The Building

9 - 1 1 R e s e a r c h

an attempt to uncover the truth about September 11th 2001

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M.I.T., Rotch Visual Collections

Visual Communications in Building Technology Project

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3.4 HEATING, VENTILATING, AND AIR CONDITIONING SYSTEM (HVAC)

With the definition of spaces and their enclosure, the designer must confront the problem of creating and maintaining a habitable environment. This tempering of the building's environment is the function ot the heating, ventilating and air-conditioning systems, or HVAC for short. A basic premise for the design of these systems in modern highrise office towers is the creating of sealed interior spaces allowing for complete environmental control.

*The design of the HVAC system must consider all sources of potential heating or cooling that may affect the building's environment. These sources will vary with the seasons and the building's usage. Typically, the designer will consider the heat given off by the building's occupants (both as sensible [temperature] and latent [moisture] heat), lighting system and miscellaneous equipment, and the gain or loss of heat through the envelope of the building. Human comfort is a function of the temperature and moisture content of the air and to some extent, the temperature of exposed surfaces. A well designed HVAC system must therefore maintain a habitable environment by controlling both the temperature and moisture content of the air within an acceptable rage. In addition, to avoid odors or a sense of staleness in the air, it is necessary to provide a continual supply of fresh air and to totally exhaust air from washrooms and elevator shafts.

Figure 3.18 PERSPECTIVE OF HVAC SYSTEMS

The HVAC system chosen for 60 State Street is based upon a decentralized concept using small mechanical rooms on each floor of the building (see Figure 3.18 for an overall perspective). This type of system was brought to Boston by I.A. Naman + Associates from Texas, where it had been first developed in the Milam Building, San Antonio, in 1928. [59] This system contrasts with the highly centralized systems usually found in the Northeastern part of the United States.

A schematic diagram of the decentralized system, for a typical office floor, is shown in Figure 3.19. In order to control the environment locally it is necessary to develop control zones, with individual thermostats throughout the office floor. There are two basic types of zones: perimeter zones and core zones. In the case of 60 State Street, the 13 perimeter zones extend approximately 15 feet in from the envelope and are partially defined by the unusual shape of the building. These perimeter zones are highly affected by the external environment and the movement of the sun, requiring heating in the winter and cooling in the summer. The 3 core zones, however, are little affected by the heat flow and radiation through the envelope and therefore require continual cooling throughout the year because of heat generated by the lighting system and the occupants.

Figure 3.19 HVAC SYSTEMS ON TYPICAL OFFICE FLOOR

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1. Diagram of air distribution and return.
2. Fire wall smoke damper.

3. .....4.
3. Air conditioning fan.

5. .....6.
5. Fan coil unit and piping.

7. .....8.
7. Vibration control for air handling and ducts.

9. .....10.
9. Ductwork awaiting assembly.

11. .....12.
11. Variable air volume controller.

13. .....14.
13. Light box diffusers.

15. .....16.
15. Detail of light and sprinkler head.
16. Finished ceiling light and sprinklers.

17. .....18.
17. Interior partitioning.
18. Distribution to perimeter.

19. .....20.
19. Variable air volume unit.
20. Flexible ductwork.

To provide the needed heating and cooling to the perimeter zones, a dual-duct constant-volume system is used. This system requires supply ducts to bring hot and cold air to a mixing box (see Figure 3.19) where they are mixed in appropriate proportions to temper the perimeter zone. This mixed air enters the perimeter zone through a series of slots located over the windows. In the core zones a single-duct variable-volume system is used. Here a single duct of cool air supplies a control box (see Figure 3.19) where the volume is controlled to provide the needed cooling. Both the dual-duct mixing box and the single-duct control box contain sound attenuation systems to minimize acoustic transmission from the building's mechanical equipment.

As the conditioned air is blown into the zones, it is necessary for some air to be removed in order to avoid a pressure buildup. This air is drawn through additional slots in the hung ceiling into what is called the ceiling plenum (see Figure 3.19). The ceiling plenum is the space between the bottom of the structural floor and the suspended ceiling. Many ducts and conduits are located in this space and light fixtures protrude upward into it, but the remaining space is sufficient to serve as a return air plenum. This return air plenum brings the air back to both the decentralized mechanical room for reconditioning and in part to a vertical duct to be exhausted (see Figures 3.18 and 3.19).

An Air Handling Unit (AHU) is located in the mechanical room to condition the air for the hot and cold supply ducts (see Figure 3.19). The mechanical room acts as a mixing plenum for the AHU by both drawing in the return air from the ceiling plenum and fresh air from a vertical supply duct (see Figure 3.19). The fresh air is supplied at a rate of 0.1 CFM/FT2 or about 10% per air change by volume of the office spaces. The mixed air is drawn through an air filter into the fan portion of the AHU and then passed over either heating or cooling coils before entering the respective supply ducts. If excessive moisture is in the air, it is removed through condensation when the air passes over the cooling coils.

The heating and cooling coils are supplied with the necessary hot or chilled water from a circulating vertical riser system (see Figures 3.18 and 3.20) . The hot water is pumped upward from the Service Level (1st Garage Level, see Figures 3.18 and 3.20). The heat for this water is obtained from steam purchased from the local utility -- Boston Edison. The hot water is thus generated indirectly through a heat exchanger (see Figure 3.20). The chilled water is also pumped upward from the service level. It is supplied by three electrical chilling units (see Figure 3.20). As a by-product of the chilling process, condenser water is heated. This heated water (carrying the heat removed from the building) is pumped in a circulating riser system to the roof, where it is cooled in one of three cooling towers (see Figures 3.18 and 3.20).

Figure 3.19 HVAC SYSTEMS ON TYPICAL OFFICE FLOOR (continued)

21.

.....22.

21. Perimeter system.
22. Perimeter diffuser awaiting installation.

23. .....24.
23. Perimeter diffusers in place.
24. Ceiling to cover perimeter system.

25. .....26.
25. Detail of finished ceiling and diffuser.
26. Interior ceiling partition--plenum anticipated.

27. .....28. .....29.
27. Return air intake ducts.
28. Small return air opening.
29. Pneumatic controls for air handling equipment.

30. .....31.
30. Air handling fan.
31. Fan and duct.

32.
32. Compressors for pneumatic controls.

The fresh air supply is drawn in at the 39th level (see Figure 3.18). This air is preconditioned by going through an air filter and then passing over a cooling coil and then a heating coil before being supplied to each floor by a central supply AHU. Before the AHU there is also a low-pressure steam humidifier available if moisture is required to be added to the air. The heating and cooling coils on the 39th level are supplied by the same risers that supply each floor. There is also a fan on the 39th level that drives the exhaust duct. With the appropriate use of heating, cooling and humidification at the 39th floor level or on each floor, it becomes possible to maintain a habitable environment in the many interior spaces that constitute Sixty State Street.