Heatcraft Refrigeration Products H-IM-72A - Manual (Page 31)

31
Liquid Drain Control Method
This method is ideal for large capacity systems since a
smaller regulator is required for liquid line than for discharge
line.
During warm ambient temperature conditions valves A and C
will be open and Valve B will be closed. When the ambient
temperature at the condenser drops, the condenser pressure
will tend to become lower. As this pressure is reduced, when
the pressure becomes as low as its setting, Valve A will begin
to close, thus causing the refrigerant to back up inside the
condenser tubes, reducing the condensing surface and
allowing the pressure to be maintained. As Valve A closes,
the receiver pressure may be reduced by the cold entering
liquid to a level below the setting of Valve B, which will begin
to open to bypass sufficient gas to maintain the receiver
pressure at the set point of Valve B. Check valve C will
prevent the high pressure from backing up to the condenser
when the receiver pressure is higher than the condenser
pressure, as would be the case during shutdown in a system
with a warm receiver.
When the condenser pressure builds up to the setting of
Valve A, it will open allowing liquid to flow to the receiver.
To describe it again, during cold ambient temperature
conditions this liquid will be considerably subcooled and will
tend to lower the receiver pressure. Valve B will sense the
drop in pressure and open to admit hot gas into the drain line,
thus pressurizing and warming the liquid and maintaining the
receiver pressure.
Recommended Valve Settings
Regulators ordered for a Condenser Pressure Control system
for use with common refrigerants will be furnished with the
ranges and factory settings shown in Table 29 below. Turning
the adjusting stem in (clockwise) will raise the set point;
turning the stem out (counterclockwise) will lower the set
point. See Table 29 below for ranges and amount of
pressure change per turn.
Table 29. Pressure Range, Set point & Change per Turn
Factory
Valve Range Set Point Change Per Turn
Type psig kg/cm 2 psig kg/cm 2 psig kg/cm 2
A4A
D
75 to 5.2 to
140 9.8 53 3.7
ARA0 280 19.7
A7A 75 to 5.3 to
A7A1 D 280 19.7 120 8.4 75 5.3
A72
A9 B
80 to 5.6 to
120 8.4 25 1.8
220 15.5
Field Adjustment
Before final field adjustment of regulators for Condenser
Pressure Control, the following should be done:
1. Install gauges to read compressor discharge, condenser
and receiver pressures.
2. Fully charge the system.
3. Have other controls and components functioning
properly.
4. Have the system as fully loaded as possible.
5. Final adjustments should be made when the outdoor
ambient is below 65
°
F. Ideally the ambient should be
near minimum system outdoor temperature to allow the
system pressures to drop below the regulator pressure
settings. Before adjusting make sure that all manual
opening stems on the regulators are set for automatic
operation. It may be necessary to temporarily deactivate
the low pressure cut out controls to keep the compressors
running during adjustment.
6. The regulators should be preset using information in
Table 29.
7. When the valves are adjusted with the system operating,
enough time must be allowed for the system to stabilize.
Check the sight glass to make sure sufficient liquid is
supplied to the evaporators.
8. Turn the condenser fans off to allow the discharge
pressure to build up. The regulator A should be adjusted
to open when the pressure reaches the desired control
point. Listen for flow through regulator A while watching
the pressure gauge.
To determine the final setting of regulator B, allow the
condenser fans to run long enough to subcool the liquid
supplied to the receiver. Adjust the regulator until the desired
receiver pressure is obtained. Flow through the valve can be
determined by listening at the valve for gas flow or by feeling
the outlet for change in temperature. The setting of regulator
B should be at least 10 psi lower than the setting of regulator
A.
Hot Gas Bypass Regulator
Adjustment
Discharge Bypass Valves (DVB) respond to changes in
downstream or suction pressure. When the evaporating
pressure is above the valve setting, the valve remains closed.
As the suction pressure drops below the valve setting, the
valve responds and begins to open. as with all modulating
type valves, the amount of opening is proportional to the
change in the variable being controlled - in this case the
suction pressure. As the suction pressure continues to drop,
the valve continues to open farther until the limit of the valve
stoke is reached.
On refrigeration systems discharge bypass valves are used to
prevent the suction pressure from going below the minimum
value determined by the job requirements.
Sporlan Valve Company
Valve Setting and Adjustment
A complete discussion on valve settings is given in Sporlan
Application Bulletin 90-40. The fully adjustable models
ADRS(E)-2, ADRP(E)-3, and ADRH(E)-6 are available with
two adjustment ranges - 0/30 and 0/80 psig. The standard
factory settings for these are 20 and 60 psig, respectively.
The ADRI(E)-1-1/4 is available with a 0/55 psig range and the
standard factory setting is 28 psig.
To adjust these valves, remove the cap an turn the
adjustment nut with a 5/16” hex wrench for fully adjustable
models ADRS(E)-2, ADRP(E)-3, and ADRH(E)-6. The
ADRI(E)-1-1/4 model has a 3/8” adjustment screw on top of
the adjustment housing. A clockwise rotation increases the
setting and a counterclockwise rotation decreases the setting.