Types of Evaporators
Evaporators are classified based on two main criteria: How they are constructed and
- 2) How the refrigerant is fed. The following flowchart outlines the primary classification system:
1. Classification by Construction & How Fluid is Cooled
A. Shell and Tube Evaporator
This is the most common type in water chiller systems. It consists of a cylindrical shell (a large pressure vessel) with a bundle of tubes inside.
- How it works: One fluid flows through the tubes, and the other flows over the tubes within the shell. Heat exchange occurs through the tube walls.
- Sub-types:
- Flooded Evaporator: The refrigerant is in the shell, and the water to be chilled is in the tubes. The shell is “flooded” with liquid refrigerant, which boils on the outside of the tubes as it absorbs heat from the water inside the tubes.
- Dry Expansion (DX) Evaporator: The opposite of flooded. The refrigerant is fed through the tubes, and the water to be chilled is in the shell. As the refrigerant travels through the tubes, it gradually evaporates (“dries out”).
B. Plate Type Evaporator
Uses a series of thin, corrugated metal plates stacked together. Alternating plates create channels for the refrigerant and the fluid to be cooled.
- How it works: Heat is transferred through the thin plates between the two fluids. They are very compact and efficient due to their large surface area.
- Applications: Common in modern, compact chillers and heat pumps.
C. Finned-Tube (Air Cooled) Evaporator
Used in systems where air is the medium being cooled directly.
- How it works: Tubes carrying refrigerant are passed through a series of fins. The fins greatly increase the surface area for heat transfer. A fan blows air across the fins and tubes, causing the refrigerant inside to evaporate and cool the air.
- Applications: Air conditioners, heat pumps, and refrigeration display cases.
Classification by Refrigerant Feed Method
This is a critical operational distinction, as highlighted in the flowchart.
A. Flooded Evaporator
- Operation: A constant liquid refrigerant level is maintained in the evaporator (typically a shell-and-tube type) by a float valve or a low-pressure receiver. Only about 70-80% of the tubes are submerged. The refrigerant boils on the outside of the tubes. The vapor generated travels to the top of the shell and then to the compressor.
- Advantages: Very high heat transfer efficiency because the entire tube surface is wetted.
- Disadvantages: Requires more refrigerant charge. Risk of liquid carryover to the compressor if not designed properly. More complex and expensive.
- Use Case: Large industrial and commercial chillers where efficiency is the top priority.
. Dry Expansion (DX) Evaporator
- Operation: A thermostatic expansion valve (TXV) meters just enough liquid refrigerant into the evaporator tubes so that it is fully evaporated by the time it reaches the end of the coil. The coil operates “dry,” meaning it is not flooded.
- Advantages: Simpler design, requires less refrigerant charge, lower cost.
- Disadvantages: Lower heat transfer efficiency compared to flooded type because not all the tube surface is used effectively.
- Use Case: Most common type, used in smaller commercial and virtually all residential AC systems, refrigerators, and smaller chillers.
C. Falling Film Evaporator
- Operation: A variation of the flooded type. Liquid refrigerant is pumped to the top of the tube bundle and is distributed over the tubes, “falling” like a film down the outside of them. The evaporation happens from this thin film.
- Advantages: Extremely efficient, requires even less refrigerant charge than a standard flooded evaporator.
- Disadvantages: Highly complex design and distribution system.
- Use Case: High-efficiency chillers and in industries where minimizing refrigerant charge is critical.