THE THEORY BEHIND KEEPING COOL

Like electricity, refrigerant thermal management theory is not that easy to comprehend, because you cannot see what is going on inside the system. This leads Rob Marshall to ask leading specialists about the fundamentals and how technician misunderstanding can lead to grief.

While dedicated air conditioning specialists are less likely to struggle, the professionals within more general repair and service workshops can have issues relating to the supposedly sealed Air Conditioning circuit (AC). Revealingly, Denso finds that the standard motor trade response to AC service is ‘We’ve re-gassed it’, which, without understanding the system, is a move that can cause additional problems. The situation is made worse if, in addition to not appreciating AC system theory, the technician thinks that an AC refilling machine will service the AC system fully.

In Waeco's experience, technicians can find it tricky to visualise the different states and pressures that the refrigerant experiences at different points within the AC circuit. The company explains that the high and low-pressure sides involve the refrigerant changing states from a liquid into a gas and back again. Waeco concludes that the relationship between pressure, temperature and the physical state of the medium refrigerant causes the most frowns on technicians' faces.

Chill dude

Do not worry – help is at hand. Denso reminds us that the compressor promotes refrigerant flow, driven usually by the engine, via a magnetic clutch, which engages and disengages the drive. Some systems employ a damper limiter pulley to drive a variable displacement type compressor continually. On high-voltage EV systems, the compressors are, of course, powered electrically. Most technicians understand this basic principle.

Clearly, the compressor experiences high pressure at the discharge port and low pressure at its suction port. What is less well-known is that the compressor sucks gaseous refrigerant from the evaporator, where this superheated refrigerant absorbs heat energy from the passenger compartment. Once arriving at the compressor, this low-pressure vapour is compressed, adding more heat energy. This high-temperature, high-pressure gas is then forced from the compressor

The latent heat of evaporation theory – in a sentence

The heart of AC relies on the refrigerant changing state from a liquid to gas (and back again), absorbing and shedding heat energy as it does so.

Unwanted moistness

Moisture equals trouble within the refrigerant system. Waeco explains that technicians appreciate that brake fluid is hygroscopic but comparatively few repair professionals understand that compressor oil shares this unfortunate characteristic.

This may be down to developments behind the scenes. Moisture ingress is not too much of an issue with systems using R134a but it has become more pressing with R1234yf. These systems are more sensitive, because moisture facilitates acid formation. A consequence of this is the acid attacking the compressor internals.

discharge port into the condenser. Condensers share their working principles with domestic radiators, because the condenser also transfers heat energy into the surrounding air. As its name implies, this causes the vapour to lose pressure and condense into a liquid, shedding heat energy as it does so.

Next, the cooled liquid refrigerant flows to the expansion valve, where it is forced through a small orifice. This causes the refrigerant to emerge as a mist, from which it expands. The expansion reduces the pressure and, therefore, the temperature further. This causes the refrigerant to change state again from a warm, high-pressure liquid to a cooler, lowpressure mist.

This low-pressure mist flows into the last major component of the AC system, the evaporator. The evaporator consists of a thin aluminium tank behind the dashboard. As the evaporator is chilled, it absorbs heat energy from the cabin, which transfers quickly to the refrigerant mist inside. As it warms, the mist vapourises, evaporating back to a gas. This vapour flows from the evaporator and back to the compressor where the whole process starts again. Phew!

The consequences

Waeco and Denso agree that technicians not appreciating AC basics tend to be a major cause of misdiagnosis. For instance, where the compressor has failed, due to a lack of refrigerant and resultant lubrication starvation, changing the compressor alone is not addressing the underlying problem. The new part, therefore, will fail very quickly and not through any manufacturing defect. Waeco argues that, dissatisfied customers aside, expensive and time-consuming repeat repairs are the consequence.

Nissens agrees that without a grasp of AC basics, and the relationship between temperature and pressure, it will be very hard to conduct AC servicing, or repairs. Even so, there will be a much higher risk of system breakdown on a newer and more complicated heat pump system. Nissens explains that the cost of heat pump system failure could be much higher, because the consequences of contamination impact many more components than a traditional AC system.

Featuring on high-voltage vehicles, heat pump systems can possess up to eight solenoid valves to control refrigerant flow. Nissens reveals that, because the system relies heavily on ambient temperature, it can switch refrigerant direction within a split second to seek the optimum energy-saving solution, to preserve battery range. This means that there can be multiple ways by which the system warms not just the cabin but also the battery pack. Technicians, therefore, need to appreciate how the system operates in every possible heating and cooling mode to perform checks of all functions as part of their diagnosis process. While Nissens says that heat pumps do not reinvent the wheel, and technicians must not forget the basic AC principles, it is still important to be trained in this relatively new system.

Getting qualified

LKQ Euro Car Parts confirms that any garage undertaking air conditioning work must be F-gas certified. It will sell R134a to customers only if they can prove that they are qualified to handle and dispose of the refrigerant properly. This is the product with which most aftermarket technicians are familiar, because it is the most common automotive refrigerant used before 2013. However, the more environmentally friendly R1234yf was used on new vehicles after 2017, which is becoming more common.

The theory of leak detection

While many consumers view leaks as causing their AC to stop working, technicians should view escaping refrigerant as the cause of compressor oil starvation and thermal overheating.

Waeco explains that the AC compressor is cooled by the refrigerant and lubricated by the oil which circulates with it. Should a leak, or incorrect filling, cause the system to possess insufficient refrigerant, lubrication gaps will occur. This situation causes the remaining AC oil to overheat and decompose, ruining its lubricating effect. To avoid this, Waeco recommends UV dyes that make it easier for you to detect leaks at an early stage. Furthermore, it is prudent to possess tools for an inert gas (such as nitrogen) pressure test, to repair vehicles that have not had UV dyes added to their refrigerant circuits.