Designing HVAC systems for government buildings covers a spread of occupancies that range from office spaces to laboratories, detention facilities, high security areas, data centers, and military bases. Long life is one common requirement of design. However, over the life of a building, its systems will need to be revised and often modernized. Many existing building are visually historic in nature, and that historic appearance must be maintained. Other buildings must be also expandable either vertically or horizontally.
Guidelines and StandardsMost governmental agencies have developed a design guideline or manual of standards for their facilities. The purpose of these documents is to aid the design team to stay on a common course of system design throughout the agency. The published materials are utilized to define systems and lists of equipment that have a history of success in meeting the client’s goals of comfort, dependability, and economy of operation. The downside of this approach is that newer technologies are not often considered. The good news is that in today’s world, high LEED® ratings are now a new design goal.
Old Historic BuildingsThe City Center Court House is a typical historic building. Historic buildings have been around for a long time and will be around for many years to come. However, these facilities often require a new communication center, a data center for the client’s computer network, open offices that will be used to take advantage of cubical flexible layouts, improved IAQ for the greater number of occupants, improvements to meet current codes, and/or reduced use of operational energy since green is often the theme throughout the renovation.
The look of modern HVAC systems must blend in with the old historic appearance of the space. The first change is often the conversion from steam heat in the radiators and built-up AHUs to a hot water system. Hot water is piped to existing cast iron radiators with the addition of either all new piping, or at least new piping, for the replacement of the old return condensate piping which is too small for the return piping. The old built-up units are replaced with new modular factory-built AHUs that feature hot-water heating coils, improved efficiency filter banks, either chilled water or DX cooling coils, variable-speed motor/fan systems, and a DDC system that can be adjusted and monitored from a central location.
The old radiators still work well, however, the client may want to improve the look of the units by covering them with a wood or metal screen. This screen addition still tells a message of being old, but it looks better than the rough dirty cast iron radiators with all their exposed piping, valves, and controls.
If new ceilings are added as part of an office modernization, hot water radiant ceiling panels may be better than replacing old radiators with fin tube radiation units. Ceiling heating panels also work well with modular office furniture.
HVAC systems in historic buildings often began just as heating/ventilation systems. When air conditioning became a need, the existing distribution ductwork was modified by adding a cooling source (chilled water or DX coils) and insulating the ductwork to prevent condensation. To maintain the old look most of the terminal air outlets were changed to bronze-colored grilles. The use of the bronze-colored outlets and return grilles should be maintained in any modernization projects.
The downside of this type of air system is that sidewall grilles tend to blow toward the occupants and often result in draft complaints. Old ceiling-type air outlets generally were decorative plaster openings. The air pattern from these terminals is a low-velocity discharge in which cool air “falls” into the occupied space. Floor air returns also were used to pull the air downward from the high ceilings used in the historic buildings. The addition of air conditioning still works with this airflow pattern. However, heating with the air system in this non-velocity-induction flow arrangement of high-supply, low-velocity discharge is not successful. The use of low-return air openings does reduce the cold feet complaint.
Another successful method of adding cooling to a space involves high-velocity small unit packages with small, round air outlets. Even though the system is noted as a high-velocity type, which would suggest a higher space noise level, the systems can be designed for very quiet operation. The source HVAC units fit well into attic spaces, the small duct diameters run through confined above-ceiling spaces, and the visual appearance is a small round outlet in the plane of the ceiling.
Reducing Energy Usage and Increasing ComfortThe traditional HVAC system in buildings constructed in the ’70s and beyond was a constant-volume zoned system, with reheat coils for adjusting the space comfort temperature for heating and cooling. As more zone control and less energy became a new design goal, the air systems of the ’80s and beyond were the VAV (flow) type.
Dual-duct systems and multi-zone systems lowered fan hp by the variable volume concept. Hot water hydronic systems further reduced energy by introducing variable flow pumping systems. The use of variable-speed controllers on the fan and pump motors did reduce energy usage. When DDC was added to the chillers, chilled water pump hp reduction was also possible. This variable pumping flow through the chillers also has reduced the number of systems using the primary/secondary design concept. Variable primary pumping systems use less pumping hp.
A constant volume reheat system can be economically modified by removing the zone reheat coils, sealing up the existing ductwork above the ceiling to reduce the duct leakage, and then joining the ends of the zones into a loop-type supply system. Then new VAV zone units are connected to the modified existing cooling loop to offer more comfort at the use of less energy. A perimeter system of heating is either added or modified to condition the building’s perimeter zones.
logging On, Heating UpData centers have been added to buildings to allow the client to use a computer network rather than having many desktop units adding heat gain into office zones. The data center units operate on a 24/7 schedule vs. the daily operational hours of the office staff. These data centers also contain a UPS system and the telephone switch, which adds to the cooling load of the space on a 24/7 basis.
Buildings in the ’90s incorporated the data center space with a raised floor for air distribution and cable management. However, buildings of the future will need to be designed to meet higher server densities. This higher-density rack concept will require the use of small chilled water fancoils within the racks to remove the high-density blade type server heat.
Small rooms that contain data center racks should be arranged to blow cooling air down in front of the racks, and the ceiling return grilles should be located behind the racks to capture the rising thermal plume. This is a simplified arrangement of the cold aisle/warm aisle arrangement in larger data centers with the raised floors.
Ventilation SecurityA terrorist attack may be a destructive blast or a chemical intrusion into the air or domestic water systems. To improve the durability of piped and ducted HVAC systems, seismic supports are recommended as standard in facilities that consider homeland security as a design goal. The outdoor air louver location was noted as the easy way for a chemical introduction into the air system. Dummy outdoor air louvers have been installed to create uncertainty as to the true outdoor air path into the facility. Chemical powder protection filters have been installed in high-security facilities.
All new buildings locate the intake louvers in places that make unauthorized access difficult. The intake dampers are also low-leakage types, connected to an emergency shutdown control/alarm system. The alarm system is also connected to gas sensors in the outdoor air intakes.
Detention FacilitiesThe range of this type of a facility is from community jails, cells in the juvenile center, holding cells in court houses to large campus prisons. The common goals of the HVAC system is to secure the ducts being used as a conversation tube, to being able to pass things through the air openings, and in larger ducts to be able to crawl through the duct to an escape. The typical duct design is to use small offset ducts to combat conversation and pass-through. Security bars are provided in the larger ducts to prevent a potential crawl/escape path.
Smoke Contol SystemsLarge government buildings often have a multi-story atrium, and the facility may also be classified as a high-rise, which will require pressurized stairwells for safe exiting. Detention facilities often require a building level to be divided into two zones, so in case of a fire, the inmates can exit into the safe zone protected by smoke/fire partitions and a pressurized air system. The atrium smoke control system comprises a calculated exhaust rate, a replacement air system, smoke/sprinkler flow alarms to activate the system, and a control network. The control network will open/close the appropriate fire/smoke dampers, recall the elevators, alert the fire department, and activate the smoke exhaust fan(s).
The smoke control systems are a life safety feature, and the International Building Code requires them to be commissioned by a special inspector or special inspection agency. Formal testing and certified reports are prepared by the special inspection team.
LaboratoriesState health and agriculture departments have extensive laboratories. A modern laboratory ductwork layout should be designed for changes that occur often in this field. The ductwork should have capacity flexibility in both the supply and return duct mains. The lab hood exhaust should be variable flow. The general laboratory working space also has a general exhaust to control the IAQ. Heat recovery is possible from the general exhaust stream. Coordination from a general team leader is required to prevent over design requests from various departments.
The exhaust fan discharge should be the high-velocity, upblast type to prevent recirculation of any contaminants from entering any outdoor air intakes in the vicinity of the building.
Energy Saving Technolgy for Today's Green DesignA currently popular open office HVAC system is the underfloor air distribution system (UFAD) in which the air is supplied into the underfloor plenum and then transferred to the space with special floor diffusers controlled by the occupant in the area.
However, these systems require coordination of the underfloor wall construction, for vapor barrier protection, use of fire/smoke rated cabling and slab cleaning/sealing for a dirt free plenum
Another increasingly used option is the dedicated outdoor air system (DOAS). The outdoor air is introduced into the air stream directly after passing through a filter and heating/cooling unit. The general underfloor air is filtered and cooled to a warmer temperature (65°F) than the normal 55°, which avoids the use of reheat for the cooler air. The air is delivered at a lower velocity which reduces the fan horsepower energy.
The building’s perimeter system takes the air from the plenum and delivers it, through fancoil units, to condition the exterior heating zones.
Radiant heating/cooling hydronic panel systems can also be used in the perimeter conditioning arrangement. The chilled water panel system can also be used in interior spaces with relative constant and stable heating/cooling loads. This new system is labeled a passive chilled beam. The chilled beam technology requires close control of the space dewpoint temperature to avoid ceiling drips. A DOAS system can be connected to the chilled beam and is noted as an active beam system.
Hot water in-slab radiant heating systems are also appropriate for a single-story government building.
Air-to-air heat recovery systems can capture heat from the toilet exhaust and general exhaust from most buildings. Water-to-water heat recovery systems can capture condensate heat from district steam systems. Water-to-water “free cooling” (winter chilled water production) is also readily available in the Northern climate areas.
Geothermal condensing loop systems should be considered for facilities constructed on sites available for wells, lakes or slinky field piping arrangements. Water–to-water and water-to-air heating/cooling units are available for connection to the geothermal condensing source.
ConclusionThe basic principle for designing modern HVAC systems in old buildings is to be able to blend the modern system types into the historic old look of the building. This process does take thought and market analysis as to what is available. The system selected should have been successful in providing comfort conditions at reasonable first and continuing operational costs. A life-cycle cost analysis should be included in the proposed design options.
The HVAC designer also needs to consider the operational abilities of the building’s maintenance staff. New systems contain many controls that need to be monitored and adjusted to obtain the planned for energy savings, however in the real world, the controls are usually “dumbed down” to the abilities of the operators. This results in energy saving design funds being wasted. Education and training of the operational staff must be incorporated in the new or remodeled project. This work is often part of system “commissioning” as well as future recommissioning efforts
Economic energy usage and wise reuse of construction materials is the environmental design goal in today’s world. The current model building codes (international and uniform) also guide the design team down this path. The goal of green facilities and gathering of LEED points for government projects is only becoming more important as a design goal in today’s engineering environment. ES