313A Evaporators & Expansion Devices

Question 1

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

Accepted Answer

To absorb heat from the refrigerated space or process fluid by evaporating the refrigerant

Answer

To compress refrigerant vapour to high pressure

Answer

To condense refrigerant vapour back into liquid

Answer

To separate oil from the refrigerant vapour

Question 2

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

Accepted Answer

To meter the flow of liquid refrigerant into the evaporator by maintaining a constant superheat at the evaporator outlet

Answer

To control compressor discharge pressure

Answer

To regulate condenser fan speed based on outdoor temperature

Answer

To bypass refrigerant around the compressor during startup

Question 3

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

Accepted Answer

A flooded evaporator is always filled with liquid refrigerant; a DX evaporator allows the refrigerant to fully evaporate before the outlet

Answer

A flooded evaporator uses water as a secondary coolant; a DX evaporator uses only refrigerant

Answer

A flooded evaporator operates at higher pressure; a DX evaporator operates at lower pressure

Answer

A flooded evaporator has a TXV; a DX evaporator has a fixed orifice

Question 4

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

Accepted Answer

Refrigerant overcharge causing the coil surface to reach 0°C or below, and moisture in the air freezing on the coil

Answer

High superheat causing dry spots on the coil surface

Answer

Low head pressure reducing the heat transfer rate

Answer

Oil logging in the coil reducing refrigerant flow

Question 5

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

Accepted Answer

A fixed-length, small-bore tube that creates a fixed pressure drop by restriction; suited for small, factory-charged systems with stable loads

Answer

A variable-flow tube that adjusts automatically to load changes; suited for large commercial chillers

Answer

A tube that measures refrigerant quality; suited for diagnostic use only

Answer

A tube used to equalize pressure during the off-cycle in heat pumps; not used for expansion

Question 6

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

Accepted Answer

To sense evaporator outlet pressure at the TXV diaphragm, compensating for pressure drop across the evaporator for accurate superheat control

Answer

To add refrigerant charge to the system remotely

Answer

To equalize refrigerant pressure between the high and low side during the off-cycle

Answer

To drain condensate from the evaporator drain pan