increases, the rate of heat transfer increases

Relationship of flow rate and rate of heat transfer

Conductors

(fill in the blanks) Good _________ transfer heat at a faster rate.

Scale

• build up of impurities

• poor conductor

Build-up of scale

slows the rate of heat transfer

True

(T/F) The extra thickness of material also restricts heat conduction and flow through the pipe

Most common Heat Exchange Media

• Air (cooling)

• Water (cooling)

• Steam (heating)

• Gas (cooling)

Main Types of Heat Exchangers

• Plate Heat Exchanger

• Air Cooled Heat Exchanger

• Double Pipe Heat Exchanger

• Shell & Tube Heat Exchanger

Plate Heat Exchanger

• made up of a series of thin, corrugated plates clamped together

• Each plate contains holes for the passage of the two exchanger fluids

• The corrugated plates are separated and sealed by gaskets, so that fluid can flow through the exchanger without mixing

Corrugated Plates

• used to promote turbulence in the flow path

• improves rate of heat transfer

• The plates can be arranged to give true counter current flow

• faster heat transfer rate (3-5x)

  • due to the greater turbulence caused by the corrugations on the plates.

• Gasket leaks will be to the outside and will be easily visible.

• takes up much less floor space (50 to 80% less) than a shell and tube exchanger of similar performance.

• There is greatly reduced servicing time due to the ease of inspection

• the reduction in disassembly and assembly time.

• Performance can be changed readily by adding or removing plates.

What are the advantages of Plate Heat Exchanger?

• The exchanger will clog easily when used with dirty fluids.

• The operating pressure is limited to 1000kPa. (10 bar)

• It is not suitable for condensing vapour (e.g. steam)

What are the disadvantages of Plate Heat Exchanger?

Air Cooled heat Exchanger (Aerial Coolers)

• the fluid is cooled or condensed by circulating inside the tubes while forcing atmospheric air across the coil sections.

2 Types of Aerial Coolers

1. Induced Drafts

2. Forced Drafts

Induced Draft

• Fan is installed on top

• inducing air across a horizontal bank of tubes located below the fan and discharging the air upwards

• The tubes carry the fluid in a closed circuit and the ends are enclosed in boxes, called headers, one for the discharge and another for the inlet.