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An innovative dedicates outdoor air application for a government office tower.

INTRODUCTION

The BRAC 133 facility, located in the City of Alexandria, Virginia is an office complex constructed for the Department of Defense. The complex consists of a 1,700,000 ft2 [158,000 m2] office tower, a 700,000 ft2 [65,000 m2] North Parking Structure, a 530,000 ft2 [49,000 m2] South Parking Structure, a 69,000 ft2 [6,400 m2] Remote Distribution Facility, a 3,000 ft2 [278 m2] Remote Inspection Facility and a 7,300 ft2 [680 m2] Visitor Center. The office tower is comprised of a 17 and 15 floor office tower connected at the 10th level. Floors 2 through 17 house the buildings multiple tenant groups. Building support areas such as a cafeteria, conference, fitness and health center, along with information technology, and security reside in the lower three floors.

The project was awarded as the result of a design/build competition in response to a government generated Request for Proposal (RFP). The RFP requires the building to qualify at a minimum USGBC LEED Silver Certification in addition to the required programming. The RFP suggested a variety of mechanical systems such as DOAS with chilled beams and Underfloor Air Distribution (UFAD). The Design / Build team ultimately chose to propose an alternate system utilizing a DOAS with Fan Powered Induction Units which assisted the team to achieve LEED NCv2.2 Gold.

FAN POWERED INDUCTION SYSTEM

The heart of the FPIU system is a Dedicated Outdoor Air System (DOAS). All OSA is provided to the building through the use of dedicated outdoor air handling units. The DOAS equipment cools and dehumidifies the OSA to provide latent cooling through-out the building1.

The BRAC 133 office tower is provided with (4) 100,000 cfm [47,200 l/s] Dedicated Outdoor Air Units (DOAU), which distribute air to the FPIU's through the use of a medium pressure duct distribution system. Each unit is provided with a MERV 8/13 Filter Bank, (3) centrifugal plenum fans, a pre-heat coil, cooling coil, and re-heat coil served with Chilled Water Return (CHWR). Two DOAU's are provided with total energy recovery units.

The FPIU is a modified series fan powered variable air volume terminal unit (Series VAV) as shown in Figure 1. Similar to a Series VAV, the FPIU utilizes a primary air damper, forward curved fan and filter. Unique to the FPIU is a chilled water coil at the induced air inlet providing only sensible cooling. Zones requiring heat are equipped with a heating hot water coil at the induction inlet. The Fan Powered Induction Unit system was chosen for the BRAC 133 building for the following reasons:

[FIGURE 1 OMITTED]

Optimal Ventilation and Control: Outside Air provided by the DOAS is precisely metered to each zone through the use of a primary air damper. Introduction of OSA directly to the zone results in reduced overall system OSA flow rates in accordance with ASHRAE 62.1, in comparison to systems that exhibit low system ventilation efficiency (Ev) such as traditional, single-duct VAV systems. Demand Control Ventilation (DCV) strategies can be more easily implemented through the modulation of the primary air damper to maintain zone level CO2 concentration. High ventilation efficiency and the use of DCV results in an overall reduction of OSA to the building when compared to multiple-zone, recirculating (VAV) systems, requiring less mechanical infrastructure and energy to condition2,3.

Sensible Only Cooling: High temperature chilled water to provide sensible only cooling, requires less energy to generate than a traditional chilled water system normally operating between 42[degrees]F to 45[degrees]F [5.6[degrees]C to 7.2[degrees]C] that are necessary to provide dehumidification. The isolation of latent loads at the DOAS and the shift of internal cooling load to the sensible chilled water system decreases the building annual energy consumption.

Variable Volume Supply Air: ECM fan motors at each FPIU can be programmed to modulate the supply air fan with the cooling and heating load to vary the airflow delivered to the zone, similar to a VAV system. In an FPIU system, only the OSA is distributed at medium pressure. Airflow providing zone level heating and cooling is recirculated locally within the space at low static pressure. The end result is an air distributions system that consumes approximately the same fan energy as a traditional conventional VAV system as shown in Table 2.

Flexibility: A Fan Powered Induction Unit equipment layout, sized to accommodate the maximum expected people loading of a typical tenant floor can minimize equipment size changes and relocation during Tenant Fit Out (TFO). The FPIU's ability to modulate supply and primary airflow minimizes equipment re-design as a result of individual tenant needs. Assuming no major changes of occupancy, the FPIU system when designed with reserve capacity can accommodate most TFO changes requiring only low pressure air distribution re-arrangement and air balancing. (4)

Architectural Benefits: In an FPIU system cooling is conveyed hydronically to each zone. In a traditional VAV system 24 inch [610mm] diameter duct is required to convey 10 tons [35 kW] of cooling capacity. The same capacity is conveyed in an FPIU system with (2), 1.5 inch [38mm] diameter chilled water pipes. Medium pressure ductwork is only used to convey OSA, therefore reducing the overall building medium pressure ductwork by approximately 80% when compared to a VAV system. Given this, utilization of the FPIU system can result in lower building floor to floor heights, reduced shaft footprint, the elimination of mechanical rooms on each tenant floor, and simplified inter-trade coordination when compared to a traditional VAV system. These benefits provide great advantage to the project team when competing for projects during bid phase.

Integration with Other DOAS Technology: The FPIU should be considered as part of an overall DOAS strategy. Chilled beams, chilled sails, radiant slabs/walls and fan powered induction units can be integrated into the same building. These systems fundamentally require that all dehumidification is accomplished with the DOAS and that the building dew-point temperature is maintained below the sensible cooling water temperature. Throughout a typical building, the heating and cooling load can vary greatly. The designer should select the terminal unit based upon its intrinsic cooling and heating capacity and the operating characteristics of the space.

Unoccupied Mode Set-Back: The FPIU does not require primary airflow to provide cooling or heating. During un-occupied periods, perimeter zones served via FPIU's can operate to a set-back temperature using the supply air fan and sensible cooling or heating coil. This is not possible with single-duct VAV systems or active chilled beams that require primary airflow to deliver cooling and heating.

Excellent Controllability: Each thermal zone is provided with an FPIU for temperature, humidity and ventilation air control. Temperature control is provided by modulating flow through either the sensible chilled water or heating hot water coil to maintain the zone temperature set-point. Temperature and humidity sensors are used to provide area dew-point calculations which modulate the primary air damper to maintain the zone level dew-point below 55[degrees]F [12.8[degrees]C]. IAQ sensors measuring CO2 concentration in open office areas and conference rooms are used to vary the required OSA quantity to the space. Private offices are provided occupancy sensors that enable the primary air damper when occupied. Figure 1 depicts the FPIU control devices and major components.

CENTRAL UTILITY PLANT

The BRAC 133 latent and sensible cooling loads are de-coupled from each other and served via different central cooling plants. Alexandria, VA, located outside of Washington, D.C., OSA requires conditioning during the spring and summer months when the OSA dew-point and dry-bulb temperatures are above the DOAS supply air setpoint of 48[degrees]F [8.9[degrees]C]. Sensible cooling loads, such as lighting, envelope, plug and people, will occur year round regardless of the season. To remove latent energy from OSA, chilled water must be generated below the leaving supply air dew-point provided by the DOAS. Sensible chilled water should be generated at a temperature above the design building dew-point so that no condensation will occur. Given this, a latent chilled water plant and a sensible chilled water plant, utilizing two different loop temperatures can be implemented.

Latent Chilled Water System

The latent chilled water plant consists of two, variable speed, water-cooled centrifugal chillers providing 2,500 tons [8,792 KW] of chilled water to the building latent loads. One redundant chiller selected to provide chilled water to either the sensible or latent chilled water loop was also provided. The latent chilled water loop primarily serves the DOAS along with miscellaneous latent loads located throughout the building. The latent chilled water loop provides 42[degrees]F [5.6[degrees]C] chilled water to the building via a variable primary pumping configuration. The associated DOAS is designed to dehumidify OSA from the greatest latent condition expected (89[degrees]F DB / 79[degrees]F WB)5, [31.7[degrees]C/ 26.1[degrees]C] to deliver primary airflow at 48[degrees]F [8.9[degrees]C] dew-point to the FPIU's throughout the two building towers. The latent chilled water plants primary function is to dehumidify OSA, therefore the latent chilled water plant is not required to operate when the ambient OSA dew-point is below the primary air set-point. During the winter and much of the fall, the Washington, D.C. weather allows for the latent chilled water plant to remain offline. During this period the DOAS provides free-cooling and dehumidification to the building. Zones within the building requiring additional dehumidification are designed with over-sized ductwork to allow the delivery of additional OSA for humidity control during free cooling periods.

Sensible Chilled Water System

The sensible chilled water plant consists of two, variable speed, water-cooled centrifugal chillers providing 2,500 tons [8,792 KW] of chilled water to the building sensible loads. Sensible loads are served by FPIU's, Air Handling Units (AHU) and Computer Room Air Handling Units (CRAH), the use of which are determined by space constraints and cooling load density. The sensible chilled water loop provides no latent cooling, which is accomplished by supplying chilled water at a higher temperature than the design room dew-point temperature.

The sensible chilled water plant generates chilled water at a supply temperature of 55[degrees]F [12.8[degrees]C] and is returned to the plant at 65[degrees]F [18.3[degrees]C]. Chilled water generated within this range allows for a significant increase in water-side economizer hours depending upon climate. In Washington D.C., when comparing against a 42[degrees]F [5.6[degrees]C] chilled water system, the BRAC 133 building will realize a 65.3 % increase in full free cooling economizer hours with an overall increase of 25% in annual water-side economizer operating hours during both full and partial cooling. When the water-side economizer is not available due to high OSA wet-bulb temperatures the sensible chiller will operate with very low power consumption at design and part load. The benefit of water-side economizer varies by climate zone as shown in Table 1 and operating schedule. Each design team must qualify if the use of a water-side economizer is applicable for their specific project.
Table 1. Hydronic Economizer Availability Hours by Sample City

 42[degrees]F CUNY 55[degrees]F CUNY
 [5.6[degrees]C] System [12.8[degrees]C] System
Location Full Cooling Partial Full Cooling Partial
 Cooling Cooling
 (hr) (hr) (hr) (hr)

Chicago, IL 2,935 2,375 4,611 1,520

Dallas, TX 475 2,728 2,497 1,801

Denver, CO 3,408 3,023 5,508 2,306

Las Vegas, 1,628 4,184 4,723 2,347
NV

Los Angeles, 0 2,530 837 3,930
CA

New York, 1,918 3,022 4,201 1,554
NY

San 2,690 3,007 4,725 1,835
Francisco,
CA

Washington, 1,434 3,567 4,115 1,554
DC

1. Weather data provided by NREL TMY3 for each example city.
2. Analysis based upon 8,760 hours of operation.
3. Analysis assumes 3[degrees]F [1.7[degrees]C] heat exchanger
approach and 4[degrees]F [2.2[degrees]C] cooling tower approach.
4. Analysis assumes 16[degrees]F [8.9[degrees]C] differential
temperature for the 42[degrees]F [5.6[degrees]C] chilled system
and a 10[degrees]F [5.6[degrees]C] differential temperature for the
55[degrees]F [12.8[degrees]C] chilled water system.


[FIGURE 2 OMITTED]

Condenser Water System

Spatial and economic constraints in providing two separate condenser water systems required the installation of a common condenser water system for the latent and sensible chilled water systems. The condenser water system provides free cooling to the sensible chilled water system through the use of a plate and frame, free cooling heat exchanger (HEX-1). The Heat Exchanger is piped in series with the chilled water return to provide both full and partial cooling of the sensible chilled water depending on the ambient wet bulb conditions. In order for the condenser water system to provide complete free cooling the system must supply condenser water with a temperature as low as 52[degrees]F [11.1[degrees]C] during low ambient outdoor air wet-bulb periods. Both systems operate with very different load profiles given the nature of the loads they serve; this provides a challenge when trying to operate each loop's chillers simultaneously with cold condenser water. As a result, the design team worked with the chiller manufacturer to provide a chiller that was capable of operating using compressor head pressure control.

The use of head pressure control allows the centrifugal chillers to accept a wide range of condenser water supply temperatures. Each chiller will analyze its inlet condenser water temperature and leaving evaporator water temperature to ensure that the compressor has the required lift to operate properly. In response to varying condenser water temperature and compressor loading, the chiller's on-board control panel will modulate the condenser water supply valve to the chiller, providing the necessary condenser water flow rate as determined by the chiller. Benefits of utilizing low temperature condenser water include reduced energy consumption due to lower compressor lift and an increase in chiller capacity (6).

To provide stable condenser water loop flow and pressure due to the modulation of chiller and heat exchanger control valves, an end-of-main bypass valve was installed. The end of main bypass valve modulates to maintain a constant differential pressure within the condenser water system header resulting in stable loop and chiller operation. To maximize the benefit from the free cooling plate and frame heat exchanger the design team selected cooling towers with a 4[degrees]F [2.2[degrees]C] design approach temperature, to produce 82[degrees]F [27.8[degrees]C] condenser water during the peak design day (78[degrees]F WB, [25.6[degrees]C]). The Building Automation system (BAS) will automatically re-set the condenser water temperature set-point in accordance with the cooling tower performance curve. Upon a decrease in condenser water supply temperature below the sensible chilled water return temperature, the sensible chilled water system will enter into free-cooling mode.

SYSTEM DESIGN AND ANALYSIS

System and equipment sizing of a dedicated outdoor air system with multiple central plants requires a unique approach by the design team. During the initial design phase, it was unclear to the design team how the client would load each tower, only that 6,400 full time employees would occupy the tenant floors in addition to the podium support facilities. Great care was taken by the design team to allow the client flexibility to distribute their multiple tenant groups per their individual needs.

To provide maximum flexibility, each of the (4) dedicated outdoor air handling units were arranged to serve approximately 1/4th of the tower area. Each air handling unit was sized to allow the maximum tenant loading of 64 ft2 [5.9 m2] per person as indicated in the programming guide. This allowed the client flexibility during tenant fit out. In addition to permanent seating for the 6,400 full time employees, approximately 10% of the tenant floor space was assumed for use as conference space, increasing the non-diversified tower population to 12,800 people. The podium support / conference space could also accommodate an additional 4,200 people, resulting in total nondiversified building population of 17,000 people.

The design team, realizing that on a typical day the full time employee population would not increase above the design number specified in the RFP utilized demand control ventilation throughout the tower. This allowed the team to apply a 50% diversity factor to the tower population, reducing design people count from 17,000 to 10,600. As a result, (4), 100,000 cfm [47,200 l/s] air handling units were selected to allow for the building sum of peak population, however it is expected that the building will operate at diversified OSA flow rate of approximately 220,000 cfm [103,800 l/s]. Given this, the latent chilled water plant was sized for the diversified OSA flow rate, with the applied population diversity resulting in a latent chilled water plant reduction of 1,250 tons [4.396 KW] (4).

A dynamic relationship exists between the sensible and latent chilled water systems. The latent chilled water system capacity is sized to cool the total diversified OSA flow on the design dehumidification day down to 48[degrees]F [8.9[degrees]C]. The 48[degrees]F [8.9[degrees]C] DOAS primary air to the building provides significant sensible cooling capacity also. An optimized and economically sized sensible chilled water system should account for the cooling provided by the 48[degrees]F [8.9[degrees]C] primary air. Reduction in cooling capacity due to DCV or OSA temperature re-set strategies required the designer to evaluate the zone level sensible cooling coil selection under these scenarios. The sensible cooling capacity needs to accommodate the fluctuation of primary air flow. The BRAC 133 design team analyzed various combinations of OSA flow rates and temperatures to determine their effect on the sensible chilled water system size.

The design team performed a chiller study to pick the optimal chiller and cooling tower configuration. The study resulted in the selection of an optimized chiller / cooling tower configuration with the lowest kW/ton NPLV and an acceptable life cycle cost payback. The study resulted in the selection of two latent chillers selected with a NPLV performance of 0.380 kW/ton and two sensible chillers selected with a NPLV of 0.229 kW/ton. The peak design power consumption rate of the latent and sensible chilled water systems is 0.710 and 0.586 kW/ton, respectively.

Dedicated outdoor air systems lend themselves to use of energy recovery devices. To properly size the energy recovery system so that it can be used to its full extent, the energy recovery system must be designed to accommodate a realistic exhaust airflow rate that the energy recovery system will normally see. Defining anticipated building leakage impacted the ability to determine the available airflow for heat recovery7. In addition, the use of extensive demand control ventilation compounds the situation further. The design team analyzed a variety of scenario's and building leakage rates to determine the effect on the latent chilled water plant and energy recovery unit sizing. Ultimately the design team selected two, 35,000 cfm [16,500 l/s] total energy recovery wheels, utilizing molecular sieve desiccant media. This reflects an average building leakage rate of 0.10 cfm/ft2 [0.51 l/s.m2] of floor area.

Exfiltration. Coincidentally this aligned closely with the building general exhaust system which allowed the design team to integrate the two systems together.

The BRAC 133 office complex utilizes a large consolidated server room along with distributed telecommunications rooms. A key energy efficiency measure was the use of sensible chilled water for all Data and IT loads, which accounted for 1/3rd of the sensible chilled water system capacity. The design team worked closely with the CRAH unit manufacturer to develop a semi-custom CRAH with chilled water coils that could accept 55[degrees]F [12.8[degrees]C] sensible chilled water. The building's inherent nature to limit interior dew-point to below 55[degrees]F [12.8[degrees]C] allowed the design team to eliminate the need for dehumidification and re-heat within each CRAH. The elimination of which was a significant factor in reducing the overall building energy consumption. Some humidity fluctuation will occur with changes in IT room temperature during summer months as the building dew-point nears 55[degrees]F, therefore this strategy should be analyzed and discussed with IT staff to ensure IT system requirements and compatibility.

The BRAC 133 project is programmed to achieve LEED NCv2.2, Gold certification. The project team committed to achieving (5) Energy and Atmosphere points requiring at least 24.5% energy cost savings compared to its respective ASHRAE 90.1-2004 baseline model. The ASHRAE 90.1-2004, Appendix G baseline system for the BRAC 133 building is a variable air volume air system, with cooling provided from a water-cooled centrifugal chilled water plant and heating from a standard efficiency gas-fired hydronic hot water boiler system8. Energy modeling of the FPIU system, along with Lighting Power Reductions, Demand Control Ventilation, Latent and Sensible Cooling Systems, Energy Recovery and a Water-Side Economizer, yields the BRAC 133 campus with an annual energy cost savings of a 24.7% when compared to the baseline system, as shown in shown in Table 2.
Table 2. BRAC 133 Energy Model Results

Load Utility Baseline Model
 Energy Percent
 of
 10^6 Btu/yr Total
 (10^6 w/yr)

Lighting Electricity 15,326.3 6.4%
 (4,490.6)

Heating Electricity 1,263.4 (370.2) 0.5%

 Gas 75,417.6 31.4%
 (22,097.2)

Cooling Electricity 27,042.1 11.3%
 (7,923.3)

Pumps Electricity 4,781.8 2.0%
 (1,401.1)

Heat Electricity 2,638.0 (772.9) 1.1%
Rejection

Fans Electricity 20,484.9 8.5%
 (6,002.0)

Process Electricity 88,989.9 37.0%
 (26,073.8)

 Gas 4280.3 1.8%
 (1,254.1)

Total Building Energy 240,224
Consumption

Annual Building Cost (US 4,170,238
Dollars)

Load Utility FPIUS System Overall
 Energy Percent Reduction
 of
 10^6 Btu/yr Total
 (10^6 w/yr)

Lighting Electricity 10,104.5 6.2% 34.1%
 (2,960.6)

Heating Electricity 425.8 (124.8) 0.3% 66.3%

 Gas 28,278.6 17.4% 62.5%
 (8,285.6)

Cooling Electricity 9,125.3 5.6% 66.3%
 (2,673.7)

Pumps Electricity 4,114.4 2.5% 14.0%
 (1,205.5)

Heat Electricity 2,325.8 (681.5) 1.4% 11.8%
Rejection

Fans Electricity 19,143.3 11.8% 6.5%
 (5,608.9)

Process Electricity 84,859.6 52.2% 4.6%
 (24,863.6)

 Gas 4280.3 2.6% 0.0%
 (1,254.1)

Total Building Energy 162,658 32.3%
Consumption

Annual Building Cost (US 3,141,937 24.7%
Dollars)


CONCLUSION

The Fan Powered Induction System can provide significant benefits when compared to a traditional VAV system such as increased thermal comfort, humidity and ventilation control and reduced infrastructure cost to the client while providing significant energy savings. These benefits can be achieved only if the design team is cognizant of basic DOAS design principals and if the FPIU system is implemented properly. With the prevalence of DOAS and chilled beam systems through-out the Heating, Ventilating and Air Condition industry, the FPIU should be given consideration alongside these technologies during the design phase.

ACKNOWLEDGMENTS

This paper is dedicated to the memory of David Peters, whose dedication to innovation and brilliant design shall always be an inspiration to all who worked with him. Thank you to Scott Winkler for his leadership and guidance throughout this project.

REFERENCES

(1.) 2008. ASHRAE Handbook-HVAC Systems and Equipment. Atlanta: American Society of Heating Refrigeration and Air Conditioning Engineers, Inc., Chapter 4, p 4.14, "Dedicated Outdoor Air"

(2.) Dieckmann, J., Roth, K., Brodrick, J., 2007. "Dedicated Outdoor Air Systems Revisited." ASHRAE Journal 49(12):127 - 129

(3.) ANSI/ASHRAE Standard 62.1-2007, Ventilation for Acceptable Indoor Air Quality

(4.) Murphy, J. 2010. "Selecting DOAS Equipment with Reserve Capacity.", ASHRAE Journal 52(4): 30-40

(5.) 2001. ASHRAE Handbook-Fundamentals. Atlanta: American Society of Heating Refrigeration and Air Conditioning Engineers, Inc., Chapter 27, p 27.21, "Table 1B"

(6.) Demma, D., "Understanding the Fundamentals of Head Pressure Control", Sporlan Valve Company

(7.) Mumma, S.A. 2010. "DOAS & Building Pressurization." ASHRAE Journal 52(8):42 - 52.

(8.) ANSI/ASHRAE Standard 90.1-2004, "Energy Standard for Buildings Except Low-Rise Residential Buildings."

Michael Hallenbeck, PE

Member ASHRAE

Michael Hallenbeck, PE is an Associate Principal Engineer with Southland Industries in Irvine, California.
COPYRIGHT 2012 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
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Author:Hallenbeck, Michael
Publication:ASHRAE Transactions
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2012
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