Expansion Devices

Shyam SunderAssistant Professor

Mechanical Engineering

BITS PilaniMarch 11, 2016

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Refrigeration SystemExpansion Devices

CONDENSER REJECTSHEAT TO THEOUTDOORS.

EVAPORATOR ABSORBSHEAT FROM SPACE.

INSIDE

COMPRESSOR PUMPSHEAT-LADENREFRIGERANT.

EXPANSION DEVICEMETERS REFRIGERANT.

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Expansion DevicesDifferent Types

Capillary tube

Automaticexpansion valve

Thermostatic expansion valve

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Expansion DevicesCapillary Tubes

Its a copper tube with a very small, calibrated,inside diameter

Simple construction, no moving parts, directlyconnects condenser and evaporator

Length and diameter of the tube depends onthe required mass flow rate and pressure drop

Equilizes pressure when ref. plant is shut-off and therefore reducesmotor starting torque when the plant is started again

Cannot adjust to load change. Usually used where the load isrelatively constant with no large fluctuations.

Small bore size tubes can get blocked with metal particles, or evenwith high viscosity oils. A strainer is required at the inlet to the device

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Expansion DevicesAutomatic Expansion Valve

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Expansion DevicesAutomatic Expansion Valve: Force Balance

Force due to Refrigerant pressure @ Evaporator

Force due to Spring Setting

Force due to Ambient pressure

ValveDiaphragm

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Expansion DevicesAutomatic Expansion Valve

These maintain a constant evaporator pressure and hence a constantevaporator temperature during the on-cycle

Adjustment spring is usually set such that during off-cycle the valve isfully closed

There is a slight variation in evaporator pressure during the on-offcycle

Time

Pe

Off On

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Automatic Expansion ValveEffect of load variation

Decrease in Refrigeration LoadI The evaporator temperature and hence the evaporator pressure

decreasesI Due to constant spring setting, the valve will open more, allowing more

amount of refrigerant into the evaporator

Increase in Refrigeration LoadI The evaporator temperature and hence the evaporator pressure

increasesI Due to constant spring setting, the valve will close more, allowing less

amount of refrigerant into the evaporator

This valve counteracts to the requirement

Result:I Flooding of evaporator with a decrease in refrigeration loadI Starving of evaporator with an increase in refrigeration load

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Automatic Expansion ValveApplications

Automatic Expansion Valves are cheaper in cost

These are used in applications where an almost constant andcontinuous load is maintained in the evaporator

Examples:I These are used in milk chilling units in dairiesI Used in small capacity water coolersI Also used in some air conditioning systems

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Expansion DevicesThermostatic Expansion Valves

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Expansion DevicesTEVs: Bulb Location

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Expansion DevicesThermostatic Expansion Valve: Force Balance

Spring pressure

Evaporator pressure

Bulbpressure

Valvediaphragm

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Expansion DevicesThermostatic Expansion Valve: Characteristics

A thermostatic expansion valve maintains a constant degree ofsuperheat at the exit of evaporator

The pressure of the bulb fluid is the saturation pressure correspondingto the temperature at the evaporator exit

If the evaporator is starving:

⇒ Degree of superheat is higher⇒ Bulb pressure is more⇒ Valve will tend to open

If the evaporator is flooding:

⇒ Degree of superheat is less⇒ Bulb pressure will be less⇒ Valve will tend to close

The refrigerant mass flow rate supplied through the expansion valve isproportional to the load

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Expansion DevicesThermostatic Expansion Valve: Effect of Load Variation

Increase in Evaporator LoadI The evaporation rate of liquid refrigerant increasesI The area available for superheating the vapour increasesI As the degree of superheat increases, bulb pressure increasesI This pressure will open the valve letting more refrigerant into the

evaporator

Decrease in Evaporator LoadI The evaporation rate of refrigerant decreasesI The degree of superheat decreasesI The bulb pressure decreases so as to close the valve

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Expansion DevicesTEVs: External Equilizer

In case the evaporator is large and has a significant pressure drop, thepressure from evaporator exit is fed directly to the bottom of thebellows by a narrow tube

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Expansion DevicesOther Types

Float Valves, Hand Valves etc.

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Thank you all for the attention...Any questions?

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