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Chapter 4

Refrigerant Control

Refrigerant in the evaporator must be at a low pressure so it will evaporate at a low

temperature. The liquid refrigerant in the condensing unit is at a relatively high pressure.

To enable the refrigeration unit to operate automatically, an automatic refrigerant flow

control must be placed in the circuit between the liquid line and the circuit between theliquid line and the evaporator. This control reduces the high pressure in the liquid line to

the low pressure in the evaporator.

There are six main types of automatic refrigerant flow controls:

NAME OF THE CONTROL ABBREVIATION

A. Automatic Expansion Valve AEV or AXV

B. Thermostatic Expansion Valve TEV or TXVC. Thermal-Electric Expansion Valve THEXV

D. Low-Pressure Side Float LSFE. High-Pressure Side Float HSF

F. Capillary Tube Cap.Tube

In the following paragraph, in addition to naming the controls, some information andterminology is listed. This will help in understanding the operation of each type of

refrigerant control. Also, the meaning of the terms “flash gas”, “superheat”, heat

exchanger”, “hunting” and the like are explained as they apply to refrigerant.

Compression System Refrigerant Controls

Modern refrigeration systems are automatic in operation. Devices have been developed

for controlling the electric motor which drives the mechanism.

The refrigerant control may be divided into three principal classes:1. Control based on pressure changes

2. Control based on temperature changes

3. Control based on volume or quantity changes

A combination of controls may also be used.

Automatic refrigerant and motor control are needed that will maintain the temperature in

the refrigerated space within specific limits.

For comfort cooling, temperatures are usually maintained not to exceed 10 to 12F. (5.5 to

6.6C).



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Automatic Expansion Valve Principles

An automatic expansion valve is a refrigerant control operated by the low-side pressureof the system. Its purpose is to throttle the liquid refrigerant in the liquid line down to a

constant pressure on the low-pressure side while the compressor is running.

The valve acts like a spray nozzle. While the compressor is running, the liquid refrigerant

is sprayed into the evaporator. A system using an automatic expansion valve is some

times called a “dry” system. This is because the evaporator is never filled with liquidrefrigerant, but with a mist or fog.

Automatic Expansion Valve Design

The diagram in Fig 4-1 is of a flexible bellows linked up to a needle valve with

evaporator pressure, P2 on the inside and atmospheric or confined gas pressure P1 on the

outside. F1, spring force tends to open the valve, while spring force, F2 tends to close the

valve.

Fig 4-1 :- Automatic expansion valve shows various pressures inside the valve which

cause it to operate. P1- Atmospheric pressure. P2 -Suction or evaporator pressure. P3

– Liquid line pressure. F1 – Adjustable spring force.F2-Nonadjustable spring force.



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Fig 4-2:- Diaphragm type pressure expansion valve. Force 1, adjustable range

opening, moves diaphragm. Force 2 moves push rod and ball assembly. Force 3 is

evaporator pressure. Valve is designed to control flow of refrigerant to evaporator,

maintaining a constant evaporator pressure.

From the illustration, it may be seen that, as the pressure in the evaporator decrease, the

difference in pressures will force the bellows toward the valve body. Since it is attached

to the needle, it will open the needle valve and some liquid refrigerant will spray into theevaporator. As the refrigerant evaporates at a constant low pressure, it keeps the

evaporator and cabinet temperature within their design limits.

The expansion valve opens only when the evaporator pressure drops. Pressure drop

occurs only when the compressor is running. The expansion valve, however will not

flood the low side when the compressor is running. As soon as the evaporatingrefrigerant, liquid, spray and vapor reach the evaporator, the motor control (sensing bulb)

attached to the suction line will cool. This opens the switch and stops the motor. The low

side pressure will then immediately build up enough to close the expansion valve.

These valves are adjustable to permit opening of the needle valve over a wide range of pressures. Expansion valves must be adjusted for atmospheric pressure, P1 which affectstheir operation. High altitudes will cause a decrease in atmospheric pressure. The

adjusting screw must be turned in to make up for the lower atmospheric pressure. Various

refrigerants have different expansion valve setting. Their evaporating pressures are not

the same.



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Automatic expansion valves are of many different designs. The flexible part is either a

diaphragm or a bellow. Usually it is made of phosphor bronze soldered or brazed to the

valve body. Flexible elements must move in and out time after time without losingflexibility. The valve body is usually drop forged brass, but sometimes it is cast. It must

be seepage (leak) proof.

The liquid inlet has either a soldered connection, a standard flange, a flared connection,

or a pipe thread. It usually has a screen designed for easy removal. The screen is made of

brass or stainless steel wire 60 to 80 mesh. (This means 60 to 80 openings per squareinch.)

An AEV with a double spring on the needle balances the forces and gives smoother

control of the refrigerant flow. Such a unit is shown in Fig 4-2.

It is important to remember that the liquid refrigerant flowing past the expansion valve

needle is the same weight as the vapor (gas) pumped by the compressor. Valve capacity

should equal pump capacity.

Use a “one ton Valve” with a one ton capacity condensing unit. An under capacity valvetends to “starve” an evaporator (too little refrigerant gets through). An over-capacity

valve will tend to allow too much refrigerant into the evaporator when the valve opens.

This may cause sweat backs or frost backs on the suction line.

Thermostatic Expansion Valve (TEV) Principles

Thermostatic expansion valves are of two basic types :

1. Sensing bulb

2.

Thermal-electric

The sensing bulb type is further divided into four types:

1. Liquid-charged2. Gas-charged

3. Liquid cross-charged

4. Gas cross-charged

In the liquid charged and the gas charged elements, the refrigerant is the same as is used

in the system. Cross charged means that the fluid in the sensing bulb is different than therefrigerant in the system.

In the automatic expansion valve, the refrigerant flow (through the valve and into theevaporator) is controlled only by the pressure in the evaporator.

In the thermostatic expansion valve, the flow through the valve and into the evaporator is

controlled by both the low-side pressure and the temperature of the evaporator outlet. The



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valve provides a rather high rate of flow if the evaporator is quite empty (warm). It slows

up the flow as the evaporator fills (cools) with refrigerant.

The sensing bulb type thermostatic expansion valve is operated by the accumulated

pressure difference or force difference between the sensing bulb bellows and the valve

low-side pressure. See Fig 4-3.

Fig 4-3:- Thermostatic expansion valve showing various pressures and temperatures

within the valve which cause it to operate. F1- Sensing bulb pressure (force) tending

to open valve. F2 – Low-side pressure (force) tending to close valve. F3 – Springforce tending to close valve. P1 – Sensing bulb pressure tending to open valve. P2 –

Suction pressure (low-side) tending to close valve. T1 – Sensing bulb temperature. T2

– Evaporator refrigerant temperature (low-side). Valve opens when F1 is greater

than combined force of F2 and F3. The valve closes when the combined F2 and F3

forces are greater than F1.

With the unit running, the refrigerant, T1, in the expansion valve sensing bulb is usually

about 10F. (5.6 C) warmer than the refrigerant in the evaporator, T2. This temperature

difference produces the different pressure and therefore the different force.

This means that the unit pressure in the sensing bulb, P1 is greater than the unit pressureP2, in the evaporator. This temperature difference is often described as the superheat of

the bulb over the refrigerant temperature inside the evaporator. It should be noted that, asthe temperature increase or decrease, the pressure will also increase or decrease.

When the compressor stops, the low-side pressure and the sensing bulb pressure tend toequalize. The total expansion valve internal force, F2 plus F3 overpowers the sensing bulb



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force, F1 and the needle is forced firmly into its seat. Refrigerant flow stops. The needle

will stay closed until the sensing bulb force overcomes the low-side force.

This valve opening action should only happen when the unit is running. If the valve is

adjusted correctly, the closing of the valve while the compressor is idle prevents the

flooding of the low-side with the liquid refrigerant.

The thermostatic expansion valve does not regulate the low-side pressure, but rather

controls the filling of the evaporator with refrigerant. The pumping action of thecompressor establishes the low-side pressure. Fig 4-4 illustrates a bellows type

thermostatic expansion valve.

Fig 4-4: Thermostatic expansion valve. A- Adjusting nut. B-Seal ring. C- Capillary

tube. D – Bellows housing. E – Housing spacer. F – Temperature sensing bulb. G-

Body bellows. H- Screen. I- Gasket. J-Refrigerant inlet. K-Needle pin. L-Sealed

Fitting. M- Needle. N- Seat. O- Evaporator connection. P-Inner spacer. Q – Spacerrod. R- Snap ring. S-Thermal bellows.

Fig 4-5: A thermostatic expansion valve low-side pressure time cycle diagram using

high superheat adjustment. A- Pressure drop on the low side between opening of the

valve and cut out point.



[4-7]

Fig 4-6:- A thermostatic expansion valve low-side pressure time cycle diagram using

a low superheat adjustment. A – Pressure drop on low side between the opening of

the valve and the cut out point. Note: Compare with A, in Fig 4-5, to understand

how superheat adjustment controls the evaporator (low side) pressure.

Fig 4-7:- A pilot-controlled thermostatic expansion valve. A-pilot valve body. B-

Temperature sensing bulb. C- Refrigerant line, in. D-Connection to the evaporator.

The adjustment enables one to set the valves so the needle can seat itself sooner while theunit is running. The needle will then close even though there is a greater temperature

difference (about 15F or 8.3C) between the refrigerant in the evaporator and that in the

sensing bulb, as shown in Fig 4-5.The evaporator liquid refrigerant will not reach thesensing bulb location in this case. The temperature of only the low-pressure vapor will becold enough to reduce the sensing bulb temperature (and therefore the pressure) to the

closing point. The needle will close before the evaporator becomes full of liquid

refrigerant droplets and the evaporator will be “starved”.

If the adjustment is turned in too much the other way (one or two revolutions) to move

the needle away from the seat, the temperature of the sensing bulb refrigerant will



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become closer to the temperature of the evaporator refrigerant 5 to 7F. (2.8 to 3.9C). See

Fig 4-6.

The evaporator must now become more than full of liquid refrigerant droplets to bring the

temperature (pressure) different down to this value. The evaporator will be completely

flooded. Some liquid droplets may even go into the suction line causing a sweating orfrosting of the suction and may harm the compressor (slugging). This adjustment is

sensitive and should never be turned more than one-quarter of a turn each 10 to 15

minutes while the unit is operating.

Some thermostatic expansion valves use diaphragms instead of bellows. In either design,

the valve is closed when the unit is not running.

In some large refrigeration installations (50 tons and over) a pilot controlled thermostatic

expansion valve may be used. In these installations, a conventional thermostatic

expansion valve is mounted on a large auxiliary valve body. The auxiliary valve provides

a larger pressure operated needle and orifice. The conventional thermostatic refrigerantcontrol (pilot) orifice control. Fig 4-7 illustrates one type of pilot controlled thermostatic

expansion valve.



[4-9]

Thermostatic Expansion Valve Design

Fig 4-8: Effect of temperature on sensing bulb of thermostatic expansion valve. A-

Sensing bulb is cold, the pressure is low, and a considerable quantity of control fluid

is shown as a condensed liquid in the bulb (solid red). B – Sensing bulb is warmer

and some of control fluid has evaporated (red dots) and is exerting pressure onexpansion valve diaphragm, which will cause it to admit more refrigerant into the

evaporator. Note that for accurate control, there is enough control fluid in the

sensing bulb to ensure control fluid in the sensing bulb at all times. Red cross-

hatching indicates suction line pressure.



[4-10]

Fig 4-9:- Diaphragm thermostatic expansion valve. Turning adjustment screw in

will starve evaporator. Turning it out will flood evaporator.

Fig 4-10:- Parts of a thermostatic expansion valve. In this valve, the thermostatic

element is threaded on body of valve.



[4-11]

Fig 4-11:- This cutaway shows a thermostatic valve orifice that has been designed

for a large capacity. Note that both valve and valve seat are formed by flat surfaces.

Arrows indicate direction of flow of refrigerant through valve seat mechanism.

Fig 4-12:- Exterior view of thermostatic expansion valve. Valve may be adjusted

after removing sealing cap.

Thermostatic expansion valves are usually used on multiple evaporator systems.However, the low side float may also be used on multiple systems. It is possible in a

multiple system using thermostatic expansion valves to provide a variety of temperaturesin the various cabinets. This valve is also commonly used on air conditioning systems.

One must choose the correct sensing element charge and the correct valve size for each

installation.

The thermostatic expansion valve consists of a brass body into which the liquid line and

evaporator line are connected. The needle and seat are inside the body. The needle is

joined to a flexible metal bellows or diaphragm. This bellows, in turn, is made to move by a rod connected at the other end to a sealed bellows or diaphragm (power element)

which is joined to the sensing bulb by means of a capillary tube.

Fig 4-8 shows the refrigerant behavior in the sensing element. Each manufacturer has a

code for identifying the fluid that charges the sensing elements. Some use letters. Others

use colors or numbers to identify the charge. Some valves are marked with the refrigerantnumber. Some valves intended for use with refrigerant R-12 are color coded yellow.



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The valve is sealed to prevent moisture seeping into any part. A strainer (screen) is

always located between the liquid line connection and the orifice to keep dirt away from

the needle and seat. See Fig 4-9.

Some large air conditioning systems may use as many as six thermostatic expansion

valves on one evaporator. In this way it is possible:

1. To maintain constant pressures and temperatures

2. To make sure that all of the evaporator has a full charge of refrigerant.3.

To reduce pressure drop through the evaporator

Fig 4-10 is an exploded view of a thermostatic expansion valve. The body is usually

made of brass.

Note that the inlet flare surface is mounted on the strainer. The pin or needle is usually

made of Stellite, Hastelloy or stainless steel. The needles are usually sharp pointed cones,

but spherical valves (balls) and flat orifice closers may also be used. The cone needle is popular for small capacity valves, while the ball type or the flat type is used in larger

capacity valves.

Fig 4-11 illustrates a large capacity flat valve seat. It is always good practice to place a

filter-drier in the liquid line immediately ahead of the thermostatic expansion valve.

Fig 4-12 shows the outside of a diaphragm type TEV with a ball type valve.

It is the single diaphragm type designed particularly for service in air conditioningapplications. It is equipped with a bleed valve for rapid pressure balancing (RPB). The

bleed mechanism works only on the off cycle. When the compressor starts up again, the

secondary bleed port closes and the valve operates in a normal manner. Rods carry thediaphragm action to the needle. The liquid inlet is on the left, the evaporator connection

on the right.

Thermostatic Expansion Valve Capacities

The capacity of a thermostatic expansion valve (TEV) varies according to:

1. Orifice size

2. Pressure difference between the high side and the low side.

3. The temperature and condition of the refrigerant in the liquid line. The amount offlash gas will increase with a rise of liquid line temperature.

The capacity of most thermostatic expansion valves may be selected from the size of theorifice and needle assembly. The some body may be used for many capacities.

The larger the orifice, the more liquid refrigerant that can be fed into the evaporator for

each unit of time.



[4-13]

Valves are rated in tons of refrigerant. However, the same orifice usually has three

different tonnage capacities. This range of capacity depends upon the different tonnagecapacities. This range of capacity depends upon the difference in pressure between the

high side and the low side.

Increasing this pressure difference will increase the rate of refrigerant flow. Therefore, if

the valve is used on an R-12 refrigerant system, a valve that has ½ ton (0.455 metric ton)

rating at 13 psi (0.9kg/cm2) pressure on the low side, it will have a ¾ to 1 ton. (0.72 to

0.98 metric ton) capacity at 5 in. (12.7cm) Hg. vacuum on a freezer unit. A 1/3 ton (0.3

metric ton) will have only a capacity at a low-side pressure of 40 psi (2.8kg/cm2) on an

air conditioning system.

In the first case, there is a 130 – 13 = 117 psi pressure difference, assuming a 130 psi (9.1

kg/cm2) head pressure. In the second case, it is a matter of 130 plus 2 ½ psi (5 in. of

vacuum = 2 ½ psi = 0.18kg/cm2) = 132 ½ psi (9.3 kg/cm

2) pressure difference. In the last

case, it is 130 – 40 = 90 psi (6.3kg/cm

2

) pressure difference.

It is important to use a valve of the correct capacity. If the valve orifice is undersized, theevaporator will be starved regardless of the superheat setting. The full capacity of the

evaporator cannot be reached.

If the orifice is oversized, the valve will “hunt” or surge. When the valve opens, too muchrefrigerant will pass into the evaporator and the suction line will sweat or frost before the

thermal element can close the valve. If one tries to correct this condition by increasing the

superheat setting, the evaporator will be starved most of the time.

Types Of Solenoid Valves

Fig 4-13:- Two way solenoid valve. A- De-energized, close position. B-Energized,

open position.



[4-14]

Fig 4-14:- Three-way solenoid valve used to close a thermostatic expansion valve

during off part of a cycle. When compressor is running, openings marked suction

and common are connected. When compressor is off, high pressure inlet andcommon are connected.

Three types of solenoid valves are in common use:

1. The two way valve which controls the flow of refrigerant through a single line.

See Fig 4-13.

2. The three way valve with an inlet which is common to either of two oppositeopenings. It controls refrigerant flow in two difference lines. This valve is

illustrated in Fig 4-14.

3. The four way reversing valve which is used often on heat pumps.

Solenoid valves may be turned on by means of a thermostat and used to control the

temperature of a refrigerator or a room.

Three way solenoid valves are used mainly on commercial refrigerating units. They may

be used to control two separate refrigerant circuits for defrosting, two temperatureevaporators and so on.

In the three way solenoid valve shown in Fig 4-14, opening A is the common opening

and is never closed. As the electromagnet is de-energized, the weight of the solenoid plunger assembly and the force of the upper spring hold the valve firmly against the

lower seat. This action closes port B to the common port and opens port C.

When the electrical circuit is closed, the solenoid becomes energized. The piston with the

two valves attached to it will move in the opposite direction. This action opens port B in

Fig 4-14 to common port A and thereby closes port C to the common port.

Four-way solenoid valves are often called reversing valves. They are used chiefly on heat

pumps to control the cycle for either heating or cooling, as needed.



[4-15]

When the solenoid is de-energized, the valve stem closes several ports and opens others

to reverse the flow of the refrigerant to the condenser and evaporator. The heat pump then becomes a cooling system.

When the solenoid valve becomes energized, the four way valve stem is drawn upward.The heat pump system becomes a heating system again.

For large commercial applications, it is desirable to use a pilot-operated solenoid valve.In these valves, the solenoid operates a pilot mechanism and the pressures in the lines

concerned operate on a piston arrangement. This causes the opening and closing of the

main or large valve.

Fig 4-15 is a cutaway of a pilot-operated solenoid valve. When solenoid A is energized,

plunger B will be pulled from its seat and the pressure in D will leave the cylinder and

will cause piston E to move up. The movement of the piston controls the opening and

closing of the main valve F.

When the solenoid valve is de-energized, plunger B returns to its seat. Pressure from Ggoes through a small opening H and builds up the pressure in control cylinder D. The

spring then closes control valve F.

Fig 4-15:- A Pilot operated solenoid valve. A- Solenoid. B- Solenoid plunger. C-

Pressure at the left end. D- Control cylinder. E- Control piston. F-Control valve. G-

Pressure at the right end. H-Opening.

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