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

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

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 COUNTERCLOCKWISE.

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 CLOCKWISE

direction until bypass occurs and suction pressure does

not fall below the predetermined set point. A CLOCKWISE

turn of the adjusting stem will increase the pressure

setting; a COUNTERCLOCKWISE 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.

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Heatcraft Refrigeration Products H-IM-72A User Manual Page 32

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