Gardner Denver TWISTAIR EWDQ D, Operating And Service Manual

13–11–602    Page 29

the first lag compressor (rotation #2) has a PSET  of 97
PSI; the second lag, 94 PSI, and so on.  But look what
happens in an eight compressor installation: The
eighth compressor will have an initial setpoint of [100
– 3(8 – 1)], or 79 PSI.  Does this mean that an eight
compressor installation must operate 21 PSI below the
desired operating point when all compressors are run-
ning?  NO!  This is where the “AUTO SENTRY

–ES”

dynamic setpoint control takes over.  This is how it
works: Whenever the system pressure is below the
programmed RESET PRESSURE, the PSET of each
lag compressor is incremented 1 PSI every thirty se-
conds.  Thus, after a short interval (about five minutes
in this example), the PSET of the last sequenced com-
pressor will climb up until either it equals the RESET
PRESSURE, or a decrease in demand causes the ac-
tual system pressure to rise above the RESET PRES-
SURE.  IT can be seen then, that except for short peri-
ods just after a sudden increase in demand, the “AUTO
SENTRY

–ES”, with its dynamic setpoint control, will

maintain system pressure between the limits of RESET
PRESSURE and SET PRESSURE.  Remember, RE-
SET and SET PRESSURE values are programmed by
the operator so the operating range is completely pro-
grammable and predictable.

Dynamic setpoint control will also work in reverse of the
operation described above.  Obviously, incrementing
setpoints will cause overlap of the compressors’ modu-
lation ranges.  While this enables us to maintain a high-
er pressure than competitor’s sequencers, overlap is
undesirable as demand decreases, because a system
could end up with several compressors running partial-
ly loaded instead of running the minimum number of ful-
ly loaded compressors.  To overcome this, as pressure
rises through the range between RESET and SET, the
lag compressors’ PSET’s are now decremented, re-
versing the effect described above during periods of
high demand.  The “AUTO SENTRY–ES” keeps track
of all functions at all times so there is never any mix–up
of setpoints and the proper rotation sequence is always
maintained.

The Automatic Sequence Change

After the master (lead) compressor has served for the
duration programmed (TRANSFER INTERVAL), it re-
linquishes control and assigns itself the highest avail-
able rotation number.  The lag compressors detect the
loss of the master and decrement their rotation num-
bers.  Number 2 becomes number 1, the new master,
number 3 becomes number 2, etc.

It should be noted also that whenever the master de-
tects a missing rotation number, such as when a com-
pressor is turned off that was previously in the rotation,
it will automatically ‘close the gap’ by decrementing the
rotation numbers of all compressors whose rotation
numbers were greater than the missing number.  Like-
wise, if for whatever reason, the master compressor

fails to carry out its role, all lag compressors begin
decrementing their rotation number until a new master
is established.  Regardless of the scenario, the end re-
sult will always be that the compressors that remain in
rotation will always end up with the lowest possible
rotation numbers.

Other Features

Any air system will exhibit pressure differences from
one point to the next.  Even a well designed multi–com-
pressor installation will show ‘minor’ pressure varia-
tions between one compressor’s discharge point and
another compressor’s discharge.  These points will
also vary from the central system (normally the air stor-
age receiver).  The “AUTO SENTRY

–ES” sequenc-

ing system is designed to tolerate minor variations.
These pressure differences wreak havoc with conven-
tional sequencers.  If a central sequencer is used, it will
be sensing a lower pressure than is seen at each com-
pressor.  With such systems, there is always a chance
that the sequencer could cause a compressor to over
pressure due to this pressure drop. The alternative has
been to set the central sequencer to a lower pressure
to prevent this or allow local override of the sequencer
by the local pressure control, neither of which is desir-
able in the scheme of maintaining plant pressure effi-
ciently with sequencing.  The “AUTO SENTRY

–ES”

sequencing system will automatically adjust the sys-
tem to prevent over pressures in any individuals.

The “AUTO SENTRY

–ES” sequencing system lets

each compressor control itself independently about a
setpoint (PSET) derived to cause staggered operation,
or sequencing.  The aforementioned pressure drops
can also cause derogatory effects (mainly skewed, or
out of sequence operation) to the sequencing algorithm
used by the “AUTO SENTRY

–ES”.

Since these pressure variations are not constant (they
will vary due to demand changes, compressor load per-
centage changes, and number of compressors run-
ning), any scheme to compensate for the pressure vari-
ations must be dynamic.  The exclusive dynamic
setpoint control feature enables this error correction
scheme to be accomplished rather easily.

Here’s how it works:  The master continually receives
system pressure values from every machine in the se-
quence rotation.  The values are averaged and this av-
erage is then distributed to all lag compressors.  All
compressors, lead and lag, then compare their local
pressure reading to the average and adjust their PSET
by the amount of error.  The effect is that all compres-
sors are controlling to a single pressure reading, a
reading that is not one that is picked up somewhere re-
moved from the compressor, but an average of actual
discharge pressures.

It should be noted that the pressure displayed on the
top line by all sequenced compressors is this average.