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shall always be stopped when the evaporator pressure is
seen to be close to the Low Side Evaporator Pressure relief
valve setting.
Chilled Water Flow Interlock
Symbio™ 800 has an input that will accept a contact
closure from a proof-of-flow device such as a flow switch.
The flow switch is to be wired in series with the chilled
water pump starter's auxiliary contacts. When this input
does not prove flow within 20 minutes relative to transition
from Stop to Auto modes of the chiller, or if the flow is lost
while the chiller is in the Auto mode of operation, the chiller
will be inhibited from running by a non-latching diagnostic.
The flow switch input shall be filtered to allow for
momentary openings and closings of the switch due to
turbulent water flow. This is accomplished with a 6 seconds
filtering time. The sensing voltage for the condenser water
flow switch is 115/240 Vac.
Important: DO NOT cycle the chiller through starting and
stopping the chilled water pump. This could
cause the compressor to shut down fully
loaded. Use the external stop/start input to
cycle the chiller.
Condenser Water Pump Control
Symbio 800 provides a contact closure output to start and
stop the condenser water pump. If condenser pumps are
arranged in a bank with a common header, the output can
be used to control an isolation valve and/or signal another
device that an additional pump is required.
The Condenser Water Pump Prestart time helps with cold
condenser water problems. In very cold outdoor ambients,
the cooling towers sump cold water would reach the chiller
some time after the low system differential pressure
protection had run through its ignore time, and result in an
immediate shutdown and latching diagnostic. By simply
starting the pump earlier, and allowing mixing of the
warmer indoor loop with the cooling tower's sump, this
problem can be avoided.
Condenser Water Flow Interlock
The Symbio 800 shall accept an isolated contact closure
input from a customer installed proof-of-flow device such
as a flow switch and customer provided pump starter
auxiliary contact for interlocking with condenser water flow.
The input shall be filtered to allow momentary openings
and closings of the switch due to turbulent water flow, etc.
This shall be accomplished with a 6 seconds filtering time.
The sensing voltage for the condenser water flow switch is
115/240 Vac.
On a call for cooling after the restart inhibit timer has timed
out, the Symbio 800 shall energize the condenser water
pump relay and then check the condenser water flow
switch and pump starter interlock input for flow
confirmation. Startup of the compressor will not be allowed
until flow has proven.
If flow is not initially established within 1200 seconds (20
minutes) of the condenser pump relay energizing, an
automatically resetting diagnostic “Condenser Water Flow
Overdue” shall be generated which terminates the prestart
mode and de-energizes the condenser water pump relay.
This diagnostic is automatically reset if flow is established
at any later time.
Note: This diagnostic would never automatically reset if
Symbio™ 800 was in control of the condenser pump
through its condenser pump relay since it is
commanded off at the time of the diagnostic. It could
however reset and allow normal chiller operation if
the pump was controlled from some external source.
Chilled Water Reset (CWR)
The controller will reset the chilled water temperature
setpoint based on either return water temperature, or
outdoor air temperature. The Return Reset option is
standard, Outdoor Reset is optional.
The following is selectable:
• RESET TYPE Setpoint. This can be set to: DISABLE,
CONSTANT, RETURN or OUTSIDE AIR. The MP shall
not permit more than one type of reset to be selected.
• RESET RATIO Setpoints. For outdoor air temp. reset,
both positive and negative reset ratios will be allowed.
• START RESET Setpoints.
• MAXIMUM RESET Setpoints . The maximum resets
shall be with respect to the chilled water setpoint.
When the chiller is running, if any type of CWR is enabled,
the controller will step the CWS toward the desired CWS'
(based on the below equations and setup parameters) at a
rate of 1 degree F every 5 minutes until the Active CWS
equals the desired CWS'. When the chiller is not running
the CWS will be fully reset immediately (within one minute).
The chiller will then start at the Differential to Start value
above a fully reset CWS or CWS' for Outdoor, Return, and
Constant Return Reset.
Equations for calculating CWR
Equation used to get Degrees of Reset:
Outdoor Air:
Degrees of Reset =
Reset Ratio * (Start Reset - TOD)
Return Reset:
Degrees of Reset =
Reset Ratio * (Start Reset - (TWE - TWL))
Constant Return:
Degrees of Reset =
100% * (Design Delta Temp - (TWE - TWL))
To obtain Active CWS from Degrees of Reset:
Active CWS = Degrees of Reset + Previous CWS
Note: Previous CWS can either be Front Panel, BAS, or
External
Reset Ratio calculation:
Installation Electrical
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