42
The algorithm first determines if corrective action is neces-
sary. This is done by checking two sets of operator configured
data points, which are the MINIMUM and the MAXIMUM
Load Points, (T1/P1;T2/P2). These points have default settings
for each type of refrigerant, HCFC-22 or HFC-134a, as defined
on the OPTIONS table, and on Table 3. These settings and the
algorithm logic are graphically displayed in Fig. 24 and 25.
The two sets of load points on this graph (default settings are
shown) describe a line which the algorithm uses to activate the
hot gas bypass. Whenever the temperature difference between
the entering and leaving chilled water is on the left side of the
line on the graph (as defined by the MINIMUM and MAXI-
MUM Load Points), the algorithm will then energize the hot
gas bypass valve to falsely load the chiller and prevent dis-
placement of oil. If the actual values are on the right side of the
line, the algorithm takes no action.
HEAD PRESSURE OUTPUT REFERENCE (See
Fig. 26) — The PIC II control outputs a 4 to 20 mA signal for
the configurable Delta P (condenser pressure – evaporator
pressure) reference curve shown in Fig. 26. The Delta P at
100% (default at 35 psi). Delta P at 0% (default at 25 psi) and
Minimum Reference Point are configurable in the EQUIP-
MENT SERVICE-OPTIONS table. When configuring this out-
put, ensure that minimum requirements for oil pressure and
proper condenser FLASC orifice performance are maintained.
Lead/Lag Control —
The lead/lag control system auto-
matically starts and stops a lag or second chiller in a 2-chiller
water system. A third chiller can be added to the lead/lag sys-
tem as a standby chiller to start up in case the lead or lag chiller
in the system has shut down during an alarm condition and ad-
ditional cooling is required.Refer to Fig. 20 and 21 for menu,
table, and screen selection information.
NOTE: The lead/lag function can be configured on the LEAD-
LAG screen, which is accessed from the SERVICE menu and
EQUIPMENT SERVICE table. See Table 3, Example 19.
Lead/lag status during chiller operation can be viewed on the
LL_MAINT display screen, which is accessed from the SER-
VICE menu and CONTROL ALGORITHM STATUS table.
See Table 3, Example 11.
Lead/Lag System Requirements:
•
all chillers in the system must have software capable of
performing the lead/lag function
•
water pumps MUST be energized from the PIC II
controls
•
water flows should be constant
•
the CCN time schedules for all chillers must be identical
Operation Features:
•
2 chiller lead/lag
•
addition of a third chiller for backup
•
manual rotation of lead chiller
•
load balancing if configured
•
staggered restart of the chillers after a power failure
•
chillers may be piped in parallel or in series chilled
water flow
COMMON POINT SENSOR INSTALLATION — Lead/
lag operation does not require a common chilled water point
sensor. Common point sensors (Spare Temp #1 and #2) can be
added to the CCM module, if desired. Spare Temp #1 and #2
are wired to plug J4 terminals 25-26 and 27-28 (J4 lower,
respectively).
NOTE: If the common point sensor option is chosen on a
chilled water system, each chiller should have its own common
point sensor installed. Each chiller uses its own common point
sensor for control when that chiller is designated as the lead
chiller. The PIC II cannot read the value of common point sen-
sors installed on the other chillers in the chilled water system.
MINIMUM
REFERENCE
OUTPUT
DELTA P
AT 100%
DELTA P
AT 0%
DELTA P
0 mA 2 mA 4 mA
(0%)
20 mA
(100%)
4 T0 20 mA OUTPUT
LEGEND
∆
P
—
Condenser Pressure-
Cooler Pressure
∆
T
—
ECW-LCW
ECW
—
Entering Chilled
Water Temperature
LCW
—
Leaving Chilled
Water Temperature
HGBP
—
Hot Gas Bypass
Fig. 24 — 23XL Hot Gas Bypass (English)
LEGEND
∆
P
—
Condenser Pressure-
Cooler Pressure
∆
T
—
ECW-LCW
ECW
—
Entering Chilled
Water Temperature
LCW
—
Leaving Chilled
Water Temperature
HGBP
—
Hot Gas Bypass
DEFAULT VALUES:
POINT
HCFC-22
HFC-134a
∆
T
1
1.4
1.4
∆
P
1
345
207
∆
T
2
5.6
5.6
∆
P
2
1379
1172
DEFAULT VALUES:
POINT
HCFC-22
HFC-134a
∆
T
1
2.5
2.5
∆
P
1
50
30
∆
T
2
10
10
∆
P
2
200
170
Fig. 25 — 23XL Hot Gas Bypass (SI)
Fig. 26 — Head Pressure Output
Reference Control