Points, (T1/P1;T2 /P2). These points ha ve default settings for each type of r efrigerant, HCFC-22
or HFC-134a, as de fined on the Servi ce1 table, or on Ta ble 4. These setti ngs and the algori thm
function are graphica lly displayed in Figure 20. The two sets of load point s on this graph (default
settings are shown) describe a line which the algorithm uses to determine the maximum lift of
the compressor. Whenever the a ctual differe ntial pressure b etween the c ooler and conde nser,
and the temperature dif ference between the ent ering and leaving ch ill ed water are above the
line on the graph (as defined by the MINIMUM and MAXIMUM Load Points) the algorithm will
go into a corrective actio n mode. If the actual valu es are below the line, t he al gorithm 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 t aken by making one of 2 choi ces. If a hot gas bypass line is
present, and the h ot gas is configu red on the Servi ce1 table, then the hot gas bypass valve can
be energized. If a hot ga s bypass if not present, then the action taken is t o hold the guide vanes.
See Table 4, Capacit y Overrides. Both of the se corrective actions wi ll reduce the lift
experienced by the comp ressor and help to preven t a surge condition. Sur g e is a condition
when the lift beco mes so high that the gas flow across t he impeller revers es. This condition can
eventually cause mach ine damage. The surge pre vention algorithm is in tended to notify the
operator that machin e ope rating conditions ar e marginal, and to take act ion to help prevent
machine damage such as low ering entering con den ser water temperat ur e.