Cold Room 3: Reading a Service Bulletin Against the Log
You are an apprentice refrigeration technician at a fish-processing plant near Larnaca. Cold room 3 must hold −18 °C. The maintenance log for the last ten days shows defrost termination at 6-hour intervals, suction pressure steady at 1.8 bar, and a room temperature that swings between −14 °C and −19 °C, settling at −14 °C each morning between 06:00 and 08:00 before recovering. The evaporator coil was cleaned four weeks ago. The door gasket was replaced last month and passes the paper test. Your supervisor hands you this extract from the manufacturer's service bulletin for the unit:
'Models fitted with the KD-40 controller ship with a factory defrost interval of 6 h. In high-humidity rooms (fish, dairy, chilled dough) the coil re-freezes before the next cycle. For these rooms set the interval to 4 h and confirm the termination sensor is clipped to the lowest coil pass. Do not extend the interval beyond 8 h.'
You must give your answer in English, using evidence from both the log and the bulletin.
1. State the most probable cause of the morning temperature rise, and explain why the two checks already carried out (coil cleaning, gasket test) do not account for it.
Read the worked answer
Most probable cause: ice re-forming on the coil between defrost cycles because the 6 h interval is too long for this room.
Reasoning chain:
- The fault is rhythmic: a rise to −14 °C every morning at the same hours, followed by recovery. A rhythmic fault points to something driven by the control cycle, not to a slow decline such as lost refrigerant.
- The bulletin says KD-40 units leave the factory at 6 h and that high-humidity rooms (fish is named explicitly) re-freeze before the next cycle. The log shows exactly 6 h. The bulletin and the log describe the same unit in the same condition.
- Coil cleaning four weeks ago only proves the coil was clean then; dirt is not the mechanism here, because a dirty coil gives a permanent loss of performance, not a daily recovery.
- The gasket passing the paper test rules out warm, moist air leaking round the door. It does not rule out a coil icing from the inside, which is what happens when moist air is carried in on product and the defrost is too short or too infrequent.
- Steady suction pressure at 1.8 bar argues against charge loss or a restriction, which would normally hold pressure low and not correct itself every 24 hours.
Confirm before changing anything: inspect the coil at 06:00 and read the controller's defrost-interval parameter.
2. What is the 'termination sensor' in the bulletin, and why does the bulletin insist it is clipped to the lowest coil pass?
Read the worked answer
The termination sensor is the temperature probe that tells the controller when to end defrost: when the probe reaches the set termination temperature, the heaters switch off and the unit returns to cooling. It is not a timer; it is the sensor that decides the defrost is finished.
Why the lowest pass matters: the coil is coldest at the bottom, so ice clears there last. If the probe sits higher up, it warms to the termination temperature while ice is still on the lower passes - the controller believes the coil is clear when it is not, and the leftover ice grows cycle after cycle. Clipping it to the lowest pass makes the probe report the last part of the coil to clear, so defrost ends only when the whole coil is clear.
This also explains why simply shortening the interval is not enough on its own: a misplaced probe can leave ice on the coil even with a 4 h setting.
3. The bulletin says 'Do not extend the interval beyond 8 h.' Explain in your own words why a longer interval would not be a sensible repair, and state one check you would make before altering the setting.
Read the worked answer
A longer interval treats the symptom by defrosting less: the coil would run iced for longer, airflow would fall further, and the room would sit above −18 °C for more of each day - a product-quality and possibly legal problem in a fish plant. Ice also builds faster once it has started, because a rough ice surface collects moisture more readily than a clean fin, so the growth is not linear: a longer interval can produce a worse coil, not a better one. Finally, a longer interval needs a longer defrost to clear the extra ice, which puts more heat into the room.
Check before changing the setting: confirm the unit really is the high-humidity case the bulletin describes - i.e. read the controller model and current parameter, and look at the coil condition at the end of a cycle. If the coil is clear at 6 h, the interval is not the problem and the fault lies elsewhere (for example the termination probe position or the defrost heater). Record the change, the date and your name in the log.