McQuay IM 743 User Manual

OM 780-2 Page 63
Figure 26. Tower Staging With Low-Limit Controlled Bypass Valve
Spt
V
Spt
V
+ Db
Spt
V
– Db
70.0
71.0
74.0
75.0
Control Temperature
72.0
73.0
76.0
77.0
78.0
21.0
23.0
22.0
24.0
25.0
Spt
1
Spt
2
(°F)(°C)
Spt
1
– Df
Spt
2
– Df
Spt
3
Spt
3
– Df
Spt
4
Spt
4
– Df
Spt
5
Spt
5
– Df
Spt
6
Spt
6
– Df
24.5
23.5
22.5
21.5
Valve Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 Stage 6
on
on
on
on on on
offoffoff
off
off
offclose
open
Legend
SptV Tower Valve Setpoint
Db Tower Valve Deadband
Spt1 Tower Stage 1 Setpoint
Df Tower Stage Differential
79.025.5
a0161
Tower Staging with Intrastage Controlled Bypass Valve
If the Tower Valve Control Option variable is “Stage Setpoint,” the tower stages are controlled as described below.
Stage-Up Control: The CSM stages up when the Control Temperature is too high. This occurs whenever the following
four conditions are satisfied:
1. The stage-up timer has expired. (See “Interstage Timers” above.)
2. The Control Temperature is greater than the Tower Stage
x
Setpoint, where
x
is the next higher tower stage (1 through
12).
3. The current tower stage is less than the Max Tower Stage setting of the chillers in the highest active sequence.
4. The bypass valve position is greater than the Maximum Tower Valve Position Setpoint. (This ensures that the valve is
sufficiently open to the tower.)
Stage-Down Control: The CSM stages down when the valve is bypassing more heat than the stage to be turned off can
reject. This occurs whenever the following two conditions are satisfied:
1. The stage-down timer has expired. (See “Interstage Timers” above.)
2. The bypass valve position is less than the Minimum Tower Valve Position Setpoint. (This setting must be determined
by trial and error.)
Figure 27. Tower Staging With Intrastage Controlled Bypass Valve
Stage 1
With
Valve
Spt
1
Spt
2
Stage 2
With
Valve
Stage 3
With
Valve
Stage 4
With
Valve
Spt
3
Spt
4
Spt
5
Spt
6
Stage 5
With
Valve
Stage 6
With
Valve
Spt
1
+ Db
Spt
1
– Db
Spt
2
+ Db
Spt
2
– Db
Spt
3
+ Db
Spt
3
– Db
Spt
4
+ Db
Spt
4
– Db
Spt
5
+ Db
Spt
5
– Db
Spt
6
+ Db
Spt
6
– Db
open
on
close
close
close
close close close
on
on
on on on
open
open
open open open
Legend
Spt1 Tower Stage 1 Setpoint
Db Tower Valve Deadband
70.0
71.0
74.0
75.0
Control Temperature
72.0
73.0
76.0
77.0
78.0
21.0
23.0
22.0
24.0
25.0
(°F)(°C)
24.5
23.5
22.5
21.5
79.025.5
a0162

McQuay IM 743 Specifications

General

Brand	McQuay
Model	IM 743
Category	Controller
Language	English