Refrigeration System Section 7
7-18 Part No. 80-1100-3
ANALYZING SUCTION PRESSURE
DURING FREEZE CYCLE
The suction pressure gradually drops throughout the
freeze cycle. The actual suction pressure (and drop rate)
changes as the air and water temperatures entering the
ice machine change. This affects freeze cycle times.
To analyze and identify the proper suction pressure drop
throughout the freeze cycle, compare the published
suction pressure to the published freeze cycle time.
“Operating Pressure” and “Freeze Cycle Time” charts
can be found later in this section.
NOTE: Analyze discharge pressure before analyzing
suction pressure. High or low discharge pressure may
be causing high or low suction pressure.
Procedure
Step Example Using QY0454A Model Ice Machine
1. Determine the ice machine operating
conditions.
Air temp. entering condenser: 90°F/32.2°C
Air temp. around ice machine: 80°F/26.7°C
Water temp. entering water fill valve: 70°F/21.1°C
2A. Refer to “Cycle Time” and “Operating
Pressure” charts for ice machine model being
checked. Using operating conditions from Step
1, determine published freeze cycle time and
published freeze cycle suction pressure.
Published freeze cycle time: Published freeze cycle suction pressure:
13.7 - 14.1
minutes 55-36 PSIG
2B. Compare the published freeze cycle time
and published freeze cycle suction pressure.
Develop a chart.
Published Freeze Cycle Time (minutes)
1 3 5 7 9 12 14
55 52 48 44 41 38 36
Published Freeze Cycle Suction Pressure (psig)
3. Perform an actual suction pressure check at
the beginning, middle and end of the freeze
cycle. Note the times at which the readings are
taken.
Beginning of freeze cycle: 59
PSIG at 1 minute
Middle of freeze cycle: 48
PSIG at 7 minutes
End of freeze cycle: 40
PSIG at 14 minutes
4. Compare the actual freeze cycle suction
pressure (Step 3) to the published freeze cycle
time and pressure comparison (Step 2B).
Determine if the suction pressure is high, low
or acceptable.
Time Into
Freeze Cycle
Published
Pressure
Actual
Pressure
Result
1 minutes
7 minutes
14 minutes
55 PSIG
44 PSIG
36 PSIG
59 PSIG
48 PSIG
40 PSIG
High
High
High
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