Emerson Fluid Chiller Guia de Resolução de Problemas descarregar pdf (Página 9)

Thermal Expansion Valves

Emerson Climate Technologies

= 45.5 PSIG

= 35 PSIG

= 10.4 PSIG

35 PSIG = 40°F

35 PSIG = 50°F

TXV with internal

equalizer on evaporator with

no pressure drop.

Fig. 6

Superheat

A vapor is said to be superheated whenever its

temperature is higher than the saturation temperature

corresponding to its pressure. The superheat equals the

temperature increase above the saturation temperature

at that pressure. For example, a refrigeration evapora-

tor is operating with R-134a at 35 psig suction pressure

(See g. 6). The R-134a saturation temperature at 35

psig is 40°F. As long as any liquid exists at this pressure,

the refrigerant temperature will remain 40°F as it evapo-

rates or boils off in the evaporator.

suring superheat, install a calibrated pressure gauge in

a gauge connection at the evaporator outlet. In the ab-

sence of a gauge connection, a tee installed in the TXV

external equalizer line can be used just as effectively.

A refrigeration type pocket thermometer with appro-

priate bulb clamp or an electric thermometer with ther-

mocouples may be used to measure gas temperature.

The temperature element from the thermometer

should be taped to the suction line at the point of remote

bulb location and must be insulated. Thermometers will

give an average reading of suction line and ambient if

not insulated. Assuming an accurate gauge and ther-

mometer, this method will provide accurate superheat

readings.

Approximate Methods

of Reading Superheat

When a gauge connection is not available and the

TXV is internally equalized there are two ways of esti-

mating superheat. Neither of these methods will yield an

exact superheat reading.

The rst is the two-temperature method, which uses

the difference in temperature between the evaporator

inlet and outlet as the superheat. The error is caused by

the pressure drop in the evaporator. When the pressure

drop between the evaporator inlet and outlet is 1 psi or

less, the two-temperature method will yield fairly accu-

rate results. But evaporator pressure drop is usually not

known and will vary with load. For this reason, the two-

temperature method cannot be relied on for absolute

superheat readings. The error in this method is negative

and always shows a lower superheat.

The second method involves taking the temperature

at the evaporator outlet and using the compressor suc-

tion pressure as the evaporator saturation pressure. The

error is caused by the pressure drop in the suction line

between the evaporator outlet and the compressor suc-

tion gauge. On packaged equipment and close-coupled

installations, the pressure drop and resulting error are

usually small. But on large built-up systems or systems

with long runs of suction lines, considerable error can

result. Since estimates of suction line pressure drop are

usually not accurate enough to give a true picture of the

superheat, this method cannot be relied on for absolute

values. The error in this method is positive and always

shows a higher superheat.

The only method for checking superheat that will

yield an absolute value involves a pressure and tem-

perature reading at the evaporator outlet.

By realizing the limitations of these approximate

methods and the direction of the error, it is often pos-

As the refrigerant moves along in the coil, the liquid

boils off into a vapor. The liquid is completely evapo-

rated at point B because it has absorbed enough heat to

change the refrigerant liquid to a vapor. The refrigerant

gas continues along the coil and remains at the same

pressure (35 psig); however, its temperature increases

due to continued absorption of heat. When the refriger-

ant gas reaches the end of the evaporator (point “C”) its

temperature is 50°F. This refrigerant gas is now super-

heated and the superheat is 10°F. (50°F minus 40°F).

The amount of superheat depends on how much re-

frigerant is being fed into the evaporator by the TXV and

the heat load to which the evaporator is exposed.

Superheat Adjustment

The function of a TXV is to control the superheat of

the suction gas leaving the evaporator. If superheat is

within reasonable limits, the TXV is operating in a satis-

factory way. If superheat cannot be checked directly, it

is important to know the size and direction of whatever

error is present.

The pressure and temperature of the refrigerant suc-

tion gas passing the TXV remote bulb are required for

an accurate determination of superheat. When mea-

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