Heatcraft Refrigeration Products H-IM-72A - Manual (Page 32)

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Adjusting these valves can be complicated because the load
must be varied during the setting procedure and it is difficult
to determine exactly when the bypass valve opens unless a
pressure gauge can be located at the valve outlet.
Therefore, sufficient load must be available in some form to
raise the suction pressure above the desired valve setting.
Once this is accomplished, the load can be slowly decreased
until the DBV opens (a hissing sound and/or an
accompanying pressure rise at the outlet connection will
indicate that the bypass valve has opened).
Alco Controls
Valve Setting and Adjustment
CPHE and DGRE regulators are commonly used to prevent
the suction pressure from falling below a predetermined set
point. Complete information about these valves can be found
in
ALCO CONTROLS Catalog 24-D.
The first step after installation is to determine the set point for
the minimum suction pressure allowable for the application.
The following procedure should be followed.
1. Start the system and determine if it is operating properly:
Install a gauge on the compressor suction line and
measure the suction pressure after the system has
stabilized. To make certain that hot gas is not being
bypassed, listen to the main regulator flow or feel the
outlet piping. If the piping is warm, this would indicate
hot gas is flowing.
2. Stop the flow of hot gas by de-energizing the hot gas
solenoid valve or by turning the power assembly
adjusting stem full C
OUNTERCLOCKWISE .
3. Reduce the evaporator load until the suction pressure
lowers to the point at which bypass is desired.
4. If the hot gas solenoid was de-energized to stop hot gas
flow, make sure it is now energized.
5. Turn the power assembly adjusting stem in a C LOCKWISE
direction until bypass occurs and suction pressure does
not fall below the predetermined set point. A C
LOCKWISE
turn of the adjusting stem will increase the pressure
setting; a C
OUNTERCLOCKWISE turn will decrease it.
Standard pressure pilot is adjustable from 0 to 80 psig,
with one complete turn equal to approximately 4 psi
change. Adjustments should be made in small
increments, allowing for the system to stabilize after each
turn.
6. Vary the evaporator load to test at various conditions that
the suction pressure does not fall below the
predetermined set point.
7. Replace the seal cap on the adjusting stem.
Control Settings
The following tables are for use when mechanical low
pressure switches are incorporated for rack pressure control.
All control settings are approximate and should be adjusted
for actual field conditions and equipment. All settings are
based on 10
°
F T.D. evaporator.
Table 30. Control settings for R404A (507)
Air Temp EPR LP C/I LP C/O
°
F psig psig psig
38-42 69 72 64
34-38 64 66 57
34-36 60 62 55
33-35 59 61 53
28-32 55 58 50
24-28 48 50 35
-10-/0 21 24 15
-10-/-5 20 22 14
-15/-10 15 17 14
-20/-15 12 15 8
Table 31. Control settings for R22
Air Temp EPR LP C/I LP C/O
°
F psig psig psig
38-42 54 57 50
34-38 50 48 43
34-36 47 50 41
33-35 45 47 40
28-32 43 45 38
24-28 36 38 25
-10-/0 13 17 8
-10-/-5 11 14 7
-15/-10 8 10 5
-20/-15 5 8 2
Low Pressure Switch Setting for RMCC
Some systems use a form of electronic control such as CPC’s
RMCC solid state controller for Rack operation. On these
systems, there is one mechanical low pressure control for
each suction group that may be on the rack. This control is
for backup emergency control in the event of a board or other
electronic component failure. This low pressure control must
be set for the minimum suction pressure that the rack would
be expected to operate at to keep from interfering with the
RMCC control.
Example, for a low temperature suction group, this control
should be set for 0 - 2 psig cutout and approximately 10 psig
cutin. A medium temperature suction group can be set
higher. In the event that this control is needed to actually
control the compressors, set the cutin and cutout for the
suction pressure that you want the compressors to operate to
maintain case or box temperatures.
NOTE: This low pressure control will gener-
ally be mounted on a compressor with
the braided stainless steel tubing
connecting to the appropriate suction
group header.