Honeywell AUTOMATIC CONTROL Engineering Manual page 324

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
CHILLED WATER PUMP OFF LONGEST IS STARTED UPON SECONDARY SYSTEM
CHILLED WATER PUMPS ARE SEQUENTIALLY STAGED ON ANYTIME THE DECOUPLER
TEMPERATURE GREATER THAN
MINUTES, BUT WITH NO LESS THAN
3.0
CHILLED WATER PUMPS ARE STAGED OFF ANYTIME THE FORWARD DECOUPLER
FLOW EXCEEDS
Features
1. Automatic start-stop sequencing of multiple decoupled
chillers.
2. User friendly monitoring and adjustment.
3. Flow calculations without costly and maintenance-prone
flow meters (see SPECIFICATION).
4. Chiller that has been off longest is next to start.
5. Constant flow through chillers with a variable flow
secondary water system.
Conditions For Successful Operation
1. Control network, software, and programming to advise
chiller plant controller of AHU chilled water demands.
2. Interlock and control wiring coordinated with the chiller
manufacturer.
3. Appropriate cooling tower and control.
4. Two way AHU control valves provide variable flow
secondary operation.
5. Precise and matched temperature sensors for accurate
flow calculation. Refer to the flow equation in Figure 16.
6. Proper and precise positioning of primary return water
well and sensor to get accurate measurement of mixed
water temperature.
7. Digital controller configuration to suit cost and reliability
requirements.
Specification
Chiller Plant Start-Up:
Anytime any secondary pump starts, the chiller plant controls
shall be enabled, and the chiller pump that has been off longest
shall start, subject to its software ON-OFF-AUTO function and
its respective chiller software AUTO–OFF function. Pump/
chiller combinations with either function OFF shall be removed
from the control sequence.
When any chiller pump flow is proven, its respective chiller
controls shall be energized. Upon a call for cooling by the chiller
controls, the chiller controls shall enable the condenser water
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER SEQUENCING
DEMAND FOR CHILLED WATER.
TEMPERATURE EXCEEDS THE PRIMARY SUPPLY WATER
GPM FOR GREATER THAN
820
NO LESS THAN
4.0
Fig. 20. Multiple Chiller Sequencing.
DEGREES FOR GREATER THAN
2.0
MINUTES BETWEEN STARTS.
7
MINUTES, BUT WITH
4.0
MINUTES BETWEEN STOPS.
system controls and, upon proof of condenser water flow, the
chiller shall start. Starting, loading, and interlock wiring shall
be as recommended by the chiller manufacturer.
Chiller On-staging:
Anytime a chiller has operated greater than 50 minutes and the
decoupler line temperature is greater than the chiller leaving water
temperature setpoint by greater than 1.0F for greater than 5
minutes, the off chiller pump that has been off longest shall start.
Chiller Off-staging:
Anytime more than one chiller is operating and the decoupler
has a supply chilled water flow in excess of the capacity of one
chilled water pump for greater than 3.0 minutes, the chiller
that has been running longest shall stop.
Chilled water flow calculations:
The primary supply water flow shall be calculated by summing
the design water flow for all operating chiller pumps (each pump
shall have a commandable value for its design flow).
F = ∑Flow(P
P
The secondary water flow shall be calculated by dividing the
product of the primary flow times the primary water differential
temperature by the secondary water differential temperature.
F P (T R P – T SP )
F S =
(T R S – T SS )
The decoupler flow shall be calculated by subtracting the
secondary return water flow from the primary supply water flow.
F D = F P – F S
The temperature sensors for flow calculation shall be platinum
and software field-matched to within 0.1 degree at 50F. The
primary return water sensor shall be in a stainless steel well
extended at least 50 percent of the distance across the pipe and
positioned as far away from the decoupler/secondary return
mixing tee as possible.
314
M10503
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