Minimum Load Valve - Carrier ComfortLink 6 Series Controls, Start-Up, Operation, Service, And Troubleshooting

MINIMUM LOAD VALVE (MLVS, Configuration mode
under sub-mode OPT1) — When this option is installed and
configured, the first stage of capacity is altered by energizing
the Minimum Load valve relay. Once the control requires
more capacity, the minimum load valve is deenergized and
normal capacity staging resumes with loaders and compres-
sors. Similarly, the Minimum Load valve relay will be ener-
gized for the last stage of capacity to be used before the circuit
is shut down.
Configure Unit for Minimum Load Control — The chiller
must be configured for minimum load control operation. This
may be done using the Navigator. Set the Enable/Off/Remote
Contact switch in the Off position.
1. Press until 'Select a Menu Item' is displayed.
ESCAPE
2. Press to illuminate the Configuration mode LED.
3. Press and to select 'OPT1'. Press
ENTER
to select 'MLV'.
4. Press and enter the Password (use arrow
ENTER
keys and press for each digit) if required.
ENTER
5. Use to change the flashing 'No' to 'Yes'. Press
and the display says 'MLV Yes'.
The chiller is now configured for minimum load valve control.
Test Minimum Load Relay Outputs — After the unit is con-
figured, test the operation of the relay and solenoid valve using
the Service Test mode.
1. Switch the Enable/Off/Remote Contact switch to the
'Off' position.
2. Press on the Navigator to display 'Select a Menu
ESCAPE
Item' and press to illuminate the Service Test LED.
3. Press and 'TEST OFF' will be displayed.
ENTER
4. Press (enter Password if required),
ENTER
to display 'TEST ON'.
ENTER
5. Switch the EOR (Enable/Off/Remote Contact) switch to
the "Enable" position.
6. Press to select 'COMP' and press
7. Press to select 'MLV OFF'. Press
by and again. The minimum load valve output
ENTER
will be turned on. Both circuits' solenoids are turned on at
the same time.
8. Press , followed by
ENTER
the valve output off.
Adjust Setting of Minimum Load Ball Valve — The mini-
mum load ball valve must be adjusted to suit the application.
Calibrate one circuit at a time as follows:
1. Adjust the ball valve so that it is approximately half open.
2. Operate the chiller in Manual Control mode, with one
circuit operating, and all compressor loaders deenergized.
3. Record the cooler ∆T (the difference between cooler
entering fluid temperature and cooler leaving fluid
temperature) at this fully unloaded condition.
4. Use the Manual Control feature to enable the minimum
load valve for the circuit that is operating.
5. Observe and record the cooler ∆T with the minimum load
valve energized.
and then
ENTER
ENTER
and then
.
ENTER
followed
ENTER
and again to turn
ENTER
10
6. Adjust the minimum load ball valve until the cooler
temperature difference reading from Step 5 is equal to
half of the temperature difference reading from Step 3.
7. Open the ball valve to decrease the temperature differ-
ence or close the ball valve to increase the temperature
difference (∆T). When the valve is adjusted correctly, the
difference between cooler entering and leaving fluid tem-
peratures when the minimum load control is energized
must be at least half of the temperature difference when
the minimum load control is deenergized. For example, if
the difference between the cooler entering and leaving
water temperature is 3° F with the valve deenergized,
then the difference between cooler entering and leaving
water temperature must be at least 1.5° F with the valve
energized.
Once the outputs have been tested and the ball valve
adjusted, the installation is complete. Disable manual control
and return chiller to desired operational status.
CAPACITY CONTROL OVERRIDES — The following over-
rides will modify the normal operation of the routine.
Deadband Multiplier — The user configurable Deadband
Multiplier (Z.GN, Configuration mode under sub-mode SLCT)
has a default value of 2.0. The range is from 1.0 to 4.0. When
set to other than 1.0, this factor is applied to the capacity Load/
Unload Factor. The larger this value is set, the longer the con-
trol will delay between adding or removing stages of capacity.
Figure 7 shows how compressor starts can be reduced over
time if the leaving water temperature is allowed to drift a larger
amount above and below the set point. This value should be set
in the range of 3.0 to 4.0 for systems with small loop volumes.
The Main Base Board (MBB) closely follows the rate of com-
pressor cycling for each circuit.
First Stage Override — If the current capacity stage is zero,
the control will modify the routine with a 1.2 factor on adding
the first stage to reduce cycling. This factor is also applied
when the control is attempting to remove the last stage of
capacity.
Slow Change Override — The control prevents the capacity
stages from being changed when the leaving fluid temperature
is close to the set point (within an adjustable deadband) and
moving towards the set point.
Ramp Loading — (RL.S, Configuration mode under sub-
mode SLCT) — Limits the rate of change of leaving fluid
temperature. If the unit is in a Cooling mode and configured
for Ramp Loading, the control makes 2 comparisons before
deciding to change stages of capacity. The control calculates a
temperature difference between the control point and leaving
fluid temperature. If the difference is greater than 4° F (2.2° C)
and the rate of change (°F or °C per minute) is more than the
configured Cooling Ramp Loading value (CRMP, Configura-
tion mode under sub-mode SLCT), the control does not allow
any changes to the current stage of capacity.
Low Entering Fluid Temperature Unloading — When the
entering fluid temperature is below the control point, the
control will attempt to remove 25% of the current stages being
used. If exactly 25% cannot be removed, the control removes
an amount greater than 25%, but no more than necessary. The
lowest stage will not be removed.
Low Discharge Superheat — If a circuit's discharge superheat
is less than 15° F (8.3° C), the control does not increase the
current capacity stage. If the discharge superheat is less than
5° F (2.8° C) and decreasing, the circuit is unloaded every
30 seconds until the superheat is greater than 5° F (2.8° C). The
final capacity stage is not unloaded unless an alarm condition
exists. This override is ignored for the first 3 minutes after a
compressor is started.