Carrier 19XRV Specifications download pdf (Page 49)

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 RAMP)

limits the degrees per minute rate at which either leaving

chilled water or entering chilled water temperature de-

creases. This rate is configured by the operator on the

TEMP_CTL screen. The lowest temperature ramp load-

ing 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).

NOTE: If chiller control power has been off for 3

hours or more, the next start-up (only) will follow the

minimum configurable temperature ramp rate regard-

less 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

(MOTOR LOAD RAMP%/

MIN) limits the percent per minute rate at which the com-

pressor motor current or compressor motor load increas-

es. The LOAD PULLDOWN rate is configured by the

operator on the RAMP_DEM screen in amps or kilo-

watts. The point name is MOTOR LOAD RAMP%/MIN.

If kilowatts is selected for the DEMAND LIMIT SOURCE,

the MOTOR RATED KILOWATTS must be entered on the

RAMP_DEM screen (information found on the chiller Requi-

sition 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.

Rampdown — The Rampdown control applies to VFD

equipped chillers. At machine startup the VFD speed is set to

“Start Speed” which is the lower of VFD MAXIMUM SPEED

or the configured value of VFD START SPEED. VFD START

SPEED is configured on the SETUP2 screen. After Ramp

Loading is complete the control begins to reduce the VFD

speed. Speed reduction continues until the leaving water tem-

perature is within the control point dead band and any condi-

tion is reached that would require the VFD speed to hold or in-

crease. The chiller is then out of Rampdown and running in

normal capacity control.

Capacity Override (Table 7) — 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. When the value of interest crosses the First Stage Set

Point into the Override Region, the guide vanes are pre-

vented from opening further, and the status line on the

ICVC indicates the reason for the override. Normal ca-

pacity control operation is restored when the value cross-

es back over the First Stage Set Point, leaving the Over-

ride 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 Ter-

mination Condition. The PIC II controls resume normal

capacity control operation after the override termination

condition has been satisfied. (In the case of high dis-

charge superheat, there is an intermediate stage.)

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 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 53.

If the chiller is VFD-equipped, the guide vanes will be al-

lowed to open during surge prevention if the VFD is at 100%

speed or forced and the chiller is in Ramp Loading or in Ramp-

down. Opening the vanes will assist in loading the compressor

to bring it out of the surge prevention range.

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.

Compressor Bearing Temperature — The thrust

bearing temperature (MTRB) is a calculated value which is the

temperature of the oil leaving the bearing plus an internal incre-

ment calculated with the THRUST BRG RESET FACTOR. The

Comp Thrust Bearing Reset is calculated using the rate of

change of the oil leaving the bearing. If the oil temperature is

stable the COMP THRUST BRG TEMP equals the COMP

THRUST LVG OIL TEMP. As the oil temperature rises, the

COMP THRUST BRG RESET is calculated to account for any

lag between actual bearing temperature and the temperature of

the leaving oil.

In the SETUP1 screen an adjustment called the THRUST

BRG RESET FACTOR is an adjustment to the COMP THRUST

BRG TEMP calculation. The default value is 1.4 and will nor-

mally be left at that value unless other wise advised by Carrier

Service Engineering.

Oil Sump Temperature and Pump Con-

trol —

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 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 temperature is less than 150 F (65.6 C) and

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 is confirmed, 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 temperature is less than the evaporator

refrigerant 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 evaporator

refrigerant temperature plus 55 F (12.8 C).

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Carrier 19XRV Specifications Page 49

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