Evaporators & Expansion Devices Flashcards

What is the primary function of the evaporator in a refrigeration system?

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.

What is the purpose of a thermostatic expansion valve (TXV)?

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.

What is the difference between a flooded evaporator and a dry expansion (DX) evaporator?

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.

What causes frost to build up on an evaporator coil in a refrigeration system?

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.

What is a capillary tube expansion device and what type of system is it most suited for?

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.

What is the purpose of the equalizer line on an externally equalized TXV?

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.