HVACR416 - Design Air Distribution 1

23.1 Air Properties and Behavior Important properties of interest include: o Weight of air. o Manner in which air absorbs heat. o Way in which air separates into layers.

Weight Gases that compose air have definite mass and air has a definite weight. The weight of air varies based on temperature and relative humidity conditions. One pound (0.454 kg) of dry air at 70°F (21°C) and standard atmospheric pressure will occupy a space of ft 3 (0.383 m 3 ) 2

Weight

Weight Air is a gas and therefore, obeys Boyle’s law and Charle’s law. As temperature of air rises, it takes more cubic feet to weigh one pound. As pressure drops, it takes more cubic feet to weigh one pound. As relative humidity increases, it takes more cubic feet to weigh one pound. Each water molecule weighs less than each nitrogen or oxygen molecule

Heat in Air Air is a physical substance and can carry heat. The specific heat of dry air is 0.24 Btu/lb °F (1.004 kJ/kg °C). Additional heat due to moisture in the air varies considerably depending on the amount of saturation. Only sensible heat needs to be considered when distribution of air is concerned. Condensing of water should not occur in ducts or in the room being conditioned. 23

Stratification Warm air tends to rise, cold air tends to settle. If air is not deliberately moved, it will assume levels according to its temperature. This is referred to as stratification. Air in occupied spaces must be kept moving to avoid stratification. Thermostats and humidstats must be placed at the proper level due to stratification. Stratification makes smoke haze hover in layers that are difficult to remove. 23.

Stratification If grilles are improperly located, air will move only in certain parts of a room and becomes stagnant in others. Furnishings may obstruct air movement. Grilles should be located 6' high on walls or located in ceilings.

Stratification A diffusion grille promotes the mixing of some room air with the entering conditioned air. 6, 7 & 8” Multivent

23.1.3

Stratification The mixing principle of a ceiling grille.

Air Circulation In warm air heating, three basic systems are used to circulate air: o Gravity: No longer popular; too much energy is lost prior to the air heating the room. o Intermittent forced air: Most commonly used, a thermostat in the furnace plenum chamber is used to control a fan. o Continuous forced air: Becoming more popular, provides a more constant temperature in rooms. 2

Air Circulation Systems that provide cooling as well as heating require additional capacity to move air. Warmed air leaves the duct many degrees warmer than room air it is replacing. Cooled air is 15 to 20 Degrees cooler than room air. Cooled air has lower relative humidity. (People have difficulty getting cool when relative humidity is high.) More airflow is required to cool. In average conditions, increased airflow is provided by: o Use of a two-speed blower motor (for directly driven blowers). o Installation of a two-speed pulley on the motor (if a belt-driven unit).

Room Air Movement Air entering the conditioned space through ducts must circulate without annoying drafts. Primary air is air delivered to the room at velocity of 150 ft. per minute or more. Primary air pushes against, and mixes with, room air. The distance from the grille that air travels prior to slowing down to 50 ft. per minute (terminal velocity) is called throw. Outlet velocity is the speed of air as it leaves the grille. The total area of air openings in the grille determines its capacity. Spread of air leaving grille very important. Locate return air grilles where the room air is slowest..

Return Air Ducts Airflow through return ducts is almost always from the “pulling” action of the fan or blower. If return airflow does not match airflow into a room, airflow may not be properly balanced. More return air than entering air results in negative room pressure. More entering air is needed. Other rooms may starve for air. During heating season, rooms starved for air will be too cold. Return air grilles should be placed in the stratified air zone of a room. In heating season, along floor. During cooling season, near ceiling. Return air grilles should be placed as far as possible from inlet grilles

Basic Ventilation Requirements Air is a mixture of gases and normally contains about 21% oxygen. A certain oxygen content is required to maintain life. In a tightly sealed room, oxygen is slowly consumed and replaced by carbon dioxide, water vapor, and impurities. This may cause drowsiness or death. Fresh air must be admitted into living spaces to provide oxygen. 23.3

Basic Ventilation Requirements Historically, fresh air entered by infiltration (leakage) through door and window openings and cracks in a structure. However, modern construction reduces air leakage. Therefore, the air conditioning apparatus must furnish fresh air. Modern units have a controlled fresh-air intake. This air is conditioned and mixed with recirculated air prior to entering the room. 23.3

Basic Ventilation Requirements Any kind of exhaust fan removes conditioned air. Replacement fresh air should be brought in through an air system. Certain areas of a building should have slightly less positive pressure than the rest of the building. This lower positive pressure (10% to 15% less than rest of building) reduces the spread of odors. 23.3

Basic Ventilation Requirements To calculate fresh air requirements and air changes per hour, consider the following factors: o Number of occupants. o Use of space. o Dry bulb temperatures. o Relative humidity. o Amount of fresh air admitted by infiltration. o Efficiency of the unit. 23.3

Basic Ventilation Requirements Basic rule for cooling is to provide at least 15 cfm of air per person. o Six people occupying a 10,000 ft 3 space require 90 cfm of fresh air (6 X 15 cfm = 90 cfm). o At this flow rate, it would take 1.85 hours (10,000 ft 3 / 90 cfm = 111 minutes) to completely replace the air in the space. 23.3

Basic Ventilation Requirements Air handling can produce positive or negative pressure in the building. o Positive pressure is higher than atmospheric pressure. o Negative pressure is below atmospheric pressure. Homes using fuel-burning furnaces need air for combustion. Combustion air leaving the chimney may create a slightly negative pressure inside the house. 23.3

Basic Ventilation Requirements 23.3

Basic Ventilation Requirements Ventilation is usually based on the number of air changes per hour that occur for the conditioned space. In a 1000 ft 3 space, three changes per hour mean that 3000 ft 3 are changed every hour, or 50 cfm. Three changes every hour is the minimum for residences during the heating season.

Basic Ventilation Requirements 23.3 Typical air changes for both heating and cooling seasons.

Basic Ventilation Requirements It is good practice to keep air blowers running at all times to provide good ventilation to all parts of a building. Variable speed blowers are sometimes used to provide more air movement when the heating or cooling system is running. Evaporation, convection, radiation, and respiration control body comfort. o The temperature of walls, floors, and ceilings must be controlled. o Enough air must be provided for good respiration, evaporation, and convection. If specific conditions are unknown, design for 2 cfm for every square foot or 12 changes/hr. 23.3

Basic Ventilation Requirements Local and state codes must be considered in all installations and servicing. Local codes may be more stringent than state codes.

Attic Ventilation Attic ventilation is important because the air within the attic affects the conditions of the structure. An unventilated attic may reach 150°F (66°C) on a hot summer day. Louvers or vents may be used to allow fresh air to enter the attic and internal attic air to escape. Buildings may use exhaust fans to remove extremely hot air that collects in attic spaces. Attic fans may bring cool evening air into a building

Basement Ventilation Basements tend to be cool and damp in the summer, resulting in mold and odors. An exhaust fan will reduce the dampness and help prevent mold growth. A fan installed to remove basement air should take air from the floor level and exhaust it outdoors