Carrier 30XW150-400 Economizer Assembly, Cutaway Views of the Electronic Expansion Valve

The EXV control logic has several overrides, which are also used to control the position of the EXV.

•Approach between SST and LWT

•Maximum Operating Pressure (MOP)

Approach — If the approach (pinch), which is the difference between leaving fluid temperature and saturated suction tem- perature, is equal to or less than the pinch set point then the EXV will not open any further even though discharge super- heat set point is not met. Pinch set point is calculated using suction superheat, discharge superheat and pinch offset. Pinch offset is used to adjust calculated pinch set point do to accuracy of transducers and thermistors.

MOP — The EXV is also used to limit cooler saturated suction temperature to 55 F (12.8 C). This makes it possible for the

EXV to maintain economizer superheat set point, which is ap- proximately 8° to 12° F (4.4° to 6.7° C). If the circuit capacity is less than 70%, the economizer EXV will be closed.

The economizer EXV has one override. If the discharge gas temperature exceeds 195 F (90.6 C) the economizer EXV will start to open. The EXV will be controlled to maintain discharge gas temperature at approximately 195 F (90.6 C).

If it appears that main EXV or economizer EXV is not properly controlling circuit operation to maintain correct super- heat, there are a number of checks that can be made using test functions and initialization features built into the microproces- sor control. See the Service Test section to test EXVs.

MAIN EXV CONNECTOR

chiller to start at higher cooler fluid temperatures without over- loading the compressor. This is commonly referred to as MOP (maximum operating pressure). If the SST is equal to or greater than the MOP set point then the MBB will try to control the EXV position to maintain the MOP set point.

The discharge superheat leaving the compressor is main- tained between approximately 18 and 25 F (10 and 14 C), or less. Because EXV status is communicated to the Main Base Board (MBB) and is controlled by the EXV modules, it is pos- sible to track the valve position. The unit is then protected against loss of charge and a faulty valve. During initial start-up, the EXV is fully closed. After an initialization period, valve position is tracked by the EXV module by constantly monitor- ing the amount of valve movement.

ECONOMIZER EXV CONTROL — The economizer EXV

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ECONOMIZER EXV CONNECTOR

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NAME PLATE

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3

is controlled by the circuit EXV board. There is an economizer gas temperature thermistor and economizer pressure transducer located in the line, which runs from the economizer assembly to the compressor. The economizer pressure is converted to satu- rated temperature and is used to calculate economizer superheat. Economizer superheat equals economizer temperature minus saturated economizer temperature. The economizer EXV only operates during normal conditions when the capacity of the circuit is greater than 70%. Once the capacity of the circuit is greater than 70% the MBB will start controlling the economizer

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LEGEND

EXV — Electronic Expansion Valve

1— Fuse Plug Adaptor

2— High Flow Access Fitting

3— Filter Drier

4— Main Expansion Valve

5— Economizer Expansion Valve

6— Brazed Plate Heat Exchanger

Fig. 42 — Economizer Assembly

1.	Cable
2.	Glass Seal
3.	Motor Housing
4.	Stepper Motor
5.	Bearing
6.	Lead Screw
7.	Insert
8.	Valve Piston	a30-4241
9.	Valve Seat
10.	Valve Port

Fig. 43 — Cutaway Views of the Electronic Expansion Valve

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