How a cup of tea on Mount Everest can help explain cooling

A few months ago, a client asked our company to develop a short training session on two-phase heat transfer for their non-mechanical engineers.

It was an interesting challenge. We’re so used to talking to clients and colleagues with some understanding of HVAC&R that we sometimes forget how complicated even the fundamentals of our work are.

We only had one shot to get it right in front of up to 200 professional engineers with varying levels of experience during a one-hour training slot. This required a lot of reflection and set off an interesting thought process for me.

Whether you trained as an HVAC&R technician or a mechanical engineer, you would’ve learnt about two-way heat transfer in quite technical terms. The explanation would’ve been something like this: “To remove heat from an environment, you increase the pressure in a system through mechanical work, thereby increasing the enthalpy. You then condense the gas before expanding it to create the cooling effect.”

After talking to some non-engineering contacts in my professional environment, I realised that this a complicated and unnatural concept to most people. We need a more approachable explanation.

The tea metaphor

Everyone I talked to could understand that atmospheric pressure decreases as you move from sea level to the top of the highest mountain. They could also understand that, as pressure falls, so does the boiling point of water.

So, I asked them to imagine a cup of tea on the top of Mount Everest – 8,848m above sea level – where water boils at around 70°C. As soon as the water hits 70°C, it evaporates, meaning the liquid will never get up to our customary 100°C.

Unfortunately, black tea brews at 90–95°C. This means that, no matter how long you boil the water or swirl the teabag around, your mountaintop tea will taste weak and bitter – not unlike if you just dunked the teabag into room temperature water. And those potatoes you boiled in the other pot? They’ll be rock solid. Just when you needed a nice hot meal and a warm cuppa!

From there, it’s quite easy to explain the idea of sensible and latent heat transfer. It makes sense that “breaking water particles into gas” requires much more energy than just raising the temperature of the water does, and as such removes more heat from the environment in the process.

It’s a sucker, sucker!

It’s challenging for most non-engineers to understand how an expansion device is used to convert high-pressure, high temperature liquid refrigerant to a low temperature liquid-gas refrigerant mixture. It’s much easier for them to understand how a vacuum pump can be used to lower the pressure inside a space to achieve a lower liquid boiling point. They can also visualise a mechanical throttling device that ensures that only a limited amount of refrigerant is replenished in the low-pressure space.

These aren’t alien concepts within the industry. We often refer to a capillary tube as a throttling device, because through its restriction in cross-sectional diameter, it will only allow a certain amount of liquid refrigerant to move into the low-pressure space. We also use a slightly modified compressor assembly in the form of a vacuum pump when technicians evacuate an HVAC&R system prior to charging it with refrigerant.

So, would it make more sense to call a capillary tube a throttle? Or to call a compressor a sucker? I’d argue that, at least for non-technical audiences, it would.

Old habits die hard

Pick up a refrigeration training manual and you’re guaranteed to see the same old basic vapour compression circuit with a high side and a low side, a compressor, a condenser, an evaporator and a TX valve. To an outsider, this diagram may as well be hieroglyphics.

If you’ve been in the HVAC&R bubble too long, you forget how difficult the vapour compression cycle is to understand. This cycle can perpetuate when senior experts – who have been immersed in the bubble longer than anyone – write teaching materials. We use these materials to teach tradespeople who have already been exposed to refrigeration during their apprenticeship and engineering students who have proven their mechanical aptitude and logical thinking. But we make it almost impossible to explain our industry to laypeople.

Are we so set in our ways that we’re not interested in making our industry more accessible? Or are we simply too busy to care?

I don’t think it’s the latter. Most long-term HVAC&R professionals understand that we’re a hidden industry that struggles to attract school leavers and engineering students. They want to make our work more accessible; they just might not be using the right vocabulary to do it.

Do I want you to throw out your training materials? Absolutely not! But instead of the jargon-heavy explanation you learnt at TAFE or uni, some real-world metaphors can make our work so much easier to understand. I’d argue that, in some cases, it makes sense to reframe refrigeration as boiling a liquid at a saturated pressure-temperature point that is lower than the ambient temperature to achieve latent heat transfer.

So, next time someone asks you how refrigeration works, offer them a cup of tea. Unless, of course, you’re on Mount Everest …

Featured image courtesy of Wirestock via Freepik

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