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Office Buildings

Load Characteristics
Office buildings usually include both peripheral and interior zone spaces. The peripheral zone extends from 3 to 3.6 m inward from the outer wall toward the interior of the building and frequently has a large window area. These zones may be extensively

subdivided. Peripheral zones have variable loads because of changing sun position and weather. These zone areas typically require heating in winter. During intermediate seasons, one side of the building may re However, the interior zone spaces usually require a fairly uniform cooling rate throughout the year because their thermal loads are derived almost entirely from lights, office equipment, and people. Interior space conditioning is often by systems that have variable air volume control for low- or no-load conditions.

Most office buildings are occupied from approximately 8:00 A.M. to 6:00 P.M.; many are occupied by some personnel from as early as 5:30 A.M. to as late as 7:00 P.M. Some tenants’ operations may require night work schedules, usually not to extend beyond 10:00 P.M. Office buildings may contain printing plants, communications operations, broadcasting studios, and computing centers, which could operate 24 h per day. Therefore, for economical airconditioning design, the intended uses of an office building must be well established before design development.

Occupancy varies considerably. In accounting or other sections where clerical work is done, the maximum density is approximately one person per 7 m2 of floor area. Where there are private offices, the density may be as little as one person per 19 m2. The most serious cases, however, are the occasional waiting rooms, conference rooms, or directors’ rooms where occupancy may be as high as one person per 2 m2. The lighting load in an office building constitutes a significant part of the total heat load. Lighting and normal equipment electrical loads average from 10 to 50 W/m2 but may be considerably higher, depending on the type of lighting and the amount of equipment. Buildings with computer systems and other electronic equipment can have electrical loads as high as 50 to 110 W/m2. An accurate appraisal should be made of the amount, size, and type of computer equipment anticipated for the life of the building to size the airhandling equipment properly and provide for future installation of air-conditioning apparatus.

About 30% of the total lighting heat output from recessed fixtures can be withdrawn by exhaust or return air and, therefore, will not enter into space-conditioning supply air requirements. By connecting a duct to each fixture, the most balanced air system can be provided. However, this method is expensive, so the suspended ceiling is often used as a return air plenum with the air drawn from the space to above the suspended ceiling.
Miscellaneous allowances (for fan heat, duct heat pickup, duct leakage, and safety factors) should not exceed 12% of the total load. Building shape and orientation are often determined by the building site, but certain variations in these factors can produce increases of 10 to 15% in the refrigeration load. Shape and orientation should therefore be carefully analyzed in the early design ges.

Design Concepts
The variety of functions and range of design criteria applicable to office buildings have allowed the use of almost every available airconditioning system. While multistory structures are discussed here, the principles and criteria are similar for all sizes and shapes of office buildings. Attention to detail is extremely important, especially in modular buildings. Each piece of equipment, duct and pipe connections, and the like may be duplicated hundreds of times. Thus, seemingly minor design variations may substantially affect construction and operating costs. In initial design, each component must be analyzed not only as an entity, but also as part of an integrated system. This systems design approach is essential for achieving optimum results. There are several classes of office buildings, determined by the type of financing required and the tenants who will occupy the building. Design evaluation may vary considerably based on specific tenant requirements; it is not enough to consider typical floor patterns only. Included in many larger office buildings are stores, restaurants, recreational facilities, data centers, telecommunication centers, radio and television studios, and observation decks. Built-in system flexibility is essential for office building design. Business office procedures are constantly being revised, and basic building services should be able to meet changing tenant needs.

The type of occupancy may have an important bearing on the selection of the air distribution system. For buildings with one owner or lessee, operations may be defined clearly enough that a system can be designed without the degree of flexibility needed for a less well-defined operation. However, owner-occupied buildings may require considerable design flexibility because the owner will pay for all alterations. The speculative builder can generally charge alterations to tenants. When different tenants occupy different floors, or even parts of the same floor, the degree of design and operation complexity increases to ensure proper environmental comfort conditions to any tenant, group of tenants, or all tenants at once. This problem is more acute if tenants have seasonal and variable overtime schedules. Stores, banks, restaurants, and entertainment facilities may have hours of occupancy or design criteria that differ substantially from those of office buildings; therefore, they should have their own air distribution systems and, in some cases, their own heating and/or refrigeration equipment.

Main entrances and lobbies are sometimes served by a separate system because they buffer the outside atmosphere and the building interior. Some engineers prefer to have a lobby summer temperature 2 to 3.5 K above office temperature to reduce operating cost and thetemperature shock to people entering or leaving the building.
The unique temperature and humidity requirements of data processing installations and the fact that they often run 24 h per day for extended periods generally warrant separate refrigeration and air distribution systems. Separate backup systems may be required for data processing areas in case the main building HVAC system fails.

The degree of air filtration required should be determined. The service cost and the effect of air resistance on energy costs should be analyzed for various types of filters. Initial filter cost and air pollution characteristics also need to be considered. Activated charcoal filters for odor control and reduction of outdoor air requirements are another option to consider. Providing office buildings with continuous 100% outdoor air is seldom justified; therefore, most office buildings are designed to minimize outdoor air use, except during economizer operation. However, attention to indoor air quality may dictate higher levels of ventilation air. In addition, the minimum volume of outdoor air should be maintained in variable volume air-handling systems. Drybulb or enthalpy controlled economizer cycles should be considered for reducing energy costs.
When an economizer cycle is used, systems should be zoned so that energy waste will not occur due to heating of outside air. This is often accomplished by a separate air distribution system for the interior and each major exterior zone. High-rise office buildings have traditionally used perimeter dual-duct, induction, or fan-coil systems. Where fan-coil or induction systems have been installed at the perimeter, separate all-air systems have generally been used for the interior. More recently, variable air volume systems, including modulated air diffusers and self-contained perimeter unit systems, have also been used. If variable air volume systems serve the interior, perimeters are usually served by variable volume or dual-duct systems supplemented with fan-powered terminals, terminals with reheat coils, or radiation (ceiling panels or baseboard). The perimeter systems can be hydronic or electric.
Many office buildings without an economizer cycle have a bypass multizone unit installed on each floor or several floors with a heating coil in each exterior zone duct. Variable air volume variations of the bypass multizone and other floor-by-floor, all-air systems are also being used. These systems are popular due to low fan power, low initial cost, and energy savings resulting from independent operating schedules, which are possible between floors occupied by tenants with different operating hours.
Perimeter radiation or infrared systems with conventional, single- duct, low-velocity air conditioning that furnishes air from packaged air-conditioning units or multizone units may be more economical for small office buildings. The need for a perimeter system, which is a function of exterior glass percentage, external wall thermal value, and climate severity, should be carefully analyzed. A perimeter heating system separate from the cooling system is preferable, since air distribution devices can then be selected for a specific duty rather than as a compromise between heating and cooling performance. The higher cost of additional air-handling or fancoil units and ductwork may lead the designer to a less expensive
option, such as fan-powered terminal units with heating coils serving perimeter zones in lieu of a separate heating system. Radiant ceiling panels for the perimeter zones are another option. Interior space usage usually requires that interior air-conditioning systems permit modification to handle all load situations. Variable air volume systems are often used. When using these systems, a careful evaluation of low-load conditions should be made to determine whether adequate air movement and outdoor air can be provided at the proposed supply air temperature without overcooling. Increases in supply air temperature tend to nullify energy savings in fan power, which are characteristic of variable air volume systems. Low-temperature air distribution for additional savings in transport energy is seeing increased use, especially when coupled with an ice In small to medium-sized office buildings, air-source heat pumps may be chosen. In larger buildings, internal source heat pump systems (water-to-water) are feasible with most types of air-conditioning systems. Heat removed from core areas is rejected to either a cooling tower or perimeter circuits. The internal source heat pump can be supplemented by a central heating system or electrical coils on extremely cold days or over extended periods of limited occupancy. Removed excess heat may also be stored in hot water tanks. Many heat recovery or internal source heat pump systems exhaust air from conditioned spaces through lighting fixtures. Approximately 30% of lighting heat can be removed in this manner. One design advantage is a reduction in required air quantities. In addition, lamp life is extended by operation in a much cooler ambient environment. Suspended ceiling return air plenums eliminate sheet metal return air ductwork to reduce floor-to-floor height requirements. However, suspended ceiling plenums may increase the difficulty of proper air balancing throughout the building. Problems often connected with suspended ceiling return plenums are as follows: • Air leakage through cracks, with resulting smudges • Tendency of return air openings nearest to a shaft opening or collector duct to pull too much air, thus creating uneven air motion and possible noise • Noise transmission between office spaces Air leakage can be minimized by proper workmanship. To overcome drawing too much air, return air ducts can be run in the suspended ceiling pathway from the shaft, often in a simple radial
pattern. The ends of the ducts can be left open or dampered. Generous sizing of return air grilles and passages lowers the percentage of circuit resistance attributable to the return air path. This bolsters effectiveness of supply air balancing devices and reduces the significance
of air leakage and drawing too much air. Structural blockage can be solved by locating openings in beams or partitions with fire dampers, where required.

Spatial Requirements Total office building electromechanical space requirements vary tremendously based on types of systems planned; however, the average is approximately 8 to 10% of the gross area. Clear height required for fan rooms varies from approximately 3 to 5.5 m, depending on the distribution system and equipment complexity. On office floors, perimeter fan-coil or induction units require approximately 1 to 3% of the floor area. Interior air shafts and pipe chases require approximately 3 to 5% of the floor area. Therefore, ducts, pipes, and equipment require approximately 4 to 8% of each floor’s gross area. Where large central units supply multiple floors, shaft space requirements depend on the number of fan rooms. In such cases, one mechanical equipment room usually furnishes air requirements for 8 to 20 floors (above and below for intermediate levels), with an average of 12 floors. The more floors served, the larger the duct shafts and equipment required. This results in higher fan room heights and greater equipment size and mass.
The fewer floors served by an equipment room, the more equipment rooms will be required to serve the building. This axiom allows greater flexibility in serving changing floor or tenant requirements.
Often, one mechanical equipment room per floor and complete elimination of vertical shafts requires no more total floor area than a few larger mechanical equipment rooms, especially when there are many small rooms and they are the same height as typical floors. Equipment can also be smaller, although maintenance costs will be higher. Energy costs may be reduced, with more equipment rooms serving fewer areas, because the equipment can be shut off in unoccupied areas, and high-pressure ductwork will not be required. Equipment rooms on upper levels generally cost more to install because of rigging and transportation logistics.
In all cases, mechanical equipment rooms must be thermally and acoustically isolated from office areas. Cooling Towers. Cooling towers are the largest single piece of equipment required for air-conditioning systems. Cooling towers require approximately 1 m2 of floor area per 400 m2 of total building area and are from 4 to 12 m high. When towers are located on the roof, the building structure must be capable of supporting the cooling tower and dunnage, full water load (approximately 590 to 730 kg/m2), and seismic and wind load stresses.
Where cooling tower noise may affect neighboring buildings, towers should be designed to include sound traps or other suitable noise baffles. This may affect tower space, mass of the units, and motor power. Slightly oversizing cooling towers can reduce noise and power consumption due to lower speeds, but this may increase
the initial cost. Cooling towers are sometimes enclosed in a decorative screen for aesthetic reasons; therefore, calculations should ascertain that the screen has sufficient free area for the tower to obtain its required air quantity and to prevent recirculation.
If the tower is placed in a rooftop well or near a wall, or split into several towers at various locations, design becomes more complicated, and initial and operating costs increase substantially. Also, towers should not be split and placed on different levels because hydraulic problems increase. Finally, the cooling tower should be built high enough above the roof so that the bottom of the tower and the roof can be maintained properly.
Special Considerations
Office building areas with special ventilation and cooling include elevator machine rooms, electrical and telephone closets, electrical switchgear, plumbing rooms, refrigeration rooms, and mechanical equipment rooms. The high heat loads in some of these rooms may require air-conditioning units for spot cooling. In larger buildings having intermediate elevator, mechanical, and electrical machine rooms, it is desirable to have these rooms on the
same level or possibly on two levels. This may simplify the horizontal ductwork, piping, and conduit distribution systems and permit more effective ventilation and maintenance of these equipment rooms. An air-conditioning system cannot prevent occupants at the perimeter from feeling direct sunlight. Venetian blinds and drapes are often provided but seldom used. External shading devices (screens, overhangs, etc.) or reflective glass are preferable. Tall buildings in cold climates experience severe stack effect. The extra amount of heat provided by the air-conditioning system in attempts to overcome this problem can be substantial.

The following features help combat infiltration due to the stack effect

: • Revolving doors or vestibules at exterior entrances

• Pressurized lobbies or lower floors
• Tight gaskets on stairwell doors leading to the roof

• Automatic dampers on elevator shaft vents
• Tight construction of the exterior skin
• Tight closure and seals on all dampers opening to the exterior




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