Air

Natural ventilation: stack ventilation

Provided by Nick Baker: Research Associate, The Martin Centre, University of Cambridge

 

What, Why, When, How, Extras

 

What is it?

Figure 1

Fig 1. Temperature difference between inside and outside creates pressure differences across the envelope which drive the airflow out of the upper part of the building and into the lower part of the building.

Stack ventilation is where air is driven through the building by vertical pressure differences developed by thermal buoyancy. The warm air inside the building is less dense than cooler air outside, and thus will try to escape from openings high up in the building envelope; cooler denser air will enter openings lower down. The process will continue if the air entering the building is continuously heated, typically by casual or solar gains.

Stack ventilation is one of the two natural ventilation mechanisms, the other being wind-induced. Since the same openings may contribute to both stack and wind pressure induced flows, they must not be considered in isolation.

The effectiveness of the stack effect, i.e. the volume of air that it drives, is dependent upon the height of the stack, the difference between the average temperature of the stack and the outside, and the effective area of the openings. The mathematical formula is given in the Design Procedure.

Stack ventilation occurs naturally whether we design it or not, and has been consciously used for centuries, in traditional and vernacular buildings ranging from Indian tepees to churches . However, modern analysis and design advice greatly extends its area of application to much larger buildings, with more exacting demands.

 

Why use it?

Natural ventilation as an alternative to mechanical ventilation has several benefits: low running cost, zero energy consumption, low maintenance and probably lower initial cost. It is also regarded as healthier, having less hygiene problems with ducts, and filters etc, and the 'naturalness' in the way that it connects with outside, often in conjunction with windows, is seen as a psychological benefit.

Ventilation in buildings has three main purposes:

  • 1. To maintain a minimum air quality (1 – 2 ac/h)
  • 2. To remove heat (or other pollutant) (2 – 15 ac/h)
  • 3. To provide perceptible air movement to enhance thermal comfort ( 0.5 – 2 m/s)

In winter, typically only (1) is required, though in highly insulated buildings (2) will also be required sometimes. (2) is the typical summer condition in the UK, and (3) would be required when the outside air temperature is already at the lower comfort limit. The air change rate (ac/h) is indicative only, and for (3), the required air movement will often lead to very high air change rates of 50 to 100 ac/h

Stack ventilation can contribute to (1) and (2), but cannot typically generate sufficient air flows for (3).

When to use it?

Stack ventilation, can operate when no wind pressure is available. (The absence of wind can occur at certain times, due to its variability, or in certain sites, due to blocking effect of other buildings or vegetation). It can also operate in deep plan buildings where the distance from openings in the perimeter, and the presence of partitions, make wind-driven cross ventilation impractical.

It must be born in mind that the stack effect can only take place when the average temperature in the stack is greater than the outside air. Three distinct situations can be identified – (1) where the stack is formed by the occupied part of the building itself, (2) where the stack exists in the occupied space but where the space is tall (such as in an atrium) and the heated air is well above the heads of the occupants, and (3) as a separate element.

When used for cooling, in case (1) the temperature increment may decrease the thermal comfort of the occupants. This may necessitate large openings to keep the temperature increment to a minimum. In case (2), since there are no occupants in the stack zone itself, the temperature increment may be larger. In case (3), there is no effect on the occupants, and the temperature may be deliberately enhanced by solar gains, as in the so-called solar chimney. It is important to reiterate that it is the average temperature of the whole column of air that matters, so any heat input must be made as close as possible to the base of the chimney.

How to use it?

Key points:

  • Typically the stack effect is quite weak, and therefore openings and ducts must be large, to minimise resistance – think big and tall!
  • The stack effect is dependent upon the difference between the average temperature of the stack over the full stack height, NOT the temperature of the air as it leaves the top of the stack.
  • The pressure difference within the stack varies with height resulting in diminishing air flows from spaces opening on to the stack, as their height above ground floor increases
  • The stack normally provides extraction of the warm air. Equal consideration must be given to the provision of openings and flow routes for the supply of fresh air.
  • Large vertical connections in buildings may conflict with noise control and fire compartmentation requirements.
  • In tall spaces (multi-room height) the temperature of the air may be hotter in the upper zone. This is referred to as stratification. For a given average temperature, this means that there is a cooler zone at the bottom, which is good news if this is the only occupied space. However it means that rooms facing the upper zone may experience unwanted heat gains, as well as reduced stack effect due to their smaller stack height.

Design procedure:

 

Volume flow calculator

Fig 2. Volume flow calculator. (click image to enlarge)

Cooling power graph

Fig 3. Cooling power graph. (click image to enlarge)

This procedure helps you to determine if stack ventilation is a realistic option to meet the requirement for sufficient heat removal to provide thermal comfort. Due to the mathematics of the problem, we have do it iteratively, first calculating the air flow rate for a desired maximum temperature increment, then checking to see if that flow rate and temperature increment has sufficient cooling power to meet the estimated heat gains from occupation and solar, if present. If it is insufficient, we then have to go back to the stack design and change the area of opening, or the height., or, reduce the gains.

 

  • Step 1: Stack configuration: Establish whether your building configuration supports type (1), type (2) or type (3) stacks. Note all 3-dimensional spaces support type (1).

 

Type 1

Type 1. (click image to enlarge)

Type 2

Type 2. (click image to enlarge)

Type 3

Type 3. (click image to enlarge)

 

  • Step 2: Stack height: IF Type (1), for single floor, determine the maximum stack height. This is the height between the centre of the lowest and the highest practicable openings.
    IF Type (2), determine if the stack effect is providing ventilation to the tall space only, or inducing ventilation in the adjacent rooms. If the latter, determine maximum stack height for each floor.
    IF Type (3), determine the maximum feasible stack height.
  • Step 3: Area of openings: For all types, determine a trial area of opening, assuming the total inlet and exit areas are the same, which is the best condition.
  • Step 4: Temperature increment: Establish a target temperature increment. For example, if you wanted to keep internal temperatures down to 29ºC maximum, and your design external temperature is 26ºC, then for a Type (1) stack (i.e. normal floor to ceiling height), where you must assume that the occupants are in the zone of the full temperature increment, you could tolerate an increment of 3ºC.
  • Step 5: Flow calculations: Using the Flow Rate Predictor (fig 2) read off the flow rate for your trial values, and convert to M3/sm2 floor area, for the floor area that the ventilation has to be provided.
  • Step 6: Total heat gains: Calculate the total internal gains for the space. This will consist of gains from the occupants, their equipment (e.g. computers etc), and lighting. It may be difficult to determine the exact values, so you might like to use a rule of thumb. For example, a typical office with low energy florescent lighting and LCD computer screens, would be about 20W/m2.
    Calculate the solar gains to the space. Bright sunlight falling on the window at an angle of incidence not greater than 45o will deliver about 350 W/m2 (of glazing) to the room. Add them to the internal gains, and convert to W/m2 floor area
  • Step 7: Balancing gains and losses: Now use the Cooling Power Predictor (fig 3) to check if the stack induced flow is sufficient.
    If it is not sufficient, first try to reduce the heat gains – shading the glass exposed to the sun, ensuring that lights are off at times of good daylight, specifying low energy equipment.
    If this fails, increase the inlet and exit areas, increase the stack height, or for types (2) and (3) consider allowing higher temperature increment. Test the cooling power again until an acceptable compromise is achieved.
  • Step 8: Ventilation efficiency: This applies to all kinds of ventilation and refers to the need for the airflow to relate spatially to the location of the source of heat (or pollution). When calculating the required air changes, it is usually assumed that there is perfect mixing of the heated (or polluted) air with the fresh air, throughout the whole volume. In reality it can be worse than this or better. Worse when the fresh air “takes a short cut” and by-passes the location of the pollution source, but better, when the pollution is vented locally before it is allowed to mix with the less polluted air in the occupied space.
    This aspect can be greatly informed by simulation studies using computational fluid dynamics (CFD) software, or physical modelling in water tanks, but some benefit can be gained by simply applying common sense to the location of inlets and outlets.
  • Step 9: Control: The thermal buoyancy driving force is not constant in that it depends on the temperature difference. Generally this will be greatest in winter when ventilation is least needed, and vice versa. In order to match supply and demand, controls to the openings must be built in. These could be activated manually, or could be under automatic control, sensing temperature or air quality. Remember, human beings are very good detectors of thermal comfort and air quality, and if they are provided with well-designed accessible and intuitive controls, they can make a good job of controlling their own environment. This principle applies to all ventilation methods.

 

Conflicting strategies

Due to the weakness of the driving pressures generated by thermal buoyancy, openings have to be large and unobstructed. This means that they will readily transmit noise. Noise attenuating techniques, often used in ductwork of mechanical systems, involve labyrinthine pathways, lined with acoustic absorber. This principle can be applied here but has to be on a large scale in order to cause a minimum flow resistance (see noise control case study).

Large openings at ground level create a security risk. If the openings are protected with bars or grids, these will cause obstruction to flow and must be taken into account when sizing the opening.

In larger buildings, the requirement for fire compartmentation will limit the interlinking of spaces and their use of common exhaust ducts and chimneys. However in tall spaces such as atria, generous roof vents under automatic control can double up as smoke vents, and normal ventilation.

Take this further

  • Natural ventilation in non-domestic buildings. CIBSE
  • Natural ventilation in buildings. F Allard and M Santamouris (Eds)
  • Ventilation of buildings. Hazim B Awbi

Case studies


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