1Prepared by: Kow Kien Woh

EP 315 Heat Transfer

Chapter 7: Heat exchangers

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Double pipe

1. The simplest type of heat exchanger consists of two concentric pipes of different diameters, called the double-pipe heat exchanger.

2. One fluid in a double-pipe heat exchanger flows through the smaller pipe while the other fluid flows through the annular space between the two pipes.

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Double pipe

3. Two types of flow arrangement are possible in a double-pipe heat exchanger: in parallel flow, both the hot and cold fluids enter the heat exchanger at the same end and move in the same direction.

4. In counter flow, the hot and cold fluids enter the heat exchanger at opposite ends and flow in opposite directions.

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Cross flow

1. Cross-flow exchangers are commonly used in air or gas heating and cooling applications.

2. Gas is forced across a tube bundle, while another fluid is used inside the tubes for heating or cooling purposes.

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Double pipe

https://www.youtube.com/watch?v=iIRbhZY8MpE

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Shell & tube

1. The most common type of heat exchanger in industrial applications is the shell-and-tube heat exchanger.

2. It contains a large number of tubes (sometimes several hundred) packed in a shell with their axes parallel to that of the shell.

3. Heat transfer takes place as one fluid flows inside the tubes while the other fluid flows outside the tubes through the shell.

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Shell & tube (S&T)

4. Baffles are usually installed to increase the convection coefficient of the shell-side fluid by inducing turbulence and a cross-flow velocity component relative to the tubes. In addition, the baffles physically support the tubes, reducing flow-induced tube vibration.

Baffle

Baffle

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Shell & Tube (assembly)

https://www.youtube.com/watch?v=hxhB3k0vh2g

https://www.youtube.com/watch?v=RXeB8BQORNo

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Numbers of S&T1. Shell-and-tube heat

exchangers are further classified according to the number of shell and tube passes involved.

2. Heat exchangers in which all the tubes make one U-turn in the shell, for example, are called one-shell-pass and two-tube-passes heat exchangers.

3. Likewise, a heat exchanger that involves two passes in the shell and four passes in the tubes is called a two-shell-passes and four-tube-passes heat exchanger.

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Shell & Tube (flow pattern)

https://www.youtube.com/watch?v=iIRbhZY8MpE

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Example: 1S-2T

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Plate & frame

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Plate & frame1. Plate & frame heat exchanger

consists of a series of plates with corrugated flat flow passages.

2. The hot and cold fluids flow in alternate passages, each cold fluid stream is surrounded by two hot fluid streams, resulting in very effective heat transfer.

3. Advantage: exchangers can grow with increasing demand for heat transfer by simply mounting more plates.

4. They are well suited for liquid-to-liquid heat exchange applications, provided that the hot and cold fluid streams are at about the same pressure.

Source: http://www.guntner.co.uk/thermowave_plate_heat_exchangers_how_it_works.html

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Plate & frame

https://www.youtube.com/watch?v=Jv5p7o-7Pms

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Evaporator1. If a vaporizing

exchanger is used for evaporation of water or aqueous solution, it is called evaporator.

2. A chemical evaporator is usually used to concentrate a chemical solution by evaporation of solvent or water.

3. The heat of vaporization is usually supplied by heating steam in tubes.

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Natural circulation evaporator

1. Liquid which was not vaporized is usually recombined with fresh feed and circulate by to the evaporator.

2. Recirculation is used to increase the liquid velocity in tubes to prevent buildup of crystals/deposits/fouling.

3. Recirculation can be accomplished by natural hydrostatic head OR force by pump.

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Forced circulation evaporator

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Forced circulation evaporator

https://www.youtube.com/watch?v=22W753joAnA

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Falling film evaporator1. In falling film evaporator, liquid feed to be evaporated

flows downwards by gravity as a continuous film. 2. The fluid will create a film along the outer walls of tubes.3. Heat is supplied by steam flows inside of tube.4. As downward vapor velocity increases, increasing the

shear force acting on the liquid film and therefore also the velocity of the solution. The result is progressively thinner film resulting in increasingly turbulent flow. The combination of these effects allows very high heat transfer coefficients.

5. Advantage of the falling film evaporator is the very short residence time of the liquid making it ideal for heat-sensitive products such as milk, fruit juice, pharmaceuticals.

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Falling film evaporator

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Falling film evaporator

https://www.youtube.com/watch?v=3T8Km9BYHeg

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1. In the some applications, the vapor leaving evaporator is condensed before discharge to the plant.

2. Instead of condensing the vapor, the hot vapor can be channeled to second evaporator as heat source to futher concentrate the liquid leaving the first evaporator as follow:

Multi-effect evaporator

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3. This is called multi-effect evaporator where every evaporator is considered as one effect. E.g. triple effect means three evaporators connected in same manner.

4. When the liquid feed flows in same direction as vapor, it is termed forward feed. If the liquid is flowing in opposite direction of vapor, it is called backward feed.

Multi-effect evaporator

Multi-effect evaporator5. Forward feed is more energy efficient. Since

vacuum is usually maintained at the last effect, the liquid flows by itself from effect to effect without pumping. In addition, ”free” evaporation occurs in succeeding effect because saturation temperature reduced at lower pressure.

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Multi-effect evaporator

6. The viscosity of liquid increases causes low heat transfer in the last effect in forward feed.

7. When backward feed is used, the most viscous liquid is heated by steam in the first effect to reduce its viscosity. However, liquid must be heated in each effect and pumped from effect to effect.

8. The steam cost will be less in forward feed if the feed liquid is at higher or about the operating temperature of the first effect.

9. The steam cost will be less in backward feed if the feed liquid is cold.

10.The greater number of effects, the greater the reduction in operating cost.

11.However, this is offset by installation and maintenance cost. Hence, there will be an optimum number of effects when total cost is plotted against number of effects.

Condenser1. Condenser is heat exchanger which the

fluids is cooled and condenses to liquid as it flows through.

2. The direct-contact condenser is one in which the coolant is brought into contact with the vapor. It has the advantage of low cost and simplicity of mechanical design, but its use is restricted to those applications in which mixing of the vapor and coolant is permissible.

3. Examples of direct contact condensers are spray condenser, tray condenser, packed column .

4. In case the vapor are not allow to contact directly with the coolant, shell-and-tube condensers are used.

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Spray Condenser

Prepared by: Kow Kien Woh

The coolant is sprayed, using nozzles, into a vessel to which the vapor is supplied. It is important that the spray nozzles and vessel are designed to produce a fine spray of liquid (to give a large interfacial area for heat transfer), and a long enough residence time of liquid droplets in the vessel.

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Tray Condenser

Prepared by: Kow Kien Woh

Coolant is directed to flow over a series of trays in a column. The vapor is supplied to the bottom of the column. It has the advantage of countercurrent flow of vapor and coolant, though care must be taken to avoid flooding. (Flooding is an unstable condition when the vapor flow is such that the downward flow of condensate is interrupted and rises up.)

Shell & tube condenser

Source: http://www.expertsmind.com/topic/large-steam-system-condensers/single-pass-condensers-99965.aspx