313A Metering Devices 2

Question 1

What is an electronic expansion valve (EEV) and what advantages does it offer?

Accepted Answer

A stepper-motor or pulse-width-modulated valve controlled by a microprocessor that adjusts refrigerant flow based on multiple sensor inputs for precise superheat control

Answer

An EEV uses a stepper motor or pulse-width modulation controlled by a microprocessor to precisely regulate refrigerant flow. It reads multiple inputs (suction temperature, evaporator pressure, discharge temperature) to optimize superheat and system performance across all operating conditions, offering superior efficiency and control compared to TXVs.

Question 2

What is the external equalizer on a TXV and when is it required?

Accepted Answer

A small tube connected from the TXV diaphragm to the suction line after the evaporator, required when evaporator pressure drop exceeds 2°C (3°F) saturation temperature equivalent

Answer

An external equalizer is a small tube that connects the underside of the TXV diaphragm to the suction line at the evaporator outlet. It provides the valve with the true evaporator outlet pressure rather than inlet pressure. It is required when the evaporator has significant pressure drop (distributor-fed or multi-circuit coils) to maintain accurate superheat control.

Question 3

What causes a TXV to hunt (oscillate)?

Accepted Answer

Improper sensing bulb placement, oversized valve, liquid in the suction line, or rapid load changes causing the valve to overcorrect repeatedly

Answer

TXV hunting is a condition where the valve repeatedly overcorrects — opening too wide then closing too much — causing oscillating superheat and unstable operation. Common causes include: poorly mounted sensing bulb, oversized valve, external equalizer problems, liquid slugging affecting the sensing bulb, or a system with rapidly changing loads.

Question 4

How does a balanced-port TXV differ from a standard TXV?

Accepted Answer

It uses a balanced piston design that eliminates the effect of changing head pressure on valve operation, providing more stable control

Answer

A balanced-port TXV uses a dual-ported piston design that neutralizes the effect of inlet (head) pressure on the valve's operation. In a standard TXV, changing head pressure affects the closing force on the valve. The balanced design maintains consistent superheat control regardless of head pressure variations, improving performance in fluctuating ambient conditions.

Question 5

What is a distributor in an evaporator system and why is it important?

Accepted Answer

A manifold that divides refrigerant from the metering device into equal portions for each evaporator circuit, ensuring even heat absorption across the entire coil

Answer

A refrigerant distributor is a precisely engineered manifold installed between the metering device and a multi-circuit evaporator. It divides the liquid-vapour refrigerant mixture into equal portions for each circuit. Even distribution is critical — if one circuit receives more refrigerant, it will be starved while others are flooded, reducing overall coil performance.

Red Seal HVAC 313A Practice Test

Red Seal HVAC 313A Practice Test

313A Metering Devices 2