Convective Flow due to Stack Effect
One way to ventilate a building that is hotter or colder on the
inside than outside is to use what is known as "stack effect". Because
of the temperature difference, the air inside the building is
either more or less dense than the air outside. If there is
an opening high in the building and another low in the building,
a natural flow will be caused. If the air in the building is
warmer than the outside, this warmer air will float out the
top opening, being replaced with cooler air from outside. If
the air inside is cooler than that outside, the cooler air will
drain out the low opening, being replaced with warmer air from
One common use for stack effect would be nighttime flushing
of a building's interior, to cool it for the next day.
The rate at which air flows depends on several factors, the
inside and outside air temperatures, the area of the openings,
and the height difference between the top and bottom openings.
The 1997 ASHRAE Fundamentals handbook gives the following
Q = 60 * Cd * A * sqrt(2 * g * (Hn - Hb) * ((Ti - To) / Ti))
Q = flow rate in cfm,
Cd = 0.65 (for unobstructed openings),
A = opening area, square feet,
Ti = indoor temp (Rankine),
To = outdoor temp (Rankine),
Hn = height of "neutral pressure point" (for simple systems, assume
1/2 way between top and bottom openings).
Hb = height of bottom opening
g = gravity.
To calculate flow rate, fill in the boxes below.
For a more complete discussion of this, and especially of unusual situations,
please see the discussion in the ASHRAE Fundamentals handbook.
You might want to compare the flow rates of this natural ventilation to
those of forced ventilation. For example, a typical "whole house fan", available
at Home Depot for $130, is rated at 4500 CFM (but consumes about 200 watts
Note also that for stack effect, flow rate diminishes as the ventilation
proceeds, bringing the temperature difference down.
The purpose of ventilation is to flush the indoor space out with outdoor
air. As the outdoor air replaces indoor air, it either adds heat to or removes heat
from the inside space. Knowing the flow rate, the specific heat of air, the
density of air, and the temperature difference, we can calculate the effective
heating or cooling rate, and express the result in BTU per hour, or BTUH (for reference,
air conditioners are often rated in "tons" of cooling capacity, one ton being
12,000 BTU per hour):
Heat Transfer Rate:BTUH,
which is about Watts.
You can see that the potential for stack effect cooling is rather low,
compared to mechanical refrigeration. However, it does work, has no
moving parts, and requires no extra energy input. Consider it as part
of a complete plan that includes a significant effort to reduce daytime
heat gain (especially from the sun).
An interesting variation on this theme is the so-called "solar chimney".
Imagine that at the top of the building, air flows through a duct that is
heated by the sun. The added height and temperature difference could combine
to significantly increase air flow, if designed carefully.
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