Handbook of air conditioning and refrigeration / Shan K

30.12 CHAPTER THIRTY
Case Study: A Fan-Powered VAV Central System
A fan-powered VAV central system was designed and constructed for the 34-story office building of
the Taipei World Trade Center, Taipei, Taiwan. It has a gross floor space of 1.05 million ft 2 (97,580
m 2 ) and a total air conditioned floor space of 880,310 ft 2 (81,810 m 2 ). The peak refrigeration load of
the building is about 2530 tons (8890 kW). The local utility rate structure strongly favors the use of
the refrigeration system during off-peak hours. This project won first place in the 1991 ASHRAE
Technology Awards for HVAC&R system designs for commercial buildings. The entrants were
Hsing-Chung Yu and Carl E. Claus.
Refrigeration System. The ice storage system was a partial storage one (ice storage systems are
discussed in the next chapter). A separate flooded liquid cooler was connected to the screw compressors
to provide direct cooling during on-peak hours as well as during after-hours operation, to
offset the significant nighttime load. During direct cooling, the compressors were controlled to operate
at a higher suction temperature of 33°F (0.56°C) to conserve energy.
A liquid overfeed system was used instead of direct expansion during the ice-making period
because of its higher heat transfer. About 20 percent of the refrigerant coil surface in the ice
builders could be saved. During ice melting, chilled water was supplied directly through a closedloop
water circuit to the storage tanks, and the tanks were pressurized. Such an arrangement obviates
the use of a heat exchanger and thus avoids the corresponding chilled water temperature
rise.
Condensing heat was efficiently rejected through evaporatively cooled condensers.
Air Systems. Because of the lower chilled water temperature from the storage tank during ice
melting, cold primary air at 45°F (7.2°C) was introduced at the parallel fan-powered unit. It was
mixed with the induced plenum air to produce a supply air of 56°F (13.3°C) at the diffuser. Such a
lower primary air temperature had the following consequences:
● Summer space relative humidity dropped to 40 to 45 percent. The thermal comfort of the occupants
was greatly improved.
● Cold primary air volume flow rate was reduced to 40 percent compared with conventional air distribution.
Two built-up centralized AHUs were installed in the basement. Each was used to supply 200,000
cfm (94,380 L/s) of primary air from the bottom to the top of the entire building. Compared with
the originally proposed conventional system with additional equipment floors at mid-level, this air
distribution system saved considerable rental space.
The centralized air system, having floor-by-floor automatic shutoff dampers, was closely
matched with the roof-mounted smoke control system. In case of a building fire, the return fan
would be stopped and the rooftop exhaust fan would be started to purge the smoke from the fire
floor through the opening and closing of dampers connected to the smoke exhaust system. At the
same time, air would be supplied to the two floors above the fire floor as well as one floor below to
pressurize these floors.
The air system also did the following:
● Provided 15 cfm (7.1 L/s) of outdoor air for each occupant
● Included cartridge-type filters and upstream prefilters with a dust spot efficiency of 85 percent
● Used controllable-pitch axial fans for supply and return air
● Included electric heaters for winter heating
A microprocessor-base DDC system was used for zone temperature; AHU operating parameters;
ice-charging, discharging, and direct cooling; and emergency smoke controls.