The following article explores the destratification of air, why it happens and what effect it has on human activities. This practical look is designed to help the reader and their organisation decide whether to allow nature to take its course which includes less than ideal conditions for humans and their goods or to intervene and improve on what nature provides.
What is Destratification of Air?
Strata are a series of layers; each layer being made up of identical or very similar characteristics. Knowledge is built in layers, societies create layers, protective layers keep out weather and many more numerous examples exist. Air, water, earth and rock are all examples of strata occurring in the natural world; the building of layers is sometimes referred to as stratifying or stratification.
Strata are most noticeable to those who see a rock face, where visible evidence of different layers exist having been built up over time. Air and water’s strata are felt by our skin yet are invisible to the naked eye, these layers are defined in terms of temperature and density. To witness the layering effect in air and water a thermometer provides a good measure.
Whereas stratification is the building of layers, destratification is the process of breaking up, taking apart that layering.
Strata layers can be any thickness. Outside air has distinctive layers called – troposphere, stratosphere, mesosphere, thermosphere, exosphere. The troposphere is the lowest layer extending to between 9 to 17 km above sea level. It is where life exists as we know it: humans, animals, plants, trees and the like live here.
In broad terms: earth’s gravity pull draws dense air toward the ground, the least dense air rises; temperature of air in the troposphere decreases by an average of 6.5OC every 1000m; most clouds and weather occur in the troposphere layer.
Wherever the air, outside or inside, it has molecular mass and density. Cold air is dense because the molecules are close together. Warm air is less dense because the molecules are further apart.
Destratification of Air in Buildings
Air inside buildings stratifies. Its behaviour follows the principle of Boyle’s law, a gas law which states that pressure and volume of a gas have an inverse relationship. Meaning that as pressure increases volume decreases, resulting in an increase in density. So, the densest heavier air falls to the bottom most part of the strata, the lightest less dense air rises to form the top of the strata. Humidity makes air heavy, increasing temperature makes air lighter.
That means the most buoyant air is the least dense because its molecules are further apart. It occupies the ceiling space.
Typically, this layering is referred to as thermal stratification it carries the principle that within a defined space the hottest air rises.
Turning to the air inside buildings.
Does this stratification matter and why?
Depending on what purpose is being fulfilled in a given envelope of space the layering is likely to carry a negative impact, for three main reasons:
1. Energy inefficiency – excess energy consumption, higher carbon footprint – extra Cost
2. Human discomfort and ill health – lower productivity, higher absenteeism – extra Cost
3. Stock and inventory degradation and spoilage – higher wastage, higher cost of sale – extra Cost
Within that given space the densest air carries most particles and moisture; the lightest air is hotter and dryer. Neither of the extremes represent ideal conditions for human health or for storage of goods. In the absence of intervention, the ideal conditions might be found somewhere in-between floor and ceiling level or found throughout with intervention, by mixing all the air.
In the article on ventilation the observation made is that breathing air laden with particles is damaging to wellbeing and good health. Comfort and energy are related to temperature and levels of carbon dioxide. Humidity levels are also critical to good health. Too dry or too humid is uncomfortable and harmful. Where people are working in multi-level buildings those in the higher levels may enjoy more warm air in the winter and not enjoy the excessive heat in the summer.
Heating systems are generally designed to maintain a temperature at the location of the thermostat which is most frequently 1.2 – 1.5m above floor level.
Many goods are labelled with ideal storage conditions that typically specify the shelf life or use-by date of the product and may include conditions such as temperature and light. Occasionally the instructions will refer to dry or moist conditions.
Wood and cardboard are common packaging materials. Observing what happens to pallets and cardboard boxes when stored in humid conditions – moisture ingress is evident, over time that moisture transfers to the contents of the box. The reverse will happen in dry conditions. If packaging continues to protect the contents that might be acceptable, however the recipient of the package may reject the goods if the package is in poor condition.
Moisture creates damp conditions that promotes the growth of mould. Unsightly and harmful to people who are susceptible to a reaction from breathing or touching mould spores.
Temperature for storage is commonly defined within a band of minimum to maximum degrees centigrade and maintaining a stable temperature is preferable to allowing constant changes.
Electronic goods, pharmaceutical supplies, paint, raw materials, food and drink – all require storage conditions specific to their packaging and the content of the packaging.
A location high or low in a warehouse containing stratified air, is not conducive to the best storage conditions – the contents of jars and cans will overheat at the high level and at low level moisture will absorb into their labels.
As the use of plastic packaging decreases and the multilayer packaging designs change to make sustainability and recycling more practical, the increased use of simpler packaging materials will result in optimum storage conditions becoming critical.
No packing can keep contents immune to the effect of cold and heat for very long. Manufacturers and their customers, will not accept their goods being stored in conditions unfit for the purpose of keeping goods in prime condition.
Destratification in the summer
In summer months the air at ceiling level will be the hottest air in a room, with a tendency to remain in that position unless moved. The temperature incline tends to be linear from ground level to ceiling height. On hot days in the UK with outside temperatures ranging from 28 to 36O C, the solar gain can increase inside temperatures to over 46O C. Air in the upper levels of a warehouse become unbearable and goods begin to perish and anyone working at height is put at increased risk of injury.
In winter months solar gain is lessened and heating appliances will aim to produce air temperatures at ground level in the mid-teens O C. Much of the heat, that heating systems work hard to deliver, creates warm air at ground level. Heating the air dries it, making it less dense and more buoyant. The heated air migrates upwards and in well-insulated buildings it will reside at ceiling height with a reducing graduation in air temperature down to floor level. Typical temperature graduations are 1O C increase for each 1m rise in height.
In poorly insulated buildings warm air still rises. When it reaches the under-roof level the cold roof will cool the air, causing it to fall. The benefit in that air circulation is detrimental to the efficiency of the heating system.
Destratification Solving Stratification
The solution to air stratification in buildings is to cause the indoor air to mix and circulate. Homogenising the air by mixing it up and making it all the same characteristics and quality will even out the temperature. That process we refer to as destratification.
Envirotec’s business is improving air, moving it, conditioning it, cleaning it. If it’s air its Envirotec’s business.
Moving air is relatively simple, moving light-weight buoyant air from ceiling level to ground level is not as easy as bailing out a boat that is taking in water. A bucket, however big, will not move air effectively. Removing the warm air by extracting it to the outside is not an ideal solution. It is a simple solution that is harmful to the environment and may in future be likened to dumping since it just moves the problem into someone else’s space.
Taking the warm air and moving it by using fans is the traditional method of breaking up air strata. Several of these methods exist:
1. Siting fans in the ceiling space and continuously blowing the air down through grilles and nozzles that either act as diffusers or creating velocity by jetting are effective methods.
2. Mixing air by using large open ceiling fans are effective mixers of air for lower ceiling applications.
3. Collecting air at high level and reintroducing it at lower levels creates circulation and is highly effective.
4. Extracting air at high level and dumping it to the outside is an effective but potentially damaging to the environment and will reduce a company’s ESG (Environment, Sustainable, Governance) score.
Newer methods remove the high-level warm air, recover the heat it contains and reintroduce the air, minus its excessive heat, back into the building at high level and under pressure so that it may drop down, thereby pulling other air with it on its way down. This uses the principle of air entrainment whereby the air being introduced draws more air from around it thereby multiplying and increasing the quantity of air in the flow, which causes more mixing.
Heat recovered from the hot air is useful for heating water to luke warm which in turn can be used for cleaning, washing, irrigation of landscape planting – grass, plants, shrubs and trees – and heating pond or lake water. Very effective method of keeping down running costs, effective air temperature and air quality management control and environmentally friendly to boost ESG scores.
Collecting the heated air from ceiling level and bringing it down to floor level to diffuse into the colder floor level air is an ideal solution because the hot buoyant dry air moves up through the coldest strata causing that stratum to mix and dry and draw in heat. The previously hot air cools down until an even balanced homogenised air space is created over time.
The ideal solution for combating inferior air quality and temperature stratification will vary with the seasonal change in weather conditions. In summer months when heating is unnecessary the indoor air will heat up and stratification is problematic. Homogenising the indoor air temperature by destratification will alleviate the stratification although it may not reduce the overall temperature sufficiently. Combining destratification with heat recovery will enable greater control of the indoor air temperature and is perhaps the best solution for hot weather and periods of high solar gain.
In winter months destratification by forcing air from high level down to floor level will provide mixing and homogenising of the indoor air temperatures. The success of that strategy depends on the height of the building envelope and the power and distribution of the destratification devices diffusing or jetting hot air towards floor level.
These solutions meet the criteria for improving indoor air quality at a cost that is equitable to not causing injury or damage to humans, stock and the environment. All stakeholders – customers, staff, suppliers, business owners, investors and the environment – should delight in benefiting from better quality of air: since all benefit by not being disadvantaged through poor quality air.
A myriad of Envirotec products lie at the heart of many systems and are largely unseen, yet they carry on working for decades, delivering solutions that make customers feel safe and comfortable.
Envirotec has been improving air in buildings for over 50 years. It has a broad range of products from destratification fans, extract units, air handling and air movement units that include a wide array of air improvement components. Alongside those specific destratification products there are support products that help maintain existing destratification installations – air curtains that help contain destratified air environments including Envirotec’s uniquely special destratification air curtains.
Tens of thousands of successful installations of Envirotec products provide the proving ground for continually creating innovative solutions to customers’ indoor air problems. From Palaces to Prisons, Leisure Centres to Shopping Centres, Schools to Hospitals, Production Facilities to Distribution Warehouses, Museums and Galleries and thousands more examples.
Envirotec frequently installs its products at customers premise and monitors their performance, which extends to including ongoing maintenance services. We can work directly with customers or alongside their contractors to optimise performance. Envirotec often makes prototype products for customers to experiment in finding the best solution to fit any application.
Envirotec is proud to support its customers with its very own British designs and British made products, from its offices and factory in the United Kingdom.
