Carrier XRV Spécifications télécharger pdf (Page 42)

DIFFUSER CONTROL — On all units with Frame 5 com-

pressors and those Frame 4 compressors with the variable (split

ring) diffuser option, the PIC III adjusts the diffuser actuator

position (DIFFUSER ACTUATOR on the COMPRESS screen)

based on the ACTUAL GUIDE VANE POSITION. This is

done in accordance with a compressor build-specific “schedule”

entered in the SETUP2 screen. The schedule consists of guide

vane and diffuser positions for three points (designated as the

25%, 50%, and 75% Load Points). In order for the schedule to

be valid, the guide vane values must be ascending and the dif-

fuser values must be descending for the three points. Diffuser

actuator output is controlled by a 4 to 20 mA output from CCM

terminals J8-3(+) and J8-4(–). Figure 22 shows the relationship

between diffuser-related parameters for a typical build.

Diffuser control output is enabled whenever the DIFFUSER

OPTION is enabled, whether the machine is running or not. As

shown in Fig. 22, 0% output corresponds to a full open diffuser.

The minimum closed position (25% Load Point value) will be

at less than 100% for most diffusers (depending upon the

model). This coordinated guide vane-diffuser operation may be

tested in the Control Test selection “IGV & SRD Actuator.”

Note that the diffuser actuator should NOT be forced to a great-

er percent than the configured 25% Load Point (maximum)

value. The diffuser opening can be incremented from fully open

to completely closed. A 0% setting is fully open; a 100% setting

is completely closed. To obtain the proper settings for Diffuser

Control, contact a Carrier Engineering representative.

DEMAND LIMITING — The PIC III controls provide a

feature for limiting AVERAGE LINE CURRENT or LINE

KILOWATTS (demand) by limiting capacity via guide vane

control. The limit applied is called ACTIVE DEMAND

LIMIT, which is equal to a BASE DEMAND LIMIT value

(set in the SETPOINTS Screen, page 26, default value 100%),

or that determined by AUTO DEMAND LIMIT INPUT (an

optional 4 to 20 mA input, described below). ACTIVE

DEMAND LIMIT may also be forced to be different from

BASE DEMAND LIMIT by manually overriding the value

(forcing) from the MAINSTAT screen or writing a value via a

CCN network device, or controlled by another chiller in Lead

Lag operation (see page 51).

The demand limit may be based on either line current or

kW, as indicated by DEMAND LIMIT SOURCE in the

EQUIPMENT SERVICE/RAMP_DEM table. The default is 0,

for demand limiting based on AVERAGE LINE CURRENT

(percent of RATED LINE AMPS, as displayed on the default

screen). Setting DEMAND LIMIT SOURCE to 1 makes

demand limiting based on PERCENT LINE KILOWATTS

(displayed in the MAINSTAT screen). LINE KILOWATTS is

measured by the VFD, and the MOTOR RATED LOAD kW

value (100% rated kW) is set in the VFD_CONF table.

If the DEMAND LIMIT SOURCE (percent line current)

exceeds the ACTIVE DEMAND LIMIT by 5% or less, in-

creases in guide vane opening will be prevented. If the DE-

MAND LIMIT SOURCE (percent line current) exceeds the

ACTIVE DEMAND LIMIT by more than 5%, the guide vanes

will be forced to close. Also, as the DEMAND LIMIT

SOURCE approaches the ACTIVE DEMAND LIMIT from a

lower value, allowable capacity increases become increasingly

more limited, beginning when the DEMAND LIMIT

SOURCE is within the DEMAND LIMIT PROP BAND (con-

figurable in the RAMP_DEM table).

Demand Limit Control Option

— The demand limit control

option (20 mA DEMAND LIMIT OPT) is externally

controlled by a 4 to 20 mA signal from an energy management

system (EMS). The option is set up on the RAMP_DEM

screen. When enabled, 4 mA will set ACTIVE DEMAND

LIMIT to 100% of the DEMAND LIMIT SOURCE (regard-

less of the value of BASE DEMAND LIMIT), and 20 mA will

set ACTIVE DEMAND LIMIT to the value configured as

“20MA DEMAND LIMIT OPT” in the RAMP_DEM table.

Wire the auto demand limit input to terminals J5-1 (–) and

J5-2 (+) on the CCM. In order to use a 1 to 5 vdc input instead

of 4 to 20 mA, install a 25-ohm resistor in series with the + lead

at terminal J5-2.

A DEMAND KILOWATTS monitoring feature is also

available. This feature provides a display of average demand

(power) in kilowatts (in the POWER screen). This value is

continuously updated and averaged over the preceding time

interval specified as DEMAND WATTS INTERVAL in the SER-

VICE / EQUIPMENT SERVICE/RAMP_DEM screen.

CHILLER TIMERS AND STARTS COUNTER — The

PIC III maintains two run time clocks: COMPRESSOR

ONTIME and SERVICE ONTIME. COMPRESSOR

ONTIME indicates the total lifetime compressor run hours.

SERVICE ONTIME is a resettable timer that can be used to

indicate the hours since the last service visit or any other event.

A separate counter tallies compressor starts as TOTAL COM-

PRESSOR STARTS. All of these can be viewed on the

MAINSTAT screen on the ICVC. Both ontime counters roll

over to 0 at 500,000 hours. Manual changes to SERVICE

ONTIME from the ICVC are permitted at any time. If the con-

troller is replaced, one opportunity, before the first startup with

the new controller, is provided to set COMPRESSOR

ONTIME and TOTAL COMPRESSOR STARTS to the last

readings retained with the prior controller. The SERVICE

ONTIME timer can register up to 32,767 hours before it rolls

over to zero.

The chiller also maintains a start-to-start timer and a

stop-to-start timer. These timers limit how soon the chiller

can be started. START INHIBIT TIMER is displayed on

the MAINSTAT screen. See the Start-Up/Shutdown/Recycle

Sequence section, page 57, for more information on this topic.

OCCUPANCY SCHEDULE — The chiller schedule, de-

scribed in the Time Schedule Operation section (page 25), deter-

mines when the chiller can run. Each schedule consists of from

1 to 8 occupied or unoccupied time periods, set by the operator.

The chiller can be started and run during an occupied time peri-

od (when OCCUPIED? is set to YES on the MAINSTAT dis-

play screen). It cannot be started or run during an unoccupied

time period (when OCCUPIED? is set to NO on the MAIN-

STAT display screen). These time periods can be set for each

day of the week and for holidays. The day begins with 0000

hours and ends with 2400 hours. The default setting for OCCU-

PIED? is YES, unless an unoccupied time period is in effect.

25 % Load Point

50 %0 %

78 %

50 % Load Point

rotating stall

less likely

surge more likely

75 % Load Point

surge less likely

2 mA

(0%)

4 mA

(0.2%)

Full

Open

Full

Closed

approx.

20 mA*

(100%)

Pinion Shaft Clockwise Rotation

DIFFUSER POSITION

Control Output

(Actuator Position)

GUIDE VANE OPENING (%)

* Diffuser full Span mA.

Fig. 22 — Diffuser Control

a19-1620



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Carrier XRV Spécifications Page 42

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