The thumping theory behind HVAC systems

by Helena Morgan
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On a searingly hot night, a novice facilities manager and heat transfer expert walk into a bar. The facilities manager is seeking an Obi-Wan Kenobi figure in the heat transfer expert, as they are wanting to master all things HVAC to keep their facility safely and sustainably cool in the summer months. 

Senior lecturer at RMIT School of Property, Construction and Project Management Dr Tom Simko is a heat transfer and HVAC expert – and possibly the Obi-Wan Kenobi figure in question. 

In response to summer energy bills that leave many of us pondering how we can affordably and successfully approach passive cooling, Facility Management picks Simko’s brilliant brain on refrigerants, cooling towers, a mysterious respiratory illness outbreak in the late 1970s, thorny thermodynamics and the power of a light-coloured roof.

How does a cooling system work?

There are several common types of cooling systems – single-split systems, multi-head split systems, ducted reverse cycle systems, packaged systems, absorption cooling, evaporative cooling and the economy cycle, which is a form of passive cooling. 

A standard complete packaged HVAC system is made up of a filter, coils (which can also be used for dehumidification), a humidifier and a fan – and possibly another filter box depending on how clean you want the air, as certain facilities such as hospitals or clinics require cleaner air. 

There are several common types of cooling systems – single-split systems are one of the most popular in Australia.

Ideally, most of the air returned from a zone is recirculated for energy conservation, however, some air is exhausted and replaced with fresh outdoor air to maintain indoor air quality.

“A cooling coil is simply a pipe with a fluid going through it – and that fluid can be a refrigerant or water. Warm air passes over that pipe and comes out colder. There’s no actual physical mixing between the air and the fluid,” says Simko. 

A powerful refrigerant 

How do we produce the all-too-familiar chill of air conditioning? What is the fluid and where does it come from? One option is to rely on a refrigerant circulating through a system that functions in essentially the same way as a kitchen fridge. 

Every fridge has a cooling coil – a heat exchanger called an evaporator – usually hidden behind a panel at the back wall that cools the inside of your fridge. A low-pressure refrigerant runs through this coil. Pressure and temperature work hand in hand – in an almost symbiotic relationship. 

One way to produce an air conditioned chill is to rely on a refrigerant that functions in essentially the same way as a kitchen fridge.

“Let’s say it’s five degrees Celsius inside your fridge. This fluid enters the evaporator in a mostly liquid state but with some vapor. At low pressure and low temperature, this refrigerant is designed to evaporate, like beads of sweat on your forehead on a summer’s day. And like with that sweat, the process of evaporation draws away heat – in this case from the inside of the fridge,” says Simko.

Low pressure = low temperature

He debunks the assumption that evaporation can only happen at 100 degrees Celsius. That’s the boiling point for water at sea level. If you drop the pressure low enough, and use a different fluid – as in this case – evaporation can occur at much lower temperatures, and cool things down.

After leaving the evaporator, ideally in a vapour-only state, the refrigerant is passed through a compressor that greatly increases the pressure. At this higher pressure, the now superheated vapour gives up the heat it draws away from inside the fridge, as it passes through another heat exchanger called the condenser. This is the black piping you see at the back of your fridge.

From there, the fluid shoots through a throttling valve to drop the pressure before returning to the evaporator and completing the cycle.

Splendid split systems

A split system exterior is a recognisable addition to many houses and virtually unmissable in Australia. But how do they truly function?

A split system essentially dumps heat from indoors to outdoors. Hot air is drawn over the indoor heat exchanger (evaporator) usually mounted high on a wall, and the refrigerant takes away that heat through pipes to another heat exchanger (condenser) in the outdoor unit – after first going through a compressor.

In the outdoor condenser, the refrigerant gives up its stored heat to the outdoor air, usually with the assistance of a fan that blows air over the condenser, similar to “blowing on a hot mug of coffee”, says Simko. 

A split system exterior is a recognisable addition to many houses and facilities in Australia.

 The ability of many split systems to be operated in reverse enhances their desirability and appeal. In winter, the indoor unit becomes the condenser and the outdoor unit becomes the evaporator. This allows the system to transfer heat from the outdoors – locked up in the evaporated refrigerant running through the piping – to the indoors. This is also called a reverse cycle heat pump.

Larger facilities and buildings with multiple rooms have a higher cooling capacity. This means that it is best to have an evaporator in different locations and rooms. However, multiple evaporators can be connected to a single outdoor unit capable of supporting a bigger capacity – a multi-head split system.

As cool as a cooling tower 

Chilled water systems are saving graces for larger buildings and facilities that subsequently have higher cooling loads, as water is a great medium for transporting and retaining heat, and you don’t have to operate it at low pressure. 

“Water is better at picking up heat than air because it packs a bigger thermodynamic punch,” says Simko. 

In larger capacity systems, usually the first fluid loop extracting heat from the air is chilled water. This water can be cooled by a conventional refrigerant system, yet there are ways to cool the condenser of the system and dump the heat to the outdoors via blowing air over it or relying on a cooling tower.

Completing the cycle

Simko acknowledges there are limitations to air-cooled HVAC systems. They work well energy-wise, but if you’re cooling with air, you can end up using double the amount of energy than if you used a water-cooled heat exchanger – also known as a cooling tower. 

Commonly recognised in the context of conventional power generation stations, cooling towers are used to reject heat to the environment to condense the water vapour coming out of the turbines. “You can’t generate electricity in a closed loop without dumping some of the heat to complete the cycle,” says Simko. 

“Returning to a chilled water HVAC system, extracted heat that has passed from the chilled water loop to the refrigerant can be given up to the outdoors by a cooling tower. So instead of having air blown over a heat exchanger, we’re going to put warm water through a cooling tower and it will come out cooler than that.”

Misting fans for schvitzing tennis fans 

Cooling towers are one of the most common cooling systems for large commercial buildings in Melbourne. They function most effectively in dry climates, but they are not restricted to such areas. 

In an open tower, water is sprayed into the top of a tower and rains down onto a fill which increases the surface area by splashing the water into smaller droplets. The fine mist of water droplets evaporates into air drawn into the tower from below by a fan at the top. Simko equates this to the misting fans that relieve Australian Open attendees – the droplets in the air evaporate, take heat away and produce a cooling effect.

Cooling towers are one of the most common cooling systems for large commercial buildings in Melbourne.

“If a droplet evaporates, it’s going to suck away the heat from the surrounding air, just like boiling water on your stove requires heat to evaporate. The air temperature drops. A small portion of the water entering a cooling tower evaporates, but that’s enough to make the surrounding area within the cooling tower cooler,” says Simko.

The water that doesn’t evaporate falls through this area of cooled air and gets colder before landing in the basin below, from where it is returned to the heat exchanger to remove heat from the cooling system. Make-up water is added to account for the water that evaporates.

The life-saving importance of well-maintained cooling systems 

Simko cites the infamous 1976 outbreak of Legionnaires disease as a cautionary tale against failing to keep cooling towers and air-conditioning units clean.

A convention at Philadelphia’s Bellevue Stratford Hotel that hosted more than 4000 members from the Pennsylvania chapter of the American Legion ended in disaster. Only days after the gathering, 12 Legion members – who had enjoyed relief from the July heat in the air-conditioned comfort of the hotel – had died from an unknown respiratory illness, and 36 other members were hospitalised. 

The majority of those who passed away were elderly men and smokers, however the youngest victim was 39 years old. Twenty nine people died in total. 

Other victims of the illness included a bank teller who had worked across the street from the hotel and a bus driver who transported cadets in one of the Legion’s parades. All presented similar pneumonia-like symptoms. 

The Centres for Disease Control and Prevention (CDC) launched a record-breaking six-month-long investigation. Microbiologist Joseph McDade eventually discovered the Legionella bacteria, also found in the water of air-conditioning units, was the culprit. 

The investigators concluded the air conditioning’s powerful fans emitted a mist of water contaminated with the pathogenic bacteria that flooded into the hotel lobby and onto the sidewalk in front of the hotel. 

Maintaining cleanliness and purity in cooling tower water is therefore essential.

“You have to make sure that they’re kept clean and in accordance with local regulations,” says Simko.

The untapped potential of the absorption approach and solar cooling

The absorption cooling system is underrated, according to Simko – the only catch is that in order to operate one, it requires a source of free or relatively low-cost heat.

“There’s a lot of promise for the absorption approach, because having a free or readily available source of heat – that would otherwise be dumped to the environment – is environmentally responsible,” says Simko. 

While an absorption cycle loosely resembles a single-split system, it relies on two fluids: a refrigerant and an absorbent. 

A common combination for an absorption system is water and lithium bromide, respectively. Water vapour leaves the evaporator after removing heat from the area to be cooled. In the absorber chamber, water vapour is dissolved into the liquid lithium bromide absorbent. 

Pumping liquid more energy-efficient

This process emanates heat that must be removed and the liquid mixture is pumped to a higher pressure and passed to the generator, where the water and lithium bromide are separated by heat supplied from an external heat source. The water then travels to the condenser and eventually returns to the evaporator to complete the cycle, while the separated lithium bromide goes back to the absorber chamber.

Simko notes an advantage of this system is pumping a liquid to a higher pressure is more energy-efficient than compressing a gas, as is done in a conventional refrigerant system.

The free or low-cost heat source to operate the generator could be from a nearby power plant, an industrial process or solar thermal energy, which denotes the system as a form of solar cooling.

Solar cooling is promising, but it has not gained much attention in Australia. Simko estimates there are relatively few active installations.

Economy cycle 

Passive cooling encompasses a myriad of economically rewarding and environmentally responsible tips, yet mainly champions a fusion of solar panels, light-coloured roofs and leaving windows open overnight.

As outlined in Facility Management’s recent interview with Smart Design Studio founder William Smart, bringing cold air into a house or facility overnight can be an effective way of chilling a space for the morning. The success of this method is enhanced when coupled with materials that have a high thermal mass, such as brick and concrete. 

Simko corroborates Smart in saying that this method is not always possible for larger facilities, as windows can be inaccessible or not fitted with protection against insects and debris such as leaves.

The effectiveness of bringing cold air into a space overnight for cooling is enhanced when paired with materials such as brick or concrete.

Reducing solar gain

Larger facilities are also notoriously internal-heat-gain intensive – people, computers, staff, kitchens and lights inevitably heat up a space. Simko says office spaces and facilities would be wise to turn off equipment at night. 

In a cooling-dominated environment, Simko recommends reducing solar gain through investing in insulation, double-glazed windows and ensuring roofs and walls are coloured white or light red rather than black, which has been noted in a climate such as Sydney’s. Insulation is often overlooked as an effective way to stall the rate of solar gain heat transfer into a building.

Simko’s top tips for overcoming heat 

If searching for alternatives to air-conditioning, Simko strongly recommends opting for double-glazed windows with low-emittance coatings – a feature he waxed lyrical about only a few months ago. 

Simko echoes Smart’s advocacy for underfloor cooling – a climate control method that allows facilities and homes to function sans all-air air conditioning – yet he also remains realistic about the likelihood of condensation forming on the floors and becoming a tripping and cleaning hazard. 

Smart Design Studio’s office space in Sydney functions sans air conditioning, instead using underfloor cooling.

Climate control comparison 

The budding facilities manager mentioned at the start of this article hopefully feels prepared to cool their facility during the summer months. They surely feel a renewed sense of awe towards HVAC and cooling systems in keeping a space happy, safe, and appropriately chilled or snuggly warm. 

However, in the ongoing pursuit of living and working sustainably, underfloor cooling and heating as a climate control method has tickled the facility manager’s fancy. 

Photography of Smart Design Studio by Romello Pereira.

If underfloor cooling has piqued your curiosity, read about it thriving in a Sydney office.

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