Emerson Fluid Chiller Anleitung zur Fehlerbehebung PDF herunterladen (Seite 8)

Thermal Expansion Valves

Emerson Climate Technologies

This change from 10°F to 11°F in the operating

superheat is caused by the change in the pressure-

temperature characteristic of R-134a at the lower

suction pressure of 25 psig.

Location of External Equalizer

The external equalizer line must be installed beyond

the point of greatest pressure drop. Since it may be

difcult to determine this point, it is best to connect the

equalizer line to the suction line at the evaporator outlet

on the compressor side of the remote bulb location.

(See g. 4 & 5). When the external equalizer is con-

nected to a horizontal line, always make the connection

at the top of the line to avoid oil logging in the equalizer

line.

On a multi-evaporator system including two or more

evaporators each fed by a separate TXV, the external

equalizer lines must be installed so that they will be free

from the effect of pressure changes in the evaporators

fed by other TXVs. At no time should the equalizer lines

be joined in a common line to the main suction line.

If individual suction lines from the separate evapora-

tor outlets to the common suction line are short, then

install the external equalizer lines into the separate

evaporator suction headers, or as described in the pre-

ceding paragraph.

When the pressure drop through the evaporator is

not substantial, install the external equalizer connection

at one of the return bends midway through the evapora-

tor. This equalizer location will provide smoother TXV

control when used in conjunction with an Evaporator

Pressure Regulator. Anytime a control valve is installed

in the suction line, the external equalizer line for the TXV

must be connected on the evaporator side of the control

valve or regulator.

Never cap or plug the external equalizer connection

on a TXV, as it will not operate. If the TXV is furnished

with an external equalizer feature, the external equalizer

line must be connected.

= 35 PSIG

= 25 PSIG

= 10 PSIG

35 PSIG = 40°F

25 PSIG = 29°F

25 PSIG & 40°F

TXV with external

equalizer on evaporator with

10 PSI pressure drop.

Fig. 4

To compensate for an excessive pressure drop

through an evaporator, the TXV must be externally

equalized. The equalizer line should be connected to

the suction line at the evaporator outlet, past the remote

bulb location so that the true evaporator outlet pressure

is exerted beneath the TXV diaphragm. The operating

pressure on the TXV diaphragm is now free from any

effect of the pressure drop through the evaporator, and

the TXV will respond to the superheat of the refrigerant

gas leaving the evaporator.

When the same conditions of pressure drop exist in a

system with an externally equalized TXV (see g. 4), the

same pressure drop still exists through the evaporator,

however, the pressure under the diaphragm is now the

same as the pressure at the end of the evaporator, point

“C”, or 25 psig.

The required pressure above the diaphragm for equi-

librium is (25 + 10) or 35 psig. This pressure, 35 psig,

corresponds to a saturation temperature of 40°F and

the superheat required is now (40°F minus 29°F) 11°F.

The external equalizer has lowered superheat from 22°F

to 11°F. The capacity of a system having an evapora-

tor with a sizable pressure drop will be increased by a

TXV with the external equalizer when compared to an

internally equalized TXV.

When the pressure drop through an evaporator is

substantial, or when a refrigerant distributor is used at

the evaporator inlet, the TXV must have the external

equalizer feature for best performance.

An externally equalized TXV is required when a liquid

distributor is used. Although a multi-circuit evaporator

may not have an excessive pressure drop, the liquid

distributor will introduce a pressure drop, because the

distributor is installed between the TXV outlet and the

evaporator inlet (See g. 5).

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