Indoor Air Quality

Ventilation is the process of changing the air in an enclosed space.

Purpose:

• To prevent the oxygen content of the ambient air from decreasing,
• To prevent excessive increase in carbon dioxide gas, body odors, cigarette smoke, and moisture content in the ambient air,
• To remove heat gain from machines, people, and lighting in the environment.,
• To remove moisture gain from machines, cooking, and people in the environment.,
• To remove toxic gases and dust from the environment,
• To reduce the number of bacteria and harmful microorganisms.

on one or more of some basic criteria such as people’s need for clean air, keeping the concentration levels of certain pollutants below limit values, pressure control, and temperature control. In the past, energy costs have been an important determining parameter in the design of the system.

Nowadays, besides energy costs, indoor air quality has become a second determining parameter, often conflicting with the first principle. Different standards may give different results for different applications. Turkish Standards are not decisive in this regard as they are not deep and detailed enough. The subject will be approached within the framework of ASHRAE standards, which are taken as a basis, and different data will be given together.

Depending on the forces that create ventilation, it is possible to divide ventilation into natural ventilation and mechanical ventilation. In natural ventilation, a building is ventilated in a controlled manner using natural forces. In mechanical ventilation, fan power is used. In return for energy consumption, air is forced into the spaces.

Unfortunately, when these natural forces are removed, natural ventilation stops. Therefore, sometimes mechanical ventilation is necessary. Today’s buildings generally depend entirely on fan-powered mechanical ventilation. However, energy saving, indoor air quality, and the concept of sustainability that has become prominent in recent years are forcing the development of new systems and solutions. Within this framework, new trends have emerged in building technology. Natural ventilation is of great importance in buildings that comply with this new approach. According to this new approach, mechanical systems should only be used when natural systems are inadequate. Intensive research and development studies are being conducted on these issues all over the world.

Wind Pressure

The wind pressure calculation method acting on any surface of a building at a certain height is given in ASHRAE. For wind coming from a certain angle on a wall surface, the wind pressure can be calculated as follows:

Pw=Cp.Pv

The wind pressure acts perpendicular to the surface. Pv can be determined by the Bernoulli equation:

Pv=ra.UH/2

UH wind speed. Cp is a coefficient that depends on local wind direction, building geometry, and land features. The exact values ​​of this coefficient can only be determined by model experiments. However, ASHRAE has an approach to determining this coefficient. Cp is found with the following expression:

Cp(in-out)=Cs-Ci

In this formula, Ci represents the internal pressure coefficient. If the air entering through the openings goes out through other openings on the same floor, then it can be taken as Ci = -0.2. Ci values ​​are always negative, meaning there is a draft inside.

Pressure Effect Due to Thermal Forces (Chimney Effect)

The chimney effect occurs when the air densities inside and outside the building are different. On cold winter days, the column of outside air, which is heavier than the warm air inside, exerts an inward pressure on the lower floors. As a result of this pressure difference, outside air enters from the lower floors in winter and rises up through the building. In summer, when the interior of the building is cooled, the opposite situation occurs. Air entering from the upper floors moves downwards. Due to the chimney effect, there is a level in the vertical direction of the building where the internal and external pressures are equal. This level is called the neutral pressure level. If the openings are evenly distributed throughout the building, the neutral plane is at the center height of the building from the ground. If there is a large vertical shaft in the middle of the building, such as an open atrium, the neutral level is pulled towards the top of the building and the air currents and infiltration patterns in the building become significantly different. The pressure difference resulting from the chimney effect can be expressed as follows:

Pth= (ro-ri).g.(H-NHPL)= ri.g.(H-HNPL).(Ti-To)/To

Here, the subscripts i and o indicate inside and outside, respectively. HNPL is the neutral axis height and H is the height of the floor under consideration. Ú and T are air density and temperature. There is resistance to the upward vertical movement of air in conventional buildings due to the chimney effect. Therefore, the actual pressure difference created by thermal forces can be calculated with the following expression:

PT=Cd.Pth

Cd is the chimney draft coefficient and its experimentally determined value in modern buildings is between 0.63 and 0.82.

Total Pressure Difference

The total pressure difference between the inside and outside of any floor is found by the algebraic sum of the pressure differences created by wind and thermal forces. That is:

Ps=Pw+PT

Calculation of Ventilation Air Volume

The ventilation air flow rate is found by equating the total pressure difference to the pressure drops (resistance to flow) in the openings and simple duct systems. Accordingly;

Ps=r.V2.R/2

The resistance created against the flow should include all local and friction losses and the resistance coefficient R should be calculated specifically for each building. This equation can be expressed in terms of air volume by taking the A1 opening dimensions into the equation instead of the air speed V. The amount of airflow in an opening and the associated duct system as a result of natural forces in a certain direction can be found in the following expression:

Here the subscript shows the angle between the wind direction and the wall surface. When the incoming air flow rate is multiplied by time, the air volume that enters in a certain period of time is found as quantity. The total amount of air entering from all directions in a certain period of time will give the total ventilation amount.

A study conducted in England has shown that in these climate conditions, there is a possibility of saving between 5% and 40% of cooling energy, depending on the building type, through night cooling. Both natural ventilation and natural cooling contribute to the protection of the environment and natural resources, as well as saving electrical energy.

Mechanical (Forced) Ventilation

In mechanical ventilation, fans are used for air exchange and movement. There are three systems in mechanical ventilation:

Natural air intake, mechanical air suction
Mechanical air supply, natural air release
Mechanical air supply and mechanical air suction, as referred to balanced ventilation

The mechanical system is not dependent on conditions as in natural ventilation. Continuous air movement is provided by force. However, this has a facility and operating cost. It also carries the risk of noise from the fan and duct system.

The air fed into living spaces by a fan must first be heated to room temperature in winter. At the same time, the fresh air taken from outside must be filtered and purified from dust and foreign substances. Conditioning the fresh air taken from outside is possible in mechanical ventilation. However, in natural ventilation, this is not yet practically possible due to the difficulties created by pressure losses. In balanced ventilation, where an additional exhaust fan is used to expel excess air in addition to the supply fan, it is possible to control the air volume and pressure better inside. Thus, it is possible to control the air movements inside the building by creating pressure differences between the spaces. This is an indispensable opportunity, especially in cleanroom applications. There are various energy recovery possibilities in mechanical ventilation.

For example, by using recuperators in the system, it is possible to transfer the energy taken from the exhausted air to the fresh air taken in.

Indoor Air Quality

Indoor air quality is related to the cleanliness of the air breathed into living spaces. Clean air can be defined as air that does not contain any known pollutants above the harmful concentration levels determined by the competent authorities and that 80% or more of the people breathing this air do not feel any dissatisfaction with the quality of the air. In recent years, concerns have been increasingly developing regarding the cleanliness of indoor air in non-industrial environments such as residences, workplaces, and schools. It can be easily predicted that people spend 90% of their time in indoor spaces and that the density of people in indoor spaces will be higher and that there will be problems arising from this. Again, in studies conducted in recent years, concepts such as sick building syndrome have emerged and diseases resulting from pollution in interior spaces have been diagnosed. Studies on the subject have increased in parallel, scientific articles have been published, scientific meetings have been held and new standards with sanctioning power have emerged. Of these standards, ASHRAE 62-89 covers the subject in the most comprehensive manner. The rules of this standard and its conflict with, for example, energy-saving principles have been one of the most discussed issues.

Sick Building Syndrome

“Sick Building Syndrome” (SBS) is the name given to health and comfort complaints of occupants in a building that has no apparent cause for illness, simply related to the time they spend in the building. Complainants may be located in a specific room or zone within the building, or they may be scattered throughout the building. Another concept related to the subject is the concept of “Building Related Disease” (BRD). In this case, the causes of diseases diagnosed in the building are clear and are caused by the building’s ventilation system.

Sick Building Syndrome Indicators

Building residents suddenly start complaining about disturbances.
These complaints may include headache, eye, nose, or throat discomfort, cough, dry or itchy skin, dizziness, nausea, concentration problems and hypersensitivity to odors
The source of these disease symptoms could not be identified.
Most complainants reported that they felt relieved immediately after leaving the building.

Building Related Disease Indicators

The majority of building residents complain of cough, chest tightness, fever, chills, and muscle pain.
The reasons for these findings are completely clinically explainable.
Even if complainants leave the building, it takes a while for them to recover.

Reasons for Sick Building Syndrome That Deteriorates Indoor Air Quality

It is possible to define harmful substances that impair indoor air quality and create pollution only by collecting them under various groups.
It is possible to list the pollutant groups that impair indoor air quality as follows:

• Carbon dioxide rate in the inhaled air (respiration of humans and living things, combustion-induced)
• Odor (human-induced)
• Microorganisms (environmental and human-induced)
• Humidity (from the environment and human activities such as cooking)
• Radon gas (soil source)
• Organic vapors (sourced from used items and building elements)
• Dust (sourced from the environment and used items)
• Allergenic substances and living things (sourced from the environment)
• Cigarette smoke (human-induced)

Other sources (there are many other factors that affect air quality other than those listed above. These include many possibilities, from electronic pollution to radiation).

Internally Sourced Chemical Pollutants

The source of polluted air in a building is often inside this building. For example: adhesives, coatings and floorings, some wooden products, copying machines, pesticides, and volatile organic compounds including formaldehyde emitted from cleaning materials found and used in the building are among the factors that cause air pollution. Cigarette smoke is a factor that plays a major role in the formation of highly volatile organic compounds, other toxic compounds, and respirable particles.

Research has shown that breathing high concentrations of certain volatile organic compounds, also known as carcinogens, can cause chronic and acute health problems. Many low to moderate levels of volatile organic compounds can also cause acute reactions.

Chemical Pollutants from External Sources

The air that a building takes in as fresh air may be air discharged from other buildings in the vicinity. Incorrect placement of intake vents, windows, and openings in buildings can easily cause exhaust gases from motor vehicles and buildings (from bathrooms and kitchens) and gases escaping from installations to enter the building. In addition, various combustion products can enter the building from nearby garages.

Biological Pollutants

Bacteria, mold, pollen, and viruses are some of the most common biological contaminants. These contaminants can grow and thrive in stagnant water that collects in drains, humidifiers, or drain pans, or in water that collects in roofs, floors, or insulation. Sometimes insect or bird droppings can also cause biological contamination. Cough, chest tightness, high fever, chills, muscle pain, stomach irritation, and upper respiratory tract congestion, allergic reactions are among the illnesses caused by biological contaminants. Legionella bacteria is also known to cause Legionnaires’ disease and fever.

Radon and Asbestos

While sick building syndrome and building-related diseases can cause acute or moderate health problems, radon and asbestos show their harmful effects long after they are ingested. These two substances should be considered in detail in a multi-faceted assessment of a building’s indoor air quality. These elements can be effective together, or they can increase the damage of other elements in conditions of insufficient temperature, humidity, or light.

Methods for Improving Indoor Air Quality

There are certain methods for improving indoor air quality. First of all, pollution sources need to be controlled and reduced. For example, banning cigarette smoking and not using materials that emit harmful gases, such as carpets, in indoor spaces can be counted among these measures.

The principle of capturing harmful substances at their source and discharging them before they mix with the environment is a principle widely used in areas such as industrial ventilation and kitchen ventilation. In such areas, the sources of pollutants are specific.

Filtering and cleaning indoor air

This method cannot be used successfully due to the large number and variety of pollutants. However, it is a developing sector. Especially in many regions where it is not possible to describe the outside air as clean, the only effective method is cleaning.

Ventilation is still the most widely used and effective method for ensuring indoor air quality. By providing sufficient amounts of fresh air to indoor spaces, indoor air quality can be brought to a satisfactory level.

Elimination or Change of Contaminant Source

This method is quite effective in solving indoor air quality problems when the source of pollution is known and can be controlled. Filters should be cleaned or replaced periodically. Other measures that can be considered on the subject include changing the building’s steel ceiling, isolating smoking rooms, placing the source of the pollutant so that it receives air from outside, storing paints, adhesives, solvents, and pesticides in well-ventilated areas, and using these harmful substances when the building’s occupants are not in the building. After the maintenance of the building is completed, the building should not be entered for a certain period of time to allow the effects of the toxic substances to wear off.

Increasing Ventilation Rate

Increasing ventilation rates and air distribution to reduce pollution levels in a building is often a very costly process. However, ventilation is the key parameter for ensuring indoor air quality. The design of the ventilation systems of the buildings should be made in a way that meets the local building standards. Ventilation above the standards can be foreseen when necessary, using the initiative. In cases where the source of high pollutants in the building is very strong, the local exhaust system is very important for the removal of polluted air. In rooms such as restrooms, photocopy rooms, and printing rooms where polluted air is concentrated in certain areas, the local exhaust system can be used, even partially.

Air Cleaning

Although air cleaning can be used as an additional method in source control, its application area is quite limited. Devices used for particle control, such as furnace filters, are inexpensive but inadequate in retaining small particles. High-capacity air filters that can be used to capture very small particles are quite expensive in terms of installation and operation. Mechanical filters are inadequate in retaining gas phase contaminants. Such gas phase contaminants can be removed using adsorbent holders, but these devices are expensive and require frequent filter replacement.

Mech. Eng. Yavuz İşman
GES Teknik
Magazine Tesisat Market 05.2009-Number 124