Unit 17
An R22 TEV system is operating with a suction pressure of 76 psig and a suction line temperature of 58 degrees, the operating superheat is
13 degrees
An R134a TEV system is operating with a suction pressure of 22 psig and a suction line temperature at the bulb of 40 degrees. What is the actual operating superheat?
15 degrees
Which of the following is the standard saturated temperature drop in the suction line
2 degrees
An R22 refrigeration system requires a superheat of 8 degrees. If the evaporator saturation pressure in 43 psig, what should the bulb temperature be?
28 degrees
What is the evaporator saturation temperature of an R410a system with a low side pressure of 107 psig
30 degrees
When applied to systems with air cooled condensers, what is the lowest outside ambient temperature that balanced-port valves can typically work without head pressure control
35 degrees
The superheat is 12 degrees. What is the suction line temperature if the evaporator saturation temperature is 35 degrees
47 degrees
What are the parts of a distributer
A nozzle, conical divider, and multiple holes around the divider cone
Refrigerant leaves the metering device
As a saturated mixture of liquid and vapor
What is the advantage of balanced port TEVs
Balanced port valves can retain good control over a wider range of operating conditions
One operating advantage of fixed restriction metering devices over expansion valves is
Can be used with low starting torque compressor motors
two types of fixed metering devices
Capillary tubes and orifices
to increase evaporator pressure, the adjustment on the top of the automatic expansion valve should be turned
Clockwise
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
Advantages of orifice metering devices over capillary tubes.
Easier to change.
two categories of metering devices.
Fixed and modulating
A system with a loose TEV bulb will most likely
Have a low superheat
Which metering device used on flooded evaporators opens when more refrigerant condenses, enabling the refrigerant to flow into the evaporator
High side float
Fixed Valves
Is a rarely used metering device that can be used in applications where the load is constant and the operator is present to make manual adjustments
A Bi- Flow expansion valve
Is designed to allow refrigerant to flow in both directions
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
Which of the following problems can be caused by restrictions in the capillary tubes
Low suction pressure
Internally equalized thermostatic expansion valves
Measure the evaporator pressure through a passageway inside the valve
Thermostatic expansion valves respond to changes in
Pressure
Valves that open and close rapidly under the control of electronic circuits are
Pulse modulated valves
The operation of a thermostatic expansion valve can best be described as
Spring pressure= bulb pressure + evaporator pressure
Valves that use motors that move in tiny increments are
Stepper 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 oriifaces 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 is the purpose of TEV sensing bulbs with a limited, or gas charge
They produce a maximum operating pressure at high operating superheat
What 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
How can moisture in the refrigeration system affect the metering device
Water can freeze up at hte metering device, causing a restriction
A TEV with a dead power element will operate
With high superheat
Where is the suction line temperature read when adjusting a TEV?
as close a s possible to the TEV bulb
When the heat load on the evaporator increases, the automatic expansion valve
closes to keep the pressure from increasing
An overfeeding metering device can cause
high suction pressure, low superheat, and low subcooling
In an air conditioner with a fixed metering device, a decrease in the outdoor ambient causes
increased system superheat
Modulating metering devices
is designed to respond to changes in system operations.
The most common application for high side float controls are
large chillers
A restricted inlet screen on a TEV can cause
low suction pressure, high superheat, and high Subcooling
Technicians should be careful when brazing lines to evaporator coils that have pre-installed thermostatic expansion valves because the heat from brazing the lines can
overheat the TEV bulb and destroy the valve
automatic expansion valves are similar in operation concept to
pressure regulators
A device used to evenly distribute mixtures of liquid and vapor refrigerant leaving the TEV to multiple evaporator circuits is called
refrigerant distributor
capillary tube metering devices are commonly used today in?
window units
