Hospital’s pretreat units keep patients healthy

With no exterior wall from which to directly access outdoor air (OA), roof-mounted pretreat units provided the correct amount of ventilation air to meet the hospital’s needs.

By J. Patrick Banse, PE, Smith Seckman Reid Inc., Houston November 18, 2015

This hospital project, completed in 2007, involved the addition of approximately 550,000 sq ft to an existing 850,000-sq-ft building. The primary components were an outpatient cardiovascular center, Level 1 trauma center/emergency department, interventional cath lab suite, and more than 400 inpatient beds including a neonatal intensive care unit (ICU), pediatric ICU, cesarian-section and labor-and-delivery rooms, and medical/surgical/critical care unit/ICU beds. Two nursing-unit towers housing the 400 beds—one 5-story and one 8-story—completed the project. Connections to the existing hospital buildings on five levels were also included.

One goal in planning for the patient towers was to minimize the floor-plate area and maximize the number of patient rooms with the required window openings at the exterior. This design created some unassigned floor space in the core, which was used for mechanical HVAC air-handling rooms and shaft space. Stacked mechanical rooms were created on each floor to house an air-handling unit to serve that floor. With no exterior wall from which to directly access outdoor air (OA), the solution was to use OA pretreat units to provide the correct amount of ventilation air to meet code-mandated minimum air-change rates, meet exhaust requirements, and provide for building pressurization.

Three dedicated outdoor air system (DOAS) pretreat (PT) units were selected to supply the air; PT-1 at 61,000 cfm for the north tower, PT-2 at 42,000 cfm for the south tower’s air handlers, and PT-3 at 28,000 cfm for critical-space air handlers serving specific areas of levels two and three. Each of the on-floor air handlers was also set up for air-economizer operation with a return/relief fan and relief-air ductwork routed vertically in shafts to the roof.

The three DOAS units were roof-mounted. Figure 4 shows PT-2 in the far position and PT-3 at the lower right. The supply ductwork from each unit was routed vertically in a shaft with duct connections supplying the minimum OA directly to each floor’s air handler. An airflow-monitor station was used to measure the required airflow. This arrangement prevented over-sizing of the building air handlers with the PT units, delivering constant OA to each floor’s air handler. A separate maximum OA duct for each unit supplied air from an OA duct shaft with modulating damper for economizer operation. The unit placement and design reduced the overall mechanical-space square footage required, maximized the functional floor space available, and eliminated exterior wall penetrations for louvers.

The pretreat units supplied air at 55 F dry bulb/54 F wet bulb conditions, which was adjustable through the BAS. Chilled water was used for cooling, and low-pressure steam used in the face and bypass heating coil. Figure 5 shows a typical indoor unit. The DOAS unit supplied the correct amount of OA to the air-handling units while minimizing over-airing of the spaces served. The multiple-zone recirculating system calculation was used to determine the OA totals. Building pressurization was maintained, which minimized untreated OA infiltration. The building pressurization and minimum air quantities were validated 1 yr after the project occupancy and have continued to operate effectively.

J. Patrick Banse has more than 35 yr of experience in the consulting engineering field and is part of Smith Seckman Reid’s technical guidance team involved with codes and standards updates and QA/QC programs.