Carrier XRV Especificaciones descargar pdf (Pagina 51)

by the amount configured in the VFD INCREASE STEP pa-

rameter. The VFD will not decrease in speed if SURGE

COUNTS is greater than zero.

The threshold at which a current fluctuation is interpreted as

a surge can be adjusted from the OPTIONS screen. The portion

of the surge threshold attributable to current fluctuations can be

changed by scrolling to the SURGE DELTA % AMPS parame-

ter and adjusting it with the INCREASE or DECREASE soft-

keys. The default setting is 10 %. The SURGE TIME PERIOD

can be adjusted from the OPTIONS screen. Scroll to the

SURGE TIME PERIOD parameter and use the INCREASE or

DECREASE softkey to adjust the surge count time interval.

The default setting is 8 minutes.

SURGE PROTECTION COUNTS are displayed in the

COMPRESS screen. Both SURGE PROTECTION COUNTS

and SURGE COUNTS are displayed in the SURGPREV

screen.

Head Pressure Reference Output (See Fig. 24) —

The PIC III control outputs a 4 to 20 mA signal for the

configurable Delta P (CONDENSER PRESSURE minus

EVAPORATOR PRESSURE) reference curve shown in Fig. 24.

The DELTA P AT 100% (chiller at maximum load condition

default at 50 psi), DELTA P AT 0% (chiller at minimum load

condition default at 25 psi) and MINIMUM OUTPUT points

are configurable in the EQUIPMENT SERVICE-OPTIONS

table. When configuring this output ensure that minimum re-

quirements for oil pressure and proper condenser FLASC ori-

fice performance are maintained. The 4 to 20 mA output from

VFD TB1 terminals 17 and 18 may be useful as a reference

signal to control a tower bypass valve, tower speed control,

condenser pump speed control, etc. Note that it is up to the site

design engineering agent to integrate this analog output with

any external system device(s) to produce the desired effect.

Carrier does not make any claim that this output is directly us-

able to control any specific piece of equipment (that is, without

further control elements or signal conditioning), although it

may be.

The head pressure reference output will be on whenever the

condenser pump is operating. It may also be manually operated

in CONTROLS TEST. When the head pressure differential is

less than the value entered for DELTA P AT 0%, the output will

be maintained at 4 mA.

VFD Start-Up Speed Control — Immediately accel-

erating to a high VFD speed improves the ability of the com-

pressor to compensate for some start-up environments that ex-

ceed condenser water design conditions. The 19XRV chillers

initially accelerate to high VFD speed and then gradually slow

the compressor, if possible, while adjusting the guide vane po-

sition until a stable operating point with improved chiller effi-

ciency is attained.

Following a start command, the PIC III controls internally

set the VFD TARGET SPEED to the smaller of the VFD

MAXIMUM SPEED or the VFD START SPEED. Provided

that the chiller has sufficient capacity, the VFD will continue to

run at the startup speed during Ramp Loading until the chilled

water temperature falls within the CHILLED WATER DEAD-

BAND surrounding the set point. RAMP LOADING ACTIVE

in the SURGPREV screen will indicate YES during Ramp

Loading. The GUIDE VANE DELTA will be equal to zero

when the chilled water temperature is in the CHILLED WA-

TER DEADBAND. The VFD speed will then be ramped

down at one half of the VFD GAIN rate until, surge conditions

are encountered, the VFD MINIMUM SPEED is reached, the

ACTUAL GUIDE VANE POS reaches the GUIDE VANE

TRAVEL LIMIT, or the TARGET VFD SPEED is forced.

VFD RAMPDOWN ACTIVE in the SURGPREV screen will

indicate YES during the rampdown process. The VFD speed

will be regulated by standard capacity control and surge pre-

vention algorithms at the conclusion of the rampdown process.

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

system 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 additional cooling is required. Refer to Fig. 16 and 17 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 4, Example 20.

Lead/lag status during chiller operation can be viewed on the

LL_MAINT display screen, which is accessed from the

SERVICE menu and CONTROL ALGORITHM STATUS

table. See Table 4, Example 14.

Lead/Lag System Requirements:

• all chillers in the system must have software capable of per-

forming the lead/lag function

• water pumps MUST be energized from the PIC III 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 USAGE AND INSTALLA-

TION — Lead/lag operation does not require a common

chilled water point sensor. However, common point sensors

(Spare Temp #1 and #2) may be added to the CCM module, if

desired.

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 III cannot read the value of common point

sensors installed on the other chillers in the chilled water

system.

If leaving chilled water control (ECW CONTROL OPTION

is set to 0 [DSABLE] TEMP_CTL screen) and a common

point sensor is desired (COMMON SENSOR OPTION in

LEADLAG screen selected as 1) then the sensor is wired in

Spare Temp #1 position on the CCM (terminals J4-25 and

J4-26).

NON-ZERO

EXAMPLE OF

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

Fig. 24 — Head Pressure Reference Output

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