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informs the operators that the average exhaust velocity at thesash opening is less than 100 fpm (0.5 m/s) and that one of thesashes needs to be lowered to silence the alarm. Exhaust aircalculations with these design diversity factors and the result-ing conditions are indicated in Table 2.

This approach reduced the design exhaust air from 13,000cfm to 7,000 cfm (6136 to 3304 L/s), reducing the size of the air-conditioning unit, exhaust fan, supply and exhaust ductwork,diffusers and piping. Now the reduced size of the air-condition-ing unit and ductwork could easily be fitted in the limited ceil-ing and floor spaces available. The revised airflows translatedinto a more reasonable 23.9 air changes per hour and 3.4 cfm /ft2

at full design flow. This produced a design diversity of 54%.

HVAC SystemA variable-air-volume system with reheat was selected to

achieve design and operating diversities. One 6,700 cfm (3162L/s) air-conditioning unit, complete with pre-filters, 85% effi-

ciency bag filters, hot water preheat coil, spray humidificationand dehumidification coil, chilled water cooling coil, hot waterreheat coil, airfoil fan with variable inlet vanes and high effi-ciency fan motor was selected to supply 100% conditionedoutdoor air to the laboratory. An infiltration of 300 cfm (142 L/s)was allowed from the adjacent areas to maintain this lab undernegative pressure at all times. All fume hoods and chemicalstorage cabinets were manifolded to a common exhaust ductand fan. The controls were designed to modulate the exhaust ateach hood from 250 to 1,250 cfm (118 to 590 L/s), while the totalexhaust from the lab was limited to 7,000 cfm (3304 L/s).

To achieve these results, a sash position-based fume hoodface velocity control system with makeup air valves operating in

the volumetric offset (flow tracking) mode, and low flow alarms,were specified. A simplified layout of the lab is shown inFigure 1.

Because the researchers were accustomed to operating thehoods without closing the sashes and were not familiar with thistype of diversity, a very conservative average exhaust airflow of5,500 cfm (2595 L/s) was used to estimate the annual operatingenergy cost. The diversified exhaust air calculations and the re-sulting conditions are indicated in Table 3.

Operating Energy and Construction CostsThe annual operating energy cost for the 13,000 cfm (6136

L/s) constant volume-reheat (CV-RH) system was estimated at$49,900. The annual operating energy cost for the proposed

variable-air-volume-reheat (VAV-RH) system, with an averageexhaust of 5,500 cfm (2596 L/s) was estimated at $29,126. Thisprovided an estimated annual energy saving of $20,774 (shownin Table 4 and Table 5).

The installed cost of the 13,000 cfm (6136 L/s) constant vol-ume-reheat system was estimated at about $155,000. The actual

bid price of the 7,000 cfm (3304 L/s) variable air volume-reheatsystem and lab controls was $172,000, or $17,000 more than theestimated cost of 13,000 cfm (6136 L/s) constant volume-reheatsystem. This excess cost was recovered in less than one year,

by the annual energy saving of $20,774. The breakdown ofthese costs and the calculation of the simple payback periodare shown in Table 4.

HVAC Systems ComparisonVarious elements of the VAV/RH design and operating con-

ditions are compared with the CV/RH in Table 5. Some of thekey elements shown in this table are:

Dramatic reduction in the design cooling loads from 74 to40 tons (260 to 141 kW). Cooling load reduction to 13 tons(45.72 kW) at the actual operating conditions.

Table 2: VAV system design diversity.

Table 3: VAV system operating diversity.

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September 1 9 9 8 ASHRAE Journal 4 1

L A B O R A T O R Y H V A C

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Dramatic reduction in the design heating loads from 1,091to 607 Mbh. Heating load reduction to 200 Mbh at theactual operating conditions.

Reduction in air changes per hour from 44.5 to 23.9 at thedesign conditions and to 6.8 at the actual operating condi-tions.

The estimated annual operating energy cost declined from$49,000 for the CV-RH system to $29,126 for the VAV-RHsystem, providing an estimated savings of $20,774. Theenergy cost at the actual operating conditions was evenlower at $20,230, resulting in higher savings of $29,670.

The reduced air change rates have also resulted in increased

safety due to reduction in the room turbulence, which wouldhave occurred with the CV-RH system.

Achievements and ResultsThe system was calibrated to the predetermined airflows and

was fully set up within two days. No unforeseen situationsoccurred during this period, and the lab was turned over to theowner in one week, including the system instructions.

After six months of operation, it was decided to confirm thedesign parameters that affected the diversity. The building man-agement system was programmed to log the trend of the HVACsystem at five minute intervals, 24 hours a day and seven days aweek. In addition, two hoods were equipped with zone presence

sensors, working in conjunction with the fume hood controls.These zone presence sensors were intended to automatically re-duce the exhaust airflow and corresponding makeup air by 40%during the absence of an operator from the detection zone.

The trend log variables included: Total makeup air system flows, Exhaust airflows from two fume hoods, Sash positions of two fume hoods, Total exhaust airflow and Fume hood exhaust alarms.The analysis of the data logged during the seven-week pe-

riod between Aug. 5, 1994 and Sept. 30, 1994 produceduseful results. The highlights of these results from the

Table 4: Construction costs and energy savings.

data logging are: Figure 2 indicates daily peak makeup airflow. It shows that

the makeup airflow did not exceed 5,600 cfm (2643 L/s) or43% of the comparable constant volume system.

Figure 3 indicates daily average makeup air. It shows thatthe makeup airflow did not exceed 2,200 cfm (1038 L/s) or17% of the comparable constant volume system.

Figure 4 indicates Ventilation System Performance. It

Figure 1: HVAC layout for medicinal chemistry lab.

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4 2 ASHRAE Journal September 1 9 9 8

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shows that the average ventilation rate or exhaust is re-

duced by 83%, compared to the constant volume system.

It also shows that the actual peak exhaust was 43% of the

comparable constant volume system.

The analysis also indicates that the typical fume hood usage

with full sash opening was about 6070 minutes per day.

ConclusionsThe on-site monitoring confirmed the assumptions and con-

clusions reached during the design. Since the actual makeup

airflow did not exceed 5,600 cfm (2643 L/s), the diversity calcu-lations used in sizing the HVAC system were confirmed. How-

ever, the highest average makeup airflow of 2,200 cfm (1038

L/s), logged by trending, was considerably lower than the esti-

mated average airflow of 5,500 cfm (2595 L/s). This means that

the researchers followed the operating procedures by lowering

sashes when they were not in front of the hoods. This has

further reduced the payback period of the premium cost of this

control system, to about six months.

The lab has now been in operation for more than two years and

the number of researchers has doubled. During this time the Di-

versity Alarm has been initiated only occasionally and the re-

searchers have responded appropriately by lowering the sashes

when they leave the hoods. The researchers have expressed satis-faction with the design and operation of the HVAC systems.

Please circle the appropriate number on the Reader ServiceCard at the back of the publication.

Extremely Helpful....................................................... 458

Helpful ..................................................................... 459

Somewhat Helpful ..................................................... 460

Not Helpful ............................................................... 461

Figure 3: Daily average makeup air.

Table 5: Comparison of HVAC system performance.

Figure 2: Daily peak makeup airflow.

Figure 4: Ventilation system performance.