Shell and Tube Evaporator
1. Basic Definition
A Shell and Tube Evaporator is a type of heat exchanger designed to evaporate a liquid into its gaseous state (or cause boiling) by transferring heat from a hot source fluid. It is the functional opposite of a condenser.
2. How It Works
Heat is transferred from the hot fluid to the liquid to be evaporated. The liquid absorbs this energy as latent heat of vaporization, causing it to boil and change phase into a vapor, without a significant rise in temperature.
- Example (Chiller): A liquid refrigerant flows through the tubes. It absorbs heat from the warmer water flowing in the shell, causing the refrigerant to boil and evaporate. In the process, it extracts heat from the water, chilling it.
3. Common Configurations (Which side is which?)
The design is highly application-specific. The fluid that boils can be on either the shell or tube side.
- Liquid to be Evaporated on the Shell Side (Very Common):
- Shell Side: Liquid refrigerant boils/evaporates.
- Tube Side: Warm water (or process fluid) is being cooled.
- Why? The shell provides a large space for the liquid to boil and for the vapor to separate. This is a classic “Flooded Evaporator” design.
- Liquid to be Evaporated on the Tube Side (Common):
- Tube Side: Liquid refrigerant boils/evaporates.
- Shell Side: Hot water or steam is providing the heat.
- Why? Allows for better control and is often used in “Direct Expansion (DX) Evaporators” where a precise amount of refrigerant is metered into the tubes.
4. Key Components
Similar to a standard Shell and Tube heat exchanger, but with features to handle boiling and vapor-liquid separation:
- Shell: Contains the fluid to be evaporated or the heating medium.
- Tubes: Provide the heat transfer surface.
- Tube Sheets: Seal the ends.
- Baffles: Support tubes and direct flow. Their design is less about creating turbulence and more about promoting even flow distribution to prevent dry spots in flooded designs.
- Liquid/Vapor Separator (in some designs): A section at the top of the shell or a separate vessel to ensure only vapor exits the evaporator, preventing liquid carryover that could damage downstream equipment (like compressors).
5. Advantages
- High Efficiency: Excellent heat transfer due to the latent heat of vaporization.
- Robust Construction: Can handle high pressures and temperatures.
- Large Capacity: Suitable for very high evaporation duties.
- Handles Fouling: Can be designed for easy cleaning if the boiling fluid causes scaling or fouling.
6. Primary Applications
- Refrigeration & Chillers: The most common application. Used as the evaporator in large HVAC systems and industrial refrigeration cycles to chill water, brine, or other secondary coolants.
- Chemical Processing: Used as reboilers for distillation columns to vaporize the bottom liquid product. They are also used in concentration processes to evaporate solvents from solutions.
- Power Plants: In some systems, like those using Organic Rankine Cycles (ORC) for waste heat recovery.
- Food and Beverage Industry: For concentrating products like fruit juices, milk, or sugar solutions by evaporating water.