Efficient, cost-effective and healthy flows with evaporative cooling

Compiled by Benjamin Brits

With patents recorded as far back as the early 1900s, the concept of this cooling mechanism was in fact developed in ancient times and used in Egypt and Persia by collecting and passing air over a subterranean water source.

Naturally, a lot has changed since then. There has been great development in all forms of technology and ducting related to the evaporative cooling process, and the result of this cooling-type today, is an enhanced way of delivering on nature.

“Evaporative cooling essentially works on the natural way of cooling air by taking outside air and passing it through cooling pads into a space while at the same time adding humidification and pushing out relief air. This creates a constant inflow of freshly-cooled air and you can keep windows, doors and louvres open to allow this relief air to get out.

“Generally speaking, with evaporative cooling you would change the air in the particular space around 30 times per hour – so essentially bringing in new air and removing the stale air. The air change rate can also vary depending on the varying circumstances. This concept of cooling is therefore different to conventional air-conditioning which relies on air recirculation to get to the temperature required,” says Stuart Karovsky, general manager at Air-Dale Engineering (Cool Breeze Airconditioning SA).

In the past, evaporative coolers were made locally by various companies and utilised in some retail stores in South Africa. These steel-casing units were generally installed on the back wall with a centrifugal fan blowing air directly into the space through a hole in the wall, and a single deflection grille.

“This design type was not ideal as the noise and air velocities associated with this type of installation were generally not acceptable, resulting in tenants or users limiting the use of their units as a result of the noise and discomfort associated with the high air velocities. But then, about 30 years ago, a new concept of unitary single stage evaporative cooling products arrived from Australia with an improved through-roof design and installation,” says Rodney Marillier, owner of Raptor Engineering (Aolan Evaporative Cooling Systems).

Rather than installing the units on the back wall and blowing air directly into the space at unacceptable velocities, the new design unit came in a down-discharge configuration utilising a dropper duct to penetrate the roof, and introducing air into the space via flexible ducting and ceiling-mounted multi-directional outlets that are commonly still be seen today.

This type of installation was a marked improvement over the previous method in that the noise levels were greatly reduced and air distribution was improved with acceptable air velocities. While this type of unit design is used predominantly for residential applications in Australia, the same types of units are used for various applications in Southern Africa.

“It is unfortunate that in the past, poor installations by some contractors resulted in owners, landlords and designers not being keen on evaporative coolers, or allowing their roofs to be penetrated and damaged as a result of poor workmanship– that in turn affected how designers and engineers perceived the product,” says Marillier.

Application and selection

Evaporative cooling can be applied as a solution in almost all settings including agriculture, residential, commercial, industrial, retail, sports facilities and stadiums.

The effectiveness of evaporative cooling depends on the correct selection which is based on a number of factors such as the average dry & wet bulb (Db and Wb) temperatures for the particular region, or location.

“This particular factor dictates what the achievable supply air temperature is, which, in turn, will dictate how many units or what air volume will be required for a given space – the lower the achievable supply air temperature, the less air will be required to achieve the target required,” says Marillier.

This is therefore calculated using the formula:

T (air out) = T (Db air in) – [eff% x (T (Db air in) -T (Wb air in))] / 100

The efficiency of the whole design is based firstly on the efficiency of the cooling system being used. Using an efficiently designed system, you can achieve temperature drops from ambient temperature of up to 22˚C in extreme hot and dry climates, where the Db temperature could be 48˚C and the Wb temperature of 20˚C as you would see in desert-type areas.

There are different types of evaporative cooling, namely:

• Direct (single stage)
• 2 stage (Hybrid system using wet core and plate heat exchanges)
• Indirect (Using an air to air heat exchange core).

For the purpose of this article the focus is mainly on direct or single stage evaporative cooling.

“Most makes of evaporative coolers today have three airflow discharge designs that can be used for a specific application.
These are: top discharge, side discharge and down discharge – the most popular being the down discharge where the unit is placed on a roof structure and the unit discharges the airflow down into the building to be cooled,” says Chris Toomey, Technical Manager at Seeley International.

A typical evaporative cooling unit installation feeding directly into ducting on the outside of the building. Image credit: Air-Dale Engineering

 “Most buildings can be cooled using evaporative cooling, from a metal sheet building (non-insulated) to a well-insulated energy-efficient building. The design of applications needs to take into consideration that there is particular ducting design, that applies to evaporative cooling solutions, understanding that evaporative cooling cools by bringing cooled air into the building and that the same amount of air needs to be exhausted out of the building. This high volume of air must be moved within the building structure using correct design ducting and diffusers. These ducts can be quite large compared to typical refrigerated units,” says Toomey.

There are, however, some applications where direct (single stage evaporative cooling) cannot be used that include closed sealed laboratories, small individual rooms, hotel rooms (where there is normally only one control per cooling unit and not individual room control), multi-storey buildings, and any application where there is a limited area to install the required ducting as this is essential for the volume of air continually being moved.

System sizing

The heat load of a zone within a building is the sum of the heat emanating from all the sources including solar radiation, convection, conduction, machines, lighting and people – it is measured in watts.

The calculation for what evaporative cooling units’ requirements is slightly different in the amount of heat load in the specific building that you are designing to. Heat load calculations can be complex and various calculation methods and software is available for this function. The calculations to thus establish what methods for sizing are going to used, need to be carried out correctly by an application engineer.

“The heat load calculations for evaporative cooling are simplified by ignoring the latent heat. Our company has developed a heat load calculation method where we are able to calculate the number of evaporative coolers needed for a specific site. There are basically three methods of sizing when using any form of evaporative cooling – they are spot cooling, air change method, and then heat load method calculations using the kW capacity of the evaporative cooler being used,” continues Toomey.

Spot cooling is the simplest method and is the method of cooling a target zone with a high velocity of cooled air that blankets the entire zone to a height of about 2-3m, so people or products within that zone are kept cool.

Spot cooling pays no regard to the actual heat-load of the zone, it relies on a fixed air change rate of around 45 air changes per hour (ac/h) to achieve its objective. The 45 ac/h is for direct, single stage units. However, the indirect cooling technology will need substantially less ac/h due to its supply temperature being lower than the ambient wet bulb.

The air change rate method of sizing for selecting evaporative coolers has long been the method of choice in the air-cooling industry. The method uses air change rates that have been established over many years by local companies in the Southern African market. The rates used by most contractors are called the empirical data rates.

There’s no argument that this method of sizing is suitable for most situations, however most application designs do not consider that efficiency could have been achieved using  better design methods.

This method also has some serious deficiencies as it assumes an ‘average’ building but an average building is hard to define – it may have no significant heat loads and could also be well-insulated. Compare a metal constructed warehouse to a warehouse built with brick, mortar and insulation – these have very different parameters.

Further, this method makes no consideration for air coolers of different efficiencies, and some manufacturers have similar airflows but vastly different efficiencies. Quality manufacturers publish air change rates for many cities around the world and these rates are calculated using the correct design criteria, altitude and cooler efficiency and therefore are quite different to empirical data.

The third is the heat load method. This method is also used in the air conditioning industry where the building’s heat-load is calculated and then the units that will be used are designed according to that particular heat load.

In the case of single-stage evaporative cooling units, this method can be more accurate than the methods already mentioned, but the correct efficiencies and the correct cooling effect capacities in kilowatts (kW) of the units must be established so that the correct number of units will be installed for that specific site. Again, possibly a complex calculation.

“When using this method of design, you must take into consideration how to remove the heat from the area concerned. Evaporative air cooling does not re-circulate air. The air is cooled once as it passes through the cooler. This cooled air must then be sized and distributed in such a way as to remove the heat-load in the building. A sufficient number of air changes must occur in order to expel that heat quickly enough to bring the indoor temperature down to the desired temperature. As the cooled air is delivered into the building, it immediately begins to heat up again until it reaches the exhaust point where it should not have heated above the set temperature. Therefore, depending on the heat-load and the set temperature, a calculation must be made for the number of air changes required,” says Toomey.

If the correct and full heat-load calculation has been made, the total volume of air required will be established, and from that the correct number of coolers required to satisfy the heat-load (kW). Of the total volume of air required, at least 90% must also be exhausted from the building. This will help in removing the heat load sufficiently and at the same time keep a positive pressure in the building.

General advantages

While evaporative cooling may have some disadvantages, there are many advantages associated with this method, including:

Lower initial capital cost for both supply and installation – depending on the application and selection, this saving could be as much as 40%.

Lower power consumption. Evaporative cooling is said to be the most cost-effective method of cooling.

Lower maintenance and spares costs – the expertise level required to service/repair these evaporative coolers is not as high as air-conditioning as the evaporative coolers referenced in the article have very few moving parts – a water pump and a fan motor.

Where many refrigerated systems have a coefficient of performance (COP) of 3-4, some evaporative systems can have COP of around 12 or more, this can only be achieved when using evaporative cooling systems that have been efficiently designed and tested at approved test facilities.

Longer lifespan – The polymer plastic casings and structure on some of the units on the market have a lifespan in excess of 10 years, when maintained properly.

Unlike air-conditioning that recycles the same air, albeit with some fresh air, evaporative cooling will provide 100% fresh air with a constant air temperature supply. This also plays a major role in the management of heat load in a space as already mentioned.

Healthy natural way of cooling air which adds to the green building concept as well as reducing the carbon footprint on the world due to lower power consumptions.

Air quality and ‘building health’

Air quality is a whole new area of testing that hasn’t typically required any verifications or certifications previously, so the testing that is performed with evaporative cooling currently is more about fan curves, performances and efficiency – there hasn’t been specific testing around this element as yet, but having said that, scientists all over the world are all currently looking into air quality and the association with airborne pathogens.

Figure 1: Engineering level controls to reduce the environmental risks for airborne transmission. Air quality is directly related to the the flow of air, especially in occurrences of pathogen outbreaks. Image credit: ASHRAE

I would assume little dispute in natural ventilation so the fact that a physical changeover of the air inside a building happens about 35 times in an hour speaks for itself – you don’t have to do anything to prove that other than the evidence that the product is in fact delivering this amount of air changes.

There has been wide recognition that Covid-19 is transmitted via aerosol form which is essentially scientifically different to droplet form and therefore the management thereof must follow an adjusted strategy.

“To add to this, more and more evidence has been revealed that spreading of the virus is taking place indoors and in areas of poor ventilation. Conventional air conditioning with the recirculation of air with aerosol contamination is said to be causing what is now referred to as ‘super-spreader environments’. But indoor air quality and what is known as building health is not only linked to Covid-19,” says Karovsky.

At the time of publishing, South Africa is heading into the summer months and as you would expect – everyone will be reaching for the remotes to ensure comfort conditions, closing windows and doors, which is in fact creating the environment that you don’t want when dealing with the current pandemic.

In President Cyril Ramaphosa’s address to the nation in July 2020, he stated: “We must immediately improve the indoor environment of public places where the risk of infection is greatest. We must increase natural ventilation, avoid the recirculation of air and minimise the number of people sharing the same space. We must do this in all health care facilities, nursing homes, shops, offices, workplaces, schools, restaurants, and public transport.”

Evaporative cooling not only provides fresh cool air, it also provides a humidification effect, and various worldwide organisations, including ASHRAE, state that one of the controlling factors in Covid-19 and other similar viruses and pathogens can be managed though the combination of fresh air and humidification to minimise the risk of spreading any infectious agents.

“Munters has also released a white paper, specifically related to Covid-19, which states that creating a cool and humid environment at specific parameters, makes viruses such as the coronavirus much less effective and even to the point it becomes inert,” says Munters South Africa managing director Phillip Dickinson.

The scope is there now for people generally to make some type of change in regard to building health and air quality especially considering the long wait until a vaccine is eventually delivered to this country.

“For many offices, factories or shops, the indoor air quality is going to become very important, because no one wants a breakout at their business as this would mean negotiating persistent shutdowns which cost money,” Karovsky adds.

Continual air changes and humidity in any building or space is proven to be significantly healthier than the re-circulated, dry air from conventional air conditioning. Physiologically, various medical studies have shown that occupants exposed to fresh air and relative humidity are healthier, more alert and show higher levels of general wellbeing.

Filtration

“The pads in an evaporative cooler have a filtration property when the pads are wet, not when dry or the system is not running, and when wet can be effective in particle reduction,” Dickinson notes further.

Even though there is speculation that evaporative cooling ‘filters the air’ because of the process with water, there is in fact no factual proof of this to-date – there is some natural filtration of the air but the process doesn’t include cleaning all particles completely from the air. You are generally not looking for filtration at a micro-particle level of the air because you have got high refresh rates.

“Filtration would come into the equation if the quality of the air that you are taking from outside is bad – for example if you are in a very polluted industrial area, or an area where the outside air quality is poor at the installation level or instances in high pollen areas – evaporative coolers do play a role with some natural filtration due the high ratio of surface area of air to water in the cooling process,” adds Karovsky.

Cooling pad media

Typically, two media types are produced – these being cellulose-based and glass-fibre based which are utilised in different applications. The cellulose material is used in general applications including agriculture, horticulture, commercially and in retail – where the glass-based material is suitable for applications where there is fire risk or pre-heating required – the glass-based media being flame retardant.

“Munters was the original designer of the evaporative cooling pad (through the Swedish founding member, inventor and entrepreneur Carl Munters) and also held an exclusive worldwide patent for many years. As an evaporative cooling pad supplier being our primary function, we manufacture CELdek in South Africa, as well as other countries around the world, which is our cellulose product, and our GLASdek product is produced in Italy. Both of these products hold registered trademarks. These products have corrugated  material where different flute-dimensions serve different outcomes based on saturation efficiency and pressure drop. These pads are produced in varying dimensions and can also be custom-produced for clients with particular needs,” says Dickinson.

Pads are produced by combining the different flute-dimension layers of the corrugated material that meet at different angles – for example 45˚ and 15˚ to make a 60˚ angle when joined, and 30˚ and 60˚ to make a 90˚ angle when joined. The layering creates larger blocks whereafter they are cured at approximately 100˚C and then cut to the required profiles.

Pad replacement is highly dependent on water quality as a major contributor because when you evaporate the water, salts and minerals remain behind so pump flow rate is important in terms of design. Pollution also plays a significant role when in a heavy polluted area – the pads may become sticky if your pump selection is not correct and then you will run the risk of clogging up the pads and reducing the cooling and natural filtration effects.

“If you are in a very aggressive area where water PH or chemicals are intense you do get a quicker degradation of the pads (they don’t last forever) but having said that, most HVAC applications would have some sort of water treatment plant that provides a good water source and this offers longevity of the product. Typically, replacement cycles are 2 to 3 years in HVAC units, and depending on particular circumstances this may be longer,” adds Dickinson.

 An installation in a vehicle holding warehouse. Image credit: Air-Dale Engineering

Considerations for the future

With the new general awareness of germs that building occupants have developed, as well as developments in terms of air quality and airborne risks, designers, engineers and contractors will no doubt be directed by market needs to adapt and adopt to different methodologies in ensuring building health through air flows, social distancing and ease of maintenance.

Further, the requirements in smaller offices or home offices also poses a different slant to the industry dynamics and one would wonder what the impact would be on peak electricity supply by installing less energy-efficient methods in cooling. I personally doubt the grid would be able to sustain a million more installations of that nature when we are already on the edge of an abyss of our electricity supply. Better alternatives must be sought out as the industry works on overall efficiencies now including the critical elements of health and wellbeing.

Sources:

This article was compiled through  information obtained from the following participants:

• Air Dale Engineering (Cool Breeze Airconditioning SA)
• Raptor Engineering (Aolan Evaporative Cooling Systems)
• Seeley International
• Munters South Africa
• Baltimore Aircoil Company