engineering

Thermal destratification

Thermal destratification is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Thermal destratification rather than just read about it. In short: Thermal destratification is the process of mixing the internal air in a building to eliminate stratified layers and achieve temperature equalization throughout the building envelope. Thermal stratification in buildings Destratification is the reverse of the natural process of thermal stratification, which is the layering of differing (typically increasing) air temperatures from floor to ceiling.

Key takeaways

Thermal destratification belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Thermal destratification to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Thermal destratification from memory before moving on to harder problems.

Reference excerpt

Thermal destratification is the process of mixing the internal air in a building to eliminate stratified layers and achieve temperature equalization throughout the building envelope.

Thermal stratification in buildings Destratification is the reverse of the natural process of thermal stratification, which is the layering of differing (typically increasing) air temperatures from floor to ceiling. Stratification is caused by hot air rising up to the ceiling or roof space because it is lighter than the surrounding cooler air. Conversely, cool air falls to the floor as it is heavier than the surrounding warmer air. In a stratified building, temperature differentials of up to 1.5°C per vertical foot is common, and the higher a building's ceiling, the more extreme this temperature differential can be. In extreme cases, temperature differentials of 10°C have been found over a height of 1 meter. Other variables that influence the level of thermal stratification include heat generated by people and processes present in the building, insulation of the space from outside weather conditions, solar gain, specification of the HVAC system, location of supply and return ducts, and vertical air movement inside the space, usually supplied by destratification fans. Computational fluid dynamics can be used to predict the level of stratification in a space.

Effects of thermal stratification In a study conducted by the Building Scientific Research Information Association, the wasted energy due to stratification increased consistently based on temperature differential from floor to ceiling (ΔT). The study indicates that stratified buildings tend to overheat or overcool based on the temperature at the thermostat, which tends to be lower than the overall heat energy present in the room. The study also showed that energy waste due to stratification was present at ceiling heights ranging from 20 ft. to 40 ft, and higher ceilings caused higher energy waste, even at the same ΔT. Since ΔT tends to be higher in taller ceilings, the effect of stratification is compounded, causing substantial energy waste in high-ceiling buildings.

Definition of destratification Since stratification and the costs associated with it are linear, the definition of destratification will differ based on opinion and use case. Full destratification, or a 0° ΔT from floor to ceiling, is unlikely to occur in any building. Since the costs of stratification decrease linearly as ΔT approaches 5.4°F, and no study has yet looked at the effects of stratification below 5.4°F, it is not uncommon to consider any space with a ΔT below 5°F to be destratified. In the United States, ASHRAE Standard 55 prescribes 3°C as the limit for the vertical air temperature difference between head and ankle levels, but has no standard recommending an ideal ΔT between floor and ceiling.

Destratification technologies Reducing thermal stratification can be accomplished by controlling the variables that are associated with increased stratification. Since many of the variables, including ceiling height, people and processes, solar gain, and outside weather conditions cannot be controlled, the most common technologies used are related to the building's HVAC (heating, ventilation, and air conditioning) system. One of the cheapest, most effective, and easiest to install technologies are destratification fans, including both axial destratification fans and HVLS (high-volume low-speed) fans.

Axial destratification fans Axial destratification fans are self-contained units that are installed in an array at the ceiling with the goal of blowing conditioned air in the ceiling down to the floor, where people live and work. Because axial fans are designed to blow air straight down at the floor, they can be used in ceiling and roof structures over 100 ft. tall. Because axial destratification fans can achieve destratification with low CFMs, it is imperative that the air leaving the nozzle achieve an air speed at the floor of between 0.2 and 0.5 m/s. The result of this level of air movement is the integration of conditioned air from the ceiling with air at the floor level. Failing to impact the floor will result in destratification of medial layers of air but not achieve destratification at the floor. Since the area around the thermostat will not be destratified in this instance, it is hypothesized that there will be little or no cost savings, as the thermostat will continue to overheat or overcool the room. An experiment in a room with a 21 ft. ceiling yielded a savings of 23.5% with the use of axial destratification fans.

High-volume low-speed (HVLS) fans Because of their size, HVLS fans are normally installed in new construction, rather than retrofits, as the roof structure may have to be redesigned to accommodate the increased weight and size. It's not uncommon to require the relocation of lights, due to strobing as large fan blades pass under them, and sprinkler systems, which typically require unobstructed access to the floor to meet fire code. When used in the summer to encourage evaporative cooling, HVLS fans are run forward, blowing air at the floor. When used for destratification in the winter, the fans are run in reverse, blowing air towards ceiling which then circulates around the room. The height at which HVLS fans can be effective is limited compared to axial destratification fans.

Benefits of destratification This method has the most benefits through its application in the heating, ventilation, and air conditioning (HVAC) industry and in heating and cooling for buildings and it has been found that "stratification is the single biggest waste of energy in buildings today."

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Thermal destratification

Start with the simplest possible case. Write down what Thermal destratification claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Thermal destratification before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Thermal destratification ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Thermal destratification

In research: Thermal destratification appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Thermal destratification in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Thermal destratification is common in secondary-school and first-year university syllabi. It links to neighbouring topics Building biology, Building engineering, Heating, ventilation, and air conditioning, so understanding it makes those chapters shorter.
In everyday life: Look for Thermal destratification outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Thermal destratification in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Thermal destratification means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Thermal destratification out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Thermal destratification in simple terms?

Thermal destratification is the process of mixing the internal air in a building to eliminate stratified layers and achieve temperature equalization throughout the building envelope. Thermal stratification in buildings Destratification is the reverse of the natural process of thermal stratification…

Why does Thermal destratification matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Thermal destratification?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Thermal destratification.