HVAC

Case study: Flexible, reliable HVAC system

A quiet, efficient, and flexible HVAC system replaced an older one in this government office building.
By Randy Schrecengost, PE, CEM, Stanley Consultants, Austin, Texas February 1, 2019
Figure 3: A preliminary dual duct air handling unit (AHU) selection elevation view. Courtesy: Stanley Consultants Inc.

A 60,000-sq-ft government administration building constructed during the mid-1950s, comprised of a basement and four floors, had two constant volume multi-zone dual-duct (MZ-DD) air handling units (AHUs) to provide both cooling and heating. The AHUs were located in separate mechanical rooms, one in the basement while the other was on the top floor.

As illustrated in Figure 6, these DD AHUs both had a hot deck (HD) and cold deck (CD), with corresponding hot and cold medium-pressure duct mains that served all floors within the building. The DD risers of both HVAC systems were interconnected near the middle floor within a vertical shaft in a central location. Both systems were supplemented with old constant volume DD induction mixing boxes for zone temperature control.

Figure 6: This schematic illustrates multiple options for component positions within a dual duct air handling unit (AHU) serving multiple zones. Courtesy: Stanley Consultants Inc.

The return air (RA) pathway was by way of plenum return, with RA traveling up or down through the open space around the hot deck and cold deck riser ductwork with the central chase. Conditioning of the airstreams was provided by chilled water and steam heating coils, but unlike Figure 6, which is a newer version of a DD system, there was only one single constant volume, double-width double-inlet (DWDI) centrifugal fan in a blow through arrangement.

This building’s mechanical HVAC systems’ AHUs and DD VAV boxes were replaced primarily due to system age and to improve airflow distribution and temperature control in the building’s HVAC zones. Because of budgetary constraints, and the low floor-to-floor heights of the existing concrete structure of columns and floors, the main duct risers and branch main ducts remained in place.

Figure 6: This schematic illustrates multiple options for component positions within a dual duct air handling unit (AHU) serving multiple zones. Courtesy: Stanley Consultants Inc.

Figure 3 is an illustration of the new VAV DD AHUs that were installed. The new units incorporated relatively new technology using an array of plenum fans, recognized as a fan-wall, that allow for quiet and more efficient operation at variable speeds, flexible configurations, and increased reliability and redundancy. The AHUs fan-wall’s function is to operate to provide the system’s required airflow at all times, however, if the fans are selected correctly, each fan will operate at less than its maximum capacity. If and when one fan is taken out of service, the remaining fans will speed up to continue to provide the AHUs airflow requirements and overcome any shortfall from the offline fan.

Figure 10: A double-duct variable air volume (VAV) box may be found in a double-duct air handling unit (AHU) system’s HVAC zones. Courtesy: Stanley Consultants Inc.

Figure 10 shows a simple control schematic of the new DD VAV boxes. The overall system renovation also included new ductwork from the branch mains to the DD VAV boxes. Also included were electrical and DDC upgrades to enhance the systems’ functionality and allow for remote monitoring and control from the central utility energy management system.

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Randy Schrecengost, PE, CEM, Stanley Consultants, Austin, Texas
Author Bio: Randy Schrecengost is the Stanley Consultants Austin mechanical department manager and is a principal mechanical engineer. He is a member of the Consulting-Specifying Engineer editorial advisory board.