UFAD Texas Style

Sustainable design is becoming a notable movement in the A/E community, as is the desire for LEED certification among more and more clients. Carter & Burgess had the good fortune to work with two such clients—Berkebile Nelson Immenschuh McDowell Architects (BNIM) and the University of Texas—to design the latter's School of Nursing and Student Community Center for its Health Scie...

06/01/2004


Sustainable design is becoming a notable movement in the A/E community, as is the desire for LEED certification among more and more clients. Carter & Burgess had the good fortune to work with two such clients—Berkebile Nelson Immenschuh McDowell Architects (BNIM) and the University of Texas—to design the latter's School of Nursing and Student Community Center for its Health Science Center at Houston (UTHSCH).

Recognizing the vital link between public health and the built environment, the university decided that the facilities themselves would play a pedagogical role by teaching the vital lesson of sustainability. As a result, this facility was designed to be the first LEED-platinum building in Houston.

Flexibility is obviously a major issue when designing a "100-year" building, so plans called for raised floors wherever possible to reduce the future cost of moving and reconfiguring spaces. To achieve this flexibility, it is best to configure the area under the floor as a supply air plenum, which has the added benefit of reducing the cost of HVAC supply ductwork.

The university project team agreed that it would be best to build and occupy a prototype to test these integrated solutions before applying them to a 194,000-sq.-ft. building. The prototype is a 14,000-gross-sq.-ft. tenant-improvement project located on the 14th floor of a privately developed office high-rise, built in the mid-1970s and now owned by UTHSCH.

The energy goal was a 40% reduction in energy consumption below ASHRAE 90.1 levels. Another principle was a "no-fly" zone, whereby perimeter glass was considered to belong to all tenants; offices would not be created along the perimeter, simplifying HVAC zoning.

The building housing the UFAD prototype typically uses two nominal 7,000-cfm, 10-hp dual-duct air-handling units (AHUs) with mixing boxes in the ceiling on each floor. The same goes for the 14th floor. To address the varying loads at the perimeter, variable-volume diffusers provide "exterior zone" control while still using the single supply-air plenum. Round, manual-twist diffusers on the interior supply air from the same common plenum achieve the LEED individual control credit at minimum cost. There is no ductwork and no inefficient fan-powered terminal units under the floor. Heating is accomplished with semi-recessed electric convectors under each window sequenced from the temperature sensor that controls the perimeter VAV diffusers. Hot-water heating was originally envisioned, but was changed to electric due to the low operating hours for the heating systems and the greater flexibility of being able to sequence an individual heater should an office be created on the exterior wall.

Moisture control

Underfloor air distribution (UFAD) systems generally operate at higher supply air temperatures than ducted systems, often at 65°F to 68°F. Unfortunately, in the humid Houston climate, 65°F air is still far too moist to introduce into a building. At 75°F it results in space conditions with a relative humidity in excess of 70%—well outside the ASHRAE 55 conditions—creating a ripe environment for microbiological growth. The prototype building has an existing outside-air pretreatment system, designed to filter and dehumidify outside air down to roughly 55°F before supplying air to the return side of the typical floor AHU. Unfortunately, these units were in poor shape. Under peak summer conditions, leaving-air temperatures from these units could be as high as 65°F.

To address these issues, we implemented three strategies:

  • Small pre-treat fan coil units were installed in each mechanical room to further dehumidify outside air before mixing it on the return side of the unit. The coil selections were based on a leaving-air temperature of 48°F with a chilled-water supply-air temperature of 44°F. We expect these units to eventually be removed when the main units are upgraded.

  • A face-and-bypass arrangement on the cooling coil was implemented for the main AHU. This allowed for further dehumidification in the main AHU as required, yet allowed for the air to be "tempered" back up to 65°F as appropriate for the raised floor without using reheat.

  • A wrap-around "heat-pipe" coil was incorporated around the cooling coil. This arrangement moves sensible heat around the chilled-water coil, lowering the coil leaving-air temperature, improving dehumidification and raising leaving-air temperature by as much as 8°F without any other energy input than friction loss. Thereby, it further increases airflow and dehumidification through the chilled-water coil.

This belt-and-suspenders approach provided us with the flexibility to dehumidify outside air before it is introduced into the space, and further dehumidify it at the main AHU without using reheat energy.

Minimizing intervention

To address the aggressive goal of reducing energy use 40% below ASHRAE 90.1, we were careful to "do the right things in the right order," as energy guru Amory Lovins would say. We started by field-measuring the actual power of lighting and convenience receptacles at electrical panels that were in the most recently constructed spaces and using the newer lighting systems and computers. Lighting energy was measured at 0.6 watts/sq. ft., and power was measured at 0.5 watts/sq. ft. But these numbers are expected to reduce over time, with the widespread application of flat screen computer displays. Moreover, the HVAC system was designed for 1.5-watts/sq. ft. electrical total.

We further agreed that design conditions for the space would be 78°F for cooling and 70°F for heating. While additional capacity was added to the passive elements, such as cooling coils and filter bank areas, every effort was made to minimize overdesign of the active elements where direct-drive fans were selected to optimize performance at the operating conditions. One benefit is that as a retrofit project, a base case is available in the original design.

Comparing it to the current design, we see that we have reduced the design airflow by roughly 25%, which essentially corresponds to the original overdesign (see Table 1). Also, note that the coil face velocity has been lowered by half to minimize fan horsepower. The most significant change due to the use of the raised floor itself is the reduction in supply static pressure from 3-in. w.g. (for the old double-duct system) to the floor plenum designed for 0.1-inch w.g. static pressure. This design decision, coupled with the low face-velocity coil design, results in a 78% reduction in fan horsepower, which in turn, results in further cooling load reduction, totaling 44% at the cooling coil.

That being said, among the numerous disadvantages of executing this scheme as a retrofit is the fact that mechanical room space is severely constrained by existing utility risers surrounding the room, thereby restricting the size of the AHU and restricting the air path out of the mechanical room. Another major issue was that a somewhat smaller AHU—350 fpm face velocity—was ultimately used to facilitate the contractor lifting the sections up the elevator shaft, further limiting the efficiency improvements.

A final disadvantage is the existence of numerous unsealed floor and wall penetrations, all of which represent supply-air leakage paths. UTHSCH's standard specification for conventional duct systems limits duct leakage to 2% with field verification. But UFAD is an entirely different story. When the time came to test and balance (TAB) the new system, the TAB contractor was unable to create more than 0.05 inches w.g. static pressure in the plenum. When the airflows were compared between the traverse of the AHUs against the total measured from the supply grilles, 24% (4,579 cfm) of the air supplied to the plenum was going somewhere other than the grilles it was designed to exit.

There were multiple major areas of leakage: electrical boxes installed in the floor tiles; the holes cut through the raised floor under the partitions used to route power up into the walls; and between the cracks and perimeter joints of the floor panels themselves. The general contractor was tasked with sealing the power and data boxes in the raised floor, as well as the penetrations up into the wall through the raised floor.

At this point, a discussion of floor manufacturer's leakage data is helpful (see Figure 1). It quickly becomes evident that the carpet serves as the main sealing system for the raised floor. It is further evident that floor leakage is significant. The leakage encountered on the 14th floor project was not so much a contractor installation problem as it is a characteristic of raised floors trying to act as a supply air plenum. This becomes an item of particular concern when applying sustainable principles to M/E/P system design. Where a conventional supply-air system in the Houston climate may be designed on the order of 1.25 cfm/sq. ft. to meet the aggressive energy goals, this project was designed closer to 0.8 cfm/sq. ft. Thus, it can quickly be seen that the raised-floor system—without carpet—will leak more air than the entire space cooling load, which begs the question: Who needs diffusers? And despite the addition of carpet, leakage is still a significant percentage of system capacity. In other words, it basically eliminates the ability of the system to act as an efficient variable-volume air distribution system.

After the contractor resealed boxes and floor penetrations, the 14th floor was retested with all diffusers wide open. The leakage was measured at 2,010 cfm or 16% of supply airflow, with an average plenum pressure of 0.03-in. w.g. The system was then retested with all of the diffusers in control as set by the tenants and the AHU's supplying design static pressure of 0.1-in. w.g. Leakage was measured at 4,153 cfm or 41% of supply airflow (Figure 2).

Clearly, because of the floor leakage, the air system that was designed to use variable volume to match the cooling load never lets the variable-speed drive on the fan slow below 80%. But there have been very few complaints, as UFAD systems are very forgiving. The building automation system confirms that temperature control in the space is very stable.

To mitigate the air distribution problems, we have set the plenum static pressure setpoint to 0.05-in. w.g., and that setpoint is reset somewhat seasonally.

Recommendations

In general, designer and owner are pleased with the occupant comfort. The area has now been occupied for six months with good customer feedback. Despite the higher 65°F supply-air temperature dictated by the raised floor system and the single-zone arrangement, the air-handling system has maintained safe and comfortable temperature and humidity conditions with significantly lower energy consumption and long-term flexibility. It is our hope that floor manufacturers continue to work with engineers to devise a system to create tighter raised-floor plenums, allowing VAV to be applied to UFAD to achieve more sustainable and efficient part-load performance.

Main Air-Handling Unit

Total

Original

%

Design Performance Conditions

Unit

Unit

Change

Airflow (CFM)

6,000

7,945

-24.5%

Entering Dry Bulb Temp. (°F)

73.5

75.8

-3.0%

Entering Wet Bulb Temp. (°F)

62.75

63.6

-1.3%

Leaving Dry Bulb Temp. (°F)

58.8

55

6.9%

Leaving Wet Bulb Temp. (°F)

54.6

53.8

1.5%

Face Area (sq. ft.)

25

16

-56.3%

Max. Velocity (ft./min)

250

500

50.0%

Max. Air-side Pressure Loss (in. w.g.)

0.4

0.85

52.9%

Coil Sensible (BTU/hr.)

97,020

175,000

-44.6%

Coil Total (BTU/hr.)

135,000

240,000

-43.8%

Coil flow (GPM)

39.5

40

-1.3%

Max Water-side Pressure Loss (ft.)

25

15 66.

7.0%

Chilled Water Entering Temp (°F)

45

42

7.1%

Chilled Water Leaving Temp. (°F)

57

54

5.6%

Filters

ASHRAE Efficiency

60

30

Max. Initial Pressure Drop (in. w.g.)

0.3

0.5

Max. Final Pressure Drop (in. w.g.)

0.8

1

Supply Plenum Static Pressure (in. w.g.)

0.1

3

-96.7%

Minimum Fan Efficiency

60%

Total Static Pressure (in. w.g.)

1.4

4.5

-68.9%

Maximum Fan Brake Horsepower

2.2

10

-78.0%

kW/ton

0.16

0.41

-60.4%





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