Carrier AquaEdge 19DV Controls Operation And Troubleshooting page 19

RAMP LOADING
The ramp loading control slows the rate at which the compressor
loads up. This control can prevent 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
(Setpoint Table). Ramp loading helps reduce electrical demand
charges by slowly bringing the chilled water to CONTROL
POINT. The total power draw during this period remains almost
unchanged. If the power outage lasts for more than 3 hours, then
Temperature Ramp Loading will be used regardless of user con-
figuration and the minimum loading rate (1°F/min) will be used.
Two methods of ramp loading are available: temperature ramp
loading and motor load ramp loading.
Temperature Ramp Loading
Temperature ramp loading limits the rate at which the controlled
water temperature decreases for cooling and increases for heat-
ing during ramping by reducing on cooling mode or increasing
in heating mode the PULLDOWN SET POINT (Maintenance

Menu Capacity) at the configured rate, until the pulldown set
point is less than the cooling mode control point or greater than
the heating mode control point. The PULLDOWN RAMP
TYPE (Configuration Menu
configured to 0 for temperature ramp loading.
Motor Load Ramp Loading
Motor load ramp loading limits the rate at which either the load
current percentage or motor kilowatt percentage increases by
incrementing the ramp demand limit at the configured rate.
The PULLDOWN RAMP TYPE (Configuration Menu
General Configuration) is configured to 1 for motor load ramp
loading.
If DEMAND LIMIT SOURCE (Configuration Menu
eral Configuration) is set to AMPS, then PERCENT LOAD
CURRENT is used for motor load ramp loading. If DEMAND
LIMIT SOURCE is set to kW, then MOTOR PERCENT
KILOWATTS is used for motor load ramp loading.
The motor load ramp loading algorithm shall be deactivated
when the Ramp Demand Limit is greater than or equal to the
ACTIVE DEMAND LIMIT (General Parameters). It is also
deactivated when Ramp Demand Limit is greater than or equal
to 80%. There will be a one-minute delay for the compressor to
be uploaded to target load (ramping load target 80% or AC-
TIVE DEMAND LIMIT if less than 80%) after ramping load
demand limit is set to 80% (or ACTIVE DEMAND LIMIT if
less than 80%).
SURGE CORRECTION CONTROL
There are two stages for surge correction: envelope control
(surge prevention) and surge protection.
Envelope Control
A surge condition occurs when the lift becomes so high that the
gas flow across the impeller reverses. This condition can even-
tually cause compressor damage. The surge prevention algo-
rithm notifies the operator that chiller operating conditions are
marginal and to take action, such as lowering entering con-
denser water temperature, to help prevent compressor damage.
If a high sound condition occurs at low guide vane position, the
EC/HGBP (Envelope Control/Hot Gas Bypass) valve is used to
decrease the sound level. The envelope control algorithm is an
operator-configurable feature that can determine if lift condi-
tions are too high for the compressor and then take corrective
action. High efficiency mode or low noise mode can be select-
ed. Lift is defined as the difference between the saturated tem-
perature at the impeller eye and at the impeller discharge. The
maximum lift a particular impeller wheel can perform varies
with the gas flow across the impeller and the size of the wheel.
If Actual Lift is higher than reference lift, a capacity inhibit
signal will be sent. If Actual Lift is higher than reference lift
plus Envelope High Deadband, a capacity decrease signal will

General Configuration) is

Gen-
be sent. If Actual Lift is lower than reference lift minus Enve-
lope Low Deadband, these 2 signals will be canceled. Capacity
Control will respond to these 2 signals and make correction on
IGV1 TARGET POSITION, VFD TARGET SPEED, and EC/
HGBP actuator. To improve system performance SURGE
PROFILE OFFSET will be incremented by 1 if no surge pre-
vention has been active in the past 5 minutes. The Reference
Lift will subtract the SURGE PROFILE OFFSET prior to com-
paring to Actual Lift.
Surge Protection
The Surge Protection algorithm will run after SURGE DELAY
TIME has elapsed when compressor has been commanded to
turn on. It compares the present PERCENT LOAD CURRENT
value with the previous value once every second. If the differ-
ence exceeds the maximum AMPS change value (SURGE
DELTA % AMPS + [PERCENT LINE CURRENT / 10]), an
incidence of surge has occurred, and the surge protection signal
will be sent.
When an incidence of surge determined in this manner has oc-
curred, the SURGE COUNTS will be incremented by one. On
receiving the surge protection signal, Capacity Control will
make corrections on IGV1 TARGET POSITION, VFD TAR-
GET SPEED, and EC/HGBP actuator. When correction is in
effect, Surge Protection Count will increase by 1 when a com-
mand for either IGV decrease, VFD speed increase, or EC/
HGBP actuator activation is required for correction. Guide
vane movement will be inhibited for 1 minute after surge pro-
tection ends.

Chiller will do 259 Alarm shutdown under the following
conditions.
• If Surge Protection Counts exceed 20 within a Surge Time
Period. Note that if VFD, then VFD target speed must
equal max before this alarm is activated.
• If IGV, VFD, and HGBP cannot be further adjusted for
surge protection when Surge Protection Counts exceed 4
within a Surge Time Period.
If IGV, VFD, and HGBP control cannot correct the problem,
the chiller will initiate a shutdown alarm when the surge count
is greater than 4.
ENVELOPE/HOT GAS BYPASS (HGBP) CONTROL
This function is used to artificially load the chiller and keep it
running under low load conditions or to prevent surge condi-
tions. Since this also reduces the performance of the machine,
EC/HGBP Control is a user-selectable option.
The EC/HGBP control valve is a modulating type controlled
by a 4 to 20 mA signal.
Envelope/hot gas bypass operation has three different modes
when installed (hgbp_opt > 0) and enabled (hgbp_sel > 0):
• Envelope control and surge protection — Each compressor
has unique lift characteristics that can be plotted to deter-
mine performance. The controller will determine operating
conditions that could result in compressor surge and acti-
vate the bypass valve to prevent surge until the chiller op-
erating parameters are in a safe area on the curve where the
valve may be closed again.
• Envelope (HGBP) low load operation — In this condition,
the valve will be opened to prevent a recycle shutdown
from occurring. The valve will remain open until this min-
imal loading condition has passed and there is no surge
condition present.
• Combination for envelope control and surge correction, as
well as low load operation — When this option is selected,
both EC for envelope control/surge protection and EC for
low load operation will be performed. Surge protection
will take higher priority if both conditions are satisfied.
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