313A Evaporators & Expansion Devices 1 — Questions and Answers
Question 1: What is the primary function of the evaporator in a refrigeration system?
- To compress refrigerant vapour to high pressure
- To absorb heat from the refrigerated space or process fluid by evaporating the refrigerant (Correct answer)
- To condense refrigerant vapour back into liquid
- To separate oil from the refrigerant vapour
Correct answer: To absorb heat from the refrigerated space or process fluid by evaporating the refrigerant
The evaporator absorbs heat from the space or product being cooled as the liquid refrigerant boils and vaporizes at low pressure, providing the refrigeration effect.
Question 2: What is the purpose of a thermostatic expansion valve (TXV)?
- To control compressor discharge pressure
- To meter the flow of liquid refrigerant into the evaporator by maintaining a constant superheat at the evaporator outlet (Correct answer)
- To regulate condenser fan speed based on outdoor temperature
- To bypass refrigerant around the compressor during startup
Correct answer: To meter the flow of liquid refrigerant into the evaporator by maintaining a constant superheat at the evaporator outlet
A TXV senses the superheat at the evaporator outlet and modulates the valve opening to maintain a near-constant superheat, ensuring the evaporator is fully utilized while protecting the compressor from liquid slugging.
Question 3: What is the difference between a flooded evaporator and a dry expansion (DX) evaporator?
- A flooded evaporator uses water as a secondary coolant; a DX evaporator uses only refrigerant
- A flooded evaporator is always filled with liquid refrigerant; a DX evaporator allows the refrigerant to fully evaporate before the outlet (Correct answer)
- A flooded evaporator operates at higher pressure; a DX evaporator operates at lower pressure
- A flooded evaporator has a TXV; a DX evaporator has a fixed orifice
Correct answer: A flooded evaporator is always filled with liquid refrigerant; a DX evaporator allows the refrigerant to fully evaporate before the outlet
In a flooded evaporator, the coil is completely filled with boiling liquid refrigerant for maximum heat transfer; in a DX (direct expansion) evaporator, the refrigerant fully evaporates within the coil, exiting as superheated vapour.
Question 4: What causes frost to build up on an evaporator coil in a refrigeration system?
- Refrigerant overcharge causing the coil surface to reach 0°C or below, and moisture in the air freezing on the coil (Correct answer)
- High superheat causing dry spots on the coil surface
- Low head pressure reducing the heat transfer rate
- Oil logging in the coil reducing refrigerant flow
Correct answer: Refrigerant overcharge causing the coil surface to reach 0°C or below, and moisture in the air freezing on the coil
When the evaporator coil surface temperature falls to 0°C or below, moisture from the air passing over the coil freezes and accumulates as frost, which insulates the coil and reduces heat transfer.
Question 5: What is a capillary tube expansion device and what type of system is it most suited for?
- A variable-flow tube that adjusts automatically to load changes; suited for large commercial chillers
- A fixed-length, small-bore tube that creates a fixed pressure drop by restriction; suited for small, factory-charged systems with stable loads (Correct answer)
- A tube that measures refrigerant quality; suited for diagnostic use only
- A tube used to equalize pressure during the off-cycle in heat pumps; not used for expansion
Correct answer: A fixed-length, small-bore tube that creates a fixed pressure drop by restriction; suited for small, factory-charged systems with stable loads
A capillary tube is a simple, fixed-restriction expansion device that meters refrigerant flow by its length and diameter; it is used in small, sealed systems like household refrigerators and window air conditioners with relatively stable loads.
Question 6: What is the purpose of the equalizer line on an externally equalized TXV?
- To add refrigerant charge to the system remotely
- To sense evaporator outlet pressure at the TXV diaphragm, compensating for pressure drop across the evaporator for accurate superheat control (Correct answer)
- To equalize refrigerant pressure between the high and low side during the off-cycle
- To drain condensate from the evaporator drain pan
Correct answer: To sense evaporator outlet pressure at the TXV diaphragm, compensating for pressure drop across the evaporator for accurate superheat control
An external equalizer connects the TXV diaphragm case to the evaporator outlet, so the valve responds to the actual evaporator outlet pressure rather than the inlet pressure, compensating for pressure drop across the evaporator and maintaining accurate superheat control.
313A Evaporators & Expansion Devices 1
What is the primary function of the evaporator in a refrigeration system?