York YCAV0157, Installation Operation & Maintenance

Page: 185 / 310

185
JOHNSON CONTROLS
FORM 201.21-NM1 (1223)
FLASH TANK DRAIN AND FEED VALVE CONTROLLER
LD10619
VALVE CONTROLLER AND CONTROL
ALGORITHM OPERATION
The Flash Tank Feed and Drain Valve PI Controller(s)
plays a dual role of supplying drive signals to control
the opening and closing of both the Flash Tank Feed and
Drain Valves. These valves control the liquid level in the
Flash Tank and the suction superheat of the compressor.
The Flash Tank Feed and Drain Valve Controller receives
analog signals from the Chiller Control Board to position
the Feed and Drain Valves. The Chiller Control Board
PI (Proportional plus Integral) control algorithm in the
Chiller Control Board software determines the open %
for the Drain and Feed valves. A D/A converter on the
Chiller Control Board converts the 0-110.0% signal to
an output voltage between 0 VDC and 10.28 VDC and
sends it to the Drain and Feed Controller. This voltage
is then converted to a valve position by the Drain and
Feed Valve Controller and a 2 phase (4 wire), signal
drives the Feed Valve open or closed. Power for the
Valve Controller comes from a 30VDC supply from the
Chiller Control Board.
The Feed Valve is a stepper motor valve that controls
the liquid flow from the condenser to assure the liquid
level in the Flash Tank is maintained at a proper level.
The Level Sensor is a rod inserted into the reservoir
connected to the side of the Flash Tank. The sensing rod
has an active range of about 12”. The control algorithm
looks at feedback from the Level Sensor and compares
it to the fixed level setpoint in the control algorithm.
This control strategy attempts to keep the level in the
Flash Tank to approx 35% of the usable portion of the
sensing rod. In reality, this is approximately a 50%
level in the Flash Tank. As the level in the Flash Tank
fluctuates, the control algorithm varies the voltage to
the Controller, which in turn sends a 2 phase stepped
drive signal to open or close the Feed Valve as needed.
As the Flash Tank level varies farther from the setpoint,
the gain of the control algorithm increases for faster
response. In some cases, the Feed Valve will fully open
or fully close if the levels become too low or too high.
When properly charged, the condenser subcooling will
be approx. 5-7°F at design conditions as the Feed Valve
controls refrigerant flow into the Flash Tank.
The Drain Valve is also a stepper motor valve. Like
the Feed Valve, the controller receives a 0 - 10.28 VDC
signal from the Chiller Control Board. The controller
then converts the signal to a valve position and a 2 phase
signal drives the Drain valve open or closed.
7