2nd Year HVAC - Chapter 21 Metering Devices
Bellows-Type AXV
In this type of valve, a flexible bellows is linked to a needle valve. Atmospheric pressure is inside the bellows, evaporator pressure is pushing against the bellows, and the adjusting screw spring force combines with the atmospheric and liquid line pressure to open the valve.
False
It is only the float chamber, not the evaporator, that is flooded in a high-side float system.
False
Liquid cross-charged sensing bulbs use same refrigerant as the system.
Low-Side Float (LSF)
Maintains a constant level of liquid refrigerant in the evaporator by adjusting flow through its valve based on the level of liquid refrigerant in the evaporator.
Sensible Heat
Measurable temperature
Top
ON systems with small diameter horizontal suction lines, the sensing bulb should be mounted on the __________ of the suction line.
Ball valve
Often used instead of a needle in TXV's designed for applications where the refrigerant capacity in the evaporator is critically important.
Heat Pump
Piston-type metering devices are most often used on __________ systems.
Sweat back/Frost back
Refers to the accumulation of condensation of the suction line due to the difference in the line temperature and ambient temperature.
PWM Solenoid EEV
Refrigerant metering devices that use a pulse width modulation signal to control the operation of a solenoid valve for variable refrigerant flow. In this type of valve, a control signal rapidly opens and closes a solenoid valve at different speeds in order to vary refrigerant flow.
True
Since altitude can affect atmospheric pressure, an AXV needs to be adjusted to account for this.
Orifice
Small opening
Evaporator
The LSF controls the flow of refrigerant into the __________.
Cools
The TXV should reduce the flow rate as the evaporator __________.
Longer
The __________ the capillary tube, the greater the pressure drop.
Smaller
The __________ the diameter, the greater the pressure drop.
1/4
The adjusting nut or screw on a TXV is sensitive and should not be turned more than __________ turn while the unit is operating.
Stable
The best superheat setting for an evaporator, called the minimum __________ signal (MSS) setting, is the point that causes the least sensing bulb temperature change while the system is running.
Minimum Stable Signal (MSS)
The best superheat setting/signal for an evaporator is the point that causes the least sensing bulb temperature change while the system is running.
-Orifice size -Pressure difference between the high side and low side -Temperature and condition of the refrigerant in the liquid line
The capacity of a TXV varies according to these characteristics:
Starving
The condition in which liquid refrigerant is present in only part of the evaporator.
Superheat
The difference in temperature between the evaporator inlet and the evaporator outlet.
Flooded
The evaporators in __________ systems have a reservoir that is always filled with liquid refrigerant.
Superheat
The function of an EEV is to maintain __________ at a desired setting.
False
The higher the temperature in a low-side float flooded system, the lower the low-side pressure.
True
The longer the capillary tube, the greater the pressure drop.
Pressure drop
The main purpose of a metering device's orifice is to produce a __________.
Superheat
The main value that influences the operation of a TXV is __________.
-Capillary tube -Metering orifice -Thermostatic expansion valve (TXV) -Automatic expansion valve (AXV) -Electronic expansion valve (EEV) -Low-side float (LSF) -High-side float (HSF)
The most common types of metering devices are:
Low-Side Pressure
The primary purpose of an AXV is to maintain constant __________.
True
The rate of flow through fixed metering devices is relatively constant when the compressor is running.
Flash Gas
The vapor that is produced by a drop in pressure.
Decreases
To maintain a constant pressure in the evaporator when the system is running, an AXV __________ refrigerant flow when evaporator pressure begins to increase.
Liquid Slugging
Too much refrigerant in the evaporator can result in liquid refrigerant being drawn into the compressor.
Clockwise
Turning the TXV adjuster __________ increases spring tension.
Counter-clockwise
Turning the TXV adjuster __________ reduces spring tension.
Filter-drier
Typically installed at the coolest location that is outside of the refrigerated space and upstream from the metering device.
-Orifice size -Pressure difference between high & low side -Temperature & condition of the refrigerant in the liquid line
What affects the capacity of a TXV?
-Tube length -Inside diameter -Number of turns
What are the three variables that affect capillary tube performance?
Capillary Tube
What is the simplest type of metering device?
Distributer
When a TXV is feeding refrigerant into an evaporator with multiple circuits, a __________ should be installed.
Liquid Line Filter-Drier
When a capillary tube is replaced, it is good practice to also replace the __________.
Adsorption
When a thin layer of fluid adheres to a solid, but the two substances are not mixed together.
High-side and Low-side pressures equalize.
When capillary tube systems cycle off, __________.
Initialization
When first powered, the controller will send the valve more steps than the valve can use. It is done to ensure that the valve is fully closed and at its zero open position.
Most
When the TXV is adjusted to give a normal 10˚F superheat, liquid refrigerant will fill __________ of the evaporator.
Longer
When the capillary tube has a large diameter, the length of the capillary tube must be __________ to serve the same purpose.
Reverses
When the refrigerant flow __________ for the heating mode, pressure is applied to the other side of the piston, causing it to slide to the center of the housing. This lifts the piston from its seat, and allows refrigerant to flow through the flutes in the piston.
Absorption
When two substances are mixed and become one substance.
In the TXV
Where is the pressure limiting element?
Oversized
With an __________ valve orifice, the TXV will hunt, or surge.
Undersized
With an __________ valve orifice, the evaporator will be starved regardless of the superheat setting.
Capillary Tubes
__________ do not have check valves or directional control valves.
Automatic Expansion Valve (AEV)
(aka a Pressure-controlled expansion valve) is a metering device that operates based on low-side pressure.
:)
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True
A TXV can adjust flow rate to compensate varying loads.
True
A TXV may be used on a system with multiple evaporators.
Thermostatic Expansion Valve (TXV)
A __________ is designed to maintain a constant superheat value.
Bleeder or bypass
A compressor must start while under load if an AXV does not have a __________.
Static Condenser
A condenser without a fan.
Dual-Pressure Regulator
A control valve with two pilot pressure regulators.
Superheat
A controller modulates the EEV through the use of an algorithm to maintain the desired __________.
Distributor
A device that splits the flow of refrigerant into several paths. Such a design is used to reduce the pressure drop in a large evaporator.
Metering Orifice
A fixed-orifice metering device that consists of a fitting with a small hole, called an orifice between the fitting's inlet and outlet.
High-Side Float (HSF)
A float-type metering device that regulates refrigerant flow into the evaporator based on the volume of liquid refrigerant in the high-side receiver.
Evaporator
A low-side float system maintains a constant level of liquid refrigerant in the __________.
Capillary Tube
A metering device consisting of a length of seamless tubing with a small and precisely formed inside diameter.
Hunting
A responsive mechanism going too far in one direction and then overcorrecting and returning too far in the other direction.
Sensing Bulb
A sealed bulb connected to the TXV by a capillary tube.
Liquid Cross-Charged Sensing Bulb
A sensing bulb charged with a different refrigerant than what is in the system, and some of it is always in a liquid state.
Gas Cross-Charged Sensing Bulb
A sensing bulb charged with a refrigerant that is different from that in the system and which is completely vaporized at the desired temperature.
Gas-Charged Sensing Bulb
A sensing bulb charged with the same refrigerant as the system and which is completely vaporized at a predetermined temperature.
Liquid-Charged Sensing Bulb
A sensing bulb charged with the same refrigerant as the system, some of which is always in a liquid state.
Adsorption Gas Cross-Charged Sensing Bulb
A sensing bulb that contains two substances: one is a noncondensing gas and the other is a solid.
Weight Valve
A simple valve consisting of a weighted needle, an adjustment spring, a valve seat, an inlet port, and an outlet port.
Stepper Motor EEV
A small stepper motor is used to drive the pin or piston that operates the valve.
Equalizer
A small tube that delivers pressure at the bottom of the diaphragm that is equal to the pressure of the evaporator outlet.
True
A solenoid valve's inability to hold the valve at an intermediate position would normally exclude it from being used to modulate refrigerant flow. But a PWM solenoid EEV can be used by holding its valve open or closed during a given interval.
Liquid Receiver
A system with a high-side float uses a float chamber to perform the function of a __________.
Thermostatic Expansion Valve (TXV)
A type of expansion valve metering device that adjusts the refrigerant flow rate based on a superheat pressure signal from a sensing bulb at the evaporator's outlet.
Sweat or frost
If the orifice of a TXV is oversized, too much refrigerant will pass into the evaporator and cause the suction line to __________ before the sensing bulb can close the valve.
Oil binding
If the unit is not level, the oil will not return to the compressor and forms a layer on the surface of the liquid refrigerant.
Heat Exchanger
Any device in which heat is exchanged between two mediums.
Weight Valve
In a high-side float system in which the evaporator is some distance from the float chamber, it is the __________ that controls the flow of refrigerant into the evaporator.
False
In a high-side float system, when the liquid level on the high side drops, the valve opens.
Oil
In evaporators equipped with a pan float design, the suction tube extends to the bottom of the pan to ensure that __________ collected in the pan is drawn into the suction line and recirculated.
Electronic Expansion Valve (EEV)
In this type of expansion valve, an electric operator is used instead of the diaphragm power assembly of a TXV or the spring assembly of an AXV.
Surging
Hunting may also be referred to as __________.
Oil
If a low-side float unit is not level, proper evaporation of refrigerant will not occur because a layer of __________ will cover the liquid refrigerant.
Vertical Suction Line
If the bulb is mounted on a __________, the capillary tube of the bulb should always enter from the from the top of the bulb, never from the bottom.
False
AXV operation is based on its sensing bulb.
Metering Orifice
Also called: •Fixed-orifice metering device •Flowrater •Flow restrictor •Accurator •Piston •Orifice
True
An AXV does not have a sensing bulb.
Diaphragm-Type AXV
An AXV that has stops to prevent too great a movement of the diaphragm. The diaphragm often has concentric corrugations (ripples) to improve its flexibility.
Electric Operator
An EEV controls its orifice using an __________.
Maximum Operating Pressure (MOP)
Can be set by using a TXV equipped with a gas-charged sensing bulb, a gas cross-charged sensing bulb, or a mechanical pressure limiter to prevent overloading the condensing unit. (Correct me if I'm wrong!)
No
Can you shorten a capillary tube system?
Bent or crimped
Capillary tube failure will most likely occur when it is __________.
Pressure Limiter
Devices within the TXV that prevent the evaporator pressure from exceeding a safe operating limit. It is placed between the sensing element and the needle valve.
False
Gas cross-charged sensing bulbs use same refrigerant as the system.
