York MILLENIUM 351-46, Руководство по эксплуатации

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JOHNSON CONTROLS
16
FORM 160.00-O1 (1020)
VSD ADAPTIVE CAPACITY CONTROL
The new York VSD utilizes a different approach to speed
reduction compared to earlier variable speed products.
There is no pre-programmed surge map - our adaptive
system experiments with the speed and vanes to find
the optimum speed for any given condition. It does not
always encounter a “Surge” in the process, but when it
does, the ACC stores into memory, the conditions sur-
rounding the Surge, and therefore remembers to avoid
the stored operating point anytime in the future. This
sounds a bit mysterious, but the process is really quite
simple. Once you have an understanding of the steps
involved, you will be able to watch the chiller adjust itself
to different conditions, and understand exactly why it is
performing in the manner it does.
Upon startup the chiller will always go to full speed. This
is different compared to earlier systems which could go
to a reduced speed if the total head across the chiller
was low enough. With the VSD, the chiller will always
run at fixed speed until two conditions are met. These
two conditions are:
Achieve Setpoint - The leaving water temp must
be 0.3 to -0.6 of a degree from setpoint.
Speed reduction will not occur until the leaving water
reaches setpoint.
Achieve Stability - The leaving water temp must
be stable, with the vanes not driving open or closed
to maintain the temperature at this point. Lack of
stability will be evidenced by the vanes hunting, the
leaving water temperature varying, and the green
LED on the ACC board will be on, to indicate insta-
bility.
Once the above conditions are met, the ACC begins to
lower the speed 1/10 of a hertz at a time. As the ACC
lowers the speed, the leaving water temperature will
begin to creep up, due to the reduction in speed. As this
occurs, you will see the vanes begin to open slightly, just
enough to keep the leaving water temperature within the
setpoint window. The ACC will continue to lower speed,
with the leaving water temperature in turn driving the
vanes to a more open position. This process will continue
until one of three situations occur:
Vanes Full Open - Once the vanes reach the full
open position, the ACC knows it can no longer
reduce speed. The ACC will maintain operation at
this point, with the vanes full-open, and the speed
at the last point reached when the vanes hit 100%.
If there is an increase in load while at this point,
the ACC will increase speed until the vanes are at
95%. The ACC will then be allowed to continue to
optimize the speed and vanes.
Surge is Detected - If in the process of dropping
speed and opening vanes the compressor should
surge, the ACC will boost the speed back up enough
to get the chiller out of surge, and will store in
memory the head and flow conditions present at
the time of the surge. The chiller will then know not
to reduce speed this low again, should the same
head and flow conditions be encountered again in
the future. As the chiller encounters more head and
flow combinations which result in surge, it will store
more points, and eventually this plotting of points
creates a “Surge Map”. Surges may be detected in
two ways, by monitoring the pressure differential
across the compressor, or by monitoring the com-
pressor motor current. Either detection will light the
Red LED on the ACC board, indicating a surge was
detected. The chiller may surge 6 to 8 times before
the ACC can raise the speed enough to get the
chiller back out of surge. Each surge is counted on
the surge accumulator, which may be called up on
the panel display. This surge counter will always dis-
play the total number of surges encountered by the
chiller, not the total number of surge points. Surging
which occurs at fixed speed will increment the surge
counter as well. We know of one chiller which ran
in continuous surge for two weeks due to a cooling
tower problem. The customer’s fixed speed chiller
was surging continuously for 2 weeks also. During
this time, the VSD surge counter accumulated over
18,000 surges.
Instability is Encountered - The ACC may begin
the process of reducing speed and opening the
vanes, but may stop speed reduction prematurely
if instability is encountered. This is the same insta-
bility discussed as one of the two conditions which
must be met to begin reducing speed initially (See
“Achieve Stability” above). Once the system again
becomes unstable, no additional speed reduction
can occur. The most common causes of instability
are:
• Valves on air-handler coils causing rapid
changes in heat-load.
• Extremely short chilled water loop.
• Parallel chiller with poor control is causing
temperature variations.
If you experience a problem with a VSD not reducing
speed at all, make certain the system is not in manual
speed control, and locked into fixed speed. Refer to the
section on “Manual Speed Control” in the “Frequently
Asked Questions” section in Form 160.00-M1. Also,