THERMOSTATIC EXPANSION VALVES - RSES.org expansion valve (sometimes also called a thermal expansion...

INTRODUCTION

The thermostatic expansion valve (frequently abbre-viated TEV or, sometimes, TXV) has played a signifi-cant role in the progress of the HVAC/R industry andits technology. Its value is based on its refrigerantcontrol capability and its adaptability to the many andvaried applications of the refrigeration cycle.

The development of many system components hasbeen a by-product of technical evolution. This appliesto the thermostatic expansion valve. In the early daysof mechanical refrigeration, a hand-operated needlevalve was used for refrigerant control. This devicegave some measure of control in applications wherethe load was constant. However, it could not respondto other conditions affecting the amount of refrigerantpassing through it—for example, changes in liquidpressure caused by variations in compressor headpressure. The manual expansion valve required con-stant supervision in locations where a varying loadcould produce starved evaporator or liquid refrigerantfloodback conditions.

The introduction of “float”-type metering valves was astep toward automatic refrigerant flow. One type wasdesigned to control refrigerant in the low side, andanother in the high side. The float-type valve oper-ated in the same manner as a boiler water-level con-trol to maintain a flooded evaporator. It was highlyefficient in serving that purpose—however, it couldnot be applied to a dry or direct-expansion evapora-tor system, which was gaining favor. Even so, thefloat-type valve did not disappear from the field. It isstill used in industrial process refrigeration systems.It is also used in large water chillers, especially thosewith centrifugal compressors.

Efforts to overcome these inadequacies producedwhat is called the “automatic expansion valve.” Amore accurate description would be a “constant

evaporator pressure control valve.” That is, it main-tains a constant outlet pressure, regardless ofchanges in inlet liquid pressure, in load, or in otherconditions.

The automatic expansion valve was a decided im-provement over the hand-operated expansion valve.It maintained temperature more evenly, and con-trolled the frost line of the evaporator better. Further-more, it closed the liquid line when the compressorstopped, and prevented excessive flooding when thecompressor started.

However, this device also had disadvantages andlimitations. It tended to overfeed refrigerant to theevaporator when the heat load was low, as at the endof the run. It also tended to starve the evaporatorwhen the heat load was high, as at the start of therun. Thus, its pull-down of temperature was slow. Itdid not take advantage of the full area and capacityof the evaporator at the start of the refrigerationcycle.

In the late 1920s, another device—the thermostaticexpansion valve—was introduced. It overcame thelimitations of both the hand-operated expansionvalve and the automatic expansion valve. Figure 1 onthe next page shows a typical example of a modernthermostatic expansion valve (sometimes also calleda thermal expansion valve). The purpose of the TEVis to control the flow of refrigerant to an evaporator.

If the evaporator were to remain active through allvariations in the heat load, it would contain only liquidand saturated vapor—no superheated vapor. In prac-tice, this is not a desirable condition. Some of theevaporator must be used to superheat the vapor, orelse there will be liquid refrigerant in the suction line,which can slug the compressor. The TEV is actuallya constant superheat valve—a more descriptive andaccurate name, though rarely used.

1

Refrigeration ServiceEngineers Society1666 Rand RoadDes Plaines, Illinois 60016

THERMOSTATIC EXPANSION VALVESPart 1

Revised by Loren Shuck, CMS

© 2001 by the Refrigeration Service Engineers Society, Des Plaines, ILSupplement to the Refrigeration Service Engineers Society.

620-40ASection 5E l

This document is a revision of prior publication 620-40.

The name of this device can give the impression thatit directly controls temperature. Some service techni-cians mistakenly have tried to adjust a thermostaticexpansion valve to control refrigerator temperature.

This chapter covers the theory of operation, properselection, and application of thermostatic expansionvalves. The control of superheat is one of the basicfunctions of thermostatic expansion valves. The dis-cussion that follows will better acquaint you with theprinciple of superheat.

TEMPERATURE-PRESSURE RELATIONSHIPS

If you are familiar with the concept of temperature-pressure relationships already, then you should havelittle difficulty understanding the basic function of athermostatic expansion valve. Figure 2 below illus-trates the changes that take place in 1 lb of water atatmospheric pressure.

ª Sensible heat is added. It raises the temperaturefrom 32°F to 212°F (from the freezing point to theboiling point).

ª Heat (sensible heat) is continually added until thetemperature reaches 212°F. At this point, thewater must absorb another 970.4 Btu/lb (latentheat of vaporization) to change completely tosteam at 212°F.

ª Additional sensible heat raises the temperatureof the steam above 212°F.

2

Figure 1. Typical thermostatic expansion valve (TEV)

SP

OR

LAN

VA

LVE

CO

MPA

NY

232

212

32Latent heat of vaporization = 970.4 Btu

Boiling point Point of complete

vaporization

200 400 600 800 1000 1200

20°F superheat (232°F – 212°F = 20°F)

Saturated steam @ 14.7 psia

Superheated vapor

Tem

per

atu

re,°

F

Enthalpy, Btu/lb

Figure 2. Water at atmospheric pressure

The temperature above 212°F is calledsuperheat. It is still measured in degrees.Figure 2 shows an example. The tempera-ture of the steam at atmospheric pressureis 232°F, which means that it has beensuperheated 20°F (232°F – 212°F = 20°F).

The principles are the same when you ana-lyze the effect of superheat on a refriger-ant. Figure 3 shows the effect of superheaton 1 lb of liquid R-22. At atmospheric pres-sure, the addition of heat makes it boil at–41°F. After all the refrigerant vaporizes,any added heat will raise its temperatureabove –41°F, or superheat it. Note that ittakes 100 Btu (latent heat of vaporization)to vaporize the entire pound of liquid R-22.Adding more heat will superheat the vapor.If the temperature is raised to –21°F, thevapor has been superheated 20°F.

Figure 3 is only an example of the principle of super-heat. In fact, service technicians do not work with anyrefrigerant at atmospheric pressure. You alreadyknow that reducing pressure on a liquid lowers itsboiling point. It then takes less heat to vaporize it.Conversely, increasing the pressure on a liquid raisesthe boiling temperature.

Figure 4 above shows the boiling points of R-22 atvarious pressures. Note the curved line. It shows howthe saturation or boiling temperature changes aspressure changes. As the pressure increases, ittakes a higher temperature for the refrigerant to boil.The curved line also represents the R-22 pressure-temperature characteristics. The horizontal lineshows the temperature in degrees Fahrenheit. The

3

0* 10 20 30 40 50 60 70 80 90 100

10

20

0

–10

–20

–30

–40

–50

–60

Tem

per

atu

re,°

F

Enthalpy, Btu/lb

Latent heat of vaporization = 100 Btu

Boiling point Point of complete vaporization

20°F superheat [(–21°F) – (–41°F) = 20°F]

Saturated R-22 @ 14.7 psia

Superheated vapor

*Based on 0 Btu for the saturated liquid at –40°F

Figure 3. R-22 at atmospheric pressure

5 10 20 40–20

–22

–40

50

60

70

40

30

20

10

68

28

9

Temperature, °F

Pre

ssu

re,p

sig

Pressure, Temperature, psig °F

68 40 28 5 9 –22

Curve shows refrigerant pressure-temperature characteristics

Figure 4. Boiling points of R-22 at various pressures

vertical line shows pressure in psig. The dotted linesin Figure 4 show some of the coordinates on thecurve at which the refrigerant will boil.

Follow this relationship up and down the pressure-temperature scale in Figure 4. You can determine aseries of boiling points—for example, 40°F at 68 psig,5°F at 28 psig, –22°F at 9 psig, etc. The curve itselfis simply a line drawn through all of these points.Other refrigerants have different curves. No tworefrigerants have identical pressure-temperaturecharacteristics.

THE EFFECTS OF SUPERHEAT ON A REFRIGERATION SYSTEM

Now that the basic principle of superheat has beencovered, let’s relate the various factors to a simplerefrigeration system. Assume that the system consistsof a bare tube evaporator, compressor, condenser,and receiver. It also has the most basic of refrigerantcontrol devices, the hand-operated expansion valve.Figure 5 shows such a basic system.

Opening the hand expansion valve slightly allows asmall amount of refrigerant to enter the exposed coil.Air around the coil is at a higher temperature than therefrigerant. When the heat from the air penetrates the

tubing wall, it causes the refrigerant in the coil to heatup and boil. Assume that the refrigerant flow is at arelatively slow rate. If this is the case, all the liquidboils into vapor close to the entry point. You can seean example of this at point A in Figure 5, where thetemperature is 20°F.

From this point on, the boiling vapor picks up super-heat. The farther the vapor travels and the longer it isexposed to heat, the greater the superheat becomes.At point B, the vapor’s temperature is 30°F with 10°Fsuperheat. At the fifth return bend, the temperature is40°F with 20°F superheat.

Now look at point C. The difference between vaportemperature (50°F) and boiling temperature (20°F)equals 30°F of superheat. At this point, the suctionpressure of the compressor is 43 psig.

Opening the hand expansion valve farther increasesthe flow of refrigerant. Figure 6 shows the result. Theincreased flow rate means that the refrigerant mustreach point B before complete vaporization takesplace. This reduces the evaporator coil surface that isavailable for building up superheat. As a result, thereis only 20°F superheat at point C. Higher suctionpressure results from the greater load placed on thecompressor by the additional refrigerant.

4

Condenser

Receiver

Liquid line

Hand expansion

valve

20°F20°F

Point of complete vaporization

30°F10°F superheat


43 psig

Head pressure

C

A

B

43 psig suction pressure 50°F gas temperature

30°F superheat

Compressor

Figure 5. Simple refrigeration system

Now look at Figure 7. The hand expansion valve isopen even farther. So much refrigerant flows that itcannot completely vaporize in the evaporator. Thiscauses liquid to flood back to the compressor. Theresult is wasted refrigerant and greatly reduced com-pressor capacity. The greatest concern is possible

damage to the compressor. The compressor isdesigned to compress a gas, not a liquid.

These examples demonstrate the ideal evaporatorcondition. The goal is to keep the point of completevaporization at the outlet of the evaporator. Note in

5

Condenser

Liquid line

Hand expansion

valve

21°F

21°F

21°F



44 psig

Head pressure

C

A

B


20°F superheat

Compressor

Receiver

Figure 6. Additional flow reduces superheat

Condenser

Liquid line

Hand expansion

valve

25°F

25°F

25°F

25°F

Flood back

49 psig

Head pressure

C

A

B


0°F superheat

Compressor

Receiver

Figure 7. Too much flow produces floodback

Figure 8 that superheat is 0°F at point V. Ideally, theentire surface of the evaporator performs its refriger-ation function. Only dry, superheated vapor reachesthe compressor.

In practice, it is impossible to obtain this condition.However, the TEV allows you to reach a level close tothe ideal. It takes only several degrees of superheat

to open the valve. The addition of several degreesproduces a steady, rated flow of refrigerant.

Figure 9 shows the level of efficiency achieved withthe TEV.You have seen that low superheat at the out-let of the evaporator is desired. Thus, you place theTEV bulb at this point to measure this condition. Inthe proper position, its temperature will be about

equal to suction gas temperature.

Before examining TEV operation indepth, first review the following terms.They are closely associated with valveoperation:

Liquid line. The piping that carries liq-uid refrigerant from the condenser tothe expansion valve.

Refrigerant saturation temperature.The temperature at which refrigerantboils in the evaporator.

Complete vaporization. The point atwhich liquid refrigerant is entirelychanged to vapor, governed by theamount of refrigerant in the evaporator.

6

Condenser

Liquid line

Hand expansion

valve

24°F

24°F

24°F

24°F

Point of complete

vaporization

29°F5°F superheat

47.6 psig

Head pressure

C

A

V

B

47.6 psig suction pressure 34°F gas temperature

10°F superheat

Compressor

Receiver

Figure 8. Proper flow results in correct superheat

Liquid line

Point of complete

vaporization

Remote bulb

Superheat Suction gas temperature

Refrigerant saturation temperature

Thermostatic expansion valve

Figure 9. TEV installed at entrance to evaporator

Superheat. The heat added to vapor beyond thepoint of complete vaporization.

Suction gas temperature. The temperature ofrefrigerant vapor measured along the suctionline between the evaporator and the compressor.

THERMOSTATIC EXPANSION VALVE OPERATION

Principal parts

Figure 10 shows the essential parts of a typicalthermostatic expansion valve—the diaphragmcase, capillary tube, bulb, seat, pin carrier,spring, spring guide, adjusting stem packing,adjusting stem, push rods, and inlet strainer.

Note that there is no direct mechanical linkagebetween the adjustment stem and the pin. Theadjusting stem serves only to vary the springpressure.

The diaphragm, inside its case, reacts to bulbpressure. It transmits its motion through the pushrods to the top of the pin carrier. When the pinmoves away from the seat, liquid refrigerantflows into the evaporator. The pin carrier is longenough to ensure straight-line movement andprecise pin-seat contact. As noted, the adjustingstem serves only to control spring pressureagainst the pin carrier.

Operation

From the positioning of its parts, it may seem like theprinciple of TEV operation is bulb pressure opposingspring pressure. But this is not the case. There arethree fundamental pressures that affect the openingand closing of this control device:

ª bulb pressure

ª spring pressure

ª evaporator pressure.

Figure 11 shows the application of these three fun-damental pressures. A thorough knowledge of therelationship among these three pressures is needed

7

Diaphragm caseCapillary

tube

Seat

Pin carrier

Spring Bulb

Spring guide

Adjusting stem packing

Adjusting stem

Push rods

Inlet strainer

Figure 10. Essential parts of a TEV

SP

OR

LAN

VA

LVE

CO

MPA

NY

Bulb pressure

Spring pressure

Bulb temperature = saturation temperature

+ superheat

➀

➂

➁Evaporator pressure

Figure 11. TEV operation

for complete understanding of valve operation. Asshown in Figure 11 on the previous page, in order forthe valve to open, bulb pressure (1) must equal thesum of the evaporator pressure (2) and the springpressure (3).

Evaporator pressure is a key factor in the operationof a thermostatic expansion valve. Unfortunately, it isoften overlooked in an analysis of valve operationproblems. Evaporator pressure is applied under thediaphragm in the direction of closing the valve, asshown in Figure 12 above. It is important because ofits relationship to refrigerant saturation temperature.Evaporator pressure always equals the pressure cor-responding to refrigerant saturation temperature.Look back at Figure 4 for a moment. It shows exam-ples of this pressure-temperature relationship. Oneexample is that a saturation temperature of 40°F forR-22 produces an evaporator pressure of 68 psig.

In addition, the spring transmits pressure through thetop of the pin carrier and the push rods. It is alsoapplied to the underside of the diaphragm. Thus,spring pressure further implements closing of thevalve.

Figure 13 shows more of the evaporator. It shows thepoints at which superheat and suction gas tempera-tures can be measured.These temperatures are used

to determine the final pressure—that of the bulb. Bulbpressure is applied through the capillary tube to thetop of the valve diaphragm. This pressure opposesthe sum of the evaporator pressure and the springpressure. It acts to open the valve. When a system isoperating, the bulb charge is part liquid refrigerantand part saturated vapor. Bulb temperature equalssuction gas temperature. Suction gas temperature ishigher than refrigerant saturation temperature. Theincrease is produced by superheat.

The higher bulb temperature applies more pressureto the top of the diaphragm than the opposing evap-orator pressure. However, opening the valve requiresenough pressure difference (increased superheat) toovercome the additional effect of spring pressure.Figure 14 provides a closer look at how the bulbpressure acts on the top of the diaphragm.

When the bulb and system are charged with thesame refrigerant, both the evaporator pressure underthe diaphragm and the bulb pressure on top followthe same saturation curve.

Consider the results of this relationship. Pressure onthe top of the diaphragm is induced by bulb satura-tion temperature. Pressure on the bottom is induced

8

Diaphragm

Evaporator pressure



Figure 12. Evaporator pressure relates to refrigerant saturation temperature

Bulb pressure

Bulb

Suction gas temperature


Superheat

Figure 13. Bulb pressure acts to open valve

by evaporator saturation temperature. They becomeequal and opposing, canceling each other out. Youcan see, then, that pressures induced by refrigerantsaturation temperature exert equal force. They act onboth sides of the diaphragm. This leaves superheatand spring pressure the two factors that regulate thevalve.These two opposing factors maintain a delicatepressure balance on both sides of the valvediaphragm. This permits the valve to operate over awide range of evaporator pressures. It can handlelight as well as heavy loads on the evaporator. Thethermostatic expansion valve is, in effect, a super-heat regulator.

Occasionally you may hear a service technician say,“I opened the expansion valve” or “I closed theexpansion valve.” Actually, the adjustment was simplya change in spring pressure. Increased spring pres-sure requires more pressure induced by superheat tooppose it. The valve throttles back. On the otherhand, reduced spring pressure requires less pres-sure induced by superheat to oppose it. The valveopens.

For any spring setting, when the load on the evapo-rator increases, the valve will underfeed the evapora-tor. As a result, suction pressure will drop or bulbtemperature will increase—or both condi-tions may exist. In any case, the valve willopen wider. If the evaporator load isreduced, the valve will overfeed the evapo-rator. As a result, suction pressure willincrease or bulb pressure will decrease, orboth. Existing spring pressure will throttlethe valve to the closed position.

Figure 15 illustrates conditions in a typicalsystem, and shows a closer relationshipbetween theory and practice. Evaporatorpressure is 52 psig, and spring pressure is12 psig. They exert a combined closingforce of 64 psig. Assume that the bulb ischarged with the same refrigerant as thesystem. A bulb pressure of 64 psig will berequired to equalize pressure on both sidesof the diaphragm. At 28°F refrigerant satu-ration temperature, a superheat of 9°F isrequired to increase suction gas and bulbtemperature to 37°F. This would producethe required 64 psig in the bulb.

9

Bulb pressure

Bulb

Spring pressure

Superheat

Evaporator pressure


Figure 14. Bulb pressure and the effect of superheat

Diaphragm

Bulb pressure 64 psi Converted to temperature 37°F

37°F

Evaporator outlet pressure

52 psi

52 psi

Spring pressure 12 psi

Evaporator inlet pressure 52 psi

52 psi

Figure 15. Superheat equals bulb temperature minussaturated evaporator temperature

In a system idle for a long period of time, pressure inthe bulb and above the diaphragm will be almostenough to overcome spring pressure and open thevalve. Compressor start-up will immediately lower therefrigerant pressure under the diaphragm. The bulbpressure will then be enough to oppose the springpressure. It will force the diaphragm down and openthe valve.

Liquid refrigerant will then flow through the openvalve as a spray. It will fill the evaporator with wet andsaturated vapor. If all of the liquid refrigerant evapo-rates and becomes superheated vapor before itreaches the suction line, the thermal expansion valveremains open.

The evaporator continues to absorb heat. The sur-rounding temperature will decrease until wet and sat-urated vapor extends to the point in the suction linewhere the capillary tube bulb is clamped. When thisoccurs, the reduced degree of superheat cools thesuction line and bulb. As a result, vapor pressure inthe bulb and above the diaphragm decreases. Thispermits spring pressure to close the valve, eitherwholly or partially.

Refrigerant flow is now stopped or reduced. Itremains so until superheat at the bulb locationincreases vapor pressure in the bulb and above the

diaphragm. When this pressure can overcome thecombined evaporator pressure and spring tensionbelow the diaphragm, the valve opens. This restoresor increases refrigerant flow into the evaporator.

When the system’s pressure or temperature controlstops the compressor, evaporator pressure immedi-ately increases. This is because it is no longer sub-ject to the compressor suction effect. It will increaseuntil the combined evaporator pressure and springtension pressure can overcome vapor pressure in thebulb and above the diaphragm. When it does, thediaphragm raises and the TEV closes.

PRESSURE DROP ACROSS THE EVAPORATOR

This discussion of TEV operation has not yet consid-ered pressure drop through the evaporator. It wasomitted for simplification. In actual system operation,however, pressure drop will exist and is a factor. Toovercome the effect of pressure drop, inlet pressureis used to operate the valve.

Inlet evaporator pressure acts on the underside ofthe diaphragm. It feeds through a passageway fromthe low-pressure side of the valve. This passagewayis called an “internal equalizer,” shown in Figure 16.(A thermostatic expansion valve also may have anexternal equalizer connection, which will be coveredin greater detail later in this chapter.)

By now, you should be able to form one firm conclu-sion. It is that the function of a thermostatic valve isto keep the refrigerating capability of the evaporatorat the highest level possible under all load conditions.It is not to be confused with a suction pressure con-trol device. Nor can it, in any way, control compressorrunning time.

REMOTE BULB CHARGES

The refrigeration service technician must know thecorrect replacement for an inoperative TEV. You mayface a situation where someone else has incorrectlyselected the existing valve. A major factor in valvereplacement is the type of remote bulb charge.

For correct valve replacement, you must be familiarwith refrigerant charges in the remote bulb and tubingof TEVs. There are three general types:

10

Push rods

Internal equalizer

Figure 16. TEV with internal equalizer

ª the liquid charge, in which the refrigerant used inthe remote bulb and tubing is the same as therefrigerant being controlled by the valve

ª the gas charge, a limited liquid charge

ª the cross charge, in which the refrigerant used inthe remote bulb and tubing is different from therefrigerant being controlled by the valve.

The liquid charge

Figure 17 shows a conventional liquid-chargedremote bulb and tube element. As you can see, thebulb is large enough and contains enough liquid sothat the control point is always in the bulb. Someupper refrigerant remains liquid regardless of bulbtemperature. The temperature of the diaphragm caseor capillary tubing may get colder than the bulb tem-perature. Some of the vapor may condense in eitherof them. However, there will always be enough liquidrefrigerant in the bulb to ensure control at that point.This becomes very important in low-temperatureapplications.

Look at Figure 18. Note the two curves, one on topof the other. You can see that, with the same refrig-erant in both the bulb and the evaporator, pressure-temperature relationships are identical. Now applythese curves to system operation. They show thatthere is always a direct relationship, across thediaphragm, of bulb pressure changes to evaporatorpressure changes. Thermostatic expansion valveswith liquid remote bulb charges normally are usedwhen there is a very narrow or limited evaporatortemperature range. This is a condition outside thescope of other remote bulb charges.

The gas charge

A TEV with a gas-charged remote bulb contains thesame refrigerant as that in the system. However, ituses a limited amount of liquid refrigerant. All of theliquid in the bulb will evaporate at a certain remotebulb temperature. The charge becomes a saturatedvapor. Further increases in remote bulb temperaturewill not cause any substantial increase in pressure.Such temperature increases will only superheat thevapor.

Thus, in a gas-charged valve, maximum pressure onthe upper surface of the diaphragm is limited by theamount of charge in the remote bulb. There is anequation that describes this relationship, which isshown in Figure 19 on the next page. Remote bulbpressure (P1) is equal to evaporator pressure (P2)plus spring pressure (P3), or P1 = P2 + P3.

This equation shows that if remote bulb pressure canbe limited to a certain level while spring pressureremains constant, evaporator pressure can be limitedto a certain level and no higher. When remote bulbpressure is limited, evaporator pressure is also lim-ited. The point at which all liquid in the remote bulb isevaporated is the maximum operating pressure(MOP) of the valve. Look at the example shown in

11

Bulb colder Bulb warmer

Control always in bulb

Figure 17. Bulb maintains control of power element in liquid-charged TEV

Opening force(b

ulbpr

essu

re)

Closing force(sp

ring pre

ssur

e

+ev

apor

ator

pres

sure

)

Temperature

Pre

ssu

re

Figure 18. Conventional liquid charge

Figure 20. Assume that the system pictured has anR-22 gas-charged expansion valve. It has a 100-psigmaximum operating pressure and a 10°F superheatsetting. Now, assume that the evaporator pressureincreases to 100 psig. The total force under thediaphragm will be 100 psig plus 11.5 psig (the springpressure setting for 10°F superheat). This equals111.5 psig. Remember that the charge in the gas-charged remote bulb and tube is limited. When theremote bulb reaches the saturation temperature thatcorresponds to 111.5 psig, the entire charge hasevaporated. Only vapor exists in the bulb. There willbe some added superheat of refrigerant gas leaving

the evaporator, but it will not produce enough pressurein the remote bulb to open the valve.

Evaporator pressure will now go down. It will dropbelow the MOP point of 100 psig. Only then can theremote bulb again control the valve and feed refrig-erant into the evaporator. Thus, a gas-charged TEValso provides positive motor overload protection onsome systems. This is because of the limiting effectof the maximum operating suction pressure. It alsoprevents possible floodback on start-up.

To lower maximum operating pressure, increase thesuperheat setting (increase spring pressure).To raiseit, decrease the superheat setting (decrease springpressure). This adjustment works because springpressure, together with evaporator pressure, actsdirectly on the remote bulb pressure through thediaphragm.

You should always install a gas-charged valve in alocation warmer than that of the remote bulb. Thereason for this is that a gas-charged power elementcontains liquid up to the MOP point. It is very impor-tant for this liquid to remain in the remote bulb, asshown in Figure 21. The remote bulb tubing mustnever contact a surface colder than the remote bulb.If it does, the charge will condense at the coldestpoint and the valve will not function properly.

Generally, the application of gas-charged thermosta-tic expansion valves is not limited. However, theywork most efficiently in such applications as water

12

P1 Bulb pressure

P3 Spring pressure

P2 Evaporator pressure

Figure 19. Evaporator pressure is limited bybulb pressure limit

111.5 psig maximum

Evaporator pressure 100 psig

Spring pressure 11.5 psig

Evaporator pressure (also suction pressure) must be less than 100 psig before valve pin will open, regardless of remote bulb temperature

Liquid is completely evaporated at 65°F and

111.5 psig

Figure 20. System with 100-psig maximum operating pressure

chillers and air conditioning units that oper-ate in a 30°F to 50°F evaporator temperaturerange.

The cross charge

With a cross-charged remote bulb, the pres-sure-temperature curve of the refrigerant inthe bulb crosses that of the refrigerant in thesystem.

When the system evaporator temperatureranges from +50°F to –100°F, the TEV in usewill incorporate one of the cross chargesavailable. This type of system often operateson what is termed a repetitive pull-downcycle. In this cycle, the system starts up at ahigh suction pressure. It shuts off when thepull-down reaches a much lower pressure.At the cutoff point, the compressor stops.Then the evaporator and remote bulb begin to warmup. The evaporator pressure rises more rapidly thanthe remote bulb pressure. As a result, the TEV closesquickly. It stays closed even though the bulb temper-ature may rise to a higher level than the evaporatortemperature.

When high suction pressure exists, it takes highsuperheat to produce enough bulb pressure to over-come the closing pressure. The evaporator and bulbpressure curves are farther apart at high pressures.As a result, valve opening requires higher than normalsuperheat. This is a desirable condition. It preventsflooding and helps limit the compressor load.

The difference between evaporator temperaturesand bulb temperatures create the operating super-heats of the cross-charged TEV. It is apparent, then,that a cross-charged valve must be adjusted for thebest superheat setting at the lowest evaporator tem-perature of the system in order to prevent floodbackof liquid refrigerant to the compressor.

Liquid cross charge. Cross charges may be eitherliquid or vapor. There are three types of liquid crosscharges. One is designed for use in the commercialtemperature range (from 40°F to 0°F). Another is forthe low temperature range (from 0°F to –40°F). Thethird is designed for the ultra-low temperature range(from –40°F to –100°F).

Vapor cross charge. Research by TEV manufactur-ers has led to the development of a vapor-chargedremote bulb. A vapor cross charge is designed forsystems that operate with evaporator temperaturesranging from +50°F to –40°F. The vapor cross-charged bulb meets all the requirements for liquid-charged and gas-charged bulbs. And it meets therequirements for several of the liquid cross-chargedbulbs available. A valve with this type of cross chargecan replace a valve with any other type of charge ifthe system operates in the evaporator temperaturerange noted. However, it is usually necessary toreadjust the superheat setting of the vapor cross-charged valve.

Surrounding temperature is not important to the loca-tion of a vapor cross-charged valve. This is becauseit cannot lose control if the valve is colder than theremote bulb.

You also can use a vapor cross-charged valve with acompressor, which requires motor overload protec-tion. It is also available without MOP, when motoroverload protection is not required. When there is norequirement for starting or overload protection, usethe non pressure-limiting type. This may be the casewith smaller commercial units or window air condi-tioners. When maximum operating pressure isrequired, you must consider correct MOP in selectinga replacement valve.

13

P2

Drop of liquid must be in remote bulb for proper control

Remote bulb loses control if charge

condenses at these points

Bulb

P3

P1

Figure 21. Control must remain in gas-charged remote bulb

14

0 50–40Evaporator temperature, °F

Su

per

hea

t,°F


Su

per

hea

t,°F

0 40

30

–40Evaporator temperature, °F

Su

per

hea

t,°F


Su

per

hea

t,°F

16° SH

9° SH

4° SH

16° SH

6° SH5° SH

20° SH

6° SH 6° SH

6° SH 6° SH

Normal range

Normal range

Normal range

Normal range

Liquid charge

1. Maintains control under cross-ambient conditions 2. Tends to allow floodback on start-up 3. MOP is not available

Gas charge

1. Loses control under cross-ambient conditions 2. Tends to prevent floodback on start-up 3. MOP is available

Liquid cross-charge

1. Maintains control under cross-ambient conditions 2. Tends to prevent floodback on start-up 3. MOP is not available

All-purpose vapor cross-charge

1. Maintains control under cross-ambient conditions 2. Tends to prevent floodback on start-up 3. MOP is available

Figure 22. Comparing the types of remote bulb charges

Comparing the types of remote bulb charges

Each of the four remote bulb charges discussed—theliquid charge, the gas charge, the liquid cross charge,and the vapor cross charge—has its own superheatcharacteristics. The curves in Figure 22 show theoperating limits of each type based on a superheat of6°F.

These curves may not reflect true values. They arepresented here simply to show the general advan-tages and disadvantages of each. From Figure 22,you can see why the vapor cross charge is a majordevelopment. It has the potential to maintain almostconstant superheat. This makes it possible to getmaximum refrigeration effect from an evaporator withtemperatures ranging from +50°F to –40°F. You canexpect consistent superheat adjustments for the fullrange of evaporator operation. This eliminates con-cern for floodback or starving of the evaporator at theextreme ends of the temperature range.

THE EXTERNAL EQUALIZER

Look back at Figure 16, which shows a TEV with aninternal equalizer. Pressure at the valve outlet to theevaporator is applied through the bottom of thediaphragm. It is applied through a hole drilled in thevalve or through the space around the valve pushrods. Now look at Figure 23. It shows a TEVwith an external equalizer. Note that packingaround the push rods keeps valve outlet pres-sure away from the evaporator side of thediaphragm.

Instead, suction pressure is applied to theevaporator side of the diaphragm through tub-ing. It is either soldered into an opening in thesuction line near the evaporator outlet or intoa reducing tee located there. This tubing isconnected to the external equalizer fitting onthe valve, as shown in Figure 23. The pre-ferred tubing location in the suction line isdownstream from the bulb.

Pressure drop between the valve outlet andthe evaporator outlet is the most important ofthe low-side pressure losses that affect valveperformance. Sizable pressure drops can becaused in the low side of systems by:

ª undersized evaporator tubing

ª extreme length of tubing per pass

ª improper joints

ª restrictive return bends

ª distribution headers of various types.

Certain pressure drops through the evaporator are ofconsequence:

ª above 3.0 psi in the air conditioning range

ª 2.0 psi in the commercial range

ª 0.75 psi in the low-temperature or food freezerrange.

These pressure drops will hold a TEV in a relatively“restricted” position. This will reduce system capacityunless a valve with an external equalizer is used. Theevaporator should be designed or selected for theoperating conditions. The valve should be selectedand applied accordingly.

The valve in Figure 24 on the next page has an inter-nal equalizer. Pressure at the valve outlet is on the

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Valve with external equalizer

Evaporator outlet pressure

Push rods

External equalizer fitting

Valve outlet

pressure

Push rod packing

Figure 23. TEV with external equalizer

bottom of the diaphragm. It is also assumed that theevaporator shown has a negligible pressure drop.Any large pressure drop through the evaporator willcause the valve to restrict refrigerant flow unless avalve with an external equalizer is used.

To understand this, look at Figure 25, which showsan example of an evaporator fed by a TEV with aninternal equalizer. As you can see, there is a sizablepressure drop of 16 psig.

The pressure at point C is 52.5 psig. This is 16 psiglower than at the valve outlet, point A. The 68.5 psigat point A is the pressure acting on the lower side ofthe diaphragm. The valve spring is set at a compres-sion equal to a pressure of 15.5 psig. Thus, therequired pressure above the diaphragm is 68.5 psigplus 15.5 psig, or 84 psig. This pressure correspondsto a saturation temperature of 50°F. Obviously, therefrigerant temperature at point C must be 50°F forthe valve to be in equilibrium.

But the pressure at this point is only 52.5 psig. Thecorresponding saturation temperature is 28°F. As aresult, a superheat of 22°F (50°F – 28°F = 22°F) isrequired to open the valve. An increase in superheatfrom 10°F to 22°F is needed. More of the evaporatorsurface must be used to produce this higher super-heated refrigerant gas. This means that the amount

of evaporator surface available for the absorption oflatent heat of vaporization of the refrigerant isreduced. The evaporator is said to be “starved.”

The thermostatic expansion valve is being held in asomewhat “restricted” position. The pressure under-neath the diaphragm (at the evaporator inlet) ishigher than the pressure at the remote bulb, point C.Remember that the pressure drop across the evapo-rator, which causes this condition, increases with theload. Thus, the “restricting” or “starving” effect isincreased when the demand on the expansion valvecapacity is greatest.

USING AN EXTERNAL EQUALIZER

The external equalizer type of valve must be used tocompensate for an excessive pressure drop throughan evaporator.The equalizer line can be connected inone of two ways:

ª It can connect into the evaporator at a pointbeyond the greatest pressure drop.

ª It can connect into the suction line at a point onthe compressor side of the remote bulb.

As a general rule of thumb, the equalizer line shouldbe connected to the suction line at the evaporator

16

P2 = 68.5 psig

P3 = 15.5 psig

A 68.5 psig = 40°F

B

68.5 psig = 40°F

84 psig = 50°FC 68.5 psig and 50°F

P1 = 84 psig

Figure 24. TEV with internal equalizer on evaporator with no pressure drop

outlet. When this arrangement is used, true evapora-tor outlet pressure is imposed under the valvediaphragm. Pressure drop through the evaporatordoes not affect operating pressures on the diaphragm.The thermostatic expansion valve will respond tothe superheat of the refrigerant gas leaving theevaporator.

Look again at the system shown in Figure 25. Nowassume that there are the same pressure conditionsin a system using a valve with the external equalizerfeature, as shown below in Figure 26. The samepressure drop still exists through the evaporator.Pressure under the diaphragm is now the same asthe pressure at the end of the evaporator (point C, or

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68.5 psig = 40°F

B

C

52.5 psig = 28°F

84 psig = 50°F52.5 psig and 50°F

P2 = 68.5 psig

P3 = 15.5 psig

A

P1 = 84 psig

Figure 25. TEV with internal equalizer on evaporator with 16-psig pressure drop

68.5 psig = 40°F

B

C

52.5 psig = 28°F

68.5 psig = 40°F52.5 psig and 40°F

P2 = 52.5 psig

P3 = 16 psig

A

P1 = 68.5 psig

Figure 26. TEV with external equalizer on evaporator with 16-psig pressure drop

52.5 psig). The pressure required above the dia-phragm for equilibrium is 52.5 psig plus 16 psig, or68.5 psig. (Note the increase in the spring pressure—from 15.5 psig to 16 psig—needed to bring the TEVinto equilibrium.) The 68.5-psig pressure correspondsto a saturation temperature of 40°F. Therefore, thesuperheat required is now 40°F – 28°F, or 12°F. Theuse of an external equalizer has reduced the super-heat from 22°F to 12°F. As you can see, the operatingsuperheat has changed from 10°F to 12°F. This is theresult of the change in the pressure-temperaturecharacteristics of R-22 at the lower suction pressureof 52.5 psig.

However, in an operating system, where more evap-orator surface is made available for boiling the liquidrefrigerant, the evaporator outlet pressure willincrease. It will be above 52.5 psig. Thus, you cansee the advantage of using a TEV with an externalequalizer, as opposed to one with an internal equal-izer. Capacity will increase in a system that has anevaporator with a sizable pressure drop.

A thermostatic expansion valve must be equippedwith the external equalizer feature in order to providethe best performance in the following cases:

ª when the pressure drop through an evaporator isin excess of the limits previously defined

ª when a pressure drop type of refrigerantdistributor is used at the evaporator inlet.

External equalizer application

The level of evaporator pressure drop that aninternally equalized valve can cope with isdetermined by two factors—the evaporatortemperature and the refrigerant used. Use therecommendations of valve manufacturers asguidelines. Generally speaking, a valve withan external equalizer is required when anevaporator is subject to a pressure drop ofany consequence. With R-22, for example, anexternally equalized valve is recommendedwhen pressure drop exceeds the followinglevels:

ª 3.0 psi in the comfort air conditioningevaporator temperature range

ª 2.0 psi in the commercial temperature range

ª over 0.75 psi in the low freezer temperaturerange.

The recommendations in Table 1 adequately coverthe requirements for most field-installed systems. Ingeneral, install an externally equalized valve whenpressure drop between the evaporator inlet and thesuction-line bulb location exceeds the maximumslisted in Table 1.

DUAL-DIAPHRAGM THERMOSTATIC EXPANSION VALVES

So far, this chapter has assumed that both sides ofan expansion valve’s diaphragm are of equal area.This is the case when the valve has a singlediaphragm. However, there are valves with twodiaphragms. One is subject to pressure developed inthe remote valve, the other is subject to pressuredeveloped in the evaporator and by the adjustingspring. The two diaphragms can be of different sizes,as shown in Figure 27.

There is an advantage in using two diaphragms, oneacted upon by evaporator pressure and the other byremote bulb pressure. By proportioning their size, avalve can operate at the same superheat at 5°F as at25°F. Constant superheat can be maintained. This isnot possible with a single-diaphragm valve.

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Evaporating temperature, °F

40 20 0 –20 –40

Refrigerant Pressure drop, psi

R-12R-500

2.0 1.5 1.0 0.75 0.5

R-22R-717 3.0 2.0 1.5 1.0 0.75R-502

Table 1. Greater evaporator pressure drop requiresexternal equalizer

PILOT-OPERATED THERMOSTATIC EXPANSION VALVES

The size of a conventional direct-operated thermosta-tic expansion valve, particularly the diaphragm size, isgenerally adequate for its design system capacity. Inlarge-tonnage systems, however, a direct-operatedvalve would require a diaphragm or bellows operatorof great proportions. It would not be economically ormechanically practical. For this reason, there arepilot-operated valves for use on large direct expansionchillers and other large-tonnage applications.

A typical application of a pilot-operated TEV is shownin Figure 28 on the next page. This assembly has aprimary regulator and a conventional direct-operatedvalve, which acts as a pilot. The pressure and tem-perature of suction gas leaving the evaporator oper-ate the pilot valve—which, in turn, modulates theprimary regulator in response to changes in suctiongas superheat. The primary regulator opens when

pressure is exerted on top of the main piston. A smallbleed orifice in the top of the piston vents this pres-sure to the outlet side of the primary regulator. Acage spring closes the primary regulator when thepressure supply to the top of the piston is cut off.

An increase in suction gas superheat reflects theneed for increased refrigerant flow. In response, thepilot-operated TEV moves in an opening direction.This action supplies greater pressure to the top of themain piston. The piston moves to open the primaryregulator, which provides increased refrigerant flow.

A reduction in suction gas superheat reflects theneed for reduced refrigerant flow. In response, thepilot valve moves in a closing direction. This actionreduces the pressure on top of the piston. As the pis-ton moves to close the primary regulator, the volumeof refrigerant flow is reduced. In normal operation,and in response to system load, the pilot valve andregulator assume intermediate or throttling positions.

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Power element bulb

Power element tube

(Power element charge)

Evaporator pressure

diaphragm

Adjustment screw

Push rod

SeatNeedle

Spring

In

Out

Power element diaphragm

Figure 27. Dual-diaphragm TEV

AMMONIA EXPANSION VALVES

Thermostatic expansion valves for ammonia refriger-ant operate much the same as those used with otherrefrigerants. Characteristics of liquid ammonia, how-ever, differ from those of fluorocarbon refrigerants.Even small quantities of ammonia vapor have a greaterosive effect on the expansion device used. High-velocity refrigerant and contaminants like dirt or scalecompound this erosive effect. As with other parts ofammonia systems, valves are made of steel andsteel alloys to combat the erosive effect.

EVAPORATOR LENGTH

To simplify this chapter, several assumptions havebeen made up to this point. When a TEV opens,refrigerant almost immediately travels through theevaporator. There, it boils and superheats to a levelof, for example, 10°F. Then it proceeds to the remotebulb location on the suction line.

The subsequent increase in remote bulb temperaturethen develops pressure. This pressure bears againstthe diaphragm, opening the valve. Opening the valveallows more refrigerant to flow into the evaporator,decreasing the superheat and causing the remote

bulb to become chilled. The result is total or at leastpartial closing of the valve.

You might be led to think that valve response tochanges in bulb temperature is almost instant, sothat superheat stays constant. This is not the case,however. It takes time for refrigerant to travel from thevalve outlet through the evaporator to the remotebulb. There is some delay before the remote bulbreacts to any change in valve seat position. Underthis condition, the valve “overshoots” on both openingand closing. This condition is called hunting. With anexceptionally long evaporator, this may be very unde-sirable. It could create alternate cycles, in which theevaporator would be overfed when there is notenough superheat, and then starved when superheatis too high.

There should be little hunting by a TEV with an evap-orator that has short passes from the inlet to the out-let. Hunting will be most pronounced in evaporatorswith long tubes. Avoid this type of evaporator in theinterest of efficient valve operation. If a large evapora-tor surface is required, it is better to use several shorttubes in parallel than one long tube. This reduces thetravel time of refrigerant. If possible, the total length ofevaporator tubing should not exceed 100 ft.

Shell-and-tube water cooler

External equalizer line

Expansion valve Main line strainer

Liquid line

Remote bulb well

Pilot thermal expansion valve

Pilot solenoid liquid valve

Pilot line strainer

When suction line rises, dimension “A” should be as short as possible

Suction line

Note:

A

Figure 28. Typical application of pilot-operated TEV

Refrigeration Service Engineers Society1666 Rand Road Des Plaines, IL 60016 847-297-6464

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