Emerson Vilter Vission 20/20 Operation And Service Manual page 238

Appendix B • Vission 20/20 Application Procedures
CONFIGURING SEQUENCING TABLE ON MASTER
COMPRESSOR
1. Select the correct compressor name from
Equipment drop down list.
2. Assign the Priority for the slave compressor
3. Assign the Step size in percentage for the slave
compressor
4. Assign Min/Max capacity values for the slave
compressor
5. Repeat steps #1-4 to configure all slave compressors.
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,
Fast load offset = 4 psig
Start offset = 2 psig
Suction pressure control setpoint = 20 psig
Fast unload offset = 4 psig
Timers:
Slow load timer = 30 sec
Fast load timer = 15 sec
Slow unload timer = 30 sec
Fast unload timer = 15 sec
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 at Interval 2 – slave 2 cap hold value = 70 %,
and since the required CFM to absorb slave 3 load is = 48.3, there is now enough available and
slave 3 will be shutdown.
B - 20
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).
Now the available CFM = (408 * (90 - 70) / 100) = 81.6 CFM
Vission 20/20 • Operation and Service Manual • Emerson • 35391SC 2.7
Machine start timer = 90 sec
Machine stop timer = 120 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.
The Master Compressor program monitors its suction
pressure value and identifies the load / unload rate band.
During loading cycle when suction pressure reaches a
value more than the configured start offset value (20+2
= 22 psig) and if the master compressor reaches its Max
Trigger value, then the master compressor starts ma-
chine start timer (90 sec). Once the machine start timer
has elapsed, the master then picks the highest priority
compressor (slave 1) from the list and starts loading the
compressor to the Min Cap value for that slave. The pro-
gram loads slave 1 as per steps configured till it reaches
its Max Cap value. Once slave 1 starts running at Max Cap
value and the suction pressure is still not within dead-
band ( i.e. > start offset value of (20+2 = 22) psig, then
the program starts machine start timer (90 sec ) for the
next priority compressor slave 2). This process is contin-
ued till either the 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 the suction control