“WATER
TEMPS”
KEYPAD (Setpoints)
Actual water temperature is displayed by pressing the “Water
Temps” keypad under the Display section.
To display the leaving chilled water setpoint temperature,
press the “Water Temps” keypad under the Setpoints sec-
tion just once. LVG EVAP SPT= XX.XF will be displayed. This
is the temperature setting to which the compressor vanes will
respond; opening and closing, loading and unloading to main-
tain the operating temperature at the setpoint temperature.
This is the temperature also that is used as a base against
which the compressor will start and stop. See the descrip-
tions for the “Start-up Temperature Difference” and “Shut-
down Temperature Difference”. These latter statements are
displayed by pressing the “Set-up Options” keypad.
Pressing the “Water Temps” (Setpoints) keypad a second
time will display’the statement “Reset LVG
SPT=ZZZ”.
The three Z’s indicate letter options which in this case would
be “N/A” for not active, CHW for chilled water, and ENT for
return water temperature reset.
See the section marked Reset Control for a description of
reset opportunities with
MicroTech.
Keep in mind that these statements and others under this
Setpoints section are statements only. To input these values
or statements, press the “Set-up Options” keypad under the
Service section, and follow the instructions given in the follow-
ing section, “Set-up Option”. A third press of the “Water
Temps” (Setpoints) key will display the statement
“RMT
Reset
SIG=N/A” or
"....
=Xx.X”.
The three X’s indicate that the
displayed value will be numbers that represent the milliamp
(mA)
value impressed.
If the option to reset based on “CHW” is elected, a 4 to
20
mA
signal is required. The signal is input at terminals 69,
70 and 71 on the Field Connection terminal strip (single unit
installation) or at the Lead/Lag Load Balance control box on
two-unit installations.
If the option to reset based on
“ENT”
is elected, no remote
Reset signal is required.
“Max Amp Limit” keypad also provides multiple statements
with each of 3 separate strokes of the keypad. The first
keystroke and statement “Max Amp Lim = XX% indicates the
maximum
%
of Rated Load Amps that the chiller will be per-
mitted to draw. The setting cannot be increased above
100%
nor lowered below 30%. This control provides a manual limit
of power consumption which may be used in mild weather
to save energy.
Automatic reset of amps is possible also by activating the
AMP reset option (“Set-up Options” key) and inputting a 4
to 20
mA
signal. If this has been done the second press of
the “Max Amp Lim” keypad will display this information. A
third press will display the currently impressed remote
mA
signal.
SOFT LOAD KEYPAD
Each centrifugal compressor must have its mechancial vane
closed switch in the “closed” (10% capacity) position to per-
mit completion of a unit start. Following start-up, it may be
desirable to limit the loading of the compressor for energy
conservation.
“Soft Load” provides that additional control. When press-
ed consecutively the “Soft Load” keypad produces the follow-
ing three statements.
First, “Soft Load Limit=N/A” (or XX%).
If activated, this limit
is the maximum percent of RLA at which the compressor will
be allowed to operate based upon the setting of the MAX AMP
LIMIT key. This value may be set anywhere between 30% and
100%.
Second, “Beginning Amp Limit = XX%“. This value is also
a percentage of rated load amps (RLA). It represents the com-
pressor ampere percentage at which the soft loading will begin
its’ time controlled increase up to the maximum percentage
Page 26 I
IM
403
of step one. This value can also range between 30% and
100%; but must be a lower setting than the setting displayed
in the first keypress, (above).
Third, “Ramp up
time=XX
min.“. This value is expressed
in minutes and defines the time period over which the com-
pressor can load from the “Beginning Amp Limit (percent)”
to the upper “Maximum Amp Limit (percent)” or “Soft Load
Limit (percent)“.
The time is adjustable from zero to 120 minutes.
If the Soft Load feature will be activated, press the Soft Load
Keypad three (3) times to display the statement “Ramp Up
Time=XX min.“. If the value displayed is
“0”
minutes, the first
keypress
of the Soft Load Key will have displayed the reading
“Soft Load Limit = N/A”.
Insert the desired ramp up time into the program memory
by following the procedure described under the section “To
Change a Setpoint”. Simply by inputting a time frame greater
than zero in the “Ramp Up Time” function, the Soft Load Limit
becomes activated. Returning to the first
keypress
of this
keypad will then display the percentage that will represent
the maximum load that the unit capacity will operate at follow-
ing completion of the time period.
If this percentage should be changed, return to the first
press of the Max Amp Limit keypad, and increase or
decrease the allowable percent.
Return to the Soft Load Limit keypad. The first press of
this keypad will display the % RLA input under the Max Amp
Limit key. The second press of the Soft Load Limit keypad
will, as described above, show the “Beginning Amp
Limit=XX%“. This later percentage value can be increased
or decreased within the 30 to 100% range by pressing the
“+/yes”
or
"
-/No”
keypads following insertion of the
password.
Note that the
%
amps consumed by a centrifugal com-
pressor are not directly proportional to the
%
capacity of the
compressor. The 30% amps figure in less than the amps nor-
mally drawn by a compressor operating at 10% of capacity.
The 30% amps ‘number’ is, therefore, representative of the
minimum compressor capacity. Actual amps drawn at 10%
capacity will vary based on compressor motor power factor
and efficiency characteristics, and by chilled water and con-
denser water operating temperatures.
Figure
25. Soft Load Performance.
100
t
90
zz
i
a
60,
5
g
70,
2
Z
60,
s
‘r
50
2
=
40
30
k
90
i
60
!h
/
4
s
70
SELECTABLE
TOP
fl
0
(%
AMPS
&
TIME)
z
60
2
AND
BOTTOM
(%
AMPS ONLY)
40
30-l
0
60
120 MAX.
RAMP UP TIME (MINUTES) ADJUSTABLE
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