SECTION 5

COMMERCIAL REFRIGERATION

UNIT 24

EXPANSION DEVICES

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.

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 device Evaporator

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 psig

30°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