In most commercial buildings, either a direct expansion or a chilled/hot water distribution system is utilized. The cooling equipment is typically located on the roof (as shown) or in an indoor mechanical room.
The air handling units are located either in the same rooftop enclosure, ceiling mounted, or in remote equipment rooms.
Many buildings will have separate airside systems for the perimeter and interior zones. The perimeter system must largely react to the outdoor temperature, exposure, and the solar effect, as well as occupant, ventilation, lighting, and equipment loads. They must provide both cooling and heating as required for occupant comfort or to meet equipment needs.
The interior zones typically only require cooling (except for top floors) and must react to occupant, ventilation, lighting, and equipment loads. If there are no occupants and the lighting and equipment are off, there is no load (except for the ventilation required).
Some systems need no distribution. These include space heaters, room air conditioners, and room heat pumps, or unitary cooling units that are located where heating or cooling is needed.
Air Handling Units
The air handling units typically consist of component sections selected to do the assigned air mixing, cleaning, cooling & dehumidifying, and heating & humidifying functions. There are numerous configurations to meet the needs of the building (i.e. horizontal, vertical, blow-through, draw-through, multi-zone, etc.).
Figure 1 from the ASHRAE HVAC Systems and Equipment Handbook chapter 1 shows a typical draw-through central system that supplies conditioned air to a single zone or to multiple zones. These units include various component sections of mixing dampers and chambers, filters, cooling and heating coils, humidifiers, and air circulating fan(s).
Space requirements and loads fix the quantity and quality of supplied air. Air gains and losses heat by contacting the cooling and heating coils (heat transfer surfaces) and by mixing with air of another condition. Some of this mixing is intentional, as at the outdoor air intake, other mixing is the result of the physical characteristics of a particular component, as when untreated air passes without contacting the fins of a coil (bypass factor).
Cooling may be direct expansion of a refrigerant in the coil or use chilled water. Heating may be electric resistance, steam or hot water. Fans may be of several types, typically centrifugal or axial types and the filters selected to clean the circulating air and dampers to mix return and outdoor air - all to the degree specified for the indoor air quality desired.
Air Handler - Indoor Units
A number of manufacturers offer units ranging from under 700 cfm up to 50,000+ cfm. Unit arrangement flexibility is available in draw-through, blow-through, 2 & 3 deck multi-zone, and stacked units, which can include features of custom design leakage rates, double sloped drain pans, high efficiency filtration options, and sound attenuators. Cabinet construction can include double walls, gasketed access-for-service panels & doors with hinged or removable options, extended coil connections, and flexibility in shipping with easy to assemble shipping sections
Other options may include multiple housed and plenum fans, coil options - face areas, rows, circuiting, fin spacing, materials, fluid types, filtration options, blenders, attenuators, plenums, and multiple base rail heights to meet condensate trapping requirements. The major makers also have ARI certified fan and coil performance.
Air systems use ductwork to distribute heated or cooled air throughout the building. The duct system is a collection of channels that distribute conditioned air to the various rooms. This system can make a big difference in both the cost and the effectiveness of heating and cooling. It also has an important effect on the comfort and health of the occupants.
Ducts can be rectangular, oval or round metallic, fibrous glass, and flexible in configuration. They may also be insulated for sound and thermal purposes.
Air Diffusing Equipment
Air diffusing equipment introduces air into a conditioned space to obtain a desired indoor atmospheric environment. Supply air outlets introduce air to the space while return and exhaust air is removed from a space through return and exhaust inlets. Various manufacturers offer a wide variety of types of diffusing equipment.
LINEAR SLOT OUTLETS
A linear slot outlet is a long narrow air supply device with an aspect ratio generally greater than 10 to 1. Linear outlets can be installed in multiple sections to achieve long, continuous lengths or installed as a discrete length in a modular ceiling. It can consist of a single slot or multiple slots. Linear diffusers are typically designed for supply applications, but they are also commonly used as a return inlet to provide a consistent architectural appearance.
Ceiling diffuser outlets usually have either a radial or directional discharge parallel to the mounting surface. Diffusers with adjustable deflectors that allow the discharge to be directed perpendicular to the mounting surface are available, as are round, square, and rectangular ceiling diffusers. A ceiling diffuser consists of an outer shell, which contains a duct collar, and an internal deflector, which defines the diffuser's performance, including the discharge pattern and direction.
Designers have a number of airside systems from which to choose to best meet their client's needs. For this discussion, these include all-air, air-water, and all-water systems.
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