UNIT 15

What is a bi-flow expansion valve?

A bi-flow expansion valve is designed to allow refrigerant flow in both directions. Some bi-flow valves meter the refrigerant in both directions while others allow unrestricted flow in the reverse direction.

How can a capillary tube that is restricted with debris be differentiated from a capillary tube that is restricted with ice?

A capillary tube that is plugged with debris will immediately pull into a very low suction pressure or vacuum. Ice normally only forms after the system has operated some and the evaporator starts getting cold.

Explain the difference between a fixed metering device and a modulating metering device.

A fixed metering device cannot change to respond to system changes, a modulating metering device can.

What is the purpose of a liquid distributor?

A liquid distributor is used to evenly distribute the mixture of liquid and vapor refrigerant leaving the TEV to multiple circuits.

What is the difference between the low side float and the high side float?

A low side float is inside the evaporator and lets refrigerant in when the liquid level falls. The high side float is in a separate chamber on the high side and lets refrigerant out when the liquid level rises.

What is the difference between a traditional TEV and a balanced port TEV?

A traditional TEV is affected by the pressure drop across the valve. Increased pressure difference tends to open the valve more. A balanced port valve is designed to counteract the effect of pressure difference across the valve, so the valve can retain good control at a much wider range of operating conditions.

What is the difference between a traditional liquid charged TEV bulb and a liquid cross-charged TEV bulb?

A traditional charge uses the same refrigerant as is in the system. A cross- charged bulb contains a different refrigerant than the system, so the pressure response of the bulb crosses the pressure response of the system at some point.

What are the two types of fixed metering devices?

Capillary tubes and orifices

Which way would the adjusting stem on an expansion valve be turned to increase the superheat?

Clockwise

Which way would the adjustment on top of an automatic expansion valve be turned to increase the evaporator pressure?

Clockwise

Which way would the adjusting stem on an expansion valve be turned to decrease superheat?

Counter clockwise

What is the difference between internally and externally equalized thermostatic expansion valves?

Internally equalized valves obtain the evaporator pressure through a passageway inside the valve. Externally equalized valves obtain evaporator pressure through a separate line that runs from the valve to the suction line.

What is the most common application of the high-side float?

Large chillers.

List types of modulating metering devices?

Low side float, high side float, automatic expansion valve, thermostatic expansion valve, electronically controlled step motor valve

How does a TEV with MOP work?

MOP valves have a gas charge. Gas charged bulbs have only a small amount of liquid in the bulb. When the valve reaches the MOP, maximum operating pressure, there is no more liquid in the bulb. This greatly limits the bulb opening pressure.

What does a thermostatic expansion valve try to regulate and keep stable?

Superheat

What types of problems could cause a TEV system to operate at too high a superheat?

System undercharge, liquid line refrigerant restriction, dead TEV power element, TEV plugged with debris from brazing

Explain the basic operation of the automatic expansion valve.

The automatic expansion valve is a refrigerant pressure regulator. It is controlled by a diaphragm that balances the evaporator pressure against an adjustable spring pressure.

List the advantages of the capillary tube.

The cap tube is simple, inexpensive, and cap tube systems can be used with low starting torque compressors.

Why must technicians be careful when brazing lines to evaporator coils that have thermostatic expansion valves?

The heat from brazing the lines can conduct through the lines to the TEV bulb and destroy the bulb by building up very high pressure in the power head.

What advantages are there to orifice type metering devices?

The orifice metering device is simple, inexpensive, and systems using an orifice can be used with low starting torque compressors. In addition, orifices are easy to change in order to meet different system requirements.

What happens to the refrigerant as it passes through the metering device?

The refrigerant drops in pressure and temperature as it passes through the metering device. A small portion of the refrigerant flashes to a vapor, cooling the remaining liquid.

Why is the automatic expansion valve not suited for systems with varying loads?

The response of the automatic expansion valve to load changes causes it to feed more refrigerant when the load decreases and less refrigerant when the load increases.

An R-22 TEV is set for 12°F superheat. The system is operating with a suction pressure of 76 psig and a suction line temperature at the bulb of 50°F. What is the actual operating superheat?

The saturation temperature for 76 psig R-22 is 45°F The superheat is 50°F - 45°F = 5°F superheat

An R-134a TEV is set for 8°F. Thesystem is operating with a suction pressure of 22 psig and a suction line temperature at the bulb of 50°F. What is the actual operating superheat?

The saturation temperature for R-134a at 22 psig is 25°F. The superheat is 50°F - 25°F = 25°F

How do electronically controlled step motor valves work?

The step motor does not spin, but moves in tiny steps to turn gears that position the valve pin. The step motor is controlled by an electronic control that senses temperature using thermistors.

What system operating characteristics would you expect from a system that has an oversized-orifice-type-metering device?

The suction pressure would be high. The head pressure would be low. The superheat and subcooling would both be low.

What system operating characteristics would you expect from a system that has a TEV with a loose sensing bulb?

The suction pressure would be high. The superheat and subcooling would be low. The system would cool some, but not well.

What system operating characteristics would you expect from a system that has a TEV with a dead power element?

The suction pressure would be very low, possibly in a vacuum. The superheat and subcooling would both be high. The system would not cool.

What system operating characteristics would you expect from a system that has a restricted-orifice-type-metering device?

The suction pressure would be very low, possibly in a vacuum. The superheat and subcooling would both be high. The system would not cool.

The electronic expansion valve is activated by a(n).

Thermistor

How do pulse modulated valves control flow?

They open and close rapidly in pulses. The overall amount of flow is controlled by how long the valve is open versus how long it is closed.

Explain how an air conditioner with a fixed metering device reacts to changes in indoor temperature.

When the outdoor temperature decreases, less refrigerant will flow through the metering device because of the reduced pressure difference across it.

Explain how an air conditioner with a fixed metering device reacts to changes in outdoor temperature?

When the outdoor temperature increases, more refrigerant will flow through the metering device because of the increased pressure difference across it.