Case study: A tale of two towers
The first example (see Figure 1) shows the penthouse of a 5-story hospital bed tower, with the two AHUs shown serving the top 3 floors. Several important design elements include:
Fan arrays for both supply and return to provide redundancy upon the failure of a single fan.
Hot-water coils for preheating supply air with a circulation pump to protect against freezing with the high percentage of outside-air required.
Chilled-water coils to provide cooling and dehumidification.
Steam humidifiers to provide humidification when needed.
Care was taken to position the AHUs and other equipment to provide the required maintenance-service clearances within the penthouse. In addition, to account for prevailing winds coming from the west, the outside-air intakes were positioned on the western side of the penthouse while the relief-air and exhaust-air discharges were located toward the east and south ends of the building. The cooling towers were located on the far east end of the building, maximizing the distance between the cooling tower discharge and the outside-air intakes. The two units were cross-connected with ductwork and motorized dampers on both supply and return ducts, providing partial redundancy in the case of a unit failure.
A second example (see Figure 2) provides a different approach for providing service clearances for AHUs installed in a penthouse of a 6-story hospital bed tower, with two AHUs—each one serving half of the building. The use of a water economizer (instead of an air economizer) and lack of a return fan resulted in AHUs nearly as wide as they are long. While both units fit in the penthouse, the same was not true of their service clearances, even while using split coils for reduced pull-space requirements.
The unit to the north had service clearances that could be accommodated within the penthouse. The unit to the south only had limited service clearances available in the penthouse; however, there was ample roof space outside to accomplish what was lacking inside. The careful placement of a removable, but otherwise nonfunctioning, louver adjacent to the AHU coils provided clear access when needed.
Matt Chandler is a project manager and senior mechanical engineer for IMEG Corp., where he leads complex projects for health care and research laboratory facilities.