Trane IntelliPak with Symbio 800, Инструкция по установке, эксплуатации и обслуживанию

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RT-SVX063G-EN 81
EXV Control (Electronic
Expansion Valve)
Electronic expansion valves (EXVs) control suction
superheat to ensure the complete and efficient evaporation
of refrigerant in the evaporator. Excess liquid refrigerant in
the compressor suction (low suction superheat) can be
harmful to the compressor, and a deficit of liquid in the
evaporator (high suction superheat) is inefficient. The
benefit of EXV control, through controlling suction
superheat, suction pressure, suction saturated
temperature, or pressure ratio, promotes energy-efficient
operation and long-term reliability of the compressor(s).
Suction superheat is the difference between suction
temperature and saturated suction temperature. A
temperature sensor measures suction temperature. The
saturated suction temperature is determined from the
measured suction pressure by a refrigerant property
calculation. Typically, the EXV control maintains a small,
desired amount of suction superheat by opening or closing
the EXV(s), which adjusts the refrigerant flow rate into the
evaporator.
Figure 41. Component location — Circuit 1 of
staggered coil unit
In certain situations such as cold ambient starts, warm
supply air starts, or DOAS applications, the EXV control
can automatically move into a suction pressure control
mode, allowing the suction superheat to deviate from the
superheat setpoint. The goal of the suction pressure
control mode is to maintain suction pressure or differential
pressure within the compressor operating map.
In situations where the superheat rapidly falls to very low
values, special control modes are necessary to dry out
excess refrigerant liquid in the evaporator, suction line, and
compressor sumps, and transition back to normal control.
Configuration
Electronic Expansion Valves are always installed on DX
refrigeration circuits. Configuration is used to specify the
EXV selection on each refrigeration circuit. EXV selection
allows picking of a correlation between EXV steps or EXV
percent open, and the flow coefficient (C
v
) at each EXV
opening.
Functional Description
EXV Calibration
An EXV Calibration (also known as overdrive closed)
procedure will be initiated in the following circumstances:
• Whenever the unit controller is power cycled. This is
under the control of the unit controller software, as part
of the software startup procedure.
Note: Under the most common scenario when both the
EXV electronic controller and unit controller are
power cycled, EXV Calibration may occur twice.
• When the circuit cycles off and the circuit has run for at
least EXV Recalibration Time since the previous
calibration.
Note: EXV calibration time is used for both HGBP and
EXV valve calibration.
• Whenever the EXV electronic controller is power
cycled. The EXV electronic controller does this
automatically.
EXV Manual Override Control
See Programming Guide
EXV Control Sequence of Operation
The EXV control algorithm can be in one of the following
states:
Table 33. EXV Control States
Control State Description
Stopped EXV closed. Overdrive closed (calibration) performed if criteria are met.
Starting EXV pre-position is calculated, and the valve arrives at the requested opening
before any compressors on the circuit start.
Stopping EXV commanded to close. Remain in state until EXVs report that they are
closed.
Running EXV operates in pre-start control, or in automatic control after the pre-position
hold time expires.
Running Mode Sub-States Description
Running – Wait for Circuit Start Time after valve arrives at pre-position value, but before a compressor on the
circuit starts.
Sequence of Operation