As was outlined in chapter 4, the central fan room and the local floor-by-floor alter- natives fall into the following three discrete alternatives. Before the development of a comparative analysis can be prepared, the details of each of the alternative possibilities must be discussed.
5.1.1 Alternative 1—Central Fan Room
In this alternative, the conditioned supply air for each office floor originates from multiple air-handling systems located in one or more central fan room(s), which are fre- quently identified as central mechanical equipment rooms (MERs). The air-handling
systems can be factory-fabricated air-handling units but usually, due to the multiplicity of floors being conditioned and the resultant air quantities for the systems, they will be field-erected units. Each air-handling system, as a function of the annual temperature and humidity and building code requirements, can be provided with an outside air econ- omizer through minimum and variable outside air dampers. The multiple systems in a given fan room can be interconnected by having supply air delivered into a common discharge header from all of the supply systems in the given fan floor.
Air from the central fan room(s) is distributed to each floor by means of vertical ris- ers located in two-hour fire-rated shafts within the core of the building. At each floor, horizontal duct taps are made into each riser. The horizontal duct tap contains a fire damper (or a fire/smoke damper as required by the local building code) when air exits the rated shaft. In addition, frequently an automatically remotely controlled, two-posi- tion damper is provided. The two-position damper, which, as discussed in chapter 10, “Life Safety Systems,” should be rated as a smoke damper, provides capability for indi- vidual floor overtime operation and for smoke control. It will be controlled with regard to its position (i.e., open or closed) by the building management system.
Return air from each floor’s hung ceiling plenum is drawn through horizontal duct taps into vertical return air shafts for return to the central fan rooms. A second two- position, remotely controlled damper is included in the duct stab that provides the con- nection between the hung ceiling plenum and the return air shaft. This two-position damper, as was the case with the damper in the supply ductwork, is used for individual floor overtime operation and for smoke control. It is also positioned by the building management system.
The return air is not ducted in the shaft, so the air is carried back to the central fan room in the two-hour rated drywall shaft. In each central fan room, multiple return air fans are located that draw the return air from the return air shafts and deliver it to a headered return air duct system within the central fan room.
Where an outside air economizer is included, the return air is either returned to the supply air system or exhausted to atmosphere as a function of the relative enthalpy of the return air and the outside air being provided to the building. The quantity of outside air and the quantity of the return air being utilized by the central air-handling system or being exhausted to the atmosphere is a function of season and the resultant outdoor tem- perature and humidity. In those warmer geographic areas, where the systems operate on minimum outdoor air at all times, the return air is always returned to the supply air sys- tem except for morning start-up or where the fans are under a smoke control operating condition as is discussed in chapter 10.
A typical central fan room arrangement and the supply and the return air shaft arrangements are shown in Figure 5-1. This figure is for a design that includes four sup- ply systems and four return systems, with the ducted supply air being distributed through two shafts above and below the fan apparatus. The return air is being brought to the MER in the same shaft within which the supply duct is routed, but it is not ducted in the shaft. The return air is only ducted within the mechanical equipment room. This arrangement is subject to multiple alternative configurations as a function of the experi- ence, judgment, and analysis of the designing HVAC engineer and, of course, the actual design details of the building.
5.1.2 Alternative 2—Floor-by-Floor Fan Rooms with Chilled Water Units
The air supply for each office floor under this arrangement originates from a local floor fan room, typically located within the building core, that contains a factory-fabri- cated, chilled water air-handling unit complete with a cooling coil, filters, and fan. Morning heating needs at start-up in cold climates can be provided by a heating coil in the air-handling unit or a unit heater installed in the local fan room. The unit on a given floor usually only supplies the floor on which the unit is installed. Typically one unit is installed on each floor except in the case of large floor areas, e.g., those in excess of
Chapter 5—Central Mechanical Equipment Room vs. Floor-by-Floor Fan Rooms | 45
Figure 5-1.
Central fan room arrangement.
approximately 25,000 ft2 (2,400 m2) where more than one unit may be required to be installed. Chilled water for the cooling coil is provided by a central chilled water plant in the building sized to meet the capacity requirements of the project. The supply air fan in the air-conditioning supply system also serves as a return air fan for returning the air from the conditioned area on the floor back to the air-conditioning supply unit. The return air is ducted in the arrangements discussed in this chapter to the fan room, but the return air is not typically ducted within the fan room. Accordingly, the fan room in most cases acts as a return air plenum, and all wire and cable within the room must be ple- num-rated or within conduit.
This system, regardless of geographic location and the outside air temperature and humidity variation during the year, operates on minimum outdoor air during all periods of occupancy.
The outside air for the system is provided by an air-handling unit located on the roof or in a central mechanical equipment room that provides outside air to the unit on each floor by means of a vertical air riser that is routed to each of the multiple local air- conditioning floor-by-floor air-handling units. The unit that provides outside air to the local floor-by-floor air-conditioning units as a function of the outdoor ambient condi- tions will usually include a cooling coil and can include a preheat coil to treat the out- side air being introduced into the building.
With the air-conditioning supply systems operating on minimum outdoor air at all times, the code-mandated economizer requirement can be achieved, in periods of low wet-bulb temperature, through the use of the condenser water from the cooling tower as a means of cooling the chilled water that is supplied to the floor-by-floor air-condition- ing units. This can be done by either of two approaches. The first would be to interpose a flat plate heat exchanger between the chilled water and condenser water and con- denser water circuits. The second would be through injecting the condenser water directly into the chilled water system after passing the condenser water through a bypass sand filter to remove any potential fouling material from the condenser water.
The ability to use this system as a means of smoke control in the event of a fire is discussed in chapter 10.
A typical local fan room arrangement showing the supply and return air arrange- ment and the outside air duct provision is shown in Figure 5-2. The unit heater shown in the figure, as was noted earlier, is to provide morning heat. It can use electric energy or hot water as its heat source. The position of the several dampers that are indicated in the figure would be their operating position during the normal operating day. The posi- tion of the dampers in a smoke condition in the building is detailed in chapter 10. The arrangement shown in Figure 5-2 is frequently altered as a function of the location of the local fan room in the core and the prior experience of the designing HVAC engineer,
but the equipment should be capable of installation in a space of about 250 to 450 ft2 (24
to 42 m2) as a function of the size of the conditioned floor and the precise capacity of
the required unit. Figure 5-2 also shows the valved, capped connection of the chilled water piping to allow its extension to a location on the floor that may require the instal- lation of supplemental air-conditioning equipment, such as information technology equipment or a small area with a concentration of data processing equipment. A full- blown data center could probably not be supported by this piped connection due to the limited volume of water that is available in this location. A major data center could only be accommodated if its needs were provided for in the original design of the piping system and the capacity of the piped system.
As shown in Figure 5-2, the walls around the local floor fan room are not rated. The vertical shaft that contains the outside air duct from the central fan room, and per- haps the smoke exhaust ducts, constitute a rated shaft. Accordingly, fire dampers are only provided at the point where ducts penetrate the shaft wall, not as they leave or enter the local floor fan room itself.
Chapter 5—Central Mechanical Equipment Room vs. Floor-by-Floor Fan Rooms | 47
5.1.3 Alternative 3—Floor-by-Floor Fan Rooms with Direct Expansion Units
A second variation of the floor-by-floor alternative would consist of a floor-by- floor air-conditioning supply system identical in its details to that in Alternative 2, except the package unit installed on the floor is a self-contained water-cooled direct expansion (DX) unit complete with one or more refrigeration compressors and water- cooled condensers. The heat of rejection from the compressor is handled by a con- denser water system and a cooling tower. If an economizer requirement is needed due to lower geographic outdoor air temperature and humidity, it is met by a free-cooling coil that is installed in the package unit that will only operate when the condenser water being delivered to the unit is below a specified temperature of approximately 48°F (9°C). The only central equipment in this alternative is a cooling tower, condenser water pumps, and the central outside air supply unit. This unit is functionally the same as the floor-by-floor air-conditioning supply unit in Alternative 2, except it would be a direct expansion unit, obviating the need for any chilled water plant in the building.
The physical arrangement of the unit would not differ from that shown in Figure 5-2 other than that the chilled water risers shown in Figure 5-2 would be used for con- denser water. One additional difference is that the unit heater will usually be electric because the electrical capacity of the unit heater can be met by the electrical needs of the refrigeration compressors, as they will not both operate simultaneously. The elec- trical design for the project must be provided with a control means that will prohibit the simultaneous operation of the refrigeration compressors and the electric unit heater.
5.1.4 Floor-by-Floor Units Located on an Outside Wall
There is a variation of Alternative 2 and Alternative 3 that has found application in tall commercial office buildings to a sufficient degree that it should be noted. This would be a package floor-by-floor unit solution where the unit is located on an outside wall. In this alternative the unit can be found in a core area where the building core is located on an outside wall. It can also be utilized where there is a central core, with the floor-by-floor unit being installed in a location remote from the core. This arrangement is not common in that exterior space with the usual inclusion of windows is a preferen- tial location for office space and locating the unit on an outside wall will utilize valuable usable space.
Figure 5-2.
Floor-by-floor air- conditioning unit lay- out (normal opera- tion).
If this alternative location of the floor-by-floor unit on an outside wall is utilized, it eliminates the need for the separate outside air unit located in a central fan room, in that outside air can be directly introduced to the floor-by-floor unit through a louver and automatic louver damper for each unit. Moreover, it may allow the use of an air-cooled condenser to handle the heat of rejection in Alternative 3. In locations requiring an economizer, that requirement with this unit location can be satisfied by the inclusion in the design of a minimum and variable air damper behind the outside air louver.
Precautions are necessary in several details of this arrangement. First, where an outside air economizer is used for either Alternative 2 or Alternative 3, care must be taken in the location of the return air spill damper to ensure that mixing of the outside air and spill air does not occur. Similar care must be taken if it is determined, in the case of Alternative 3, to use an air-cooled condenser to allow the outside air to be brought to the air-cooled condenser and to be spilled to atmosphere with no possibility of mixing the heated discharge air with either the condenser intake air or the outside air being brought into the building by the supply air-conditioning unit. This can become a com- plicated alternative arrangement that may necessitate the air-cooled condenser being located at a point remote from the local fan room.