Emerson Fluid Chiller Instrukcja Naprawy Pobierz pdf (Strona 7)

Thermal Expansion Valves

Emerson Climate Technologies

Changes in load cause the TXV pin to move:

• Increasing the superheat will cause the TXV to open

• Decreasing the superheat will cause the TXV to close

Factory Settings of TXVs

The factory superheat setting of TXVs is made with

the TXV pin just starting to move away from the seat.

The superheat necessary to get the pin ready to move is

called static superheat.

TXVs are designed so that an increase in superheat

of refrigerant gas leaving the evaporator is needed for

the TXV pin to open to its rated position.

This added superheat is known as gradient. For ex-

ample, if the factory static is 6°F superheat, the operat-

ing superheat at the rated stroke or pin position (full load

rating of TXV) will be 10°F to 14°F superheat (See

g. 2).

Manufacturers usually furnish the adjustable type

TXV with a factory static superheat setting of 6°F to

10°F unless otherwise specied.

When using non-adjustable TXVs, it’s important that

they are ordered with the correct factory superheat

setting. For manufacturer’s production lines it is recom-

mended that an adjustable TXV be used in a pilot model

lab test to determine the correct factory superheat set-

ting before ordering the non-adjustable type TXV.

If the operating superheat is raised unnecessarily

high, the evaporator capacity decreases, since more of

the evaporator surface is required to produce the super-

heat needed to operate the TXV.

A minimum change of superheat to open the TXV is

important because it provides savings in rst cost of the

evaporator and cost of operation.

The TXV described so far is internally equalized,

where the evaporator pressure at the TXV outlet is

admitted internally and allowed to exert its force beneath

the diaphragm. In the next section the externally equal-

ized TXV will be discussed.

External Equalizer

A TXV with an external equalizer is required when the

pressure drop through the evaporator is substantial:

• 3°F for residential air conditioning

• 2°F for commercial air conditioning

• 1°F for refrigeration low temperature range

This is because the pressure drop will hold the TXV

in a fairly “restricted” position and reduce system capac-

ity. The evaporator should be designed or selected for

the operating conditions and the TXV selected and ap-

plied accordingly.

For example, an evaporator is fed by a TXV with an

internal equalizer, where a sizable pressure drop of 10

psi is present (See g. 3). The pressure at point “C”

is 25 psig or 10 psi lower than at the TXV outlet, point

“A”, however, the pressure of 35 psig at point “A” is the

pressure acting on the lower side of the diaphragm in a

closing direction. With the TXV spring set at a compres-

sion equivalent to 10°F superheat or a pressure of 10.4

psig, the required pressure above the diaphragm to

equalize the forces is (35 + 10.4) or 45.4 psig. This pres-

sure corresponds to a saturation temperature of 50°F.

The refrigerant temperature at point “C” must be 50°F

if the TXV is to be in equilibrium. Since the pressure at

this point is only 25 psig and the corresponding satura-

tion temperature is 28°F, a superheat of (50°F - 29°F) or

21°F is required to open the TXV.

This increase in superheat, from 10°F to 21°F means

that more of the evaporator surface needs to be used

to produce this higher superheated refrigerant gas. The

evaporator surface available for absorption of heat is re-

duced and the evaporator is starved before the required

superheat is reached.

= 45.4 PSIG

= 35 PSIG

= 10.4 PSIG

35 PSIG = 40°F

25 PSIG = 29°F

25 PSIG = 50°F

TXV with internal

equalizer on evaporator with

10 PSI drop.

Fig. 3

Since the pressure drop across the evaporator in-

creases with load, the restricting effect becomes worse

when the demand on the TXV capacity is greatest.

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