Ramp Loading — The ramp loading control slows down 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 two methods of ramp loading with the PIC II. Ramp loading can be based on chilled water temperature or on motor load. Either method 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 tempera- ture decreases. This rate is configured by the operator on the TEMP_CTL screen. The lowest temperature ramp rate will also be used if chiller power has been off for 3 hours or more (even if the motor ramp load is selected as the ramp loading method).
2.Motor load ramp loading (LOAD PULLDOWN) limits the degrees per minute rate at which the compressor mo- tor current or compressor motor load increases. The LOAD PULLDOWN rate is configured by the operator on the RAMP_DEM screen in amps or kilowatts. The point name is MOTOR LOAD RAMP%/MIN.
If kilowatts is selected for the DEMAND LIMIT SOURCE, the MOTOR RATED KILOWATTS must be entered (informa- tion found on the chiller Requisition form).
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 MOTOR LOAD RAMP %/MIN may be viewed or modified on the RAMP_DEM screen.
Capacity Override (Table 4) — Capacity overrides can prevent some safety shutdowns caused by exceeding the motor amperage limit, refrigerant low temperature safety limit, motor high temperature safety limit, and condenser high pressure limit. In all cases there are 2 stages of compressor vane control.
1.The vanes are prevented from opening further, and the status line on the CVC/ICVC indicates the reason for the override.
2.The vanes are closed until the condition decreases to be- low the first step set point. Then the vanes are released to normal capacity control.
Whenever the motor current demand limit set point (ACTIVE DEMAND LIMIT) is reached, it activates a capacity override, again, with a
The compressor high 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 by- pass valve will open instead of holding the vanes. See the Surge Prevention Algorithm section, page 39.
High Discharge Temperature Control — If the discharge temperature 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 tempera- ture is then brought 5° F (2.8° C) below the control set point temperature, the PIC II will bring the chiller into the recycle mode.
Oil Sump Temperature Control — The oil sump temperature control is regulated by the PIC II, which uses the oil heater relay when the chiller is shut down.
As part of the
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 temperature is less than the cooler re- frigerant temperature plus 53° F (11.7° C). The oil heater is turned off when the oil sump temperature is either
•more than 152 F (66.7 C), or
•more than 142 F (61.1 C) and more than the cooler refrigerant temperature plus 55° F (12.8° C).
The oil heater is always off during
The oil pump is also energized during the time the oil is be- ing heated (for 60 seconds at the end of every 30 minutes).
Oil Cooler — The oil must be cooled when the compres- sor is running. This is accomplished through a small,
NOTE: The TXVs are not adjustable. The oil sump tempera- ture may be at a lower temperature during compressor operations.
Remote Start/Stop Controls — A remote device, such as a timeclock that uses a set of contacts, may be used to start and stop the chiller. However, the device should not be pro- grammed to start and stop the chiller in excess of 2 or 3 times every 12 hours. If more than 8 starts in 12 hours (the STARTS IN 12 HOURS parameter on the MAINSTAT screen) occur, an excessive starts alarm displays, preventing the chiller from
starting. The operator must press the RESET softkey on the CVC/ICVC to override the starts counter and start the chiller. If the chiller records 12 starts (excluding recycle starts) in a sliding
RESET softkey followed by the LOCAL or CCN softkey. This ensures that, if the automatic system is malfunctioning, the chiller will not repeatedly cycle on and off. If the automatic restart after a power failure option (AUTO RESTART OPTION on the OPTIONS screen) is not activated when a power failure occurs, and if the remote contact is closed, the chiller will indi- cate an alarm because of the loss of voltage.
The contacts for remote start are wired into the starter at ter- minal strip J2, terminals 5 and 6 on the ISM. See the certified drawings for further details on contact ratings. The contacts must have 24 vac dry contact rating.
Spare Safety Inputs — Normally closed (NC) discrete inputs for additional
Analog temperature sensors may also be added to the mod- ule (SPARE TEMP #1 and #2). The analog temperature sen- sors may be configured to cause an alert or alarm on the CCN network. The alert will not shut the chiller down. Configuring for alarm state will cause the chiller to shut down.
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