Specifications

36 Centrifugal Chillers IMM WMC-2
The speed control and vane control work in conjunction. As load decreases, compressor speed is reduced as low
as possible but above the point where stall might begin. If further capacity reduction is required, the guide vanes
will close to whatever position is required to match the compressor capacity to the load.
Surge and Stall
Stall and surge are a characteristic of all centrifugal compressors. These conditions can occur at low load
conditions when the operating point moves to the left of the compressor surge line on the performance curve.
In surge, the discharge gas alternately flows backward and forward through the impeller reversing about every
two seconds. Increased noise, vibration and heat occur and motor current varies widely. Surge can damage a
compressor. The compressors are equipped with safety features that help prevent surge from occurring.
Another instability is stall or incipient surge, which occurs a little to the left, or before, the surge condition.
Discharge gas in the diffuser forms rotating stall pockets or cells. The compressor sound level will change and
the impeller starts to heat up. Motor current remains steady.
Normal Unit Startup/Shutdown
Startup and shutdown, other than seasonal shutdowns, are considered to be normal operation and the following
procedures apply (assuming that the equipment room temperature are above freezing). The procedures would be
used for a weekend shutdown, for example.
Note that the chiller is part of an entire building heating and cooling system that are usually unique to a
particular site. For example, the chilled water loop and chilled water pump can also be used for heating and
therefore must be operational year-around. The cooling tower can be used for other equipment besides the
chiller and may have to remain functional even though the chiller is not. The following procedures, therefore,
must take the peculiarities of the entire system into account.
Shutdown
If the unit is to be secured for several days, and is already off due to lack of load, the UNIT switch in the Unit
Control Panel (and the remote Start/Stop switch, if used) should be placed in the OFF position. If the chilled
water pump and cooling tower are not required for other purposes, they too can be turned off. If the pumps are
controlled by the WMC unit controller, they will shut down after the compressors.
If the chiller is running, the chilled water and condenser water pumps must remain on until the compressors are
stopped. This is true regardless of how the unit is turned off, whether by the local switches or through a remote
signal. The compressors go through a short shutdown sequence, shutting guide vanes and performing other
functions, before it finally stops. The pumps must remain on during this shutdown period.
Once the compressors and pumps have stopped, no further action is required other than opening disconnects, if
so desired.
Startup
Any disconnects that were opened must be closed. The chilled water pump and cooling tower should be turned
on and flow verified. The chiller can then be started by placing the UNIT switch (and the remote Start/Stop
switch, if used) in the ON position. There is no lube warm-up period required. The compressors go through a
starting sequence and may not start immediately. Once started, it is prudent to observe unit operation on the
operator interface screen for several minutes to check for normal functioning.
Start/Stop Switching
There are four ways to start/stop the chiller. Three are selected in SETPOINT\ MODE\SP3, the fourth way is
through panel-mounted switches:
1. Operator Interface Panel, (LOCAL) Home Screen 1 has AUTO and STOP buttons that are only active
when the unit is in "LOCAL CONTROL". This prevents the unit from being accidentally started or stopped