by Jacques Gordon
To anyone who has been following along, it looks more and more like the auto industry will never have one universal air conditioning refrigerant. That means we may see some radically different A/C technology too. Let’s look at why the choice of refrigerant influences the design of an air conditioning system.
Most people know that refrigerant evaporates at some point inside the system, but that’s only part of how it works. Pressure and temperature are related: when one decreases or increases, so does the other. When liquid refrigerant passes through the expansion valve, its pressure is reduced downstream of the valve. That reduces its temperature, and if it’s colder than its surroundings, liquid refrigerant will absorb heat.
The lower pressure also reduces the liquid’s boiling point. When a liquid evaporates, it absorbs heat from its surroundings, lots of heat. The greater the mass of liquid evaporating, the greater the amount of heat absorbed. However, when liquid refrigerant evaporates in an air conditioning system, it doesn’t just vaporize like steam from a cup of tea; it remains captured in the system as a vapor and it continues absorbing heat.
Just about any liquid or gas will work as a refrigerant, even air, but some work better than others. In broad terms, a liquid that has a higher mass and vapor pressure will carry more heat, while a gas with a lower mass and vapor pressure will compress more easily to a liquid and therefore require less energy to run the compressor. Real-world refrigerants are a balance between those two extremes, but to put automotive A/C into perspective, R-134a can still be a vapor even at the highest pressure generated by the compressor. To condense this high-pressure vapor to a liquid, we need to remove heat. That’s done in the condenser.
No matter which refrigerant is chosen, the rest of the system must be designed around it. One of the primary design criteria of every air conditioner and refrigeration system, from cradle to grave, is its potential contribution to global warming. Therefore, certain refrigerants are on a world-wide schedule to be phased out at some point, including R-134a. Right now the only replacement that’s ready is another class of fluorocarbon refrigerants called hydrofluoro-olefin (HFO), which we know as R-1234yf. It has similar physical characteristics to R-134a, so its primary advantage, other than its low global warming potential, is that it can be used with existing refrigeration system designs and most of the same technology. This is good for the auto industry and their customers, but it’s still a fluorinated gas, among the most potent and long-lived greenhouse gases used in human activities (according to U.N. climate scientists). There’s growing political pressure to regulate it out of production and force the industry to develop an even lower-impact refrigeration technology. So the search for the perfect refrigerant continues.
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When having your mobile A/C system professionally serviced, insist on proper repair procedures and quality replacement parts. Insist on recovery and recycling so that refrigerant can be reused and not released into the atmosphere.
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