It’s Not Rocket Science #6: The Stack Effect

The Stack Effect is when air is moved into and out of buildings by means of a difference in the buoyancy of the air on the inside and on the outside – between air that is colder and air that is warmer, in other words. Buoyancy can be either positive or negative – in the sense that warmer air can rise to be replaced at the bottom by cooler air, or cooler air can sink to replace warmer air at the bottom. Positive buoyancy is when warm air rises, and negative is when cool air something sinks. Got that? Both are good, if you’re trying to keep a building cool.

Or warm. Before I go any further, let’s not forget chimneys, sometimes called chimney stacks. Here’s a link to the history of the chimney. The first known English example of a chimney is at Conisbrough Castle from the 12th century, but they didn’t become common in houses until the 16th. Until then, life without chimneys was very very unhealthy.

It still is, in many parts of the world. According to a World Health Organization report, “Nearly 2 million people die prematurely from illness attributable to indoor air pollution from household solid fuel use.”

Early chimneys were an improvement, but weren’t perfect. Count Rumford, whom you’ve already met, invented the Rumford Fireplace in 1797. He made two important improvements – one was to change the shape to direct more radiant heat back into the room, and the other was to restrict the size of the chimney at the bottom to increase the speed of the smoke going up the chimney. The streamlined smoke flow generating the stack effect had the added advantage of sending the non-smoky but heated air back into the room.  Importantly, existing fireplaces could be easily modified.

To many, these new and better fireplaces looked strange. They were shallow and their sides were angled and they were taller than they were wide.

Lloyd Alter, over at treehugger, can tell you more about modern Rumford fireplaces. Interestingly, a fireplace has an optimum burn rate that is set to something like “roaring” – the heat should be burning off the soot at the front of the fireplace, as in the photo above. (Thanks for that Lloyd.)

Finally, to end this introduction to the stack effect, here’s a photograph you’ve seen before, of some chimneys that may have been admired by Walter Gropius for their stack effect. Or maybe not.

Conventional chimneys make use of positive buoyancy from the bottom, as do some solar chimneys that have been recently proposed to generate electricity.

Here’s a schematic configuration of a solar chimney power generation plant. [Thanks climateandfuel.] It’s usually proposed that the space beneath the heat trap be used to grow tomatoes. Whatever.

Other solar chimneys use the sun to create a positive buoyancy in the air at the top of a chimney and thereby draw cooler air into the building to replace it.  Here’s a CAD model that’s easy to understand. Chimney heats air, causing it to rise and draw air through the building albeit at different rates for each floor.

This principle is used by the University of Washington’s University of Phase 1 Molecular Engineering Building (MEB) except that the rates are the same as each floor now has its own solar chimney. You’ll find more facts and performance data here. These stacks are really turbine ventilation inducers in which

a glazed panel was incorporated in the west-southwest orientation of the stack, inducing a solar assist to airflow by increasing the buoyancy of exhaust air during peak summer months.

Here’s a schematic of a building that uses passive energy to draw passively cooled air through a building.

It describes exactly the same principles known in Persia in the first millennium BC.

Here, this windcatcher is coupled with a qanat which, you’ll remember from the post on yakhchal, is an underground canal (aka underground heat exchanger). Windcatchers come in many types depending upon their function and the direction or directions of the wind. They also trap a lot of heat because the µ-value of mud brick isn’t that high. This means that they function as solar chimneys when there’s no wind.

A windcatcher doesn’t have to be made out of mud brick to catch wind. However, if mud brick is what you have to build with, then you also have a solar chimney and a stack effect. This stack effect still exists when the tower is wind-driven but is insignificant compared with air movement generated by the pressure difference caused by the Bernoulli Effect. Given their names, it’s unsurprising that the stack effect of windtowers and windcatchers is overlooked but, even if wind had nothing to do with driving them, they would still be a tribute to human ingenuity and intelligence, providing cooling when and where it was needed.

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The term ‘solar chimney’ threw up this image, from www.jeremylevine.com. I’m including it as an endnote because it seems to link the stack effect with recent concerns such as courtyards, gardens, biophilia, and SANAA.

Pocket Courtyard

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