THE WIND FROM HEAVEN

What is destratification and why is it so important in tall buildings.

Heating energy savings of up to 50% for the tallest spaces.

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I was quite good at science when I was at school. Instead of blotting out the seemingly incomprehensible experiments in physics, I have a good memory of the one demonstrating convection. Convection is the technical term for when heat rises and warm air or water climbs above the cold.

The teacher took a large beaker of water and put a permanganate dye crystal against the side at the bottom. When heated gently with a Bunsen flame, the warm water rose up the side of the beaker carrying a thread of dye with it. We could all see the movement of the warm water to the top of the beaker, and how the dye stayed at the surface until it re-cooled and sank back down.

Now consider. What if the beaker were a tall building and the permanganate dye was the  money being spent on trying to heat it..

Where there is heating in a workshop or warehouse, it often takes the form of gas-fired air heating, sometimes referred to as ‘forced convection’ heating. But as soon as it is blown out of the heater unit, the hot air will start to rise, and the benefit if the heating will be lost at floor level.

At a vehicle showroom in South Wales there was a car display area some 7m tall, and at the back were two storeys of offices. The ground floor level had the customer facing operations and the upper floor, with a balcony for access, had the admin. The admin people would often complain how hot it was (mid 20s ^oC ) compared to the display area below, where the temperature was in the high teens. The air heating had ‘stratified’, with the hottest air at the top and the coolest at the bottom where the customers are!

In this instance, and most commonly, the thermostat and controls are also in the lower 2m or so of the space. Down here it’s still quite cool, and so the thermostat calls for more heat. More gas is burned and more heat provided, but it does not affect the lower areas because the newly heated air is the warmest and rises to the very top of the building: it becomes the highest stratum in the space.

Let’s go back to the beaker of water and the dye crystal for a moment. What if the teacher had taken a spoon and stirred the water in the beaker. Obviously the dye would have become more evenly distributed, with the bottom third of the beaker being the same colour as the rest. This is de-stratification – the antidote to convection.

In a tall building, the ‘spoon’ is a de-stratification fan: a downward-pointing fan placed near the roof, which blows the topmost layers of air back down toward the floor. By this means the bottom third of the space gets the same heating as the rest, the occupants are more comfortable, and the thermostat regulates the temperature more accurately.

Nowadays the benefits of de-stratification are well known and many buildings have de-strat fans. The savings are significant in reducing energy consumption and the benefit increases with the height of the building. (Greater height offers more space for stratification and leaves the ‘occupied’ area near the floor of the building as a smaller proportion of the volume.)  i-SBEM is the model used in calculation of EPC energy ratings, and including de-stratification in such a model indicates

Heat energy savings of between 5% and 50% for the tallest buildings.

A large warehouse I visited had de-stratification fans installed. The business sold shoes on the internet, and the warehouse was the shop store; scores of people finding pairs of shoes in huge racks. BUT the de-strat fans never worked because no-one knew where the switch was. My articles are full of examples where good systems are ruined by bad control. This is one of the worst.

Even when available, manual control of de-strat fans is not ideal. The fans should be ‘slaved’ to the operation of the heating, so that when the heaters are on the fans are on. An acceptable second choice is for temperature sensors on the fans and at ground level to work out if there is a significant temperature difference between them, implying the presence of stratification. If there is more than a few degrees, put the fans on.

Finally a rule-of-thumb about how ‘much’ de-stratification is required. The rule is that in an hour, the de-strat fans should move the twice the volume of air in the zone. So if your building was 20m x 20m x 10m high, the fans should move the volume of 4,000 m3 twice per hour. A square array of 9 fans might be used in the square building. Each fan must therefore have a capacity of 2 x 4,000  / 9 = ~ 900m3 per hour or approx. 250L/sec. An array of 16 fans could be smaller capacity of 500m3 per hour or approx. 140L/sec.  Plenty of suppliers will be delighted to do calculations and give details.

In summary, it need not be a strong one, but a wind from heaven blowing down into your heated warehouse or workshop can save a good deal energy. Like the dye in the experiment, a good ‘stirring’ will spread the heat evenly throughout the zone and provide more warmth for those working at floor level.