Ramp Loading; Capacity Override; High Discharge Temperature Control; Oil Sump Temperature And Pump Control - Carrier 19XRV Start-Up, Operation And Maintenance Instructions Manual

The ramp loading control slows down
Ramp Loading —
the rate at which the compressor loads up. This control can pre-
vent the compressor from loading up during the short period of
time when the chiller is started and the chilled water loop has to
be brought down to CONTROL POINT. This helps reduce
electrical demand charges by slowly bringing the chilled water
to CONTROL POINT. The total power draw during this period
remains almost unchanged.
There are several methods of ramp loading with the PIC III.
Ramp loading can be based on LEAVING CHILLED WATER,
ENTERING CHILLED WATER, PERCENT LINE CURENT,
or PERCENT MOTOR KILOWATTS. PULLDOWN RAMP
TYPE is selected from the RAMP__DEM screen.
1. Temperature ramp loading (TEMP PULLDOWN DEG/
MIN) limits the degrees per minute rate at which either
LEAVING CHILLED WATER or ENTERING CHILLED
WATER temperature decreases. This rate is configured by
the operator on the TEMP_CTL screen.
NOTE: If chiller control power has been off for 3 hours or
more, the next start-up (only) will follow temperature ramp
loading using the minimum rate regardless of the ramp loading
method and rate which are configured in the screens. This is
used to maximize oil reclaim during start-up.
2. Motor load ramp loading (AMPS OR KW RAMP %/
MIN) limits the rate at which the compressor motor cur-
rent or compressor motor load increases. The AMPS OR
KW RAMP %/MIN rate is configured by the operator
on the RAMP_DEM screen in line current or motor
kilowatts.
If kilowatts is selected for the DEMAND LIMIT SOURCE,
the MOTOR RATED LOAD KILOWATTS must be entered in
the VFD_CONF screen.
The TEMP PULLDOWN DEG/MIN may be viewed or
modified on the TEMP_CTL screen which is accessed from
the EQUIPMENT SERVICE screen. PULLDOWN RAMP
TYPE, DEMAND LIMIT SOURCE, and AMPS OR KW RAMP
%/MIN may be viewed or modified on the RAMP_DEM
screen.
Capacity Override (Table 7) —
prevent some safety shutdowns caused by exceeding the motor
amperage limit, low evaporator temperature safety limit, high
motor temperature safety limit, and high condenser pressure
limit. In all cases, there are two stages of compressor capacity
control applied by guide vane operation:
1. When the value of interest crosses the First Stage Set
Point into the Override Region, the guide vanes are
prevented from opening further, and the status line on the
ICVC indicates the reason for the override. Normal
capacity control operation is restored when the value
crosses back over the First Stage Set point, leaving the
Override Region. See Table 7.
2. When the value of interest is in the Override Region and
further crosses the Second Stage Set Point, the guide
vanes are closed until the value meets the Override
Termination Condition. The PIC III controls resume
normal capacity control operation after the override
termination condition has been satisfied. (In the case of
high discharge superheat, there is an intermediate stage.)
Capacity overrides can
42
Whenever the motor current demand limit set point
(ACTIVE DEMAND LIMIT) is reached, it activates a capacity
override, again, with a 2-step process. Exceeding 110% of the
rated load amps for more than 30 seconds will initiate a safety
shutdown.
The high compressor lift (surge prevention) set point will
cause a capacity override as well. When the surge prevention
set point is reached, the controller normally will only prevent
the guide vanes from opening. If so equipped, the hot gas
bypass valve will open instead of holding the vanes. The hot
gas bypass will only open if the compressor is at 100% speed.
See the Surge Prevention Algorithm section, page 45.
High Discharge Temperature Control —
COMP DISCHARGE TEMP increases above 160 F (71.1 C),
the guide vanes are proportionally opened to increase gas flow
through the compressor. If the LEAVING CHILLED WATER
temperature decreases 5° F (2.8° C) below the control set point
temperature, as a result of opening the guide vanes, the PIC III
will bring the chiller into the recycle mode.
Oil Sump Temperature and Pump Con-
The oil sump temperature is regulated by the PIC III,
trol —
with the oil heater relay when the chiller is shut down.
As part of the pre-start checks executed by the controls, the
oil sump temperature (OIL SUMP TEMP) is compared to
the cooler refrigerant temperature (EVAPORATOR REFRIG
TEMP) if the OIL SUMP TEMP is less than 150 F (65.6). If
the difference between these 2 temperatures is 50 F (27.8 C) or
less, the start-up will be delayed until either of these conditions
is no longer true. Once this temperature criteria is satisfied, the
start-up continues.
The oil heater relay is energized whenever the chiller com-
pressor is off and the oil sump temperature is less than 140 F
(60.0 C) or the OIL SUMP TEMP is less than the EVAP
REFRIG TEMP plus 53° F (29.4° C). The oil heater is turned
off when the OIL SUMP TEMP is either:
• more than 152 F (66.7 C), or
• more than 142 F (61.1 C) and more than the EVAP
REFRIG TEMP plus 55° F (30.6° C).
The oil heater is always off during start-up or when the
compressor is running.
The oil pump is also energized during the time the oil is be-
ing heated (for 30 seconds at the end of every 30 minutes).
The oil pump will not operate if the EVAPORATOR PRES-
SURE is less than –5 psig (–34.5 kPa).
The oil must be cooled when the compres-
Oil Cooler —
sor is running. This is accomplished through a small, plate-type
heat exchanger (also called the oil cooler) located behind the
oil pump. The heat exchanger uses liquid condenser refrigerant
as the cooling liquid. Refrigerant thermostatic expansion
valves (TXVs) regulate refrigerant flow to control the oil tem-
perature entering the bearings. The bulbs for the expansion
valves are strapped to the oil supply line leaving the heat
exchanger, and the valves are set to maintain 110 F (43 C).
NOTE: The TXVs are not adjustable. The oil sump temperature
may be at a lower temperature during compressor operation.
If the