Control; Figure 88 - Head Pressure Control Flow Diagram - Johnson Controls YORK OPTIVIEW YK Series Service Instructions Manual

SECTION 28 - HEAD PRESSURE CONTROL
The evaporator and condenser transducers are connect-
ed to the microboard as shown in Figure 87 on Page
217. This provides the head pressure (condenser mi-
nus evaporator) to the microboard. The Head Pressure
Control Valve is driven from the LTC I/O Board 031-
02895-000. The operation of this board is explained in
SECTION 26 - LARGE TONNAGE CHILLER (LTC)
I/O Board. The microboard communicates with this
board using RS-485 Modbus serial communications. It
sends the valve positioning command, via this serial
link, to the I/O Board where it is converted to a 0 to
10VDC or 4 to 20mA positioning signal that is applied
to the valve as described below.

CONTROL

When Head Pressure Control is enabled, all the follow-
ing programmable settings and applicable parameters
are available on the HEAD PRESSURE CONTROL
Screen (or HEAT RECOVERY Screen, if Heat Recov-
ery is enabled).
Setpoints
• Head Pressure setpoint (15.0 PSID to 60.0 PSID;
default 23 PSID) - Sets the Head Pressure that the
control will control to. Value entered is displayed
as Head Pressure setpoint.
• Control Valve Output Settings:
TYPE – Makes the control signal to the Head
Pressure Control Valve either 0 to 10VDC or
4-10mA (default 0 to 10VDC).
PID OUTPUT – Makes the control signal to
the Head Pressure Control Valve either Di-
rect or Reverse acting (default Direct). Set to
Direct for this application.
Head Pressure
Setpoint
Head Pressure
(Delta P)

FIGURE 88 - HEAD PRESSURE CONTROL FLOW DIAGRAM

218
+
PID
Error
Control
-
P I
(PID Gains)
SET – Is used to manually set the Head Pres-
sure Control Valve to any position between
0.0% and 100%.
AUTO – Returns the Head Pressure Control
Valve to Automatic Control Mode (default
Auto).
• Head Pressure Control – Sets the proportional
gain of the control:
P- Sets the proportional gain of the control
over the range of 0.00 to 5.00(default 2.00).
I – Sets the integral gain of the control over
the range of 0.0 to 5.00 (default 2.00).
D – Sets the derivative gain of the control
over the range of 0.0 to 5.00 (default 0.00).
When Head Pressure Control is enabled, the Control
Valve Output is modulated by a direct acting PID con-
trol. The inputs to the PID control are the chiller Head
Pressure (Condenser Pressure – Evaporator Pressure)
and the Head Pressure setpoint. The PID control er-
ror is defined as Head Pressure setpoint – Head Pres-
sure. The PID control's proportional, integral and de-
rivative gains are programmable using the CHANGE
SETPOINTS key. The Control Valve position is dis-
played as 0.0% to 100% in the box labeled CONTROL
VALVE OUTPUT. The control output to the valve is
direct acting (as the Head Pressure increases above the
setpoint, the valve is driven more open to provide less
tower bypass or more flow through the condenser).
The following diagram shows the PID controller.
PID Control
Output
Conditioning
(0.0–100.0%)
D
Type
(0-10V,
4-20mA)
FORM 160.54-M1
Analog
Analog
Output
Output
(0-10V,
10-0V,
4-20mA [2-10V],
20-4mA [10-2V])
Output
(Direct,
Reverse)
JOHNSON CONTROLS