D e c e m b e r 1 9 9 9 A S H R A E J o u r n a l 4 1

A SHRAE JOURNAL

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The original building in Tampa, Fla.was constructed in 1984 with additionaltenant spaces added later. Retail shops, afood court and restaurants were locatedon the second and third levels, overground-level parking. This original designprovided upscale shops and restaurantsin downtown Tampa with panoramicviews of the harbor area. Unitary or blockair-conditioning systems were installedpiecemeal as tenants occupied theirspaces. With the exception of commoncondenser water provided by the landlord,no central air-conditioning system existed.

Because there were no guidelines orrestrictions on type, style or configurationof air conditioning for individually leasedspaces, each tenant designed and installeddifferent systems. The original air-condi-tioning equipment is listed in Table 1.

The final re-configuration of the ex-isting building situated the computer datacenter, telephone technical communica-tion center and leased office space on theupper level. The lower level (or piazzalevel) is occupied by restaurants, a foodcourt and service shops. Table 2 showsnew use of the space.

One of the initial design decisions wasto reuse existing equipment that was still

About The Authors

Warren G. Hahn, P.E., is president of Hahn Engineeringin Tampa, Fla. He is a member of ASHRAE TechnicalCommittee (TC) 9.1, Large Building Air-ConditioningSystems.

Michael J. Hahn, P.E. is senior vice-president at HahnEngineering. He is a member of TCs 9.1 and 9.8, LargeBuilding Air-Conditioning Applications.

Fast-Track HVAC ConversionBy Warren G. Hahn, P.E.andMichael J. Hahn, P.E.

his article describes the challenges of converting an existing,fragmented retail mall HVAC system into a dual-use systemserving a combination computer data center and retail mall.Difficulties included a drastically reduced design-construction

schedule and a major building function change.

functional and had a reasonable remain-ing life expectancy.

The original 600 ton (2110 kW) cool-ing tower was replaced before the deci-sion to convert the building was made.Size, building configuration and locationon the roof required the installation of themaximum size cooling tower available.This 750 ton (2638 kW) dual-cell vari-able speed cooling tower would remainand become integrated into the design. Allother existing equipment was at two-thirds to three-quarters of its life expect-ancy. The following factors were used toevaluate each piece of equipment to de-termine if re-use was practical:

• What was the remaining life expect-ancy?

• What was its energy efficiency?• Was removal, storage and relocation

required?• Would its capacity “dovetail” into a

new use?• Could the physical size of the equip-

ment “footprint” be accommodated in thenew equipment layout?

• Did it fit into the overall “redundant”cooling plan?

New Air-Conditioning SystemOwner requirements for the converted

building included:1. Provide redundant mechanical and

electrical systems for the upper level com-puter center, including ancillary tenant

office space, corridors, restrooms andbreak areas serving the offices and datacenter.

2. Use the new 750 ton (2638 kW),two-cell cooling tower in the design.

3. Provide 72°F (22°C) temperatureand 45% relative humidity control in theupper level computer areas and 75°F(24°C) temperature and 50% relative hu-midity control in the upper level officesand lower level service shops, food courtand restaurants.

4. Provide adequate dehumidified out-door air, even under part load conditions.

5. Pipe condensate from equipmentdrain pans without jeopardizing computerroom equipment.

6. Maintain airflow through computers.7. Use the same base air-conditioning

system for the restaurant, food court, andservice shops to be located in the lowerlevel common areas.

8. Protect all air-conditioning and elec-trical equipment during a hurricane.

9. Minimize noise and vibration in theoffice tenant area on the upper level.

10. Locate all air-conditioning equip-ment in the two existing mechanicalrooms.

11. Provide a central building manage-ment system for HVAC, electrical, firealarm, security, computer room air con-ditioning and lighting control systems.

12. Electrical and mechanical equip-ment selected cannot impact the overallfloor to ceiling area because of the com-puter room raised floor.

13. Select equipment that could meetshortened manufacturing and deliverytime requirements.

TTTTT

4 2 A S H R A E J o u r n a l D e c e m b e r 1 9 9 9

14. Minimize chances of electrical and air-conditioning sys-tem failures due to piping leaks from plumbing, fire protection,roof drainage, chilled water or condenser water piping leaks.

15. Maintain an operational computer data center (upperlevel) during and immediately after a hurricane.

16. Provide uninterruptable power for the computer data cen-ter (upper level) at all times.

17. Operate in accordance with the hourly usage as outlinedin Table 3.

Choosing a SystemMany different HVAC systems were evaluated with consid-

eration given to energy efficiency, physical limitations, inte-gration with existing equipment and product availability. Thesystems evaluated are listed in Table 4 along with their advan-tages and disadvantages.

An analysis of the data shown in Table 4 resulted in a selec-tion of a water-cooled chiller. The system would provide chilledwater to computer room units, VAV units for the upper leveloffices, and VAV units and single-zone constant-volume unitsfor the lower level service shops, food court and restaurants.

The new central air-conditioning system consists of two 375ton (1319 kW) water-cooled screw chillers, one 75 hp (56 kW)chilled-water pump for the constant chilled-water load and oneredundant 75 hp (56 kW) chilled water pump. Each chiller hasits own condenser-water pump with common header manualvalves and a standby condenser-water pump. Each cooling towercell has its own cooling tower fan with modulating variable-speed drive and a common cooling tower sump. Considerationwas given to a primary/secondary-chilled water system with avariable speed pump for the secondary distribution system.

Because of the constant cooling load required, space constric-tion for additional pumps and piping, and a first cost versusoperating costs savings analysis, a single constant flow chilled-water system was chosen.

Central station VAV air-handling units are provided for of-fice, retail, and food court/vendor tenants and common areas.Each tenant is provided with VAV box(es) and return duct tapaccess points. The large VAV air-handling units are located inthe existing mechanical room.

24-Hour Standby GeneratorOf critical importance is the availability of chilled water,

air-handling unit operation and computer room AC unit opera-tion in spaces that must be on-line at all times. Two 1,500 kVA

Table 1: Original AC system with 600 ton cooling towerand condenser water.

Table 2: New use, function and characteristics.

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D e c e m b e r 1 9 9 9 A S H R A E J o u r n a l 4 3

H V A C S Y S T E M C O N V E R S I O N

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generators provide redundant backup of the critical air condi-tioning, lighting, and uninterruptible power supply (UPS). Aseparate generator building was constructed well above the floodplain of the Tampa area to allow operation during electricaloutages and immediately after a hurricane. During a Level 3hurricane or greater, the work area would be evacuated, butreoccupied immediately following the hurricane. It was man-datory that electrical power be available at all times, even atthe height of a hurricane.

An 18 in. (45.7 cm) raised floor was provided to allow forelectrical distribution, communication cables and air-condition-ing airflow supply from the computer room units to the variouscomputers. The raised floor had little impact on the overheadduct system because of the large area available above the ceil-ings in the former mall shops. However, creative duct designwas required around the perimeter of the building where aboveceiling height was at a minimum due to the sloped roof.

To prevent “missile” debris damage, minimize the air-con-ditioning load, and maximize efficiency, many of the floor toceiling windows on the south side of the building were walled-in and insulated with an R12 product. Previously exposed win-dows on the upper northwest side of the building were treatedwith horizontal venetian blinds and an extended canopy wasrefurbished. Ventilation and exhaust air ducts were routed tothe lower level through chases in the upper level data centerarea and office area for future food court vendor use.

Hurricane ProtectionSpecial hurricane protection was required for the cooling

tower because of the facility’s location and operational re-quirements. A metal “cage” made of ¾ in. (1.9 cm) steel rodwas constructed around the cooling tower to prevent 2 ft by 4ft (0.6 m by 1.2 m) projectiles from hurtling into the coolingtower. This protection, along with a standby generator sizedto handle the chilled-water air-conditioning load, enables thechiller plant to remain operational during and immediatelyafter a hurricane. Observation after more than one year ofoperation indicates no apparent detrimental effect of the “cage”on the efficiency of the tower. Additional hurricane protec-tion for the building includes automatic wind and projectileresistant shutters for all of the windows on the upper leveloffice.

System performanceSpecial data processing area requirements such as precise

temperature and humidity control, 100% redundant computerroom AC unit backup, and central monitoring/control are ef-fectively provided by the central chilled water system. A re-view of the data monitoring system during the first year indi-cates that the building temperature and humidity is effectivelycontrolled within 1°F and 3% RH. One benefit of this newsystem is that ample conditioning capacity is available andbeing produced at a kW/cost rate of about 35% lower thanthe previous air-conditioning systems. Payback of the newmechanical system, especially in light of the building’s ex-tended operational and load requirements, is expected in lessthan three years. This calculation is based on cooling avail-

ability to all areas of the building under all load requirementsand the measured cooling efficiency rate approximately 0.85kW/ton versus the previous 1.3 kW/ton.

Construction ChallengesThe system design allowed less than 4.5 months from autho-

rization of design to initial occupancy for one tenant and only6.5 months for computer equipment installation. Additional fasttrack design constraints included an accelerated equipment or-dering and installation program for the air-conditioning sys-tem. These requirements necessitated early order of chillers,pumps, electrical switchgear, UPS systems, diesel generators,and automatic transfer switches. Tenants occupied portions ofthe building while construction took place in other areas, caus-ing out-of-sequence construction to occur. This required someduct terminations to be sealed because VAV boxes were not yetin place. Temporary balancing dampers were provided on othersections of ductwork. To keep the chiller and air-handling unitsunder load and provide air conditioning for the relatively smallarea occupied by initial tenants, some of the construction areasreceived “free” air conditioning.

The fire sprinkler and fire alarm systems also were affectedby out-of-sequence construction. Both systems were requiredto be operational for the initial tenant occupancy.

A truck-mounted diesel generator was required for two weeksto keep the air conditioning and lighting operational during theinitial tenant occupancy while the electrical service on the west

Table 3: Hours of operation per day.

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4 4 A S H R A E J o u r n a l D e c e m b e r 1 9 9 9

Table 4: HVAC systems considered.

side of the building was upgraded from a2,000 amp service to a 4,000 amp service.

Lessons LearnedLessons LearnedLessons LearnedLessons LearnedLessons LearnedThe facility has been in operation for

more than a year now. This fast track reno-vation required overcoming manyhurdles, including:Problem: Water treatment. The con-

tractor was responsible for cooling towerwater treatment and operating the chillerfor tenants who had moved into finishedspaces during construction.Cause: Because little attention was

paid to water treatment during this four-month period, the chiller tubes becamescaled and required cleaning.Recommendation: While the obvious

solution to this is to ensure that properwater treatment takes place on a continualbasis, future projects should include amaintenance alarm that is connected tothe building management system for the

condenser water condition. This alarmwould monitor pH, conductivity and anyother conditions that may be consideredessential in water treatment.Problem: Noise caused by ductwork

constriction. Because of some construc-tion coordination difficulties, the foodcourt/restaurant area ceiling (partial grid/partial drywall) was completed before theduct work was installed.Cause: The sheet metal contractor was

required to install the ductwork throughand above the newly installed ceiling, andso, the resulting configuration caused air-flow noise. Reconfigured ductworksolved the airflow noise problem, butcaused both extra engineering and sheetmetal efforts.Recommendation: Provide HVAC

system commissioning. A total HVAC sys-tem commissioning process, instead ofmonthly visits, would have prevented this.Problem: Jumping chilled water pipe.

The chilled water pipe leaving the chilledwater pump made an abrupt 90° turn tofeed the water into the chillers becauseof the low ceiling area in the mechanicalmezzanine. Additionally, the restrictedspace prevented the use of thrust blocksfor proper steel anchors.Cause: When the constant-speed

chilled-water pumps were started, the pip-ing “jumped” what appeared to be 2 in.(5.1 cm) or 3 in. (7.6 cm) and caused ex-perienced steam fitters to scramble. A vari-able-speed drive was installed on the pumpto allow the pump to gradually ramp up tofull speed, eliminating the “jumping.”

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