Powerpoint: Metering Devices Flashcards
A metering device is a _____ device
control
A Metering device does 2 things:
- Meters refrigerant into the _______
- Provides a pressure drop to change _______ temp and produce _____ gas
- evaporator
2. saturation temp, flash gas
A metering device separates the _____ side from the _____ side
high side from the low side
A metering device is installed at the ____ of the _________
inlet of the evaporator
Operation:
- Refrigerant enters the metering device as a ____ pressure, ______ temp ___ _____ liquid
- high pressure, medium temperature sub-cooled liquid
Operation:
- The liquid refrigerant flows through a ________ orifice where the pressure will be decreased, as a result;
- Refrigerant will leave the MD as a ____ pressure, ____ temperature saturated liquid vapour mix of ___% vapour and ___% liquid
- The ideal saturated temperature for air conditioning is ___F
- restricting
- low, low, 25% vapour 75% liquid
- -40F
Types of Metering devices;
- _____ expansion (needle valve)
- fixed _____
- _______ tube
- Acc-____
- automatic ________ (not easy to work with)
- thermostatic _______ (most popular)
- electric ______ valve
- electronic ______ valve
- ____ side float (mostly in chillers)
- ____ side float
- hand expansion (needle valve)
- fixed orifice
- capillary tube
- Accuator
- automatic expansion (not easy to work with)
- thermostatic expansion (most popular)
- electric expansion valve
- electronic expansion valve
- high side float (mostly in chillers)
- low side float
Name 3 pressure operated MD types;
- hand expansion valve (needle)
- fixed orifice
- capillary tube
- automatic expansion valve
Name 2 volume operated MD types;
- high side float
2. low side float
Name 3 temperature operated MD types:
- thermostatic expansion valve
- electric expansion valve
- electronic expansion valve
Hand expansion valve:
- It is a needle valve that is _____ operated
- A change in load requires _____ adjustment of the valve
- Now commonly used where another device will control flow to the valve automatically( _____)
- hand
- manual
- solenoid
Fixed Orifice (cap tube):
- A small accurate inside diameter tube that offers high _______ resistance to refrigerant flow
- There is no _____ change along the tube as flashing should not occur until ref leaves capillary tube and enters the evaporator
- frictional
2. temperature
Fixed Orifice: Capacity affected by any change in;
- Length: longer means ____ refrigerant
- Inside diameter: larger means ____ refrigerant
- Pressure difference: increase means _____ ref
- less refrigerant
- more refrigerant
- more refrigerant
Capillary Tube Characteristics:
- Space temp controlled by __________
- Off cycle equalization as there is no actual separation between _______ and ________
- Critically charged system- meaning it has just enough refrigerant for the _____ it was designed for
- thermostat
- high side and low side
- load
Critical charger systems usually have a fairly _______ load
constant
critically charged systems have no _______ of refrigerant for load changes
reserve
______ down is a procedure to relocate refrigerant from the low side to the receiver.
Pump
Pump down systems have a liquid line _______ valve to prevent migration
solenoid valve
Capillary Tube Advantages:
- Inexpensive and only requires a low starting torque _________
- No ______ parts
- _______ design
- compressor
- moving
- simple
Capillary tube disadvantages:
- does not respond well to _____ changes
- The small ______ diameter will plug easily with any ______ in the system
- load changes
2. inside diameter, debris
Fixed Orifice Actuator:
- works basically the same as the _____ tube
- It is more compact and can be used as a ______ valve in _____ pump applications
- It requires careful selection to match _______ with installed system
- It is a ________ charged system
- No separation between _____ side and ______ side
- capillary tube
- check valve, heat pump
- orifice
- critically charged
- high side and low side
Automatic Expansion Valve:
One of the first automatic expansion valves is designed to maintain a constant _______ in the evaporator
pressure
Automatic Expansion Valve:
Operates under 3 pressures; name all 3
Spring pressure (opening force)
Atmospheric pressure (opening force)
Evaporator pressure (closing force)
Automatic Expansion Valve:
- When evaporator pressure drops, spring pressure and atmospheric pressure will cause the valve to _____
- When evaporator pressure increases the evaporator pressure will overcome the spring and atmosphere pressure cause inflation the valves to ______
- open
2. close
Automatic Expansion valve characteristics:
- Space temp controlled by a _________
- not recommended for varying _____ applications
- As load increases the valve tends to ______
- As load decreases the valve tends to ______
- It is a ________ charged system
- Suction accumulators should be used due to possible _________
- thermostat
- load
- close
- open
- critically
- floodback
Low side Float:
Is a liquid level control located on the ____ side of the system it operates on the rate of ________
low side, rate of evaporation
Low side float:
As the load increases, more _________ evaporates, the liquid level in the evaporator decreases and the valve will open on decrease of liquid level
refrigerant
Low side Float:
May be used on _____ or ________ systems
direct or indirect
Low side Float:
- If not critically charged may have a ______ to store ref for variations in load
- Used on ______ coils
- Simple and ______
- Low side float are common on ______ systems
- low side float may be arranged with a ____, hand expansion valve, high/low level alarms. This arrangement is commonly called ________ float switch
- receiver
- flooded
- reliable
- ammonia
- LLS (liquid line solenoid), electric float switch
High side Float:
- Liquid drops, they _____. Liquid rises, they _____
- A liquid level control, located on the _____ side of the system operates at rate of _________
- As the load increases more refrigerant evaporates, more ref condenses as the liquid level in the condenser increases the valve will _____ on increase of liquid level in the condenser
- May be used on _____ or _______ systems
- Close, Open
- high side , rate of condensation
- open
- direct or indirect
High side Float:
- Medium temp controlled by ______ and _________
- It is a _______ charged system
- Evaporator may be ______ or ______ expansion
- pressure and temperature
- critically
- flooded or dry (DX)
Pilot operated HSF valve:
- Pilot float valve follows the ______ level
- As the condensing load increases the _____ ball rises, slowly closing the pilot orifice
- liquid level
2. float
- Thermostatic Expansion Valve:
1. Operation: Refrigerant flow control for ___ evaporators
2. Controls the flow of refrigerant by maintaining a constant _______
- DX (dry)
2. constant superheat
*TXV main parts (thermostatic expansion valve)
- _________ element (top)
- Push ____ (mid)
- external ________ connection (mid/low)
- _____ body (low)
- thermostatic element
- push rods
- external equalizer conn.
- valve body
TXV valve body:
- machined ____ or stainless ____
- holds __________ together
- provides means to connect valve to the _______ circuit
- fastened by _____, ______ or flange
- has an _____ screen to stop any small particulate matter from entering the _____
- machined brass or stainless steel
- components
- to the piping circuit
- flare, solder, or flange
- inlet screen, the valve
TXV Diaphragm:
- Moves the needle in and out of the seat in response to ______ ______ changes
- Flexes downward to _____ the valve
- Flexes upward to _______ the valve
- made of thin flexible _______ ______
- located at the ___ of the valve
- system load changes
- open
- closes
- stainless steel
- top
TXV Valve: Needle and Seat;
- Controls ref _____ through the valve
- Needle is pushed into the seat to reduce refrigerant flow to the ________
- Made of ________ steel
- Greater _______ difference across the needle and seat the greater the amount of flow through the valve
- flow
- evaporator
- stainless steel
- pressure
TXV Valve: The Spring;
- One of the valves _______ forces
- Acts to push the needle into the seat causing the valve to ______
- Spring pressure determines the evap ________
- spring tension can be _____ adjusted
- closing
- close
- superheat
- field adjusted
The Sensing Bulb and Transmission Line:
- Senses temperature at outlet of _______
- this temp is converted to a pressure and is transmitted to the top of the ________
- fluid in the bulb responds to a ____/____ relationship
- when the suction line temp goes up the bulb pressure goes ____
- The bulb pressure is the only opening pressure that controls the _____
- evaporator
- diaphragm
- press/temp
- up
- valve
TXV Valve:
Name the three pressures and what they do to the valve (open or close)
Spring (closes)
Evaporator (closes)
Bulb (opens)
TXV Valve:
As the load on the evaporator increases the superheat will increase. TXV senses change in superheat through the ____
bulb
TXV Valve:
Bulb pressure increases putting more pressure on the diaphragm of the valve overcoming spring pressure and evaporator pressure to force the valve to ______ allowing more liquid/vapour to enter the evaporator
open
TXV Valve:
As the load on the evaporator decreases the superheat will ________. TXV senses the change in superheat through the bulb
decrease
TXV Valve:
Bulb pressure decreases putting less pressure on the diaphragm of the valve spring pressure and evaporator pressure overcomes bulb pressure to force the valve to _____
close
TXV Equalization: Internally equalized;
- Used on ______ evaporators
- Push rods are _______ shaped
- evaporator inlet pressure has direct access to the bottom of the _______
- single
- triangular
- diaphragm
TXV Equalization: Externally equalized;
- used of an evap has more than ____psig drop from inlet to outlet
- evap press is sensed at the _____ of the coil instead of the ______
- Used on multi circuit evaporators equipped with a _______
- 2.5psig
- outlet instead of the inlet
- distributor
TXV control one thing;
superheat
Why are multi-circuit evaporators used on external equalizers?
It is more efficient because it reduces a large pressure drop
TXV Sensing Bulb:
The _____ charge provides the temp operating characteristics for the valve
bulb
TXV Sensing bulb:
Types of bulb charges;
- _____ charge
- ___ liquid charge
- _____ charge
- _____ vapour charge
- liquid charge
- cross liquid charge
- vapour charge
- cross vapour charge
The Liquid Charge Bulb;
- bulb contains the ______ refrigerant as the refrigerant system
- contains enough liq ___/____for a wide range of temperatures
- The ref in the bulb will always follow the ____/____ relationship of the system
- same
- ref/vap
- press/temp
The Cross-Liquid Charge Bulb;
- Bulb contains ______ ref than system
- Contains enough ref/vap for a wide range of ________
- the bulb _____ ___ follow the press/temp relationship of the system
- different
- temperature
- does not
The Vapour Charge Bulb:
- Bulb contains ____ ref as system
- bulb contains just enough _______ refrigerant for valve operation within its temperature range
- Also called a _______ charge bulb
- At some predetermined temp, all of the liquid in the bulb will _____ until only ______ remains
- Any further increase in bulb temp will have no effect on bulb _______
- Also known as ______ limiting or ____ value
- same
- liquid
- critical
- boil, vapour
- pressure
- pressure, MOP
Pressure Limiting or MOP Value:
- Used to limit _____ loading (suction pressure) during hot pulldown or after defrost
- This press limiting charge causes the ____ to remain closed until the system evap pressure is reduced below ____ of charge
- Due to design of pressure limiting charges, the valve diaphragm and cap tubing must be kept at a temp _______ than the bulb during system operation.
- Otherwise, migration of the charge away from the bulb will occur, and cause loss of _____ _______
- compressor
- TXV, MOP
- warmer
- valve control
The Cross Vapour Charge Bulb:
- bulb contains ______ ref than system
- bulb contains just enough _____ ref for valve operation within its temperature range
- bulb does not follow the ____/_____ relationship of the system
- operating characteristics are similar to the _____ charged bulb
- different
- liquid
- press/temp
- gas
TXV Sensing Bulb Mounting:
- bulb should be mounted on the ______ line as close to _______ as possible
- suction line should be ____ and _______
- bulb must be mounted _______
- bulb must be well ________
- for small suction lines the bulb is usually secured to the ____ of the line
- suction , evaporator
- clean and straight
- securely
- insulated
- top
TXV Sensing Bulb Mounting:
- For lines 7/8” OD and larger, locate bulb between __ and __ o’clock
- Bulb must be located atleast ___” from where suction line exits the conditioned space
- Where possible, bulb should be mounted ______ of the equalizer connection to the suction line
- Access tube in the down position to prevent trapping of ______ charge
- Coil excess capillary tube on top of the thermostatic element to ______ it
- Do not locate the bulb on the coil _______ header
- 8 and 4 o’clock
- 18”
- upstream
- liquid
- protect
- suction
What does MOP stand for?
Maximum operating pressure
Expansion Devices:
- Often called _______ devices
- Meter the correct amount of refrigerant to the _________
- Installed in the ______ line between the ________ and the __________
- Three types are; ____ , auto_____ expansion valve, ______ bore
- metering
- evaporator
- liquid line between the cond and evap
- TXV, automatic expansion valve , fixed bore
Thermostatic Expansion Valve (TXV):
- Meters the refrigerant to the evaporator by using a _________ _______ element
- Maintains a constant evaporator ___________
- If the evap superheat is high, the valve will ______
- _________ ensures no liquid ref leaves the evap
- Low superheat _______ the net refrigeration effect
- thermal sensing element
- superheat
- open
- superheat ensures
- increases
