Ventilation design involves managing how air enters and circulates through a project. This is done through openings such as windows, facade perforations, doors, solar chimneys, and wind towers.
Flow is driven by a breeze or a difference in temperature between indoor and outdoor air, or by buoyancy. The best ventilation designs take advantage of these forces.
Inlet and Exhaust Openings
Natural ventilation uses prevailing winds and thermal buoyancy forces due to indoor and outdoor air density differences to drive natural circulation within a space. This can occur through purpose-built openings such as windows, doors and solar chimneys, or through naturally occurring nooks and crannies in building envelopes. The rate at which natural ventilation occurs depends on climate, building design and human behaviour.
Ventilation rates for general spaces are generally determined by ASHRAE standards such as ANSI 62.1, for non-residential spaces and ASHRAE 62.2, for residential spaces. These standard publications determine the ventilation rate per occupant, with the goal of achieving acceptable contamination levels in the occupied space. However, a more accurate and more cost-effective method of determining the ventilation requirements for spaces is to use a room contamination factor that considers both occupant-based and area-based contamination factors.
In health-care facilities, dilution ventilation — where fresh air is circulated to general spaces to dilute infectious droplet nuclei — has long been considered one of the most effective ways to prevent the spread of infection. However, it requires that climatic conditions be favourable for natural ventilation to work, and that the inlet and exhaust openings are kept open.
Mechanical ventilation systems include fans for supplying air into, or exhausting air from, rooms. These can be mounted in walls and ceilings, or through ducting. Ventilation fans can be powered by electrical motors, or by natural energy sources such as wind, sun or heat. They may be controlled automatically or manually, and are used to control contaminants in spaces where a higher contamination level is required such as cleanrooms, hospital rooms and laboratory facilities.
Marine through-hull openings should have an inlet for cool air to enter the engine room, and a discharge outlet for hot exhaust air on each side of the hull. The inlet and discharge openings must be sized to maintain the air velocity through them below 610 m/min (2000 ft/min).
Ventilation air exhaust fans should be mounted or ducted at the highest point in the engine room, directly over the heat source. This ensures that a high percentage of the engine heat is dissipated with the exhaust. In engine applications with multiple engines, the cooling air inlets should be designed to circulate between the engines for adequate cooling.
Cross Ventilation
The orientation of inlet and outlet openings is crucial to maximizing cross ventilation. Accepted design avoids putting inlet and exhaust windows directly across from each other, as this blocks the flow of air between them. For the best results, the distance between inlet and outlet windows should be at least twice as far as the width of the window frame, so that the wind can blow freely through the space.
Ventilation can be influenced by a wide range of factors, including the size of the building and internal thermal loads, the location of ventilation openings (typically windows), the wind direction at different times of the day and year, and the configuration of the openings. In addition, ventilation rates are impacted by the occupants’ activities and the types of pollutants they produce. The ASHRAE space-by-space ventilation rate recommendations are based on these factors, as well as occupants’ comfort and energy costs.
While the effectiveness of cross ventilation is largely dependent on the weather and geographic conditions, some specialized equipment can increase its performance. For example, specialty wall louvers exploit the natural progression of wind and significantly increase airflow through space without the need for electricity.
Stack ventilation is another effective method of cross cooling, especially for taller buildings with central atriums. This process involves drawing cool outdoor air into the building at a lower level, where it will lose its temperature compartmentalization and stale moisture content as it rises and is ventilated out at a higher level.
In addition to the benefits of cross ventilation, mechanical cooling can be used as a supplement to natural ventilation. This combination allows for a more uniform distribution of temperatures throughout the building, reducing the need for cooling equipment in occupied spaces.
Natural ventilation is a cost-effective and environmentally friendly way to cool down buildings, and it’s an excellent alternative to mechanical cooling. However, it’s important to remember that it doesn’t work unless the exterior air is colder than the interior, so it can be difficult to achieve a comfortable indoor climate in some climates. Additionally, it may take some time for the ventilation to effectively cool a space.
Vertical Ventilation
When a fire spreads quickly, vertical ventilation can make it easier for firefighters to enter and work in the burning structure. This is a very important part of firefighting as it improves interior tenability for victims and firefighters, assists in search and rescue operations and helps fire control operations (forcible entry and access to concealed spaces).
Vertical ventilation involves opening existing or creating roof cuts and channeling heat and smoke upward to escape the building through these openings. It is normally performed with the use of fans to enhance the flow of air and speed up ventilation. Ideally the venting is done in the direction of the prevailing wind as this will allow fresh air to replace the air that is being expelled from the building. This will also prevent a churning effect that can occur if the flow of air makes many turns in a building.
It can be extremely difficult to create effective vertical ventilation in many types of buildings. This is particularly true in attic and balloon construction fires where it is often too dangerous to vent above the seat of the fire. However, this is still an important operation to consider for early fire control as it can help stop horizontal flame propagation and save the rest of the building.
When performing a vertical ventilation operation the inlet and outlet openings must be positioned properly. The inlets should be as close to the seat of the fire as possible and should be positioned on the opposite side of the structure from the exhaust openings. It is important that the openings are not too close to one another as this can reduce the efficiency of the system. The door to uninvolved rooms must be controlled during a vertical ventilation to ensure that occupants do not accidentally open the door and enter the fire compartment.
The use of fans can also greatly enhance the efficiency of vertical ventilation. This can be done by placing the fan in front of an inlet opening or directly on top of the roof where the prevailing wind is blowing. This is a common way of implementing positive pressure ventilation and should be used where appropriate to help with fire attack, particularly in high rise buildings.
Ventilation Distribution
The airflow distribution produced by natural ventilation depends on the size and placement of inlet and exhaust openings. The physics of wind, buoyancy caused by temperature and humidity differences and thermal inertia cause dense, evaporatively cooled air to rise, leaving the building through openings on the windward side and roof and entering via lower openings in the wall. This effect, known as the stack effect, provides effective cooling in buildings with high humidity levels or warm temperatures. The flow of air into and out of a space is also influenced by the shape and position of the building, with sloping walls and ceilings increasing the velocity of airflow (see Figure below).
Aesthetic considerations, security or cultural criteria may dictate that windows remain closed, which reduces ventilation rates. The use of energy-consuming equipment that uses room air for combustion, leaky duct systems or other external factors can also limit the performance of a natural ventilation system.
Ventilation rates are often higher for hospital rooms in older buildings than in newer ones, but the actual ventilation rates have been difficult to determine as climatic conditions are rarely consistent enough to be analyzed. Furthermore, the ventilation rates in many studies are often based on measurement of carbon dioxide in air, rather than measured directly in the patient room.
Natural ventilation can work well in hospitals, particularly if occupants open windows or doors frequently, which increases the ventilation rate and mixes the contaminant-free fresh air with the contaminated patient air. However, if there are specific contaminants such as metal fumes or dusts that require high-efficiency filters to be controlled, it is more effective to use mechanical ventilation.
In cases where natural ventilation is insufficient to control infection, exhaust fans (with adequate pre-testing and planning) can be used to increase the flow rate of a room for a period of time. This type of hybrid or mixed-mode ventilation is generally considered to be the best option for maintaining high flow rates in rooms occupied by patients with airborne infections. The OSH Answers document Indoor Air Quality – General outlines the types of ventilation systems that can be employed in hospitals.