SECTION 5

COMMERCIAL REFRIGERATION

UNIT 24

EXPANSION DEVICES

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

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

Compressor Condenser

Metering deviceEvaporator

Direction of Refrigerant Flow

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

Thermostatic Expansion Valve

Evaporator

Direction of Refrigerant Flow

Liquid Line

Transmission Line

Thermal Bulb

TXV COMPONENTS• Valve body• Diaphragm• Needle and seat• Spring• Adjustment and packing gland• Sensing bulb and transmission tube

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

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

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

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

Diaphragm

Seat

Needle

Push Rods

Diaphragm pushed up

Needle pushed into the seat, closing the valve

Diaphragm pushed down

Needle pushed out of the seat, opening the valve

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

The spring pushes up on the push rods to close the

valve

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

Liquid refrigerant from condenser or

receiver

Valve body

Saturated refrigerant to the evaporator

Superheat spring adjusting screw

Transmission Line Thermal Bulb

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

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

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

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

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

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

R-134a

Evaporator pressure 18.4 psig

26.1 psig30°F

Spring pressure = ?

26.1 psig = Ps + 18.4 psig

Ps = 7.7 psig

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Diaphragm

Spring pressure pushes down to open the valve

Evaporator pressure pushes up to close the valve

Two pressures control the automatic

expansion valve

Diaphragm pushed up

Needle pushed into the seat, closing the valve

Caused by an increase in evaporator pressure

Diaphragm pushed down

Needle pushed out of the seat, opening the valve

Caused by a decrease in evaporator pressure

Diaphragm

Needle and Seat

Spring

Liquid refrigerant from condenser or receiver

Saturated refrigerant to the evaporator

Evaporator pressure

Spring pressure

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

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

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

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

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

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