FNBO Technology Center
1: First National Bank of Omaha Technology Center, South Exposure
Building Design Features
Air Distribution System Characteristics
Technology Center houses the corporate offices and check processing
center for the First National Bank of Omaha. The building is occupied for 24/7.
Under normal operations, it is expected that 500 people will
National Bank of Omaha
18,580 m2 (200,000 ft2)
61 m by 114 m (200 ft by 375 ft) footprint
Completed in September 1999.
views of the technology center are shown in Figures 1 and 2. The
building facade is constructed of glass curtain walls on the south end commons
areas and offices, and is of precast concrete panel construction with six-foot
high strip windows and granite panel veneer on the north two-thirds of the
building. Approximately 45% of the building exterior skin is glass curtain wall
overall U-value = 0.26 W/m2.K (0.046 Btu/hr.ft2.°F).
Window overall U-value = 1.64 W/m2.K (0.29 Btu/hr.ft2.°F)
with a 0.43 shading coefficient.
2: Technology Center - Western Exposure
underfloor air distribution (UFAD) system serves 3,340 m2 (36,000 ft2)
of office space and check processing equipment on the middle level and 2,790 m2
(30,000 ft2) of office space and computer environment on the top
level, for a total of 33% of the floor areas. The
remaining occupied spaces (67%) are served by traditional overhead VAV systems.
spaces are primarily open plan using modular walls and furniture except at the
perimeter where private offices are located. Lighting is provided by overhead
recessed ceiling lighting fixtures. Underfloor
electrical consists of hard conduit to distribution boxes from which flexible
whips emanate to serve workstations.
Underfloor Air Distribution (UFAD) System Characteristics
considered during the design of this state of the art facility included
integration of diverse building functions, critical HVAC requirements for
certain areas, and responsiveness to ‘green’ building design practices and
energy consciousness. These design
issues were to be achieved in conjunction with, and not at the expense of, the
functional demands of the Technology Center. The
overall goal of this design was to produce an integrated, straightforward, and
efficient mechanical system that optimizes the performance of the people and
building it serves. Key design
parameters were interior reconfiguration flexibility and avoidance of equipment
and power outages by providing redundancy in mechanical and electric power
m (18 in.) in all areas except computer and card processing areas which
are 0.61 m (24 in.).
areas and cubicles: Constant volume swirl, Titus TAF-R and Titus passive
linear bar grilles (aisles).
Private offices: Constant volume swirl diffusers plus active fan-powered
diffusers, Tate-TAM with linear bar grille diffusers.
m (24 in.) Tate supplied concrete-core panels using a stringer-less post
17°C (63°F), varies with load
UFAD systems are constant air volume-variable temperature (CAV-VT),
however volume can be adjusted manually at the AHU as needed.
systems - Conditioned air is supplied through fourteen modular packaged
air-handling units (AHU’s) located within the top-level mechanical penthouse
area and a ground level mechanical room. Constant
volume, variable temperature systems are used to serve the computer and
equipment intensive areas of the building. Variable volume systems with hot water reheat are provided to serve the
more standard office areas throughout the building. The forced air systems maintain space and overall building
pressurization to control airflow and contaminate movement within the building.
cooling and heating for the Technology Center is provided by chilled water and
steam purchased from Energy Systems Company (ESC). The
chilled water system is sized to provide approximately 800 tons of cooling
capacity to the building. In addition, 250 tons of back-up cooling capacity is
available via on-site chillers to serve critical cooling requirements in the
event of a failure of the primary cooling source.
heating, medium pressure steam is piped directly to integral face and bypass
preheat coils serving the AHU’s, and to heat exchangers serving hot water
heating and humidification systems. The
hot water heat exchanger serves a hot water loop that supplies the perimeter
heating terminal units and hydronic radiant ceiling panels. The
heating system is sized to provide approximately 1,052 MW (3,600 MBtuh) of
heating capacity to the building.
– The basic configuration typical of these systems is shown in Figure 3. There
are seven UFAD pressurized underfloor plenums zoned to accommodate individual
space requirements and different exterior exposures. Walls between adjacent functional areas partition the plenums between
one another. Supply air is ducted
to multiple plenum inlets from five separate air handling units through the
return air space above the ceiling. This was necessary because of the remote
location of the AHUs relative to the underfloor plenums. Air
is supplied to the underfloor plenum at multiple locations to facilitate even
air distribution. Ducting through
the UFAD plenum was ruled out to reduce obstructions and congestion for the
cabling system. Reheat coils in
the branch ducts allow individual temperature control of each underfloor
plenum. Although the underfloor
supply air plenums are operated as CAV-VT systems, each individual plenum
system is capable of delivering varying amounts of air via a variable speed
drive on the AHU. Return air flows
freely through the ceiling return air plenum and then is ducted back to the air
delivered from the pressurized underfloor plenums into the space through three
types of diffusers. In computer environments and equipment spaces typical
perforated raised access floor panels are used. Approximately half of the panels have adjustable dampers to accommodate
airflow balancing. In open/common
areas with carpeted access flooring heavy-duty rectangular floor registers are
provided, as shown in Figure 4. Small
round floor diffusers are used to serve individual cubical workspaces, as shown
in Figure 5.
4: Typical Common Area
5: Typical Cubicle
6: Typical Perimeter Office, showing
fan powered rectangular grille, and passive floor diffuser.
majority of spaces served by the underfloor air distribution systems are
‘open’ plan areas. However,
the enclosed private offices at the perimeter require special attention due to
varying loads. Conference rooms
and perimeter offices are equipped with fan powered rectangular diffusers
located on the interior side of the room in addition to the passive (no local
fan) round floor diffusers (Figure 6). In the underfloor areas typical ceiling
return air grilles are used. Hydronic radiant ceiling panels provide heating in perimeter
areas. Fan powered diffusers are
also provided at high heat producing equipment (large production laser
printers) to deliver ‘spot cooling’ from the underfloor supply plenum.
UFAD System Performance
individual AHU supply air temperature is controlled by a voting procedure based
on individual zone cooling calls (zone temperature greater than set point);
i.e., SAT is reset over a specified range (typically 59-62°F (15-16.7°C)
based on number of calls for cooling. Reheat
coils for each plenum provide variable temperature control in response to space
temperature sensors located in the interior of the zone (some are located at
the ceiling level). The east and
west office area zones are controlled on an exposure basis and encompass both
the interior and perimeter portions of the zone. AHU
supply volume is controlled by duct static pressure in a manner similar to
traditional VAV systems. Space
pressurization is maintained by controlling the return fans to maintain an
offset from supply volume.
offices and conference rooms accommodate variations in load by allowing
occupants to manually control a floor fan and adjust a passive floor diffuser. The
fans are controlled by a wall switch and may be activated to deliver increased
cooling and ventilation air as needed due to interior load or solar gain. A hydronic radiant ceiling panel provides heat.
humidity levels are controlled and adjustable at each AHU primarily to meet the
needs of the computer or paper processing equipment served. The
hot water distribution system is designed to provide a variable supply water
temperature to the floor reheat coils and radiant ceiling panels based on a
signal from a solar compensated outside air sensor. This allows the supply temperature to be reset to efficiently match the
building heating load.
building mechanical systems are monitored and controlled by an integrated
building automation system (BAS). This
computerized system operates over a dedicated network using distributed
controllers and sensors. The BAS
provides scheduling, trend logging, and energy management capabilities along
with system and maintenance alarms. This
system can also connect to other control systems such as security and fire
protection to monitor status and receive alarms. A PC-based operators workstation is located in the facilities management
it will take another cooling season and some final “wringing out” to fully
determine how well the system is performing, indications are that overall
performance is good. Facility management (FM) personnel prefer the simplicity of
the UFAD system and the fact that it has fewer active mechanical components to
service than the overhead VAV systems.
comments reflect the views of CBE researchers based on a limited study of the
building and do not necessarily represent those of the designers and/or
diffusers – During commissioning it was discovered that many interior
diffusers were located too close to occupants. However, it was considered to be too much effort to move the panels
because of the necessity of working with glued down carpet tiles. Therefore,
most diffusers located in cubicles were either shutoff or reduced to about 20%
flow. Leakage from electrical
floor plates in many cases was enough to compensate for the reduced diffuser
flow, along with aisle diffusers that were left at full flow. Reducing
the overall airflow may ameliorate the low stratification that was found
during initial operation. Furthermore, it is not clear how the use of linear
diffusers in the aisles impacts room air distribution. These diffusers generally have lower induction rates than swirl
diffusers and the effect of this on stratification performance is not well
adjustment – The FMs do not encourage occupants to adjust diffusers.
However, since there are multiple shifts some diffusers are adjusted
frequently due to individual differences. The diffusers can get stuck due to dirt, which makes adjustment
difficult. In other cases the
diffuser flow plates were too loose usually due to the baskets having been
systems – It is too early to tell how well this configuration works;
there are no automatic temperature controls in the perimeter offices since the
original design did not include perimeter offices.
During the first cooling season many perimeter offices were too hot
during the peak solar gain part of the day. In some zones the return air was found to be restricted, thus
reducing the airflow from the small fan-driven diffusers. The
FM reports that few occupants actively switch the fans off and on so most are
left on at all times. However,
energy performance could be significantly impacted by this strategy due to
small fans running continuously and from simultaneous heating and cooling in
the perimeter zones. It is
estimated that these offices require from 117.5 to 136.3 l/s (250 to 290 cfm)
cooling air for about 9.3 m2 (100 ft2) offices during
peak solar conditions. This is
somewhat greater than expected based on previous experience and results of
laboratory experiments. Locating
the fan-powered diffusers on the inside wall and operating them at high volume
may result in a high degree of mixing that might compromise the stratification
extensive use of ductwork in this design seems be a conservative solution,
at least for the plenum supply. This
extensive ductwork reduces the fan energy savings potential for UFAD
systems. CBE research has
demonstrated that multiple plenum inlets may not be necessary since even
distribution can be achieved in large plenums supplied at one end.
dirt is a maintenance issue, especially in the processing areas where a
significant amount of debris is generated. The
plenum is cleaned routinely every six months along with the fire detectors.
building codes dictated that hard conduit be used in the plenum instead of a
modular wiring system. Flexible
conduit (armored cable) whips are connected to the hard-wired junction boxes
to provide power to floor connection boxes.
Date Reviewed: September 2000 – September
2001 (Case Study, April 2002)