Metering Devices Flashcards
Metering devices can be orifices of fixed size or ____, which can vary flow depending on loads.
valves
Metering devices are always in stalled between the liquid line and the ____.
evaporator
The main purpose of a metering device’s orifice is to produce ____.
a pressure drop
What are the most common types of metering devices.
Capillary Tube (cap tube) Metering Orifice Thermostatic Expansion Valve (TXV) Automatic Expansion Valve (AXV) Electronic Expansion Valve (EEV) Low-Side Float (LSF) High-Side Float (HSF)
Capillary tubes are used extensively in ____ systems.
refrigerators and window ac unit
(T/F) The limited amount of refrigerant allowed to pass through a metering device into the evaporator is at high pressure.
False
Low pressure due to the pressure drop
(T/F) The rate of flow through fixed metering devices is relatively constant when the compressor is running.
True
(T/F) Because modulating metering devices can change refrigerant flow based on load, they operate more efficiently than fixed metering devices.
True
For increased system efficiency, a capillary tube is usually positioned so that a portion of it is in contact with the ____.
suction line
An advantage of capillary tubes over modulating metering devices is that capillary tubes have no ____.
moving parts
Capillary tube failure will most likely occur when it is ____.
bent or crimped
When a capillary tube is replaced, it is good practice to also replace the ____.
liquid line filter-dryer
Piston-type metering orifices are most often used on ____ systems.
heat pump
The main value that influences the operation of a TXV is ____.
superheat
If the pressure in a sensing bulb is ____ the combined pressure from the evaporator and the spring, a TXV’s valve is forced open.
greater than
When a thin layer of fluid adheres to a solid, but the two substances are not mixed together, ____ occurs.
adsorption
Describe the difference between adsorption and absorption.
Adsorption occurs when an thin layer of fluid (gas or liquid) adheres to a solid, but are not mixed together.
Absorption when two substances are mixed and become one substance.
(T/F) By using an external equalizer, a TXV is able to provide the correct refrigerant flow, even when there are large pressure drops through the evaporator.
True
A sensing bulb charged with the same refrigerant as the system, some of which is always in a liquid state.
Liquid-charged
A sensing bulb charged with a different refrigerant than what is in the system, and some of it is always in a liquid state.
Liquid cross-charged
A sensing bulb charged with the same refrigerant as the system and which is completely vaporized at a predetermined temperature.
Gas-charged
A sensing bulb charged with a refrigerant that is different form that in the system and which is completely vaporized at the desired temperature.
Gas cross-charged
A sensing bulb that contains two substances: one is a non-condensing gas and the other is a solid.
Adsorption gas cross-charged
On systems with small-diameter horizontal suction lines, the sensing bulb should be mounted on the ____ of the suction line.
top
A sensing bulb must not be affected by the air or liquid being cooled. it should be wrapped in insulation so that only ____ affects the bulb.
suction line temperature
Before a sensing bulb is installed, use steel wool to clean ____.
the suction line and sensing bulb
(T/F) Once all of the refrigerant is a gas cross-charged sensing bulb is vaporized, the valve is at its maximum operating pressure.
True
If the orifice of a TXV is over sized, too much refrigerant will pass into the evaporator and cause the suction line to ____ before the sensing bulb can close the valve.
sweat or frost
**Will also hunt
If the superheat setting is increased to correct for an over sized TXV orifice, the evaporator will be ____ much of the time.
starved
(T/F) Automatic expansion valve (AXV) operation is based on its sensing bulb.
False
Work off of low side pressure
To maintain a constant pressure in the evaporator when the system is running, an AXV ____ when evaporator pressure begins to increase.
decreases refrigerant flow
