Evaporators & Condensers
Evaporator types, condenser design, approach temperature, flooded systems, and defrost methods.
- Identify evaporator types and their applications
- Explain flooded vs direct-expansion evaporator operation
- Describe condenser design and approach temperature
- Compare hot gas, electric, and off-cycle defrost methods
- Explain EPR and head pressure control valve functions
Lección 1
Evaporator Types & Flooded Systems
Evaporator Classification
Evaporators are classified by their construction type and how refrigerant is fed. The two main feed methods are direct expansion (DX) and flooded. Understanding the difference is critical for the Red Seal exam.
Direct Expansion (DX)
Feed method: Metering device controls flow
Refrigerant state: Enters as low-pressure liquid/vapor mix
Superheat: Required at outlet (6-12F typical)
Oil return: Good - refrigerant velocity carries oil
Flooded Evaporator
Feed method: Float valve maintains liquid level
Refrigerant state: Shell is mostly liquid
Superheat: None - liquid always present
Oil return: Poor - requires oil separator
DX Evaporator Operation
In a direct expansion evaporator, the metering device (TXV or EEV) feeds a controlled amount of refrigerant into the evaporator. The refrigerant absorbs heat from the air or fluid passing over the coil and fully evaporates before reaching the outlet. The superheat measured at the evaporator outlet confirms all liquid has boiled off, protecting the compressor from liquid slugging.
Flooded Evaporator Operation
A flooded evaporator maintains a pool of liquid refrigerant inside a shell. The tubes carrying the fluid to be cooled pass through this liquid pool. Because the tubes are always surrounded by liquid refrigerant, heat transfer efficiency is significantly higher than DX systems. A float valve maintains the liquid level.
Exam Fact
Flooded evaporators have higher heat transfer efficiency than DX evaporators because the tubes are always submerged in liquid refrigerant. However, they require more refrigerant charge and an oil recovery system.
Evaporator Construction Types
| Type | Application | Medium Cooled |
|---|---|---|
| Bare tube | Walk-in coolers | Air |
| Finned tube | Forced-air cooling | Air |
| Plate | Beverage coolers | Liquid contact |
| Shell-and-tube | Chillers | Water/brine |
| Shell-and-coil | Small chillers | Water |
DX evaporators use a metering device and require superheat at the outlet. Flooded evaporators maintain a liquid pool for higher efficiency but need more charge and oil recovery. The exam tests the difference between these two feed methods.