Unit 15

One operating advantage of fixed restriction metering devices over expansion valves is

(that they) can be used with low starting torque compressor motors

An R134a TXV system is operating with a suction pressure of 22 psig and a suction line temperature at the bulb of 40 degrees F. What is the actual operating superheat?

15 degrees F P/T chart says: 22 psig = (approx) 25 degrees F 40 degrees F - 25 degrees F = 15 degrees F

An R22 TXV system is operating with a suction pressure of 70 psig and a suction line temperature at the bulb of 59 degrees F. The actual operating superheat is:

19 degrees F P/T chart says: 70 psig = (approx.) 40 degrees F 59 degrees F - 40 degrees F = 19 degrees F

Which is the standard saturation temperature drop in the suction line?

2 degrees F

an r22 refrigeration system requires a superheat of 8°F. If the evaporator saturation pressure is 43 psig, what should the bulb temperature be?

28°F P/T chart says: 43.1 psig = 20 degrees F 20 degrees F + 8 degrees F (superheat) = 28 degrees F

What is the evaporator saturation temperature of an R410a system with a low side pressure of 107 psig?

35 degrees F

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°F

The superheat is 12 degrees F. What is the suction line temperature if the evaporator saturation temperature is 35 degrees F?

47 degrees F 35 degrees F + 12 degrees F = 47 degrees 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

What is the advantage of balanced port TXVs?

Balanced port valves can retain good control over a wide range of operating conditions

the operation of a thermostatic expansion valve can best be described as

Bulb pressure = evaporator pressure + spring pressure

Technicians should be careful when brazing lines to evaporator coils that have pre-installed thermostatic expansion valves because the heat from the lines:

Can overheat the TXV bulb and destroy the valve

Two types of fixed metering devices are

Capillary tubes and orifices

When the heat load on the evaporator increases, the automatic expansion valve:

Closes to keep the pressure from increasing

The system capacity of an air conditioner with a fixed metering device:

Decreases when the indoor wet bulb temperature rises

The two general categories of metering devices are:

Fixed and modulating

A system with a loose TEV sensing bulb would most likely

Have a low superheat

An overfeeding metering device can cause?

High suction pressure, low superheat, and low subcooling

Which type of metering device, used on flooded evaporators, opens to admit more liquid refrigerant into the evaporator when the liquid 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

Which type of metering device is able to respond to changes in system operations?

Modulating metering devices

Valves that open and close rapidly under the control of electronic circuits are:

Pulse modulated valves

Valves that use motors that move in tiny increments are

Stopper motor valves

Thermostatic expansion valves are designed to maintain a constant:

Superheat

Thermostatic expansion valves respond to changes in

Superheat

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 TXV superheat?

The suction line temperature and pressure

what senses the line temperature in electronic expansion valves?

Thermistors

what advantages do orifice type metering devices have over capillary tubes?

They are easy to change.

What is the purpose of TXV sensing bulbs with a limited or gas charge?

They produce a maximum operating pressure at high operating superheat

To increase the evaporator pressure, the adjustment on top of an automatic expansion valve should be

Turned clockwise

capillary tube metering devices are commonly used today in

Window units

A TXV with a dead power element will operate

With a high superheat

Where is the suction line temperature read when adjusting a TXV?

as close as possible to the TXV bulb

Besides distributing the refrigerant evenly, a refrigerant distributor also

creates a pressure drop across the distributor

As refrigerant passes through the metering device it

drops in pressure and temperature

which of the following is rarely used metering device that can be used in applications where the load is constant and an operator is present to make manual adjustments?

fixed valves

In an air conditioner with a fixed metering device, a decrease in the outdoor ambient causes:

increased system superheat

A bi-flow expansion valve:

is designed to allow refrigerant flow in both directions

the most common application of the high side float control is:

large chillers

which metering device used on flooded evaporators, opens when more refrigerant evaporates, enabling the refrigerant to flow into the evaporator?

low side float

a restricted inlet screen on a TXV can cause

low suction pressure, high superheat, and high subcooling

Automatic expansion valves are similar in operating concept to:

pressure regulators

a device used to evenly distribute mixtures of liquid and vapor refrigerant leaving the TXV to multiple evaporator circuits is called a:

refrigerant distributor

automatic expansion valves are good at maintaining a constant

temperature

which metering device can respond to changes in system conditions?

thermostatic expansion valve

which 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 the metering device, causing a restriction