NATURAL AND ARTIFICIAL VENTILATION OF BUILDINGS

AR-461: BUILDING SCIENCE

By:

RAVINDAR KUMAR

Assistant Professor

Department of Architecture and Planning

NED University of Engineering and Technology

Karachi

LECTURE NO. 06

TOPIC:                         NATURAL AND ARTIFICIAL VENTILATION OF BUILDINGS

INTRODUCTION:[1]

Ventilation is the process by which fresh air is introduced and ventilated air is removed from an occupied space. The primary aim of ventilation is to preserve the qualities of air. Sometimes, ventilation may also be used to lower the temperature inside an occupied area.

NATURAL VENTILATION

Natural ventilation is the process of supplying and removing air by means of purpose-provided aperture such as open-able windows, ventilators and shafts and the natural forces of wind and temperature-difference pressures. Natural ventilation may be divided into two categories:

Controlled natural ventilation is intentional displacement of air through specified openings such as windows, doors, and ventilations by using natural forces (usually by pressures from wind and/or indoor-outdoor temperature differences). It is usually controlled to some extent by the occupant.

Infiltration is the uncontrolled random flow of air through unintentional openings driven by wind, temperature-difference pressures and/or appliance-induced pressures across the building envelope. In contrast to controlled natural ventilation, infiltration cannot be so controlled and is less desirable than other ventilation strategies, but it is a main source of ventilation in envelope-dominated buildings.

MECHANICAL VENTILATION:

Mechanical or forced ventilation is the process of supplying and removing air by means of mechanical devices, such as fans. It may be arranged to provide supply, extract or balanced ventilation for an occupied space. There are also specialized areas in which ventilation is vital, such as ventilation for industrial processes, mines, tunnels and underground developments. However, in this lecture we will focus only on natural ventilation.

DEFINITION OF NATURAL VENTILATION:[2]

It is the process of supplying and removing air through an indoor space by natural means. There are two types of natural ventilation occurring in buildings: wind driven ventilation and stack ventilation. The majority of buildings employing natural ventilation rely primarily on wind driven ventilation, but stack ventilation has several benefits. The most efficient design for a natural ventilation building should implement both types of ventilation.

PURPOSES OF NATURAL VENTILATION:[3]

Maintaining human comfort and health are two key reasons for providing ventilation in buildings. To achieve these purposes, a ventilation system should be able to meet the following criteria:

• Provide sufficient supply of air/oxygen for the physiological needs of human beings (a minimum of 0.2 l/s/person is required for breathing purpose) and/or livestock;
• Provide sufficient supply of air/oxygen for industrial, agricultural and other processes (for example, provision of oxygen for burning and combustion processes);
• Remove the products of respiration and bodily odour (including those from smoking) of human and/or animal occupants;
• Remove contaminants or harmful chemicals generated by processes or from building materials;
• Remove heat generated by people, lighting and equipment inside the occupied space;
• Create some degree of air movement which is essential for feelings of freshness and comfort (usually a velocity of 0.1 to 0.3 m/s is required).

PROCESS OF NATURAL VENTILATION:[4]

The static pressure of air is the pressure in a free-flowing air stream and is depicted by isobars in weather maps. Differences in static pressure arise from global and microclimate thermal phenomena and create the air flow we call wind. Dynamic pressure is the pressure exerted when the wind comes into contact with an object such as a hill or a building and it is related to the air density and the square of the wind speed.

The impact of wind on a building affects the ventilation and infiltration rates through it and the associated heat losses or heat gains. Wind speed increases with height and is lower towards the ground due to frictional drag. The impact of wind on the building form creates areas of positive pressure on the windward side of a building and negative pressure on the leeward and sides of the building. Thus building shape is crucial in creating the wind pressures that will drive air flow through its apertures. In practical terms wind pressure will vary considerably creating complex air flows and turbulence by its interaction with elements of the natural environment (trees, hills) and urban context (buildings, structures). Vernacular and traditional buildings in different climatic regions rely heavily on natural ventilation for maintaining human comfort conditions in the enclosed spaces.

DESIGN OF NATURAL VENTILATION:[5]

Typical building design relies on rules of thumb for harnessing the power of wind for the purpose of natural ventilation. Design guidelines are offered in building regulations and other related literature and include a variety of recommendations on many specific areas such as:

• Building location and orientation
• Building form and dimensions
• Window typologies and operation
• Other aperture types (doors, chimneys)
• Construction methods and detailing (infiltration)
• External elements (walls, screens)
• Urban planning conditions

Wind driven ventilation has several significant benefits:

• Greater magnitude and effectiveness
• Readily available (natural occurring force)
• Relatively economic implementation
• User friendly (when provisions for control are provided to occupants)

Some of the important limitations of wind driven ventilation:

• Unpredictableness and difficulties in harnessing due to speed and direction variations
• The quality of air it introduces in buildings may be polluted for example due to proximity to an urban or industrial area
• May create strong draughts, discomfort.

WIND DRIVEN VENTILATION:[6]

Wind driven ventilation or roof mounted ventilation design in buildings provides ventilation to occupants using the least amount of resources. Mechanical ventilation drawbacks include the use of equipment that is high in embodied energy and the consumption of energy during operation. By utilising the design of the building, Wind driven ventilation takes advantage of the natural passage of air without the need for high energy consuming equipment. Wind catchers are able to aid wind driven ventilation by directing air in and out of buildings. Wind driven ventilation depends on wind behavior, on the interactions with the building envelope and on openings or other air exchange devices such as inlets or chimneys. The knowledge of the urban climatology i.e. the wind around the buildings is crucial when evaluating the air quality and thermal comfort inside buildings as air and heat exchange depends on the wind pressure on facades. The air exchange depends linearly on the wind speed in the urban place where the architectural project will be built. CFD (Computational Fluid Dynamics) tools and zonal modeling are usually used to calculate pressure. One of these CFD tools, called Urban Wind. Urban Wind makes the link between this pressure and the real urban climatology.

It computes with a macroscopic method the mass flow rate incoming the building for each wind characteristic incidence and velocity magnitude, to finally give cross ventilation statistics according to the wind statistics of the considered urban location. It helps quantifying the natural cross ventilation induced by the wind flow crossing the buildings.

STACK DRIVEN VENTILATION:[7]

Stack effect is temperature induced. When there is a temperature difference between two adjoining volumes of air the warmer air will have lower density and be more buoyant thus will rise above the cold air creating an upward air stream. Forced stack effect in a building takes place in a traditional fire place.

Passive stack ventilators are common in most bathrooms and other type of spaces without direct access to the outdoors. In order for a building to be ventilated adequately via stack effect the inside and outside temperatures must be different so that warmer indoor air rises and escapes the building at higher apertures, while colder, denser air from the exterior enters the building through lower level openings. Stack effect increases with greater temperature difference and increased height between the higher and lower apertures. The neutral plane in a building occurs at the location between the high and low openings at which the internal pressure will be the same as the external pressure (in the absence of wind). Above the neutral plane, the air pressure will be positive and air will rise. Below the neutral plane the air pressure will be negative and external air will be drawn into the space.

Stack driven ventilation has several significant benefits:

• Does not rely on wind
• It can take place still on hot summer days when it is most needed.
• Natural occurring force (hot air rises)
• Stable air flow (compared to wind)
• Greater control in choosing areas of air intake
• Sustainable method

Limitations of stack driven ventilation:

• Lower magnitude compared to wind ventilation
• Relies on temperature differences (inside/outside)
• Design restrictions (height, location of apertures) and may incur extra costs (ventilator stacks, taller spaces)
• The quality of air it introduces in buildings may be polluted for example due to proximity to an urban or industrial area

Natural ventilation in buildings relies mostly in wind pressure differences but stack effect can augment this type of ventilation and partly restore air flow rates during hot, still days. Stack ventilation can be implemented in ways that air inflow in the building does not rely solely on wind direction. In this respect it may provide improved air quality in some types of polluted environments such as cities.

For example air can be drawn through the backside or courtyards of buildings avoiding the direct pollution and noise of the street facade. Wind can augment the stack effect but also reduce its effect depending on its speed, direction and the design of air inlets and outlets. Therefore prevailing winds must be taken into account when designing for stack effect ventilation. Examples of stack effect ventilation can be seen on aluminum smelters, steel mills, and glass plants. Stack effect ventilators have undergone numerous evolutionary steps in recent years to correspond to new safety standards for protection against weather penetration, air hygiene for plant workforce and methodology of construction to reduce total installed costs of Greenfield and Brownfield projects.

NATURAL VENTILATION OF BOILER ROOMS AND INDUSTRIAL BUILDINGS:[8]

Due to high internal heat loads, natural ventilation of boiler rooms, warehouses, and other similar spaces is often employed. Often, conventional or overhead doors are manually opened to provide ventilation. When natural ventilation does not suffice alone, large box fans are often employed to enhance air movement. But to provide security, and cooling-by-ventilation, some buildings have two sets of overhead doors in hot boiler and equipment rooms. The second sets of doors are custom-made grilles with bird screens, similar to the security grilles used by some stores at indoor shopping malls. Some of the custom grilles have solid slats in the lowest section to reduce the amount of trash that might blow into the rooms. During hot weather the grilles help secure the opening while the solid doors are fully open. During cool and cold weather the solid doors can be partially or fully closed.

PRINCIPLES OF NATURAL VENTILATION:[9]

For air to move into and out of a building, a pressure difference between the inside and outside of the building is required. The resistance to flow of air through the building will affect the actual air flow rate. In general, controlled natural ventilation and infiltration are driven by pressure difference across the building envelope. The pressure difference is caused by:

• Wind or wind effect;
• Difference in air density due to temperature difference between indoor and outdoor air stack or chimney effect; or
• Combination of both wind and stack effects.

GUIDELINES FOR NATURAL VENTILATION:[10]

The following guidelines are important for planning and designing natural ventilation systems in buildings:

• A natural ventilation system should be effective regardless of wind direction and there must be adequate ventilation even when the wind does not blow from the prevailing direction;
• Inlet and outlet openings should not be obstructed by nearby objects;
• Windows should be located in opposing pressure zones since this usually will increase ventilation rate;
• A certain vertical distance should be kept between openings for temperature to produce stack effect;
• Openings at the same level and near the ceiling should be avoided since much of the air flow may bypass the occupied zone;
• Architectural elements like wing walls, parapets and overhangs may be used to promote air flow into the building;
• Topography, landscaping, and surrounding buildings should be used to redirect airflow and give maximum exposure to breezes;
• In hot, humid climates, air velocities should be maximized in the occupied zones for bodily cooling;
• To admit wind air flow, the long façade of the building and the door and window openings should be oriented with respect to the prevailing wind direction;
• If possible, window openings should be accessible to and operable by occupants;
• Vertical shafts and open staircases may be used to increase and generate stack effect;
• openings in the vicinity of the neutral pressure level may be reduced since they are less effective for thermally induced ventilation;
• If inlet and outlet openings are of nearly equal areas, balanced and greater ventilation can be obtained.

BARRIERS TO THE APPLICATION OF NATURAL VENTILATION:[11]

A successful application of natural ventilation strategies is only possible when there are no problems in many areas at various levels from the design stage to actual operating demands placed on the building users. These potential barriers include:

• Barriers during building operations
• Safety concerns
• Noise from outdoor
• Dust and air pollution
• Solar shading covering the openings
• Draught prevention
• Knowledge of the users about how to take the best advantage of natural ventilation

• Barriers during building design
• Building and fire regulations
• Need for acoustic protection
• Difficult to predict pattern of use
• Devices for shading, privacy & Daylighting may hamper the free flow of air
• Problems with automatic controls in openings
• lack of suitable, reliable design tools

• Other barriers
• Impact on architectural & envelope design
• Fluctuation of the indoor conditions
• Design a naturally ventilated building requires more work but could reduce mechanical system (design fee on a fixed percentage of system's cost)
• Increase risk for designers
• Lack of suitable standards

REFERENCES:

1.  http://www.arch.hku.hk/teaching/lectures/airvent/sect01.htm (retrieved April 2, 2011)
2. http://en.wikipedia.org/wiki/Natural_ventilation (retrieved April 2, 2011)
3. http://www.arch.hku.hk/teaching/lectures/airvent/sect02.htm (retrieved April 2, 2011)
4. http://www.arch.hku.hk/teaching/lectures/airvent/sect03.htm (retrieved April 2, 2011)

---

[1] http://www.arch.hku.hk/teaching/lectures/airvent/sect01.htm (retrieved April 2, 2011)

[2] http://en.wikipedia.org/wiki/Natural_ventilation (retrieved April 2, 2011)

[3] Ibid 1

[4] Ibid 2

[5] Ibid 2

[6] Ibid 2

[7] Ibid 2

[8] Ibid 2

[9] http://www.arch.hku.hk/teaching/lectures/airvent/sect02.htm (retrieved April 2, 2011)

[10] http://www.arch.hku.hk/teaching/lectures/airvent/sect03.htm (retrieved April 2, 2011)

[11] Ibid

MECHANICAL AND STATIC METHODS OF HUMIDITY AND TEMPERATURE CONTROL

AR-461: BUILDING SCIENCE

By:

RAVINDAR KUMAR

Assistant Professor

Department of Architecture and Planning

NED University of Engineering and Technology

Karachi

LECTURE NO. 05

TOPIC:    MECHANICAL AND STATIC METHODS OF HUMIDITY AND TEMPERATURE CONTROL

INTRODUCTION:

The term mechanical may refer to as using machines instead of manual means of doing some work. The phrase static method may refer to as, a method that does not need an explicit object reference. The mechanical method is the method in which machines are used whereas; the static method is that which is based on the nature or a work done through natural means. Humidity is a term used for the amount of water vapor in air; whereas temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot. Thus; the theme of current lecture is quite explicit that it is a discussion about the natural and manmade methods of controlling the hot and cold air as well as moisture in the air within an internal space or in other words it is the discussion about HVAC systems.

In order to learn the mechanical and static method of humidity and temperature control at first it is important to understand some basic themes or ask some basic questions such as; what is humidity? What is temperature? How humidity and temperature is controlled in an internal space and in buildings of small and large scale? Then; what are the mechanical and static methods of humidity and temperature control? In the following all these questions shall be answered in an amicable and appropriate manner.

WHAT IS HUMIDITY?[1]

Humidity is a term for the amount of water vapor in air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of air and water vapor, and humidity is defined in terms of the water content of this mixture, called the Absolute humidity. In everyday usage, it commonly refers to relative humidity, expressed as a percent in weather forecasts and on household humidistats; it is so called because it measures the current absolute humidity relative to the maximum. Specific humidity is a ratio of the water vapor content of the mixture to the dry air content. The water vapor content of the mixture can be measured either as mass per volume or as a partial pressure, depending on the usage.

In meteorology, humidity indicates the likelihood of precipitation, dew, or fog. High relative humidity reduces the effectiveness of sweating in cooling the body by reducing the rate of evaporation of moisture from the skin. This effect is calculated in a heat index table, used during summer weather.

TYPES OF HUMIDITY:

Absolute humidity (Volume basis)

Absolute humidity on a volume basis is the quantity of water in a particular volume of air. The most common units are grams per cubic meter, although any mass unit and any volume unit could be used. Pounds per cubic foot are common in the U.S. and occasionally even other units mixing the Imperial and metric systems are used.

If all the water vapor in one cubic meter of air were condensed into a container, the mass of the water in the container could be measured with a scale to determine absolute humidity. The amount of water vapor in that cube of air is the absolute humidity of that cubic meter of air. More technically, absolute humidity on a volume basis is the mass of dissolved water vapor, m_w, per cubic meter of total moist air, V_net:

Absolute humidity ranges from 0 grams per cubic meter in dry air to 30 grams per cubic meter (0.03 ounce per cubic foot) when the vapor is saturated at 30 °C.

The absolute humidity changes as air pressure changes. This is very inconvenient for chemical engineering calculations, e.g. for clothes dryers, where temperature can vary considerably. As a result, absolute humidity is generally defined in chemical engineering as mass of water vapor per unit mass of dry air, also known as the mass mixing ratio (see below), which is much more rigorous for heat and mass balance calculations. Mass of water per unit volume as in the equation above would then be defined as volumetric humidity. Because of the potential confusion, British Standard BS 1339 (revised 2002) suggests avoiding the term "absolute humidity". Units should always be carefully checked. Most humidity charts are given in g/kg or kg/kg, but any mass units may be used. The field concerned with the study of physical and thermodynamic properties of gas-vapor mixtures is named Psychrometrics.

Relative humidity

Relative humidity is defined as the ratio of the partial pressure of water vapor (in a gaseous mixture of air and water vapor) to the saturated vapor pressure of water at a given temperature. In other words, relative humidity is the amount of water vapor in the air at a specific temperature compared to the maximum water vapor that the air is able to hold without it condensing, at that given temperature.

Relative humidity is an important metric used in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, a rise in relative humidity also increases the apparent temperature to humans (and other animals) by hindering the evaporation of perspiration from the skin as the relative humidity rises. For example, according to the Heat Index, a relative humidity of 75% at 80°F (27°C) would feel like 83.574°F ±1.3 °F (28.652°C ±1.7 °C) at ~44% relative humidity.

Specific humidity

Specific humidity is the ratio of water vapor to air (including water vapor and dry air) in a particular mass. Specific humidity ratio is expressed as a ratio of kilograms of water vapor, m_w, per kilogram of total moist air m_t.

MEASURING HUMIDITY:

There are various devices used to measure and regulate humidity. A device used to measure humidity is called a psychrometer or hygrometer. A humidistat is used to regulate the humidity of a building with a dehumidifier. These can be analogous to a thermometer and thermostat for temperature control. Humidity is also measured on a global scale using remotely placed satellites. These satellites are able to detect the concentration of water in the troposphere at altitudes between 4 and 12 kilometers. Satellites that can measure water vapor have sensors that are sensitive to infrared radiation. Water vapor specifically absorbs and re-radiates radiation in this spectral band. Satellite water vapor imagery plays an important role in monitoring climate conditions (like the formation of thunderstorms) and in the development of future weather forecasts.

EFFECTS OF HUMIDITY ON HUMAN COMFORT:

Humans control their body temperature mainly by sweating and shivering. The United States Environmental Protection Agency cites the ASHRAE Standard 55-1992, Thermal Environmental Conditions for Human Occupancy, which recommends keeping relative humidity between 30% and 60%. At high humidity, sweating is less effective, and we feel hotter. Air conditioning works by reducing humidity in summer. In winter, heating cold outdoor air can decrease relative humidity levels indoor to below 30%, leading to discomfort such as dry skin and excessive thirst.

EFFECTS OF HUMIDITY ON BUILDING CONSTRUCTION:

Traditional building designs typically had weak insulation, and it allowed air moisture to flow freely between the interior and exterior. The energy-efficient, heavily-sealed architecture introduced in the 20th century also sealed off the movement of moisture, and this has resulted in a secondary problem of condensation forming in and around walls, which encourages the development of mold and mildew. Additionally, buildings with foundations not properly sealed will allow water to flow through the walls due to capillary action, notably cement which is a good conductor of water. Solutions for energy-efficient buildings that avoid condensation are a current topic of architecture.

HUMIDITY CONTROL:[2]

Refrigeration air conditioning equipment usually reduces the humidity of the air processed by the system. The relatively cold (below the dew point) evaporator coil condenses water vapor from the processed air (much like an ice-cold drink will condense water on the outside of a glass), sending the water to a drain and removing water vapor from the cooled space and lowering the relative humidity. Since humans perspire to provide natural cooling by the evaporation of perspiration from the skin, drier air (up to a point) improves the comfort provided. The comfort air conditioner is designed to create a 40% to 60% relative humidity in the occupied space. In food retailing establishments, large open chiller cabinets act as highly effective air dehumidifying units. A specific type of air conditioner that is used only for dehumidifying is called a dehumidifier. A dehumidifier is different from a regular air conditioner in that both the evaporator and condenser coils are placed in the same air path, and the entire unit is placed in the environment that is intended to be conditioned (in this case dehumidified), rather than requiring the condenser coil to be outdoors. Having the condenser coil in the same air path as the evaporator coil produces warm, dehumidified air. The evaporator (cold) coil is placed first in the air path, dehumidifying the air exactly as a regular air conditioner does. The air next passes over the condenser coil re-warming the now dehumidified air. Note that the terms "condenser coil" and "evaporator coil" do not refer to the behavior of water in the air as it passes over each coil; instead they refer to the phases of the refrigeration cycle. Having the condenser coil in the main air path rather than in a separate, outdoor air path (as in a regular air conditioner) results in two consequences—the output air is warm rather than cold, and the unit is able to be placed anywhere in the environment to be conditioned, without a need to have the condenser outdoors.

Unlike a regular air conditioner, a dehumidifier will actually heat a room just as an electric heater that draws the same amount of power (watts) as the dehumidifier. A regular air conditioner transfers energy out of the room by means of the condenser coil, which is outside the room (outdoors). This is a thermodynamic system where the room serves as the system and energy is transferred out of the system. Conversely with a dehumidifier, no energy is transferred out of the thermodynamic system (room) because the air conditioning unit (dehumidifier) is entirely inside the room. Therefore all of the power consumed by the dehumidifier is energy that is input into the thermodynamic system (the room), and remains in the room (as heat). In addition, if the condensed water has been removed from the room, the amount of heat needed to boil that water has been added to the room. This is the inverse of adding water to the room with an evaporative cooler.

Dehumidifiers are commonly used in cold, damp climates to prevent mold growth indoors, especially in basements. They are also sometimes used in hot, humid climates for comfort because they reduce the humidity which causes discomfort (just as a regular air conditioner, but without cooling the room). They are also used to protect sensitive equipment from the adverse effects of excessive humidity in tropical countries. The engineering of physical and thermodynamic properties of gas-vapor mixtures is named Psychrometrics.

WHAT IS TEMPERATURE?[3]

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot. Quantitatively, temperature is measured with thermometers, which may be calibrated to a variety of temperature scales. Much of the world uses the Celsius scale (°C) for most temperature measurements. It has the same incremental scaling as the Kelvin scale used by scientists, but fixes its null point, at 0°C = 273.15K, the freezing point of water. Historically, the Celsius scale was a purely empirical temperature scale defined only by the freezing and boiling points of water. Since the standardization of the Kelvin in the International System of Units, it has subsequently been redefined in terms of the equivalent fixing points on the Kelvin scale. A few countries, most notably the United States, use the Fahrenheit scale for common purposes, a historical scale on which water freezes at 32 °F and boils at 212 °F. Macroscopically, temperature is related to the thermal energy held by matter. An immediate way of sensing this is by touching the material and deciding whether it is hot, warm, or cold. A thermometer precisely measures the temperature and indicates a numerical value for the temperature.

TEMPERATURE CONTROL:[4]

Temperature control is a process in which change of temperature of a space and objects collectively there within is measured or otherwise detected, and the passage of heat energy into or out of the space is adjusted to achieve a desired average temperature.

Control loops:

A home thermostat is an example of a closed control loop: It constantly assesses the current room temperature and controls a heater and/or air conditioner to increase or decrease the temperature according to user-defined setting(s). A simple (low-cost, cheap) thermostat merely switches the heater or air conditioner either on or off, and temporary overshoot and undershoot of the desired average temperature must be expected. A more expensive thermostat varies the amount of heat or cooling provided by the heater or cooler, depending on the difference between the required temperature (the "setpoint") and the actual temperature. This minimizes over/undershoots. The process is called PID and is implemented using a PID Controller.

Energy balance:

An object's or space's temperature increases when heat energy moves into it, increasing the average kinetic energy of its atoms, e.g., of things and air in a room. Heat energy leaving an object or space lowers its temperature. Heat flows from one place to another (always from a higher temperature to a lower one) by one or more of three processes: conduction, convection and radiation. In conduction, energy is passed from one atom to another by direct contact. In convection, heat energy moves by conduction into some movable fluid (such as air or water) and the fluid moves from one place to another, carrying the heat with it. At some point the heat energy in the fluid is usually transferred to some other object by means conduction again. The movement of the fluid can be driven by negative-buoyancy, as when cooler and therefore denser air drops and thus upwardly displaces warmer less-dense air natural convection, or by fans or pumps forced convection. In radiation, the heated atoms make electromagnetic emissions absorbed by remote other atoms, whether nearby or at astronomical distance. For example, the Sun radiates heat as both invisible and visible electromagnetic energy. What we know as "light" is but a narrow region of the electromagnetic spectrum. If, in a place or thing, more energy is received than is lost, its temperature increases. If the amount of energy coming in and going out is exactly the same, the temperature stays constant—there is thermal balance, or thermal equilibrium.

REFERENCES:

1. http://en.wikipedia.org/wiki/Humidity (retrieved April 2, 2011)
2. http://en.wikipedia.org/wiki/Humidity_control#Humidity_control  (retrieved April 2, 2011)
3. http://en.wikipedia.org/wiki/Temperature (retrieved April 2, 2011)
4. http://en.wikipedia.org/wiki/Temperature_control (retrieved April 2, 2011)

VENTILATION

AR-461: BUILDING SCIENCE

By:

RAVINDAR KUMAR

Assistant Professor

Department of Architecture and Planning

NED University of Engineering and Technology

Karachi

LECTURE NO. 04

TOPIC:                                                     VENTILATION

INTRODUCTION:

Ventilating is the process of "changing" or replacing air in any space to provide high indoor air quality (i.e. to control temperature, replenish oxygen, or remove moisture, odors, smoke, heat, dust, airborne bacteria, and carbon dioxide). Ventilation is used to remove unpleasant smells and excessive moisture, introduce outside air, to keep interior building air circulating, and to prevent stagnation of the interior air.

Ventilation includes both the exchange of air to the outside as well as circulation of air within the building. It is one of the most important factors for maintaining acceptable indoor air quality in buildings. Methods for ventilating a building may be divided into mechanical/forced and natural types. [1]

MECHANICAL VENTILATION:

"Mechanical" or "forced" ventilation is used to control indoor air quality. Excess humidity, odors, and contaminants can often be controlled via dilution or replacement with outside air. However, in humid climates much energy is required to remove excess moisture from ventilation air.

Kitchens and bathrooms typically have mechanical exhaust to control odors and sometimes humidity. Factors in the design of such systems include the flow rate (which is a function of the fan speed and exhaust vent size) and noise level. If ducting for the fans traverse unheated space (e.g., an attic), the ducting should be insulated as well to prevent condensation on the ducting. Direct drive fans are available for many applications, and can reduce maintenance needs.

Ceiling fans and table/floor fans circulate air within a room for the purpose of reducing the perceived temperature because of evaporation of perspiration on the skin of the occupants. Because hot air rises, ceiling fans may be used to keep a room warmer in the winter by circulating the warm stratified air from the ceiling to the floor. Ceiling fans do not provide ventilation as defined as the introduction of outside air.

NATURAL VENTILATION:

Natural ventilation is the ventilation of a building with outside air without the use of a fan or other mechanical system. It can be achieved with openable windows or trickle vents when the spaces to ventilate are small and the architecture permits. In more complex systems warm air in the building can be allowed to rise and flow out upper openings to the outside (stack effect) thus forcing cool outside air to be drawn into the building naturally through openings in the lower areas. These systems use very little energy but care must be taken to ensure the occupants' comfort. In warm or humid months, in many climates, maintaining thermal comfort solely via natural ventilation may not be possible so conventional air conditioning systems are used as backups. Air-side economizers perform the same function as natural ventilation, but use mechanical systems' fans, ducts, dampers, and control systems to introduce and distribute cool outdoor air when appropriate.

VENTILATION IN ASHRAE STANDARDS:

Ventilation is the intentional movement of air from outside a building to the inside. Ventilation air, as defined in ASHRAE Standard 62.1[2] and the ASHRAE Handbook, [3] is that air used for providing acceptable indoor air quality. It mustn't be confused with vents or flues; which mean the exhausts of clothes dryers, and combustion equipment such as water heaters, boilers, fireplaces, and wood stoves. The vents or flues carry the products of combustion which have to be expelled from the building in a way which does not cause harm to the occupants of the building. Movement of air between indoor spaces, and not the outside, is called transfer air.

CLOSED VENTILATION:

In commercial, industrial, and institutional (CII) buildings, and modern jet aircraft, return air is often recirculated to the air handling unit. A portion of the supply air is normally exfiltrated through the building envelope or exhausted from the building (e.g., bathroom or kitchen exhaust) and is replaced by outside air introduced into the return air stream. The rate of ventilation air required, most often provided by this mechanically-induced outside air, is often determined from ASHRAE Standard 62.1 for CII buildings, or 62.2 for low-rise residential buildings, or similar standards.

NECESSITY OF VENTILATION IN BUILDINGS:

When people or animals are present in buildings, ventilation air is necessary to dilute odors and limit the concentration of carbon dioxide and airborne pollutants such as dust, smoke and volatile organic compounds (VOCs). Ventilation air is often delivered to spaces by mechanical systems which may also heat, cool, humidify and dehumidify the space. Air movement into buildings can occur due to uncontrolled infiltration of outside air through the building fabric (see stack effect) or the use of deliberate natural ventilation strategies. Advanced air filtration and treatment processes such as scrubbing, can provide ventilation air by cleaning and recirculating a proportion of the air inside a building.

VENTILATION TYPES:

Mechanical or Forced Ventilation: through an air handling unit or direct injection to a space by a fan. A local exhaust fan can enhance infiltration or natural ventilation, thus increasing the ventilation air flow rate.

Natural Ventilation occurs when the air in a space is changed with outdoor air without the use of mechanical systems, such as a fan. Most often natural ventilation is assured through operable windows but it can also be achieved through temperature and pressure differences between spaces. Open windows or vents are not a good choice for ventilating a basement or other below ground structure. Allowing outside air into a cooler below ground space will cause problems with humidity and condensation.

Mixed Mode Ventilation or Hybrid ventilation: utilises both mechanical and natural ventilation processes. The mechanical and natural components may be used in conjunction with each other or separately at different times of day. The natural component, sometimes subject to unpredictable external weather conditions may not always be adequate to ventilate the desired space. The mechanical component is then used to increase the overall ventilation rate so that the desired internal conditions are met. Alternatively the mechanical component may be used as a control measure to regulate the natural ventilation process, for example, to restrict the air change rate during periods of high wind speeds.

Infiltration is separate from ventilation, but is often used to provide ventilation air.

VENTILATION RATE

The ventilation rate, for CII buildings, is normally expressed by the volumetric flowrate of outside air being introduced to the building. The typical units used are cubic feet per minute (CFM) or liters per second (L/s). The ventilation rate can also be expressed on a per person or per unit floor area basis, such as CFM/p or CFM/ft², or as air changes per hour.

For residential buildings, which mostly rely on infiltration for meeting their ventilation needs, the common ventilation rate measure is the number of times the whole interior volume of air is replaced per hour, and is called air changes per hour (I or ACH; units of 1/h). During the winter, ACH may range from 0.50 to 0.41 in a tightly insulated house to 1.11 to 1.47 in a loosely insulated house.[4]

ASHRAE now recommends ventilation rates dependent upon floor area, as a revision to the 62-2001 standard whereas the minimum ACH was 0.35, but no less than 15 CFM/person (7.1 L/s/person). As of 2003, the standards have changed to an addition of 3 CFM/100 sq. ft. (15 l/s/100 sq. m.) to the 7.5 CFM/person (3.5 L/s/person) standard. [5]

VENTILATION STANDARDS

In 1973, in response to the 1973 oil crisis and conservation concerns, ASHRAE Standards 62-73 and 62-81) reduced required ventilation from 10 CFM (4.76 L/S) per person to 5 CFM (2.37 L/S) per person. This was found to be a primary cause of sick building syndrome.

Current ASHRAE standards (Standard 62-89) states that appropriate ventilation guidelines are 20 CFM (9.2 L/s) per person in an office building, and 15 CFM (7.1 L/s) per person for schools. In commercial environments with tobacco smoke, the ventilation rate may range from 25 CFM to 125 CFM.

In certain applications, such as submarines, pressurized aircraft, and spacecraft, ventilation air is also needed to provide oxygen, and to dilute carbon dioxide for survival. Batteries in submarines also discharge hydrogen gas, which must also be ventilated for health and safety. In any pressurized, regulated environment, ventilation is necessary to control any fires that may occur, as the flames may be deprived of oxygen.

ANSI/ASHRAE (Standard 62-89) sets maximum CO₂ guidelines in commercial buildings at 1000 ppm, however, OSHA has set a limit of 5000 ppm over 8 hours.

Ventilation guidelines are based upon the minimum ventilation rate required to maintain acceptable levels of bioeffluents. Carbon dioxide is used as a reference point, as it is the gas of highest emission at a relatively constant value of 0.005 L/s. The mass balance equation is:

Q = G/(C_i − C_a)

• Q = ventilation rate (L/s)
• G = CO₂ generation rate
• C_i = acceptable indoor CO₂ concentration
• C_a = ambient CO₂ concentration^[9]

VENTILATION EQUIPMENT

A Fume Hood Or Fume Cupboard is a type of local ventilation device that is designed to limit exposure to hazardous or noxious fumes, vapors or dusts. A fume hood is typically a large piece of equipment enclosing five sides of a work area, the bottom of which is most commonly located at a standing work height. Two main types exist, ducted and recirculating. The principle is the same for both types: air is drawn in from the front (open) side of the cabinet, and either expelled outside the building or made safe through filtration and fed back into the room. Other related types of local ventilation devices include: clean benches, biosafety cabinets, glove boxes and snorkel exhausts. All these devices address the need to control airborne hazards or irritants that are typically generated or released within the local ventilation device. All local ventilation devices are designed to address one or more of three primary goals:

• protect the user (fume hoods, biosafety cabinets, glove boxes and pictures are now);
• protect the product or experiment (biosafety cabinets, glove boxes);
• protect the environment (recirculating fume hoods, certain biosafety cabinets, and any other type when fitted with appropriate filters in the exhaust airstream).

Secondary functions of these devices may include explosion protection, spill containment, and other functions necessary to the work being done within the device. A general but non-specific term for some of these local ventilation devices is Laminar flow cabinet. This category may include clean benches, biosafety cabinets and other devices characterized simply by the laminar nature of their airflow. The term laminar flow cabinet, however, is insufficient to identify their actual design and use - some will protect the product but not the user, and others will protect both.

Terminologies for local ventilation devices has been, and remain, unclear and non-specific, and the reader is advised to take special care in their selection and specification based upon which of the three primary goals (listed above) are to be met. Fume hoods typically protect only the user, and are most commonly used in laboratories where hazardous or noxious chemicals are released during testing, research, development or teaching. They are also used in industrial applications or other activities where hazardous or noxious vapors, gases or dusts are generated or released.

Because one side (the front) of a fume hood is open to the room occupied by the user, and the air within the fume hood is potentially contaminated, the proper flow of air from the room into the hood is critical to its function. Much of fume hood design and operation is focused on maximizing the proper containment of the air and fumes within the fume hood.

As most fume hoods are designed to connect to exhaust systems that expel the air directly to the exterior of a building, large quantities of energy are required to run fans that exhaust the air, and to heat, cool, filter, control and move the air that will replace the air exhausted. Significant recent efforts in fume hood and ventilation system design have focused on reducing the energy used to operate fume hoods and their supporting ventilation systems.

A biosafety cabinet (BSC), biological safety cabinet, or microbiological safety cabinet is an enclosed, ventilated workspace for safely working with materials contaminated with (or potentially contaminated with) pathogens in the laboratory. Several different types exist, differentiated by the specifics of construction.

ROOM AIR DISTRIBUTION

Characterizing how air is introduced to, flows through, and is removed from spaces is called room air distribution. HVAC airflow in spaces generally can be classified by two different types: mixing (or dilution) and displacement.

MIXING SYSTEMS

Mixing systems generally supply air such that the supply air mixes with the room air so that the mixed air is at the room design temperature and humidity. In cooling mode, the cool supply air, typically around 55 °F (13 °C) (saturated) at design conditions, exits an outlet at high velocity. The high velocity supply air stream causes turbulence causing the room air to mix with the supply air. Because the entire room is near-fully mixed, temperature variations are small while the contaminant concentration is fairly uniform throughout the entire room. Diffusers are normally used as the air outlets to create the high velocity supply air stream. Most often, the air outlets and inlets are placed in the ceiling. Supply diffusers in the ceiling are fed by fan coil units in the ceiling void or by air handling units in a remote plant room. The fan coil or air handling unit take in return air from the ceiling void and mix this with fresh air and cool, or heat it, as required to achieve the room design conditions. This arrangement is known as 'conventional room air distribution'.

OUTLET TYPES

• Group A: In or near ceiling, horizontal discharge
• Group B: In or near floor, vertical non-spreading discharge
• Group C: In or near floor, vertical spreading discharge
• Group D: In or near floor, horizontal discharge
• Group E: In or near ceiling, vertical discharge

DISPLACEMENT VENTILATION

Displacement ventilation systems supply air directly to the occupied zone. The air is supplied at low velocities to cause minimal induction and mixing. This system is used for ventilation and cooling of large high spaces, such as auditoria and atria, where energy may be saved if only the occupied zone is treated rather than trying to control the conditions in the entire space. Displacement room airflow presents an opportunity to improve both the thermal comfort and indoor air quality (IAQ) of the occupied space. It also takes advantage of the difference in air density between an upper contaminated zone and a lower clean zone. Cool air is supplied at low velocity into the lower zone. Convection from heat sources creates vertical air motion into the upper zone where high level return inlets extract the air. In most cases these convection heat sources are also the contamination sources (e.g., people, equipment, or processes), thereby carrying the contaminants up to the upper zone, away from the occupants.

The displacement outlets are usually located at or near the floor with the air supply designed so the air flows smoothly across the floor. Where there is a heat source (such as people, lighting, computers, electrical equipment, etc.) the air will rise, pulling the cool supply air up with it and moving contaminants and heat from the occupied zone to the return or exhaust grilles above. By doing so, the air quality in the occupied zone is generally superior to that achieved with mixing room air distribution. Since the conditioned air is supplied directly into the occupied space, supply air temperatures must be higher than mixing systems (usually above 63 °F or 17 °C) to avoid cold draughts at the floor. By introducing the air at supply air temperatures close to the room temperature and low outlet velocity a high level of thermal comfort can be provided with displacement ventilation.

NATURAL VENTILATION

Natural ventilation involves harnessing naturally available forces to supply and removing air through an enclosed space. There are three types of natural ventilation occurring in buildings: wind driven ventilation, pressure-driven flows, and stack ventilation. [10] The pressures generated by 'the stack effect' rely upon the buoyancy of heated or rising air wind driven ventilation relies upon the force of the prevailing wind to pull and push air through the enclosed space as well as through breaches in the building’s envelope (see Infiltration (HVAC)). Natural ventilation is generally impractical for larger buildings, as they tend to be large, sealed and climate controlled specifically by HVAC systems. [11] Both are examples of passive engineering and have applications in renewable energy.

DEMAND-CONTROLLED VENTILATION (DCV)

DCV makes it possible to maintain proper ventilation and improve air quality while saving energy. ASHRAE has determined that: "It is consistent with the Ventilation rate procedure that Demand Control be permitted for use to reduce the total outdoor air supply during periods of less occupancy. CO₂ sensors will control the amount of ventilation for the actual number of occupants. During design occupancy, a unit with the DCV system will deliver the same amount of outdoor air as a unit using the ventilation-rate procedure. However, DCV can generate substantial energy savings whenever the space is occupied below the design level.

LOCAL EXHAUST VENTILATION

Local exhaust ventilation addresses the issue of avoiding the contamination of indoor air by specific high-emission sources by capturing airborne contaminants before they are spread into the environment. This can include water vapor control, lavatory bioeffluent control, solvent vapors from industrial processes, and dust from wood- and metal-working machinery. Air can be exhausted through pressurized hoods or through the use of fans and pressurizing a specific area. A local exhaust system is composed of 5 basic parts

• A hood that captures the contaminant at its source
• Ducts for transporting the air
• An air-cleaning device that removes/minimizes the contaminant
• A fan that moves the air through the system
• An exhaust stack through which the contaminated air is discharged

VENTILATION AND COMBUSTION

Combustion (e.g., fireplace, gas heater, candle, oil lamp, etc.) consumes oxygen while producing carbon dioxide and other unhealthy gases and smoke, requiring ventilation air. An open chimney promotes infiltration (i.e. natural ventilation) because of the negative pressure change induced by the buoyant, warmer air leaving through the chimney. The warm air is typically replaced by heavier, cold air. Ventilation in a structure is also needed for removing water vapor produced by respiration, burning, and cooking, and for removing odors. If water vapor is permitted to accumulate, it may damage the structure, insulation, or finishes. When operating, an air conditioner usually removes excess moisture from the air. A dehumidifier may also be appropriate for removing airborne moisture.

SMOKING AND VENTILATION

ASHRAE standard 62 states that air removed from an area with environmental tobacco smoke shall not be recirculated into ETS-free air. A space with ETS requires more ventilation to achieve similar perceived air quality to that of a non-smoking environment. The amount of ventilation in an ETS area is equal to the amount of ETS-free area plus the amount V, where:

V = DSD × VA × A/60E; V = recommended extra flow rate in CFM (L/s); DSD = design smoking density (estimated number of cigarettes smoked per hour per unit area); VA = volume of ventilation air per cigarette for the room being designed (ft³/cig]; E = contaminant removal effectiveness

PROBLEMS CONCERNING VENTILATION:

In hot, humid climates, unconditioned ventilation air will deliver approximately one pound of water each day for each cubic foot per minute of outdoor air per day, annual average. This is a great deal of moisture, and it can create serious indoor moisture and mold problems.

• Ventilation efficiency is determined by design and layout, and is dependent upon placement and proximity of diffusers and return air outlets. If they are located closely together, supply air may mix with stale air, decreasing efficiency of the HVAC system, and creating air quality problems.
• System imbalances occur when components of the HVAC system are improperly adjusted or installed, and can create pressure differences (too much circulating air creating a draft or too little circulating air creating stagnancy).
• Cross-contamination occurs when pressure differences arise, forcing potentially contaminated air from one zone to an uncontaminated zone. This often involves undesired odors or VOCs.
• Re-entry of exhaust air occurs when exhaust outlets and fresh air intakes are either too close, or prevailing winds change exhaust patterns, or by infiltration between intake and exhaust air flows.
• Entrainment of contaminated outside air through intake flows will result in indoor air contamination.
• There are a variety of contaminated air sources, ranging from industrial effluent to VOCs put off by nearby construction work.

AIR QUALITY PROCEDURES

Ventilation Rate Procedure is rate based on standard, and “prescribes the rate at which ventilation air must be delivered to a space and various means to condition that air.” Air quality is assessed (through CO₂ measurement) and ventilation rates are mathematically derived using constants.

Indoor Air Quality Procedure “uses one or more guidelines for the specification of acceptable concentrations of certain contaminants in indoor air but does not prescribe ventilation rates or air treatment methods.” This addresses both quantitative and subjective evaluation, and is based on the Ventilation Rate Procedure. It also accounts for potential contaminants that may have no measured limits, or limits are not set (such as formaldehyde off gassing from carpet and furniture).

REFERENCES:

1. Ventilation and Infiltration chapter, Fundamentals volume of the ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA, 2005
2. ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality, ASHRAE, Inc., Atlanta, GA, USA
3. The ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA, USA
4. Kavanaugh, Steve. Infiltration and Ventilation In Residential Structures. February 2004
5. http://epb.lbl.gov/Publications/lbnl-54331.pdf
6. ASHRAE, Ventilation for Acceptable Indoor Air Quality. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc, Atlanta, 2002.
7. Department of the Navy. Navy Safety and Occupational Health Program Manual. 30 May 2007.
8. Apte, Michael G. Associations between indoor CO₂ concentrations and sick building syndrome symptoms in U.S. office buildings: an analysis of the 1994-1996 BASE study data.” Indoor Air, Dec 2000: 246-258.
9. http://www.wapa.gov/es/pubs/techbrf/co2.htm
10. How Natural Ventilation Works by Steven J. Hoff and Jay D. Harmon. Ames, IA: Department of Agricultural and Biosystems Engineering, Iowa State University, November 1994.
11. ASHRAE Handbook of Fundamentals, Chapter 26 by American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE). Atlanta, GA: 2001.
12. http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10631
13. US EPA. Section 2: Factors Affecting Indoor Air Quality. http://www.epa.gov/iaq/largebldgs/pdf_files/sec_2.pdf
14. ASHRAE Standard 62

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