Orion System VCM Controller - Manual (Page 38)

Technical Guide
VCM Controller 38
No matter which Sensor is configured for the HVAC Mode Enable or if
the Remote BAS sets the Mode through Remote Forced Cooling, the
Supply Air Temperature is always controlled to the Active Supply Air
Temperature Setpoint while in the Cooling Mode..
Stage Control Window
The Cooling Stage Control Window Setpoint determines when the com-
pressors start to stage up and stage down. In the Cooling Mode, as the
Supply Air Temperature rises above the Active Supply Air Temperature
Setpoint, the Cooling Stages will begin to stage on based on the Cool-
ing Stage Up Delay setting. The Cooling Stages will continue to run
until the Supply Air Temperature drops below the Active Supply Air
Temperature Setpoint minus the Cooling Stage Control Window. For
example if the Supply Air Temperature Setpoint is 55° and the Cooling
Stage Control Window is 5°, as the Supply Air Temperature drops be-
low 50°, the Cooling Stages will begin to stage off based on the Cool-
ing Stage Down Delay setting.
Cooling Staging Delay
Minimum Off Time
A Cooling Stage cannot be activated unless it has been off for
this amount of time.
Minimum Run Time
After a Cooling Stage has been activated, it must remain on for
this amount of time.
Staging Up Delay
After the first Cooling Stage has been activated, this delay
prevents additional stages from activating too quickly before
they are needed to achieve the Active Supply Air Temperature
Setpoint.
Staging Down Delay
After a Cooling Stage has met its Minimum Run Time and is not
needed, this delay prevents additional stages from deactivating
too quickly in case they are needed to maintain the Active
Supply Air Temperature Setpoint Temperature.
Modulating Cooling
The Modulating Cooling Proportional Window is used to determine
the signal to the Modulating Cooling Source and is user adjustable.
The Modulating Cooling signal is calculated based on the differential
between the Supply Air Temperature and the Active Supply Air Tem-
perature Setpoint based on the Modulating Cooling Proportional Win-
dow. The Maximum Signal Adjustment per Time Period is 10% and is
not user adjustable. The Minimum Signal Adjustment per Time Period
is based on the Modulating Cooling Proportional Window. The larger
the Modulating Cooling Proportional Window, the smaller the signal
adjustment will be per Time Period. The Time Period is the delay be-
tween another increase or decrease in the Modulating Cooling Source
Signal and is user adjustable. For example, if the Modulating Cooling
Proportional Window is 5
°
F, the signal would be adjusted 2% per
°
F
each Time Period above or below the Active Supply Air Temperature
Setpoint. When the Supply Air Temperature is above or below the
Active Supply Air Temperature Setpoint by 5
°
F or more, the signal
would adjust 10% each Time Period.
The VCM can control one of two Modulating Cooling sources, such as
a Chilled Water Valve or a Digital Compressor. Whichever source is
used, the VCM will control the Modulating Cooling source to maintain
the Active Supply Air Temperature Setpoint.
A Digital Compressor is a Variable Capacity Compressor that has a 10
to 1 turn down ratio. The VCM is capable of handling a single stage
Cooling unit with a Digital Compressor as its only stage. It is also ca-
pable of handling multistage Cooling units with a Digital Compressor.
The Digital Compressor is always the first stage of Cooling. On multi-
stage Cooling units with a Digital Compressor, Fixed Capacity Com-
pressors will stage up while the Digital Compressor modulates to achieve
the Active Supply Air Temperature Setpoint. A three compressor Cool-
ing unit with a Digital Compressor is the only special staging arrange-
ment.
On three compressor Cooling units with a Digital Compressor, the first
stage of Cooling is the Digital Compressor. The second stage of Cool-
ing is a Fixed Capacity Compressor, equal to the maximum capacity of
the Digital Compressor. The third stage of Cooling is a Fixed Capacity
Compressor with double the capacity of the second compressor. When
the third stage of Cooling activates, the second stage (compressor) of
Cooling deactivates. The fourth stage of Cooling is when the second
stage (compressor) of Cooling is reactivated. In this way, a three com-
pressor Cooling unit with a Digital Compressor becomes four stages of
Cooling.
If the Supply Air Temperature rises above the Active Supply Air Tem-
perature Setpoint, the Digital Compressor signal will modulate as needed
up to 100%. When the Digital Compressor signal reaches 100% the
Cooling Stage Up Delay begins. If the Digital Compressor signal re-
mains at 100% and the Cooling Stage Up Delay expires, an additional
Fixed Capacity Compressor will activate if equipped.
If the Supply Air Temperature falls below the Active Supply Air Tem-
perature Setpoint, the Digital Compressor signal will modulate as needed
down to 0%. When the Digital Compressor signal reaches 0% the Cool-
ing Stage Down Delay begins. If the Digital Compressor signal remains
at 0% and the Cooling Stage Down Delay expires, the Digital Com-
pressor, or a Fixed Capacity Compressor will be deactivated. The Digi-
tal Compressor is always the last compressor to be deactivated.
Economizer Operation
This section assumes you have configured your HVAC unit to control
the Outdoor Air Dampers in an Economizer Mode of operation.
The Economizer is used as the first stage of Cooling if the Outdoor Air
or Wetbulb Temperature is below the Economizer Enable Setpoint. For
Wetbulb control of the Economizer, an Outdoor Air Humidity Sensor
must be installed. If the unit is equipped with a Return Air Temperature
Sensor, the Outdoor Air or Wetbulb temperature must be at least 5°F
colder than the Return Air Temperature to allow the Economizer to modu-
late. The same is true if the unit is only equipped with a Space Tempera-
ture Sensor. By using the Return Air and/or Space Temperature refer-
ence, it allows the VCM to calculate whether the Outdoor Air Tempera-
ture will assist in Free Cooling.
The VCM Controller can monitor an Outdoor Air Humidity Sensor and
combine that reading with the Outdoor Air Temperature reading to cal-
culate a Wetbulb Temperature. If this Wetbulb Temperature is not avail-
able, just the Outdoor Air Temperature will be used. Whichever tem-
perature is available, it must be below the Economizer Enable Setpoint
by 1°F to enable the Economizer during the Cooling Mode of opera-
tion. When the temperature rises 1°F above the Economizer Enable
Setpoint, the Economizer will be disabled and return to Minimum Posi-
tion.
Sequence Of Operations