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Lesson 15 Heat Exchangers PowerPoint Presentation

Lesson 15 Heat Exchangers

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Lesson 15Heat Exchangers

- DESCRIBE the difference in the temperature profiles for counter-flow and parallel flow heat exchangers.
- DESCRIBE the differences between regenerative and non-regenerative heat exchangers.
- Given the temperature changes across a heat exchanger, CALCULATE the log mean temperature difference for the heat exchanger.
- Given the formulas for calculating the conduction and convection heat transfer coefficients, CALCULATE the overall heat transfer coefficient of a system.

Heat Exchangers

- Transfers thermal energy between fluids
- Common applications include boilers, fan coolers, cooling water heat exchangers, and condensers.
- Classifications
- Ordinary heat exchanger
- Single-phase
- Both of the fluids (cooled and heated) remain in their initial gaseous or liquid states
- Usually of the tube-and-shell type

- Two-phase
- Either of the fluids may change its phase during the heat exchange process
- Includes steam generator and main condenser of nuclear facilities

- Single-phase
- Regenerators
- Cooling towers

- Ordinary heat exchanger

Heat Exchanger Components

- Shell
- Tubes
- Relief Valves
- Vacuum Breakers

Parallel and Counter-Flow Designs

- Heat exchangers modes of operation and effectiveness are largely determined by the direction of the fluid flow within the exchanger.
- Most common arrangements for flow paths
- Counter-flow - the direction of the flow of one of the working fluids is opposite to the direction to the flow of the other fluid
- Parallel flow. - both fluids in the heat exchanger flow in the same direction

- More heat is transferred in a counter-flow arrangement than in a parallel flow heat exchanger.

Parallel-flow Design

- Advantageous when two fluids are required to be brought to nearly the same temperature.
- Disadvantages
- Large temperature difference at the ends causes large thermal stresses.
- The temperature of the cold fluid exiting the heat exchanger never exceeds the lowest temperature of the hot fluid.

Counter-flow Design - Advantages

- More uniform temperature difference between the two fluids minimizes the thermal stresses throughout the exchanger.
- Outlet temperature of the cold fluid can approach the highest temperature of the hot fluid (the inlet temperature).
- More uniform temperature difference produces a more uniform rate of heat transfer throughout the heat exchanger.

Parallel or Counter Flow

- In both parallel or counter-flow, heat transfer within the heat exchanger involves both conduction and convection.
- Process takes place over the entire length of the exchanger
- Temperature of the fluids as they flow through the exchanger is not generally constant
- Non- constant temperature causes variation in the rate of heat transfer along the length of the exchanger tubes

Non-Regenerative Heat Exchanger

- Non-regenerative application is the most frequent heat exchanger application
- Involves two separate fluids.
- One fluid cools or heats the other with no interconnection between the two fluids.
- Heat that is removed from the hotter fluid is usually rejected

Regenerative Heat Exchanger

- Typically uses the fluid from a different area of the same system for both the hot and cold fluids.
- The water returning to the primary system is pre-heated by the water entering the purification system.
- Minimizes the thermal stress in the primary system piping due to the cold temperature of the purified coolant being returned to the primary system.
- Reduces the temperature of the water entering the purification system prior to reaching the non-regenerative heat exchanger, allowing use of a smaller heat exchanger to achieve the desired temperature for purification.

- Primary advantage of a regenerative heat exchanger application is conservation of system energy (that is, less loss of system energy due to the cooling of the fluid).
- Can work in conjunction with non-regenerative heat exchanger
Example : the purification system of a reactor facility. (see next slide)

Heat Exchanger Failure Mechanisms and Symptoms

- Air Binding
- Tube Leaks
- Heat Transfer Reduction

Cooling Towers

- Cools the water of a steam power plant
- Steady-state process like the heat exchange in the ordinary heat exchanger.
- Large chambers loosely filled with trays or similar wooden elements of construction.
- Water to be cooled is:
- pumped to the top of the tower
- sprayed or distributed by wooden troughs.
- falls through the tower, splashing down from deck to deck.
- part of it evaporates into the air that passes through the tower.

- Enthalpy needed for the evaporation is transferred to the air,
- Air flow is either horizontal due to wind currents (cross flow) or vertically upward in counter-flow to the falling water.

- Water to be cooled is:

Log Mean Temperature Difference Application To Heat Exchangers

- To solve certain heat exchanger problems, a log mean temperature difference (LMTD or ΔTlm) must be evaluated before the heat removal from the heat exchanger is determined.
- Example

Overall Heat Transfer ExchangersCoefficient

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