CH-21 Metering Devices
Advantage of a capillary tube (maintenance)
***NO Moving parts
What type of Metering orifice might you find in a heat pump ?
*Fluted piston Moves one direction to meter refrigerant and move in other direction to allow Free flow without metering
What determines the length and diameter of a capillary tube NEEDED?
1. Capacity of compressor 2. Type of refrigerant 3. Desired target temperature 4. Type of condenser user
TYPES of metering devices
1. Capillary tube 2. Metering Orifice 3. TXV 4. AEV 5. EEV
5 types of sensing bulb charges
1. Liquid charged 2. Gas charged 3. Liquid cross charged 4. Gas cross charged 5. Adsorption gas cross-charged
Capillary Tube PREFORMANCE VARIABLES
1. Tube Length 2. Inside diameter 3. Number of turns ***Determines how much pressure will drop between the (condenser) and the (evaporater) ***LONGER the greater the pressure drop ***Smaller diameter greater the pressure drop
Other names for a fixed meter device??
1.fixed orifice metering device 2. Flowraters 3. Flow restrictors 4. Accurators 5. Piston 6. Orifice
How far should you turn the ADJUSTMENT screw at a time ?(TXV)
1/4 turn
What would be needed if a compressor was not suitable to start under a LOAD ?
A special TXV equipped with a BLEED PORT or valve that does equalize pressure in the OFF CYCLE
where would a filter-drier be PLACED in reference to a TXV?
BEFORE it COOL spot
METERING DEVICE installation
BETWEEN the Liquid Line and EVAPORATER
Common cause of failure
Bent Crimped Plugged
what prevents refrigerant from flowing BACKWARDS ?
CHECK VALVE
What is meant by starving the EVAPORATOR and what is TXV doing ?
CLOSING to much needs to OPEN a little
If a TXV is closing to SLOW refrigerant flow, what temp is the sensing bulb ?
COLD
what does TXV superheat refer to ?
Difference in TEMPERATURE between the evaporator INLET and OUTLET
HUNTING ( TXV)
ERROR TXV responds and OPENS valve too much then OVERCORRECTS and closes too much
TXV
Expansion valve metering device Adjust refrigerant flow Based on superheat pressure signal From sensing bulb ***mounted at the EVAPORATOR
Where are capillary tube metering devices used Vs Fixed metering devices
Extensively in fractional HP motors Like refrigerators, freezers , windows AC (FIXED) Heat pumps and split systems
What is a metering office ?
Fixed orifice that consist of a SMALL hole Between inlet and outlet Produces high and low side pressure which can be removed for cleaning
When liquid in a capillary tube (Evaporates) what's it called ?
Flash gas
What allows for a low TORQUE (compressor) to be used with a capillary tube ?
High and low side Equalize when compressor is OFF . ****Reducing torque needed to start up
A sensing bulb sensing HEAT will respond with what TYPE of pressure ?
High-pressure opening valve INCREASING flow rate DECREASING superheat to "flood" evaporator
If the TXV superheat adjuster is turned (clockwise) it WILL
INCREASE tension STARVING the evaporator increasing SUPERHEAT
under what circumstances will a TXV NOT remain closed in OFF cycle ?
If valve is ADJUSTED incorrectly
What does a Capillary tube also function as ?
LIQUID LINE
A sensing bulb that senses a COLD TEMP will respond with what type of temp ?
LOW pressure, closing valve reducing flow rate INCREASING superheat to STARVE evaporater
Under what condition might you need to change orifice SIZE??
Length of line set (suction & Liquid line) To account for extra refrigerant
Capillary tube
Length of seamless tubing with a SMALL precise INSIDE diameter
Advantage of using LARGER capillary tubes
Less likely to get plugged with dirt and WAX or ICE
Which 2 charges use the SAME refrigerant that is in the SYSTEM ?
Liquid , Gas Charged
Whats more EFFICIENT ? Fixed metering or Modulating metering device
MODULATING because they can VARY flow bases on LOAD
What does MSS stand for ?
Minimum stable signal
How is a sensing bulb mounted on HORIZONTAL section of suction line on a small system ?
Mounted on top
IF you have a TXV that does not equalize how does that change COMPRESSOR selection?
NEED a compressor capable of starting under a LOAD
DO pressures BALANCE off in OFF cycle ?
NO because its a TXV
Are (check valve) used to prevent refrigerant from flowing the wrong way in DIRECT capillary tubes ?
No refrigerant free to flow anywhere ***Pressure equalizes in off cycle
ROLE of a (filter-drier) in a capillary tube system ?
PREVENTS any contaminants from entering and CLOGGING the capillary tube PLACEMENT: between the liquid line and capillary tube
Sensing bulb
Pressure inside bulb changes with TEMPERATURE at the evaporator outlet Causing fluid inside to pressurize and depressurization
Dangers of having an overcharged (capillary tube system) in OFF CYCLE?
Pressure will equalize pushing EXTRA liquid into **Evaporator** Compressor will work harder on (start-up) ***Risk in liquid Slugging
What effect in the OVERALL system does a CAPILLARY tube and a Suction line heat exchange have ?
Provides low-side superheat and high-side subcooling Overall improving system efficiency
What does RPB ?
RAPID PRESSURE BALANCING secondary port opens during compressor OFF cycle allowing pressures to equalize allowing a lower torque compressor to be USED
What changes the pressure inside a sensing bulb ?
Refrigerant temperature in suction line at the ***evaporator outlet
Upon replacing (capillary tube) what else should be done ?
Replace (filter drier)
What is the DANGER of a HUNTING TXV ?
SLUGGING
Explain what pressure exist in a TXV when the compressor is OFF ?
Sensing bulb and Evaporator pressure pushing AGAINST each other in DIAPHRAGM CANCEL each other out which leaves spring pressure to close and hold the valve CLOSED
What pressure difference closes a TXV ?
Sensing bulb de-pressures from a cool suction line Evaporator and superheat adjustment spring pressure is greater that the sensing bulb pressure
What pressure difference opens a TXV ?
Sensing bulb pressurizes from warm SUCTION LINE Is greater than the combined pressure of the evaporator and superheat adjustment spring
Visual clue that a system is (overcharged)
Severe Frosting Of SUCTION LINE
When a compressor running what is the DELTA T between the (input) and (output) of the evaporator called ?
Superheat
What practice do manufacturers use to help INSTALLERS for having proper metering device ??
Supply **several** orifice sized
Why is proper Mounting critical ?
TEMP of refrigerant will be a few degrees colder than the suction line surface because the insulating quality of OIL
What type of valve receives a pressure signal from sensing bulb , modulating refrigerant ??
TXV
If a LOW torque compressor was in a UNIT, what type of TXV would we NEED ?
TXV a BLEED port so HIGH side pressures can EQUALIZE in the OFF cycle
Why are fixed metering devices LESS efficient than TXV ?
Unable to adjust refrigerant FlOW
How does TXV respond when all liquid vaporizes TOO late in evaporator?
Valve CLOSES to provide a lower flow rate
IF a TXV is opening to allow higher refrigerant flow, What temp is the sensing bulb relative to the input of the EVAPORATER?
WARM
HOW does it EQUALIZE ?
a SMALL passage connects the chamber under the VALVE diaphragm to the valve OUTLET
What is MSS ?
best SUPERHEAT setting for an evaporator that results in the smallest variation in sensing bulb temp CHANGE
How is a sensing bulb mounted on a VERTICAL line
capillary tube on bulb must be on TOP
MODULATING metering device
changes ORIFICE size to account for changes in COOLING load
What is a downside of an INTERNALLY equalized TXV ?
do not provide accurate CONTROL when used with EVAPORATOR, that have a large pressure drop between the INLET and OUTLET
What is an EXTERNALLY equalized TXV ?
has a TUBE connecting to the EVAPORATOR outlet to a chamber inside the TXV just below the DIAPHRAGM
What happens inside CAPILLARY TUBE ?
pressure decreases some liquid evaporates (flashgas) drops temperature last 1/4 of tube to evaporate temp
what does the ORFICE in metering device do ?
provides small opening which lets a **certain** amount of refrigerant flow THROUGH
How is the pressure difference CREATED ?
refrigerant EXPANDS dropping to a low pressure upon entering the EVAPORATER. Yet high-side remains the SAME
Primary Function of a METERING DEVICE ?
to create a PRESSURE difference between LOW-SIDE and HIGH-SIDE
What is an INTERNAL equalized TXV ?
uses evaporator INLET pressure to create the "closing" force one the VALVE
What does an EQUALIZER do ?
uses the EVAPORATOR outlet pressure to provide the closing force COMPENSATING for any pressure drop through the EVAPORATOR
How does the TXV respond when all liquid VAPORIZES too EARLY in the evaporator ?
valve opens to provide a higher flow rate
Under what CIRCUMSTANCE would an equalizer be USED ?
when EVAPORATOR pressure drop is more than 4 PSI
