Frost-free refrigerator that only cools for eight minutes after a rest
A 260-litre frost-free refrigerator using R-600a, rated 0.75 A at 230 V, is reported as 'freezer cold but not freezing, fridge compartment warm'. The compressor runs continuously. After 25 minutes of running you measure: suction pressure 0.1 bar gauge, compressor current 0.50 A, condenser uniformly warm across its whole area, compressor shell hot to touch, evaporator fan working. The evaporator is frosted only on the first two rows near the capillary inlet; the rest is bare and the suction line is not cold. Winding resistance from the common terminal is 12 ohm (run) and 30 ohm (start), with 42 ohm across run-start, and insulation resistance is above 100 Mohm. Six months ago another shop simply 'topped up gas' and the machine worked for two weeks. When you switch the unit off for 40 minutes and restart it, the whole evaporator frosts for about 8 minutes and then the frost recedes to the capillary inlet again. The customer asks for another gas top-up.
1. Using the readings, distinguish between a refrigerant leak, a moisture/ice blockage and a mechanically blocked or kinked capillary. State which observations rule out each alternative.
Read the worked answer
First, the compressor is electrically sound. 12 + 30 = 42 ohm, matching the run-start reading, so the windings are not shorted or open, and insulation above 100 Mohm rules out an earth fault. The fault is in the sealed system.
What the data show: low suction (0.1 bar), current of 0.50 A against a 0.75 A rating, a uniformly warm condenser and frost confined to the capillary inlet all indicate low refrigerant mass flow. That is consistent with undercharge or with a restriction - so the pattern alone does not separate them.
The discriminator is the rest test:
- Moisture/ice blockage: water frozen at the capillary outlet, the coldest point, blocks flow during running. During 40 minutes off, the ice melts, so the full evaporator frosts on restart until ice re-forms after about 8 minutes - exactly what you saw.
- Refrigerant leak: pressure and current stay low all the time. A leak does not clear with a rest period, so a full frost pattern after standing is not expected. Look for oil traces at joints and confirm with a leak test.
- Kinked or mechanically blocked capillary: a fixed restriction does not clear when the compressor stops, so there would be no eight-minute recovery.
Conclusion: moisture/ice blockage is the leading diagnosis; the earlier 'top-up' gave two weeks of relief, which fits refrigerant being added without proper evacuation and drier replacement. A leak may also exist and must be leak-tested after repair.
2. Write the repair procedure and acceptance tests before handover, including the R-600a safety points.
Read the worked answer
Safety first - R-600a is flammable. Work in a ventilated area, no open flame or sparking tools, and no brazing until the charge is removed and the circuit is purged with nitrogen. Recover the refrigerant with hydrocarbon-rated equipment into a suitable cylinder rather than venting it into the shop.
Procedure
- Recover the charge; cut out and replace the filter-drier - never reuse a drier, since it holds the moisture you are trying to remove.
- Blow dry nitrogen through the capillary and evaporator to clear debris and ice; if the capillary is waxed or damaged, replace it.
- Leak test with dry nitrogen at the manufacturer's test pressure (typically around 10 bar on the high side, lower on the low side), using soap solution at every joint.
- Triple evacuation with nitrogen breaks, then evacuate to 500 microns (about 67 Pa) and hold for 15 minutes with a micron gauge - a manifold gauge cannot show this.
- Charge by weight exactly as on the nameplate; R-600a charges are small, so an overcharge of even a few grams costs capacity and raises current.
Acceptance tests: correct frost pattern across the whole evaporator; freezer reaching -18 degrees C and fresh-food compartment 4 degrees C on pull-down; compressor current within nameplate; no leaks; no icing back on the suction line; and a 24-hour monitoring run before handover.
Record the evacuation figure, charge weight and temperatures on the job card.
3. The customer insists on 'just another gas top-up, it is cheaper'. Explain the consequences in terms of cost, safety and repeat failure, using the service history.
Read the worked answer
Use the history as evidence. The top-up six months ago bought two weeks of operation. Another top-up will follow the same curve: short relief, then repeated warming, and each visit costs money without fixing the cause.
Cost: two or three top-ups at Rs 1,200-1,500 each soon exceed one correct repair, and the machine runs longer, consuming more electricity while the food spoils. Repeated topping up a moisture-contaminated system also introduces more air and moisture, so the blockage worsens.
Safety: running with low suction and a hot compressor shortens compressor life; adding the wrong refrigerant or overcharging raises discharge pressure and current, which can damage the compressor. Hydrocarbon refrigerant in a system with a sparking relay or a nearby flame is a genuine fire risk - this is not a place to guess.
What to offer: a written quotation showing the correct repair, the parts, the warranty period and a callback commitment, set against the cost of repeated top-ups. Explain that a top-up is legitimate only after a leak test shows the system is leak-free and only as a short-term measure with the customer's informed consent, recorded on the job card.