Emerson Fluid Chiller Guia de Resolução de Problemas descarregar pdf (Página 46)

Regulators

Emerson Climate Technologies

Figure 7: Direct acting hot gas regulator admitting ow

between TEV and venturi distributor.

Applications: Hot Gas Bypass to

Compressor Suction Line

Figure 6 shows the most common hot gas bypass

system. In this system, the bypass line is taken directly

from the compressor discharge line, through a bypass

regulator, and into the suction line at the compressor.

Although the hot gas bypass regulator is considered a

downstream control, there is a big difference in function

between a Crankcase Regulator and a hot gas regulator.

Pilot operated bypass valve main regulators have

a long stroke stem with a restrictor plug characterized

by either a parabolic or vee port restrictor plug design.

This prevents the valve from operating close to the seat

where pressure differential unbalance may occur, elimi-

nating the need for a balanced port design.

The characterized port will provide smooth bypass

ow modulation. Pilot operated valves usually have the

extra features of a manual opening stem for testing or

emergency operation, anged connections, synthetic

tight seating seats, and replaceable parts. Hot gas

bypass valves can be applied to a system in several

ways, differing only in the point to which the hot gas is

to be bypassed. Several mixing methods are available.

The one recommended is piped so that discharge gas is

admitted to the suction line to ow against the direction

of the suction gas as in gure 6.

Applications: Bypass to Evaporator Inlet

Another method is to bypass the hot discharge gas to

the evaporator inlet, usually between the Thermal Valve

and the refrigerant distributor (see gure 7). This pro-

vides distinct advantages. The articial load imposed on

the evaporator causes the Thermal Valve to respond to

the rise in superheat, eliminating the need for the liquid

injection valve. The evaporator serves as an excellent

chamber to provide homogeneous mixing of the gases

before reaching the compressor.

Hot gas bypass into the evaporator is suggested

when the evaporator elevation is below the compres-

sor, to prevent oil trapping caused by low velocity at low

loads. This assures proper oil return. Although there

are many advantages to this system, it is not used on a

multiple coil system, or where the evaporator sections

may be located a distance from the compressor. The

coil should be a free draining circuiting design to prevent

the increase in velocity, due to forcing a large quantity of

trapped liquid out of the low side, which in some cases

may have enough volume to ood the compressor

crankcase. Separate regulators must be used for each

evaporator when bypassing to multiple evaporators

located below the compressor to help oil return.

Figure 6: Hot gas bypass using type LCL

liquid injection valve.

liQUid

inJection

solenoid VAlVe

Bypass to ooded evaporators and suction line

accumulators also present special cases. Contact the

equipment manufacturer or the bypass control valve

manufacturer for specic, detailed information.

Solenoid Valve for Positive Shut-off &

Pump-down Cycle

It is recommended that a solenoid valve be installed

ahead of the bypass regulator. This permits the system

to operate on an automatic pump-down cycle.

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