COMMERCIAL REFRIGERATION

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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

• List and describe the three most popular
  expansion devices
• Explain the operating characteristics of various
  expansion valves
• Explain how various expansion devices respond to
  load changes
• Describe the operation of balanced port, dual
  port and electronic
• expansion valves
• Explain how electronic controllers are used to
  control expansion
• valves

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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
Direction of Refrigerant Flow
Evaporator
Metering device
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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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Transmission Line
Evaporator
Thermal Bulb
Thermostatic Expansion Valve
Liquid Line
Direction of Refrigerant Flow
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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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Bulb pressure pushes down to open the valve
Diaphragm
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
Push Rods
Needle
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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 valves 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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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 30F, PBULB 26.1 psig
• PSPRING PEVAPORATOR PBULB
• Spring pressure 18.4 psig 26.1 psig
• Spring pressure 7.7 psig

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26.1 psig Ps 18.4 psig Ps 7.7 psig
R-134a
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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
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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 manufacturers instructions
• For small suction lines, the bulb is usually
  secured to the top of the line

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Thermal bulb mounted on top of the line
Use strapping material supplied with the valve to
hold bulb securely to the suction line
Thermal bulb located 45 below horizontal
Suction line smaller than 3/4
Suction line larger than 3/4
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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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Spring pressure pushes down to open the valve
Diaphragm
Two pressures control the automatic expansion
valve
Evaporator pressure pushes up to close the 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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Spring pressure
Spring
Diaphragm
Needle and Seat
Saturated refrigerant to the evaporator
Liquid refrigerant from condenser or receiver
Evaporator 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