HVACR UNIT 15
Which of the following is the standard saturated temperature drop in the suction line?
2°F
Two types of fixed metering devices are:
Capillary tubes and orifices.
A system with a loose TEV sensing bulb would most likely:
Have a low superheat.
Thermostatic expansion valves respond to changes in:
Superheat
An R-22 refrigeration system requires a superheat of 8°F. If the evaporator saturation pressure is 43 psig, what should the bulb temperature be?
28°F
What is the evaporator saturation temperature of an R-410A system with a low-side pressure of 107 psig?
35°F
When applied to systems with air-cooled condensers, what is the lowest outside ambient temperature at which balanced-port valves can typically work without head pressure control?
35°F
The superheat is 12°F. What is the suction line temperature if the evaporator saturation temperature is 35°F?
47°F
What are the parts of a distributor?
A nozzle, conical divider, and multiple holes around the divider cone
Refrigerant leaves the metering device:
As a saturated mixture of liquid and vapor
Where is the suction line temperature read when adjusting a TEV?
As close as possible to the TEV bulb
What is the advantage of balanced-port TEVs?
Balanced port valves can retain good control over a wider range of operating conditions.
The operation of a thermostatic expansion valve can best be described as:
Bulb pressure = evaporator pressure + spring pressure.
One operating advantage of fixed-restriction metering devices over expansion valves is:
Can be used with low starting torque compressor motors.
Technicians should be careful when brazing lines to evaporator coils that have preinstalled thermostatic expansion valves because the heat from brazing the lines:
Can overheat the TEV bulb and destroy the valve.
When the heat load on the evaporator increases, the automatic expansion valve:
Closes to keep the pressure from increasing.
Besides distributing the refrigerant evenly, a refrigerant distributor also:
Creates a pressure drop across the distributor.
The system capacity of an air conditioner with a fixed metering device:
Decreases when the indoor wet bulb temperature rises.
As refrigerant passes through the metering device, it:
Drops in pressure and temperature.
The two general categories of metering devices are:
Fixed and modulating.
Which of the following is a rarely used metering device that can be used in applications where the load is constant and an operator is present to make manual adjustments?
Hand-operated expansion valves
Which metering device used on flooded evaporators opens when more refrigerant condenses,enabling the refrigerant to flow into the evaporator?
High-side float
In an air conditioner with a fixed metering device, a decrease in the outdoor ambient causes:
Increased system superheat.
A biflow expansion valve:
Is designed to allow refrigerant flow in both directions.
The most common application of the high-side float controls is:
Large chillers.
Which of the following problems can be caused by restrictions in the capillary tubes?
Low suction pressure
A restricted inlet screen on a TEV can cause:
Low suction pressure, high superheat, and high subcooling.
Which type of metering device used on flooded evaporators opens to admit more liquid refrigerant into the evaporator when the liquid level in the evaporator drops?
Low-side float
Internally equalized thermostatic expansion valves:
Measure the evaporator pressure through a passageway inside the valve.
Which type of metering device is able to respond to changes in system operations?
Modulating metering devices
Automatic expansion valves are similar in operating concept to:
Pressure regulators
Valves that open and close rapidly under the control of electronic circuits are:
Pulse-modulated valves.
A device used to evenly distribute mixtures of liquid and vapor refrigerant leaving the TEV to multiple evaporator circuits is called a:
Refrigerant distributor
Valves that use motors that move in tiny increments are:
Step motor valves.
Thermostatic expansion valves are designed to maintain a constant:
Superheat
Automatic expansion valves are good at maintaining a constant:
Temperature
Why is it dangerous to drill out orifices in metering devices?
The orifices are conical in shape.
Why do standard-design thermostatic expansion valves need a minimum pressure drop to operate correctly?
The pressure difference across the pin acts as an opening force
What two pieces of information are necessary to measure the TEV superheat?
The suction line temperature and pressure
What senses the line temperature in electronic expansion valves?
Thermistors
Which of the following metering devices can respond to changes in system conditions?
Thermostatic expansion valve
What advantages do orifice-type metering devices have over capillary tubes?
They are easy to change.
What is the purpose of TEV sensing bulbs with a limited, or gas charge?
They produce a maximum operating pressure at high operating superheat.
Which of the following best describes the purpose of a distributor?
To equally distribute refrigerant
To increase superheat on a thermostatic expansion valve, the adjusting stem should be:
Turned clockwise
To increase the evaporator pressure, the adjustment on top of an automatic expansion valve should be:
Turned clockwise.
How can moisture in the refrigeration system affect the metering device?
Water can freeze up at the metering device, causing a restriction.
Capillary tube metering devices are commonly used today in:
Window units.
A TEV with a dead power element will operate:
With a high superheat.
