Carrier PC211 Start-Up, Operation And Maintenance Instructions Manual page 94

Points, (T1/P1;T2/P2). These points have default settings for each type of refrigerant, HCFC-22
or HFC-134a, as defined on the Service1 table, or on Table 4. These settings and the algorithm
function are graphically displayed in Figure 20. The two sets of load points on this graph (default
settings are shown) describe a line which the algorithm uses to determine the maximum lift of
the compressor. Whenever the actual differential pressure between the cooler and condenser,
and the temperature difference between the entering and leaving chilled water are above the
line on the graph (as defined by the MINIMUM and MAXIMUM Load Points) the algorithm will
go into a corrective action mode. If the actual values are below the line, the algorithm takes no
action. Modification of the default set points of the MINIMUM and MAXIMUM load points is
described in Figure 21.
Corrective action can be taken by making one of 2 choices. If a hot gas bypass line is
present, and the hot gas is configured on the Service1 table, then the hot gas bypass valve can
be energized. If a hot gas bypass if not present, then the action taken is to hold the guide vanes.
See Table 4, Capacity Overrides. Both of these corrective actions will reduce the lift
experienced by the compressor and help to prevent a surge condition. Surge is a condition
when the lift becomes so high that the gas flow across the impeller reverses. This condition can
eventually cause machine damage. The surge prevention algorithm is intended to notify the
operator that machine operating conditions are marginal, and to take action to help prevent
machine damage such as lowering entering condenser water temperature.