Prepared by : Lokesh Thakur Er.no : 150010721012

ME Thermal engineering- 2nd semesterA.D Patel institute of technology

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

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

Valve bodyDiaphragmNeedle and seatSpringAdjustment and packing glandSensing bulb and transmission tube

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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