OM AGSB-5
15
Compressor Control
Multiple Compressor Staging
This section defines which compressor is the next one to start or stop. The next section
defines when the start, or stop, is to occur.
Functions
1. Can start/stop compressors according to an operator-defined sequence.
2. Can start compressors based on the least number of starts (run hours if starts are equal)
and stop on most run hours.
The above two modes can be combined so that there are two or more groups where all
compressors in the first group are started (based on number of starts/hours) before any in the
second group, etc. Conversely, all compressors in a group are stopped (based on run hours)
before any in the preceding group, etc.
Required Parameters
1. Sequence number setpoint for all compressors. Possible settings = (1-3). Compressors
will start in the specified order. Default operation sequence is 1 for all compressors
(meaning they will start based on number of starts). That is, if all are 1s, the controller
will look at number of starts and run-hours
2. Maximum Number of compressors ON setpoint. Possible settings = (1-3).
3. Number of starts for all compressors.
4. Number of run hours for all compressors.
5. Status of all compressors (Available/Unavailable, Pumping down, Running, etc.).
Multiple Compressor Start/Stop Timing
This section defines when a compressor is to start, or stop, and the scenario for doing so.
Starting
Staging up, no compressors on the unit are running:
The first compressor can start when the LWT is more than the sum of the active LWT
setpoint and the Startup Delta-T. For example, with default settings, the active setpoint
would be 44
°
F and the startup delta would be 10 degrees F. In this case, the LWT must be
greater than 54
°
F. This is a necessary, but may not be a sufficient, condition for starting the
first compressor. The Startup Delta-T is adjustable from 10 degrees down to 0 degrees F.
Staging up, at least one compressor is already running:
Additional compressors can start when the LWT is more than the sum of the active LWT
setpoint and the Stage Delta-T. With the default settings, the active setpoint is 44
°
F, and the
Stage Delta would be 2 degrees F. So, one necessary condition for staging an additional
compressor on is that the LWT must be higher than 46
°
F.
If in Cool Mode, an additional requirement is that all running compressors are running at
their maximum capacity, or at least 75% slide position and the Stage Up Delay Timer (5
minute default) has timed out. Also, a compressor is considered to be at its maximum
capacity if it is in an inhibit or unload situation due to low evaporator pressure, high
condenser pressure, or low discharge superheat. If a compressor is set for manual slide
control, or the slide target has reached the maximum allowed by the Max Slide setpoint, it
will also be flagged as being at maximum capacity.
Summary of Contents for AGS 206A
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