SOLAS Refrigeration common mistakes
SOLAS Refrigeration · Understand the traps and how to avoid them.
Reading a gauge manifold's low-side and high-side readings without first confirming which scale (compound vs. pressure-only) and unit (bar, psig, kPa) the gauge face uses, then stating suction pressure in the wrong unit.
Candidates learn gauge reading on one instrument type during workshop practice and assume all faces are labelled identically; compound gauges often carry two concentric scales and the inner one is easily misread under time pressure.
What to do instead
Before any reading, state aloud the unit and scale you are using and the gauge's zero reference (gauge vs. absolute). Practise converting the same reading across bar, psig and kPa until the conversion is automatic, and always record the unit alongside the number in your answer.
Diagnosing a system as undercharged purely from low suction pressure, without checking superheat, subcooling, and condenser approach temperature together.
Low suction pressure is the most visually obvious symptom and candidates anchor on the first abnormal reading they see, treating one symptom as a diagnosis.
What to do instead
Adopt a fixed diagnostic sequence: record suction and discharge pressures, convert to saturated temperatures, calculate superheat and subcooling, then compare condenser approach. Only conclude undercharge when superheat is high and subcooling is low simultaneously.
Confusing superheat with subcooling and applying the superheat charging method to a system with a thermostatic expansion valve that should be charged by subcooling.
Both are temperature differences from a saturation point, and the two terms are learned close together; candidates memorise the definitions but not which metering device each method belongs to.
What to do instead
Tie each measurement to its location and device: superheat is measured at the evaporator outlet for fixed-orifice and capillary systems; subcooling is measured at the condenser outlet for TXV systems. Draw the circuit and mark both measurement points before answering.
Stating the refrigerant's boiling point at atmospheric pressure when the question asks for the saturation temperature at the actual operating pressure, or vice versa.
Candidates memorise a single boiling point per refrigerant and treat it as a fixed property rather than a pressure-dependent one.
What to do instead
Always link pressure to saturation temperature through a pressure-temperature chart for the specific refrigerant. When a question gives an operating pressure, look up the corresponding saturation temperature rather than quoting the atmospheric boiling point.
Describing the refrigeration cycle with the wrong component order or omitting the metering device, for example placing the compressor between the condenser and evaporator.
The cycle is learned as a diagram and candidates recall it visually rather than as a sequence of state changes, so the order collapses under recall pressure.
What to do instead
Recite the cycle as four state changes with pressure and phase at each point: compressor (low-pressure vapour to high-pressure vapour), condenser (to high-pressure liquid), metering device (pressure drop), evaporator (to low-pressure vapour). Write it out from memory daily until the sequence is fixed.
Treating a refrigerant leak as a simple top-up job and not accounting for the need to recover, evacuate and leak-test before recharging.
In workshop exercises the emphasis is often on getting the system running, so the legal and procedural steps around recovery and evacuation get skipped in written answers.
What to do instead
Answer leak questions in the full sequence: locate and repair the leak, recover remaining refrigerant, evacuate to the required vacuum and hold it, then charge by weight or by the correct superheat/subcooling method. Name the recovery step explicitly every time.
Mixing up the safety hazards of different refrigerants, for example describing ammonia as an asphyxiant only and ignoring its toxicity and flammability classification, or treating all HFCs as flammable.
Safety classifications are learned as a table and candidates recall the refrigerant name but not its assigned class, then generalise from one refrigerant to another.
What to do instead
Learn each refrigerant by its ASHRAE safety group letter and number and what each means for toxicity and flammability. When answering, state the class explicitly and the practical consequence: ventilation, personal protective equipment, leak detection method, and first-aid response.
Calculating system capacity or coefficient of performance using temperatures in degrees Celsius directly in formulas that require absolute temperature in kelvin.
Most workshop measurements are taken in Celsius and candidates substitute the numbers straight into thermodynamic relationships without converting.
What to do instead
Before any thermodynamic calculation, convert every temperature to kelvin by adding 273.15 and show the conversion in your working. Check the result for plausibility: a coefficient of performance below one on a standard cycle usually signals a unit error.
Ignoring the effect of non-condensable gases and describing high discharge pressure as caused only by a dirty condenser or overcharge.
Non-condensables are an abstract fault with no visible component, so candidates list the physical faults they can see and stop there.
What to do instead
For any high-discharge-pressure question, work through the full cause list: high ambient or restricted airflow, fouled condenser, overcharge, and non-condensable gases. Describe how non-condensables raise condenser pressure and how they are removed by recovery and re-evacuation.
Answering a control or electrical fault question by replacing components rather than tracing the circuit, so the answer names a part without stating the test that proves it faulty.
Practical training rewards getting the unit running, and candidates transfer that shortcut into written answers where the reasoning is what is being assessed.
What to do instead
Structure every fault-finding answer as symptom, test, expected reading, actual reading, conclusion. Name the instrument used at each step, such as a multimeter for continuity or a clamp meter for current draw, and only then state the faulty component.