Emerson Fluid Chiller Guide de dépannage télécharger pdf (Page 12)

Thermal Expansion Valves

Emerson Climate Technologies

Large uctuations in superheat in the suction gas are

usually the result of trapped liquid at the remote bulb

location. Even on properly designed suction lines, it is

sometimes necessary to move the remote bulb a few

inches from the original location to improve TXV perfor-

mance.

On multi-circuit evaporators fed by one TXV, install

the remote bulb at a point where the suction gas has

had an opportunity to mix in the suction header. Tighten

clamps so that the remote bulb makes good contact with

the suction line. NEVER APPLY HEAT NEAR THE

REMOTE BULB LOCATION WITHOUT FIRST RE-

MOVING THE REMOTE BULB.

Hunting

“Hunting” of TXVs is dened as the alternate over-

feeding and starving of the refrigerant ow to the

evaporator. Hunting is characterized by extreme cyclic

changes in the superheat of the refrigerant gas leaving

the evaporator and the evaporator or suction pressure.

Hunting is a function of the evaporator design, length

and diameter of tubing in each circuit, load per circuit,

refrigerant velocity in each circuit, temperature differ-

ence (TD) under which the evaporator is operated, ar-

rangements of suction piping and application of the TXV

remote bulb. “Hunting” can be reduced or eliminated by

the correct rearrangement of the suction piping, reloca-

tion of the bulb and use of the recommended remote

bulb and power assembly charge for the TXV.

Operation at Reduced Capacity

The conventional TXV is a self-contained direct oper-

ated regulator which is inherently susceptible to hunting

because of its design and the design of the system to

which it is applied.

The ideal ow rate would require a TXV with perfect

dynamic balance, capable of instantaneous response

to any change in evaporation (anticipation) and with a

means of preventing the TXV from over shooting the

control point because of inertia (compensation). With

these features a TXV would be in phase with the system

demand at all times and hunting would not occur.

A conventional TXV does not have built in anticipating

or compensating factors. A time lag will exist between

demand and response, along with the tendency to over

shoot the control point. The conventional TXV may get

out of phase with the system and hunt. An example of

overshooting occurs when the load increases, causing

the superheat of the suction gas to increase. The time

interval between the instant the remote bulb senses the

increase and causes the TXV pin to move into opening

direction allows the superheat of the gas to increase still

further.

In response to the rising superheat during the time

lags, the TXV has moved further in the opening direc-

tion, overshooting the control point and allowing more

refrigerant to ow to the evaporator than can be boiled

off by load.

When the TXV nally responds to the over-feeding of

the evaporator coil, it closes and will tend to again over-

shoot the control point and remain overly throttled until

most of the liquid refrigerant has left the evaporator.

The ensuing time delay before the TXV moves in the

opening direction allows superheat of the suction gas

to again rise beyond the control point. This cycle, being

self-propagating, continues to repeat.

Experience has shown that a TXV is more likely to

hunt at low load conditions when the TXV pin is close to

the valve seat. This is because of an unbalance be-

tween the forces which operate the TXV.

Besides the three main forces that operate the TXV,

the pressure difference across the TXV port also acts

against the port area and depending on TXV construc-

tion, tends to force the TXV either open or closed.

When operating with the pin close to seat, the follow-

ing will occur:

With the TXV closed, there is liquid pressure on the

inlet side of the pin and evaporator pressure on the

outlet.

When the TXV starts to open allowing ow to take

place, the velocity through the TXV throat will cause a

point of lower pressure at the throat, raising the pressure

difference across the pin and seat.

This sudden rise in pressure differential while acting

on the port area will tend to force the TXV pin back into

the seat. When the TXV again opens, the same type of

action occurs and the pin bounces off the seat with a

rapid frequency. This phenomenon is more frequently

encountered with the larger conventional ported TXVs

as compared to balance ported TXVs as the force

caused by the pressure differential is magnied by the

larger port area.

Most TXVs, when properly selected and applied, will

overcome these factors and operate with virtually no

hunting over a fairly wide load range.

Conventional ported TXVs will perform well to some-

what below 50% of nominal capacity depending on

evaporator design, refrigerant piping, size and length of

evaporator, and rapid changes in loading.

Nothing will cause a TXV to hunt quicker than un-

equal feeding of the parallel circuits by a distributor or

unequal air loading across the evaporator circuits.

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Emerson Fluid Chiller Guide de dépannage Page 12

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