Prescribing VAV Diffusers
CUH2A was fortunate to be the AE firm for this project, participating in the selection and design process. A review of that process will explain the goals particular to Pfizer’s work, the other factors which led to VAV diffusers, and why fintube perimeter heat was featured in the final design. We should also mention that when CUH2A began this project, we had no prior experience designing an entire project with VAV diffusers. CUH2A had used them in conference rooms, and had no predisposition toward them.
Background with PfizerCUH2A began by understanding how the client, in this case Pfizer, uses the space. Before bringing a drug to market, Pfizer is required to run clinical trails, working with doctors who conduct trials on a small scale. The studies require Pfizer personnel to continually communicate with these doctors to monitor how the drugs are performing. Teams working on the same study are grouped together in the building. To keep a study pure, phone conversations must be very private. If overheard, they could influence a person in a neighboring office working on the same study. Therefore, these people needed individual offices for privacy.
When a study is completed, the teams are disbanded and the members assigned to new study teams. This requires relocating them into the office grouping for the new study. As a result, individual offices periodically change occupants.
Our first Pfizer project in 1993 was an 80,000-sq-ft building for these study teams in Groton, CT. Pfizer had experience with water source heat pumps (WSHP) so it was natural that we used them in this building. However, the WSHPs used were plagued by electronic control problems and they did not provide the environment Pfizer wanted. As a result, when the next building was discussed, Pfizer was no longer comfortable with WSHPs.
In 1996, the study teams outgrew their building and a 200,000-sq-ft facility was built in an existing warehouse. Pfizer continued to want individual temperature control or close to it. Chilled water became available to the site so fancoil units were used. One fancoil unit served four offices. This resulted in disagreements on the thermostat setpoint, causing Pfizer to look at other options when the Pfizer Global Research and Development (PGRD) building was discussed.
Selecting the PGRD SystemBy 1998 the study teams needed more space, and a 750,000-sq-ft building was planned with 1,800 private closed offices. It was built on a brownfield site in New London, CT, across the river from Groton. Because it was their third facility in ten years, Pfizer was very involved in the construction and design process. Their goal was to produce the most productive work environment possible.
This quest started with a simple 9- by 12-ft office. Approximately 1,800 of these offices would be divided into smaller groups or villages. Everything, including amenities, was worked around the villages.
The most productive work environment was defined as having cost-effective design, plus reliable services, plus consistent environment.
As CUH2A continued to talk with Pfizer, the focus remained on the key items of energy efficiency, minimum operating cost, and low maintenance. The consensus was that if we could create good HVAC systems, we would have created the enhanced PGRD workspace we were seeking. Twelve HVAC systems were proposed and evaluated. Four of these systems used VAV diffusers. Also included were two- and four-pipe fancoil systems and WSHP systems.
To evaluate the various HVAC systems, criteria were listed and given weightings. Although some were modified as we went through the evaluation process, the criteria that everyone kept discussing were level of comfort summer, level of comfort winter, and size of thermal zones.
VAV DiffusersCUH2A turned some of its own offices into a test facility. VAV diffusers from several manufacturers were tested. The thermally powered versions proved to be simpler and less expensive to install. They gave us an increased level of comfort, individual temperature control, and eliminated subcooling of unoccupied offices.
Pfizer also wanted to see a working facility with thermally powered VAV diffusers, so we visited another pharmaceutical project with VAV diffusers. While in one of their huddle rooms, maintenance was asked, “What is the maintenance on this diffuser?” He answered, “What maintenance? I haven’t touched it in the ten years since it was installed.” This answer was a big factor in everyone’s increased comfort level with VAV diffusers.
Final SelectionThe team was now ready for the final decision. The scope was narrowed to six HVAC systems. Floor-by-floor AHUs, fancoil units, and WSHPs were dropped because in preliminary tallies they did not score well. The weightings were also revised based on the accumulated experience of Pfizer’s facility and maintenance groups. What did not change was level of comfort summer, level of comfort winter, and size of thermal zone being the top weighted criteria.
Each of the six systems was rated, and the scores were totaled. Central built-up AHUs with VAV diffusers and hot water perimeter fintube radiation had the highest score. This system was found to be the most economical, most maintenance friendly and, most importantly, provided the most comfortable environment for the users.
Note that central built-up AHUs with VAV diffusers and perimeter air system scored second. Fintube radiation was preferred over an air system, since the former is traditionally used in the Northeast because of colder winters.
PGRD DesignBecause fintube perimeter heat handles the skin heating load, the only other heating necessary was morning warm-up. Therefore, we used VAV diffusers, which modulated when cooling and provided constant volume heating. To allow individual users to adjust their personal environment, each office has an adjustment dial on the wall to adjust the temperature setpoint of the VAV diffuser. One dial can adjust several diffusers in the open areas.
Each group of six offices is served by one static pressure control damper/pressure independence module (PIM). The PIMs also serve VAV diffusers in the open areas. CUH2A limited the number of diffusers on one PIM to no more than twelve. Each PIM controls static pressure measured in the duct downstream to 0.15 in. wg.
The ducts between the PIMs and the diffusers are sized at a pressure drop of 0.05 in. wg per 100 ft, so that 0.10 in. wg static pressure can be maintained at the last diffuser. This low pressure ensures that the static pressure remains below a level where the VAV diffusers become noisy.
Each AHU has a VFD to maintain static pressure in the supply ductwork upstream from the PIMs.
The ResultOnce the PGRD building was occupied, it became apparent that the actual loads were less than the design loads. This was primarily due to the use of more efficient laptop computers instead of workstations, fewer centralized printers combined with copy machines, and high-efficiency lighting. Leakage became an issue and CUH2A found leaks around PIM damper frames and damper linkages that tended to stick open. (The dampers had not been supplied by the PIM manufacturer.)
All of these items were corrected, along with some miswiring between the temperature adjustment dial and the VAV diffuser and other minor problems. In such cases, the consulting firm and owner rely on the support of all parties involved to remedy the occasional problem; between the engineers (CUH2A), the manufacturer (Acutherm), and its local reps, all was made right and the installation was successful.