Emerson Vission 20/20 Operation And Service Manual page 173

Appendix B • Vission 20/20 Application Procedures
Auto sequencing can be started (from the master com-
pressor) by selecting the green Unit Start button and
pressing the "Auto Seq" button.
WALK-THROUGH OF SEQUENCING LOADING AND
UNLOADING
(Assume Suction Pressure Control)
Example:
Pressure control setpoints for setpoint 20 psig,
load rate 2 offset (outer) = 4 psig
load rate 1 offset (inner) = 2 psig
suction pressure control setpoint = 20 psig
unload rate 1 offset (inner) = 2 psig
unload rate 2 offset (outer) = 4 psig
Timers:
load rate 1 (inner) = 30 sec
load rate 2 (outer) = 20 sec
unload rate 1 (inner) = 25 sec
unload rate 2 (outer) = 20 sec
machine start timer = 120 sec
machine stop timer = 150 sec
Priorities:
Master -> slave 1 - > slave 2 - > slave 3.
Sequencing Loading mode operates in the following
way:
The slave compressors are placed into Remote mode.
The Master Compressor is started in "Auto Seq" mode.
An example of partial loading of slaves, and shutting one off:
Slave 3 – CFM (483) – running with Min Cap = 10%,
so the required CFM needed to handle slave 3 load = 483 * 10 / 100 = 48.3 CFM.
Now slave 2 is told to unload.
Slave 2 – CFM (408) – running at max cap = 90%, step = 10%
so at Interval 1 – slave 2 receives a cap hold value = 80 %,
So, the available CFM = (408 * (90 - 80) / 100) = 40.8 CFM
and the required CFM to absorb slave 3 load = 48.3 ( which is more than is available).
so at Interval 2 – slave 2 cap hold value = 70 %,
Now the available CFM = (408 * (90 - 70) / 100) = 81.6 CFM
and since the required CFM to absorb slave 3 load is = 48.3, there is now enough available and slave 3 will be
shutdown.
Vission 20/20 • Operation and Service Manual •Vilter/Emerson • 35391SC 2.0
The Master Compressor program monitors its suction
pressure value and identifi es the load / unload rate band.
During loading cycle when suction pressure reaches a
value more than the confi gured load rate 1 offset value
(20+2 = 22 psig) and if the master compressor reaches
its Max Cap value, then the master compressor starts
machine start timer (120 sec). NOTE: The master com-
pressor is allowed to load to 100% capacity (beyond its
Max Cap value.) Once machine start timer has elapsed,
the master then picks highest priority compressor (slave
1) from the list and starts loading compressor to the Min
Cap value for that slave. Program loads slave 1 as per
steps confi gured till it reaches its Max Cap value. Once
slave 1 starts running at Max Cap value and suction pres-
sure is still not within deadband ( i.e. > load rate 1 offset
value of 20+2 = 22 psig, then program starts machine
start timer (120 sec ) for next priority compressor slave
2. This process is continued till either setpoint is achieved
or all compressors are running at their Max Cap values.
OVERVIEW OF COMPRESSOR UNLOADING
The compressor unloading scheme incorporates an in-
telligent algorithm to identify when it is possible to turn
a compressor off. When a slave compressor has been un-
loaded to its Min Cap value of capacity and the suction
pressure is still less than a value of unload rate 1 offset
for the time period of the unload rate 1 timer, then a cal-
culation of the operating CFM of the slave compressor
is made. This value is compared against the available
CFM of the other running compressors. If enough CFM is
available, then the machine stop timer is started. When
it times out, and provided there is still enough CFM avail-
able from the remaining running compressors, the slave
compressor is stopped. Continue to example of "partial
loading of slaves, and shutting one off".
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