AIR QUANTITY REQUIRED TO TRANSFER HEAT IN A FORCED AIR SYSTEM In all four mechanical systems considered, the transfer of heat, either by taking it out of the room or adding heat to a room, air was the medium by which heat is moved. So, what is the proper quantity of air? The physical movement of air must be controlled so that it does not become a nuisance for persons within the space. Constant movement of air across one’s face to the extent it can be felt is uncomfortable. Movement of air within a work space such that it moves papers around is a distraction. Yet too slow movement of air to condition a space and provide the proper amount of heat added or removed, could result in the conditioning air being too cold or too warm, which has an adverse affect on the thermostat, resulting in short-cycling the system.

Remember that one of the main criteria that tells if a system is successful, is that no one notices when the system is on. The space is comfortable. Experience in using systems has resulted in a steady change of volume of air within a space, such that the movement is slow enough to not be noticeable, and at the same time, air temperature is proper that abrupt changes in room temperature do not change. Supply air quantity for comfort conditioning should require that the air be completely changed in a space approximately 8 times in one hour. Since air quantity is measured in units of CUBIC FEET per MINUTE, (CFM) the amount of air for a room can be easily calculated. In larger areas, since large volumes of air are involved, an even distribution with incremental supply points is desired, so the velocity of air movement is not annoying, nor does it create noise due to turbulence. So air quantity, and air velocity must be considered.

A good rule of thumb to considered in the velocity of air delivery to a space, is that velocity no more than 800 feet per minute will not create turbulence. Realize that as soon as air leaves an outlet in the room, its velocity suddenly slows, because the volume of space occupied by the air abruptly increases. Velocity of air in ducts immediately adjacent to habitable space should be limited to 800 FPM. Velocity in ducts once removed from habitable space can be increased to 1000 FPM, and velocity in ducts twice removed can be increased to 1200 FPM. It is important to consider the even distribution of air is essential to reach all areas of a space, so large areas and irregular shaped areas may require multiple outlets. Consider that the movement of air is caused by a fan that creates an uneven pressure from one end of a system of delivery duct; the velocity is rapid through the supply, then velocity is slow through the space, but then the air must increase in velocity as it returns to the suction side of the fan mechanism. Even distribution of air is not a factor of return air from a room..

CALCULATION OF AIR VOLUME IN CFM Consider a room to change the air 8 times in one hour: Say a room is 20 ft. x 15 ft. x 9 ft. What is the volume in cubic feet? 20 x 15 x 9 = 2,700 cubic feet. Eight times that volume = 2,700 x 8 = 21,600 cubic feet in one hour. To find the volume per minute, 21,600 cu.ft. x 1 hour = 360 cu.ft. hour 60 minutes minute In the example plan the quantities of air have been calculated for each room. The next step in a design is to locate register outlets for the best air distribution. Then connect the register outlets with a line that represents air duct. Then determine duct size.

The following duct size chart shows the direction for determining a section of duct that is round in cross section. There are two methods by which air velocity, air quantity, and air pressure in duct is considered. Remember that for the air to move, there must be air pressure inside the duct. The air velocity method involves a quantity of air at a limited velocity. If velocity is controlled, air noise due to turbulence can be eliminated. In the left side, see the column that says “air quantity in CFM,” represented by horizontal lines. Then observe lines in the chart that extend from upper left to lower right. Sloping lines from lower left to upper right represent diameter of round duct. Example: Say one requires 400 CFM air quantity, and limit of velocity to 800 fpm. Find 400 on the left side, follow the horizontal line to the right until it intersects the line that represents 800 fpm. At that point observe the intersection occurs between the duct diameter lines of 9” round and 10” round. A 10” duct would be proper if the air is limited to 800 fpm.

The duct air pressure method is called the ”equal friction” method, which is a measure of limiting the friction loss between the air and the sides of the duct. Always maintain no more pressure than 0.08 inches of water per 100 feet. This is how air pressure in duct is illustrated. Example: Note the vertical lines on the chart extending from the bottom where numbers are shown that represent pressure in “inches of water per 100 feet.” The same 400 CFM, except this time find duct diameter based on a limit of.08 inches of water. Find 400 on the left side and follow the horizontal line to the right until it intersects the vertical line from the bottom at.08. Note again that the intersection occurs between the 9” and 10” round duct. Either method is acceptable, except the control on air noise is not obvious with the equal friction method. If air velocity is limited from the beginning, then air noise is automatically checked.

The next step is to select register and return air grille sizes. In the interest of technical accuracy, do not be guilty of calling an air supply register or grille a vent. It is NOT a vent. A vent is a device that removes unwanted air from a space. Realize that registers are initially placed in order to achieve even distribution of air. Registers are so constructed as to expel air in a pattern, so as to further complete the distribution. Most registers have adjustable fins on the exterior, and registers should be equipped with a damper. A damper enables a technician to balance the overall supply of air in an installation, because duct size alone is not precise enough to provide an exact balance of air quantity.

Ducts are placed in the system to provide a complete circuit for air circulation, and to provide a certain measure of safety during a fire. Building codes prohibit the practice of allowing air from rooms in a building to return through corridors and other rooms because of the potential rapid spread of smoke. A return air grille is not required to have adjustable fins, because air distribution is not a factor in return air. The grille is primarily cosmetic, in that it covers an opening into a duct that, for a finished space may be unsightly. But grilles must have sufficient free area to allow the required volume of air to pass through without causing turbulence that would result in noise.

The last subject in this example plan is how ventilation air is treated. Recall in the heat gain/loss chart, a certain amount of ventilation was involved, which would bring outside “unconditioned” air into the space. This quantity of air must be heated or cooled in order to blend with the total quantity. Ventilation air is exhausted directly to the outside. But ventilation air must be replaced – from the exterior. A duct is installed to draw outside air into the system using the suction side of the return. The unconditioned air is drawn into the return air duct adjacent to the mechanical unit so it goes directly into the system to be heated or cooled before it enters habitable space.

END OF AIR QUANTITY AND DISTRIBUTION AND END OF HVAC SECTION OF ENVIRONMENTAL SYSTEMS NEXT SUBJECT IS PLUMBING