The amount of ventilating air required in a space generally depends on the number of people occupying the space, their activities, the volume of the space, and the duration of the stay of the occupants in that space. According to ASHRAE Standard 62-1989, the air exchange rate should be 15 cfm per person to maintain acceptable indoor air quality in a reception area. In office space and conference rooms, the standard is 20 cfm per person. Improper layout of air-distribution devices may cause an otherwise properly designed HVAC system with adequate capacity to under-produce.
In warm-air heating, ventilating, and air-conditioning, the object is to properly combine temperature, humidity, and air motion in the occupied zone of the room, from the floor to six feet above the floor. Conditioned air introduced into a space must be distributed so that horizontal and vertical temperature variations within the space are minimized.
When fresh air is introduced into a room,
- The air should be evenly spread over the whole area so there are no stagnant pockets of air.
- The air should not strike the occupants directly; this may be perceived as a draft.
The proper quantity of air needed to heat and cool must be delivered to all spaces without creating drafts. A draft is an objectionable air movement in an occupied space. There should be just enough air motion to carry away heat and moisture. Air velocity below approximately 25 fpm in a room is considered still air. Room air movement over 50 fpm is generally considered to be a draft. To avoid both still air and drafts, air should be distributed at a velocity between 25 and 50 fpm.
The type of occupancy, arrangement of the room, acceptable noise level, and degree of activity all have a bearing on the permissible velocity of air in a conditioned space. Locating supply air devices and return air devices in close proximity to each other will cause short cycling. This results when the conditioned air flows directly from the supply device to the return device without the conditioned air getting mixed with the room air.
Over the years, small dust particles work past the filters and get deposited on the inside of ducts. They usually remain there until they’re disturbed by a renovation or an increase in duct air velocity that is caused by a change in the use of the space. The disturbance is usually temporary, but may require additional cleaning.
If a duct made of galvanized (zinc-coated) steel requires painting, first pickle the metal. This means to treat the metal surface with vinegar or some other weak acid so the paint will cover the surface more readily.
Ducts must be protected from the corrosion that can result from condensate forming in them or being carried over into them. Insulating the duct can prevent condensation. Because of the potential for mold to grow on insulation material, ducts should have their insulation material sandwiched between an inner and an outer layer of metal or should be insulated on the exterior only.
Supply Air Outlets
A supply air outlet is an opening in the wall, ceiling, or floor through which the conditioned air is delivered to a room. Outlets can be placed high on the wall or in the ceiling to deliver cool air and are placed low or on the floor to deliver warm air. Residential units can have outlets in ceiling, walls, and floor locations, whereas commercial units tend to use ceiling-installed outlets. To cool a space, supply air is introduced at 15°F to 30°F below the ambient room air temperature and at a duct velocity of 300 to 800 fpm before it reaches the distribution device. To effectively mix this conditioned supply air with the air in the space, a well-designed air-distribution device must perform as follows:
- entrain (integrate) the room air with the colder supply air so that the mixture will not cause discomfort to the occupants
- reduce the air velocity before it reaches the occupied zone and produces drafts
- provide a turbulent, eddying motion within the occupied zone to prevent stagnation of air
- maintain air outlet noise below an objectionable level
The four most common air-distribution outlets are sidewall grilles, slot diffusers, ceiling diffusers, and air-distributing ceilings. These outlets have different physical configurations that control the way they diffuse or disperse the conditioned supply air into the conditioned space and entrain room air into the supply airstream.
Maintenance of Supply Air Outlets
Smudging is a problem with ceiling and straight line diffusers. Dirt particles held in suspension in the room air are subjected to turbulence at the outlet face. Smudging can be expected in areas of high pedestrian traffic, such as lobbies and stores. Anti-smudge rings will help reduce the deposit, but the best solution is to select outlets with lower velocities.
Smudging of the ceiling is a problem frequently encountered with slot-type diffusers. Supply air is generally clean air since it has been filtered. However, room dirt particles are picked up by the supply air and deposited on the ceiling surfaces. The smudging caused by this dirty room air entrained in the supply airstream is more pronounced on highly textured ceilings, in rooms with high dust content, and in areas with high pedestrian traffic.
Return Air Devices
In air-conditioning systems, a large proportion of the air supplied to a room must be returned to the unit. The location of the return air system is as critical to the performance of an air-distribution system as the location of the air supply. It is not necessary to locate a return grille in each space because central returns may be sufficient if the air can move to the return without appreciable draft or resistance. Air should generally move toward the return air grille at a velocity of 15 to 30 fpm. Avoid collecting large quantities of air from a single return because this can cause drafts or noise.
Locating supply air devices and return air devices in close proximity to each other will result in short cycling. This happens when the conditioned air flows directly from the supply device to the return device without being mixed into room air. This prevents complete mixing of supply air with return air. Floor returns, if used, should be reinforced if they are in a heavy traffic area. Be certain that low-wall returns are not located near areas where occupants will be seated, as these low-wall returns may cause drafts.
Types of Return Air Inlets
Return air inlets usually have as large a face area as possible and should be chosen to make as little noise as possible. Maintaining and cleaning these inlets is similar to maintaining and cleaning supply air devices, except that more dirt will be getting picked up from the room and accumulating on the device.
This article is adapted from BOMI International’s Air Handling, Water Treatment, and Plumbing Systems course, part of the SMA and SMT designation programs. More information regarding this course or BOMI International’s new High-Performance Sustainable Buildings credential (BOMI-HP™) is available by calling 1-800-235-2664. Visit BOMI International’s website, www.bomi.org.