section 5 commercial refrigeration unit 24 expansion...
TRANSCRIPT
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SECTION 5
COMMERCIAL REFRIGERATION
UNIT 24
EXPANSION DEVICES
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UNIT OBJECTIVES
After studying this unit, the reader should be able to
• Describe the three most popular types of expansion devices.
• Describe the operating characteristics of the three most popular expansion
devices.
• Describe how the three expansion devices respond to load changes.
• Describe the operation of a balanced-port expansion valve.
• Describe the operation of a dual-port expansion valve.
• Describe how electronic expansion valves and their controllers operate.
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EXPANSION (METERING) DEVICES • Meters the correct amount of refrigerant to the evaporator
• Installed in the liquid line at the inlet of the evaporator
• Common devices: Automatic expansion valve, thermostatic expansion valve, fixed bore (capillary tube)
• Less common devices: High-side float, low-side float
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Compressor Condenser
Metering device Evaporator
Direction of Refrigerant
Flow
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THERMOSTATIC EXPANSION
VALVE (TXV) • Maintains a constant evaporator superheat
• If the evaporator superheat is high, the valve will open
• Superheat ensures that no liquid refrigerant leaves the evaporator
• Low superheat increases the net refrigerant effect
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Thermostatic Expansion
Valve
Evaporator
Direction of Refrigerant
Flow
Liquid Line
Transmission Line
Thermal Bulb
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TXV COMPONENTS
• Valve body
• Diaphragm
• Needle and seat
• Spring
• Adjustment and packing gland
• Sensing bulb and transmission tube
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THE VALVE BODY
• Machined brass or stainless steel
• Holds components together
• Provides means to connect valve to the piping circuit
• Fastened by flare, solder, or flange
• Has an inlet screen to stop any small particulate matter from entering
valve
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THE DIAPHRAGM
• Moves the needle in and out of the seat in response to system
load changes
• Flexes downward to open the valve
• Flexes upward to close the valve
• Made of thin, flexible stainless steel
• Located at the top of the valve
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Diaphragm
Bulb pressure pushes down to open the valve
Evaporator pressure
pushes up to close the
valve
Spring pressure
pushes up to close
the valve
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NEEDLE AND SEAT
• Control refrigerant flow through the valve
• Needle is pushed into the seat to reduce refrigerant flow to the evaporator
• Made of stainless steel
• The greater the pressure difference across the needle and seat, the greater
the amount of flow through the valve
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Diaphragm
Seat
Needle
Push Rods
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Diaphragm pushed up
Needle pushed into the seat,
closing the valve
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Diaphragm pushed down
Needle pushed out of the seat,
opening the valve
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THE SPRING
• One of the valve’s closing forces
• Acts to push the needle into the seat, causing the valve to close
• Spring pressure determines the evaporator superheat
• Spring tension can be field adjusted
• Only EXPERIENCED field technicians should do adjustments on the valve
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The spring pushes up on the
push rods to close the valve
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THE SENSING BULB AND
TRANSMISSION LINE • Senses temperature at the outlet of the evaporator
• This temperature is converted to a pressure and is transmitted to the top of the diaphragm
• The fluid in the bulb responds to a pressure / temperature relationship
• When the suction line temperature goes up, the bulb pressure goes up
• The bulb pressure is the only opening pressure that controls the valve
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Liquid refrigerant from
condenser or receiver
Valve body
Saturated refrigerant to the
evaporator
Superheat spring adjusting
screw
Transmission Line Thermal Bulb
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TYPES OF BULB CHARGE
• Bulb charge is the type and amount of refrigerant contained in the thermal
bulb transmission line and the space above the diaphragm
– Liquid charge
– Vapor charge
– Cross liquid charge
– Cross vapor charge
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THE LIQUID CHARGE BULB
• Bulb contains the same refrigerant as the refrigeration system
• Under all conditions, the bulb will ALWAYS contain some liquid
• The refrigerant in the bulb will always follow the
pressure/temperature relationship of the system
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THE CROSS LIQUID CHARGE
BULB • Bulb contains a different refrigerant than the system
• Under all conditions, the bulb will ALWAYS contain some liquid
• The bulb does not follow the pressure/ temperature relationship of
the system
• Valve closes during the compressor off cycle
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THE VAPOR CHARGE BULB
• Bulb contains the same refrigerant as the system
• Bulb only contains a small amount of liquid
• Also called a critical charge bulb
• At some predetermined temperature, all of the liquid in the bulb will boil until
only vapor remains
• Any further increases in bulb temperature will have no effect on the bulb
pressure
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THE CROSS VAPOR CHARGE BULB
• Bulb contains a different refrigerant than the system
• Bulb only contains a small amount of liquid
• Also called a critical charge bulb
• At some predetermined temperature, all of the liquid in the bulb will boil until
only vapor remains
• Any further increases in bulb temperature will have no effect on the bulb
pressure
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EXAMPLE OF A TXV WITH INTERNAL
EQUALIZER – LIQUID-FILLED BULB • Normal load conditions – medium temperature application, R-134a, valve is
in equilibrium
• Suction pressure 18.4 psig
• Suction line temperature 30°F, PBULB= 26.1 psig
• PSPRING + PEVAPORATOR = PBULB
• Spring pressure + 18.4 psig = 26.1 psig
• Spring pressure = 7.7 psig
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R-134a
Evaporator pressure 18.4 psig
26.1 psig
30°F
Spring pressure = ?
26.1 psig = Ps + 18.4 psig
Ps = 7.7 psig
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LOAD CHANGES WITH FOOD
ADDED TO COOLER
• Addition of warm food increases evaporator load
• Refrigerant boils faster and suction pressure rises
• Evaporator superheat rises
• Valve opens to feed more refrigerant to the evaporator
• Increased evaporator superheat causes temperature of remote bulb to rise
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LOAD CHANGES WITH FOOD
REMOVED FROM THE
COOLER • Removal of food reduces load on the evaporator
• Refrigerant boils slower and suction pressure drops
• Evaporator superheat drops
• Valve closes to feed less refrigerant to the evaporator
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TXV WITH EXTERNAL EQUALIZER
• Used if an evaporator has more than a 2.5 psig drop from inlet to outlet
• The evaporator pressure is sensed at the outlet of the coil instead of the inlet
• Used to prevent the coil from starving
• Connected to the evaporator outlet after the thermal bulb
• Used to compensate for pressure drop in the evaporator
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Saturated refrigerant to the
evaporator
Liquid refrigerant to the
expansion valve
External equalizer line
connected to the outlet of the
evaporator coil
Evaporator pressure pushing up on
the diaphragm
Diaphragm
Solid brass divider
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TXV RESPONSES TO LOAD
CHANGES • When load increases
– Refrigerant boils faster and the suction line temperature increases
– Valve opens to feed more refrigerant to the evaporator
• When load decreases
– Refrigerant takes longer to boil
– Valve closes to feed less refrigerant to the evaporator
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BALANCED PORT TXV
• Designed to operate in low ambient conditions
• Used if any of the following conditions exist
- Large varying head pressures
- Large varying pressure drops across the TXV
- Widely varying evaporator loads
- Very low liquid line temperatures
• Have larger-than-normal orifices
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DUAL PORT TXV
• Used when systems need a larger TXV for short periods of time
• Dual-port valves have two independent capacities
- Larger port for periods of high load
- Smaller port for periods of normal load
- TXV capacity is doubled when larger port is open all the way
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PRESSURE LIMITING TXV • Allows evaporator pressure to only reach a predetermined
pressure
• If the evaporator pressure exceeds this pressure, the valve will
close
• Desirable on low-temperature applications
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SENSING ELEMENT (BULB)
INSTALLATION • Bulb should be mounted on the suction line as close to the evaporator as
possible
• Suction line should be clean and straight
• Bulb should be mounted securely
• Follow manufacturer’s instructions
• For small suction lines, the bulb is usually secured to the top of the line
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Suction line smaller than 3/4”
Thermal bulb
mounted on top of
the line
Suction line larger than 3/4”
Thermal bulb located 45°
below horizontal
Use strapping material supplied with the valve to hold
bulb securely to the suction line
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THE SOLID-STATE CONTROLLED
EXPANSION VALVE • Uses a thermistor as a sensing element
• Electrically controlled
• When coil is energized, the valve opens
• Responds very quickly to temperature changes
• Suitable for heat pump applications
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STEP MOTOR EXPANSION
VALVES
• Uses a small motor to control the valve port
• Valve port controls evaporator superheat
• Temperature sensor sends a signal to the controller
• The controller sends a signal to the motor
• The motor turns a fraction of a rotation for each controller signal
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ALGORITHMS AND PID CONTROLLERS • Proportional Controllers
- Generate an analog output signal
- Difference between actual superheat and superheat set point is the “offset” or “error”
• Integral Controller Modes
- Helps reduce the “error” or “offset”
- Calculates error size and the length of time the error exists
• Derivative Controller Modes
- Estimate rate of change of temperature/time curve
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AUTOMATIC EXPANSION VALVE
• Maintains constant pressure in the evaporator
• When the evaporator pressure drops, the valve opens
• The spring pressure pushes to open the valve
• The evaporator pressure pushes to close the valve
• Turning the adjustment screw into the valve increases the spring
pressure
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Diaphragm
Spring pressure pushes down to open the valve
Evaporator pressure
pushes up to close the
valve
Two pressures control the
automatic expansion valve
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Diaphragm pushed up
Needle pushed into the seat,
closing the valve
Caused by an increase in evaporator pressure
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Diaphragm pushed down
Needle pushed out of the
seat, opening the valve
Caused by a decrease in evaporator pressure
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Diaphragm
Needle and Seat
Spring
Liquid refrigerant from
condenser or receiver
Saturated refrigerant to the
evaporator
Evaporator pressure
Spring pressure
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AUTOMATIC EXPANSION VALVE
RESPONSE TO LOAD CHANGES
• Responds in reverse to load changes
• If the load increases
– Refrigerant boils faster in the evaporator
– The evaporator pressure increases
– The valve closes
• Used where the load is fairly constant
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THE CAPILLARY TUBE
METERING DEVICE • Controls refrigerant flow by the pressure drop across it
• Diameter and length of the tube determine flow at a given pressure
• Does not maintain evaporator pressure or superheat
• Used when the load is relatively constant
• No moving parts to wear out
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OPERATING CHARGE FOR THE
CAPILLARY TUBE SYSTEM
• Capillary tube systems are critically charged
• All refrigerant in the system circulates at all times when the system is
running
• Capillary tube sometimes fastened to the suction line for heat
exchange
• Responds very slowly to system load changes
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UNIT SUMMARY - 1 • Expansion devices meter the correct amount of refrigerant to the evaporator
according to system operating conditions
• Common expansion valves include the capillary tube, automatic expansion
valve and the thermostatic expansion valve
• The thermostatic expansion valve is designed to maintain constant
superheat in the evaporator
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UNIT SUMMARY - 2 • Three pressures control the operation of the TXV: the bulb pressure, the
spring pressure and the evaporator pressure
• Thermal bulb can be liquid-charged, vapor-charged, cross liquid-charged, or
cross vapor-charged
• Internally equalized TXVs get the evaporator pressure from the inlet of the
coil, while externally equalized TXVs get the evaporator pressure from the
outlet of the coil
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UNIT SUMMARY - 3
• Special TXVs include the balanced port TXV, the dual port TXV and the electronic TXV
• The automatic expansion valve maintains a constant evaporator pressure
• Two pressure control the AXV: the spring pressure and the evaporator pressure
• The capillary tube is a fixed bore metering device
• The capillary tube meters refrigerant depending on the pressure drop across the tube