Appendix D

D.7.3 Mechanical System Design

The HVAC systems maintain a comfortable and healthy indoor environment by responding to the loads imposed by the building’s envelope design, lighting system design, and occupant activities. Proper design of the control schemes for the systems that heat and cool the interior spaces, provide fresh air for the occupants, and remove contaminants from the building will ensure that the HVAC system operation complements the architectural and lighting designs and minimizes building energy consumption.

D.7.3.1 HVAC System Zones

Determine the minimum conditions, such as temperature and humidity requirements, for all spaces in the buildings. Group spaces having similar space conditioning requirements into one zone, then separate the HVAC systems into zones based on these expected loads. When identifying HVAC system zones:

Separate systems serving office areas from those serving laboratory or process areas.
Separate areas with relatively constant and weather-independent loads (e.g., interior offices or interior light laboratory spaces) from perimeter spaces.

Identify the space loads to determine the capacity of the HVAC equipment for each zone. Typically, envelope loads dominate the heating and cooling loads of perimeter zones and equipment loads dominate the interior zone loads.

A peak load analysis may show the largest cooling loads occurring late in the afternoon because of solar gains through west-facing windows. It may be that changing the specified glass characteristics (to those that reduce the amount of solar gains entering a space, see Appendix D, Section 6.3.3 (Glazing Selection)) of these windows will help reduce the cooling load. Another solution could be to shade the windows from the outside with an architectural screen. Or, the solution may be to reduce the glazing area on the west facade. The following points exemplify why peak load analysis is important:

When using daylighting to offset lighting loads, the peak cooling month will often shift from a summer month (such as for a conventional building) to October. This non-intuitive peak loading occurs because the sun is low in the sky during October so that overhangs no longer shade the building, yet the daytime outdoor temperatures are still high enough that cooling will be needed. One solution to this late-season cooling load is to use outside air to cool the building, by means of economizer, natural ventilation, or precooling the building by night flushing.
A winter morning peak load may occur during the building warm-up period. One solution is to design a heating system to accommodate this peak load, but this system will then operate at part load for most of the time. Another is to downsize the heating system so that it is operating near full capacity during a typical heating day. Begin the morning warm-up period earlier in the day to decrease the system peak load to that which the system can handle. The system then has a longer time to heat the building before the occupants arrive. Compare the lifetime operating costs of these and other scenarios before determining the best solution.

D.7.3.1.1 Perimeter Zones

Placing the windows deep in the south-facing wall helps to shade the window to minimize direct solar gains and reduce perimetere cooling loads .

The wall, roof, and floor insulation and the heat transfer characteristics of the window glass will affect the perimeter zone heating and cooling loads. In a well-designed building, the architectural features of the envelope will shade the building to minimize direct solar gains and reduce perimeter-zone cooling loads.

It is likely that daylighting will be available in the perimeter zones. Interior zones may have daylighting if clerestories, roof monitors, light tubes, or other strategies are used to bring daylighting to the space (see Appendix D, Section 6.4 (Designing for Daylighting)). Remember to accurately evaluate how day-lighting will affect the zone loads. A good daylighting design will decrease the internal heat gains from operating electric lighting systems and introduce little or no adverse solar gains. Reducing the internal and solar heat gains decreases cooling loads and potentially increases heating loads.

D.7.3.1.2 Interior Zones

Internal Loads and Supply Air Flow

Internal loads from people occupying the space and waste heat from operating electric lighting systems and equipment in the space typically determine the interior-zone heating and cooling loads. Reduce internal loads as much as possible by reducing the lighting loads. Use efficient lighting fixtures and control the lighting to operate only to provide the lighting level needed when the space is occupied. Also, select energy-efficient equipment (e.g., office or process equipment), or recommend specific energy-efficient equipment for the user to purchase.

Plug loads are loads from the individual pieces of electrical equipment that can be removed from the building and powered through electrical wall outlets. Process loads are the loads produced by the process equipment in a space. When operating, both plug and process loads introduce heat to the space and increases load on the building’s cooling system. The first step to reducing plug and process loads is to ensure that the equipment is turned off or set to a “sleep” mode when not in use. For example, always enable the power saving features for computer equipment. In addition, it is best that all new equipment be ENERGY STAR® rated or of the highest available efficiency.

Be sure to accurately calculate the magnitude of plug and process loads. Consider the average instead of the peak energy use for this equipment when determining the maximum internal gains, unless all the equipment will be simultaneously operating at peak power draws. If the equipment to be used in a new building is similar to equipment in an existing building, base the estimated loads on the measured average energy use of that existing equipment.

D.7.3.1.3 Ventilation Systems for Zones

Ventilation air requirements often vary between zones. Ventilation is the use of outdoor air for controlling containment concentration by dilution or sweeping the contaminants from their source. Ventilation should meet the recommended values of ANSI/ASHRAE Standard 62-1999, 15 to 20 cubic feet per minute (cfm) per person, or the performance criteria described in the Standard using demand-control ventilation systems. It is also important to note that according to ANSI/ASHRAE Standard 62-1999, “Where peak occupancy of less than three hours duration occurs, the outdoor air flow rate may be determined on the basis of average occupancy for buildings for the duration of operation of the system, provided the average occupancy used is not less than one-half the maximum.” Spaces having intermittent or variable occupancy may then have lower ventilation rates than would be required if the peak occupancy were used to determine the amount of ventilation (e.g., a conference room). It is better to use CO2 sensing to control the amount of ventilation air needed during any particular period versus supplying 15-20 cfm per person for the average expected occupancy. Changing the ventilation rates with the changing occupancy will result in lower energy consumption and improved occupant comfort.

Demand-controlled ventilation reduces outside air requirements to the minimum needed for the actual zone occupancy when the zone would not benefit from economizer operation. Demand-controlled ventilation can greatly reduce the heating and cooling required for treating outside air. Carbon dioxide sensors are a useful indicator of the concentration of human bioeffluents and work well for regulating ventilation air rates. Use multiple sensors to ensure proper ventilation in densely versus lightly occupied spaces. As a general rule, place one sensor for the return air stream of the air handling unit and one sensor for each densely occupied space to ensure proper ventilation per minimum requirements and provide opportunities for increased energy savings.

Use higher levels of ventilation (e.g., economizers) as a substitute for mechanical cooling when ambient conditions allow for this “free” cooling. During the heating season, continue to use cool outdoor air to offset cooling loads that occur in interior zones. For the zones that require heating, reduce the ventilation air rates to the lowest volume possible and still maintain adequate indoor air quality to minimize the amount of cold air that must be heated before delivering it to the space.
It is common for separate zones within a building to experience opposite loads during the same period. For example, an interior zone may call for cooling while a perimeter zone requires heating, or an office zone may require very different conditions than a laboratory zone. To satisfy the space conditioning requirements of all spaces using the least amount of energy, separate the HVAC systems to serve zones with dissimilar heating and cooling patterns. Also, keep control zones small, especially when expecting a large difference in internal loads.

D.7.3.2 HVAC System Selection

Select the system type after competing a thorough analysis of the heating and cooling loads and the varying ventilation requirements of each zone. Evaluate several types of HVAC system options to identify the system that will satisfy the zone’s temperature and humidity requirements using the least amount of energy.

D.7.3.2.1 VAV Systems

VAV systems moderate space conditions by varying the amount of air delivered to the space. For most Fort Carson buildings, variable-air-volume (VAV) systems will best meet space conditioning requirements of each zone. This is true for both office spaces and laboratory spaces. For example, occupants of each office or group of offices may have varying temperature demands compared to their neighbors. Also, one laboratory may call for high levels of exhaust air flow, while a neigh-boring laboratory may be unoccupied and require very little exhaust airflow.

Use induction VAV units for interior zones and fan-powered VAV units with hot-water reheat coils for perimeter zones. Be sure to select a reheat coil rated for low-airside and low-waterside flow resistances. In addition, recommended perimeter zone VAV units are parallel flow” fan-powered (the VAV box fan is only on during the heating mode and is off during the cooling mode). Control the perimeter heating system operation so that it can maintain minimum space conditions during unoccupied hours without requiring the main air-handling unit (AHU) fan to also operate.
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D.7.3.2.2 Air-Handling Units (AHU)

The following are air-handling unit (AHU) design guidelines:

Fan selection - In most cases, vane-axial and backward-curved centrifugal fans are the most efficient AHU fan choice. Consider the rated acoustical properties, space limitations, inlet and outlet conditions, and air quantities/pressure requirements of the fan before identifying the best fan for the application.
Coil and filter selection - Select AHU coils for low airside and waterside flow resistance, low water flow rates, and operation at warmer chilled water or cooler hot water temperatures. Specify coil control strategies that will minimize water flow and maximize heat transfer. Pay special attention to the pressure drop of coils and filters. Limit face velocity to 450 feet per minute (fpm) for VAV systems and 400 fpm for constant air volume systems.
Hot and chilled water piping systems - Increasing the system pipe diameters and specifying low-friction valves reduces flow resistance through the piping and coils and decreases the system pumping energy.
Air distribution systems - Select air distribution components that offer the lowest pressure drop through the system. Large duct sizes provide low pressure drop and future flexibility if increased airflow is required. Try to minimize fittings such as elbows and transitions, since they have large pressure drops.

D.7.3.2.3 Air Distribution Systems

The two types of air distribution systems that are likely to be considered for most Fort Carson buildings are overhead and under-floor air distribution systems.

Overhead air distribution systems deliver conditioned air through ducts above the ceiling and then to the space through overhead diffusers. These conventional distribution systems typically deliver 50°F to 55°F supply air. They rely on the properties of the diffuser to throw the conditioned air to mix with the room air and maintain comfort at the occupant’s level (e.g., at desk level in a typical office space).

Well-designed overhead air distribution systems have little variance in the floor-to-ceiling space temperatures.

Under-floor air distribution systems use a plenum under a raised floor to distribute air to a space. The systems typically deliver 60°F to 65°F supply air through diffusers in the floor. The systems then rely on stratification to move the warm air above the occupant’s level to be replaced by the cooler conditioned air. There is typically a large temperature variance between the conditioned air temperature at the floor and the warmer air temperature at the ceiling. Under-floor air distribution systems can be installed with little or no first-cost penalty, and operational savings will occur over the life of the systems.

Under-floor air distribution systems can lead to improved thermal comfort, improved indoor air quality, reduced HVAC system energy use, and increased flexibility of the office space.

Increased personnel comfort is possible because personal control of airflow can be relatively easily incorporated into the system design.
HVAC system energy savings result from reduced ventilation air rates. Cleaner air can be delivered directly to the occupants without dilution with existing room air (displacement ventilation), compared to overhead systems that deliver air at the ceiling and then mix with room air.
The higher supply air temperatures of under-floor air distribution systems allow more hours of economizer operation, increase chiller plant efficiency, and decrease the run-time of mechanical cooling equipment.
The under-floor air distribution plenum often doubles as a wire management corridor, increasing the space reconfiguration flexibility.

D.7.3.2.3.1 Under-Floor Air Distribution Systems

The following are under-floor air distribution system design guidelines:

Evaluate under-floor air distribution systems early in the design process during the conceptual design phase.
Consider under-floor air distribution systems for spaces having a high density of information technology equipment, office spaces, or spaces that are expected to undergo frequent reconfigurations.
Minimize pressurized plenum air leakage by sealing all plenum penetrations and specifying low-air-leakage (tight) raised floor systems. Use VAV controls for all under-floor air systems to control plenum pressure. Use variable speed fan units and reheat in the perimeter zones if additional heat is needed in these zones. Deliver adequate supply air quantities to meet the loads. Supply air quantities do not differ greatly between conventional over-head and under-floor air distribution systems.

D.7.3.2.4 “Free” Cooling Systems

The high diurnal temperature swings and low humidity levels prevalent at Fort Carson are ideal conditions for “free” cooling . Free cooling is accomplished by delivering outdoor air to cool buildings instead of relying on mechanical cooling systems. These systems can significantly decrease compressor, cooling tower, and condenser water pump energy requirements as well as tower makeup water use and the related water treatment. Free cooling has the added benefit of providing a high level of ventilation air to a space, often resulting in improved indoor air quality.

Air-side economizer systems - A mixing box capable of handling 100 percent outside air integrated with the HVAC system is an economizer system. The amount of outside air brought in to the building through these systems is limited by the outside air conditions (usually just temperature in Fort Carson) or requirements for ventilation air (based on indoor CO2 levels). Note that laboratories requiring 100 percent outside air are always in “economizer” mode.
Nighttime precooling (night purge) systems - Use of nighttime precooling (night purge) reduces daytime mechanical cooling requirements. Flushing the building at night with outside air cools the building mass, which will stay cool through the beginning hours of building occupancy. When operating night purge system, let the building temperature float during the first part of the night then run the system fans for the few hours prior to occupancy to precool the building to the desired temperature.
Natural ventilation systems - Natural ventilation relies on the air movement through the space without the use of fans to cool the building. Consider natural ventilation early in the design process to ensure that the architectural design incorporates strategically placed, operable windows to accommodate natural ventilation systems. Many times, operable windows are automatically controlled to promote natural ventilation only when the outdoor conditions are suitable and to ensure that all operable windows close if fire or smoke are detected in the building. Using natural ventilation whenever an economizer is operating would also be appropriate.

D.7.3.2.5 Evaporative Cooling Systems

In evaporative cooling, the sensible heat in an air stream is exchanged for the latent heat of water. Many buildings at Fort Carson could be cooled by evaporative cooling methods alone.
Direct evaporative coolers (also known as swamp coolers) introduce some moisture to the air stream, subsequently reducing the dry bulb temperature of the outside air to within 5°F to 10°F of the wet bulb temperature. Indirect evaporative coolers provide sensible cooling only. Air cooled by water sprayed on the backside of a heat exchanger is separate from the air delivered to the occupied space.

Indirect/direct evaporative cooling systems pass air through an indirect evaporative cooling system heat exchanger to provide sensible cooling to the air stream before it reaches the direct evaporative cooling section of the unit. These systems are often sized so that small to medium cooling loads can be met with the indirect section operating alone. The indirect/direct sections operate together to meet larger cooling loads.

Both direct and indirect evaporative cooling systems can be modified to improve the performance of other cooling systems. Direct evaporative cooling systems can extend the range of economizer cycles by pre-cooling the air stream. An adaptation of indirect evaporative cooling systems is to circulate cooling tower water through a coil installed in an AHU to provide sensible cooling. This type of indirect system if often augmented with a chiller to provide enough cooling capacity to meet peak loads.

The Coolerado Cooler™ is a new evaporative cooling technology that can deliver cooler supply air temperatures than either direct or indirect evaporative cooling systems, without increasing humidity. The technology has been described as an “ultra cooler” because of its performance capabilities relative to other evaporative cooling products.

The Coolerado Cooler™ evaporates water in a secondary (or working) airstream, which is discharged in multiple stages. No water or humidity is added to the primary (or product) airstream in the process. This approach takes advantage of the thermodynamic properties of air, and it applies both direct and indirect cooling technologies in an innovative cooling system that is drier than direct evaporative cooling and cooler than indirect cooling. The technology also uses much less energy than conventional vapor compression air-conditioning systems and therefore can be a cost- and energy-saving technology for many Federal facilities in the United States.

Performance tests have shown that the efficiency of the Coolerado Cooler™ is 1.5 to 4 times higher than that of conventional vapor compression cooling systems, while it provides the same amount of cooling. It is suitable for climates having low to average humidity, as is the case in much of the western half of the United States.

D.7.3.2.6 Ventilation and Exhaust Systems

Rooms with exhaust air systems, such as kitchens and restrooms, can draw air from adjacent occupied spaces to replace the exhausted air. This approach has two benefits:

Rooms where odors may be an issue are kept at lower pressure than surrounding spaces, minimizing the potential for odors to spread.
Ventilation air is distributed to occupied spaces before being exhausted, thereby reducing the required ventilation air.

Consider potential contamination sources when locating outside air intakes for building ventilation air. Exhaust fan discharge, plumbing vents, cooling towers, and combustion products from vehicles and equipment e.g., boilers and generators) are examples of contamination sources. Perform effluent plume models using wind tunnels or Computational Fluid Dynamics (CFD) software programs to predict plume paths and help locate air intakes and exhausts.

D.7.3.2.7 Air-to-Air Energy Recovery Systems

Air-to-air energy recovery opportunities exist at Fort Carson in buildings with high ventilation loads. In air-to-air energy recovery systems, exhaust air and outdoor air both pass through a heat exchanger where the exhaust air pre-conditions the outdoor air entering the building. These systems reduce the heating and cooling peak energy demands and can reduce the heating energy consumption of buildings by 30 percent to 50 percent.

Air to Air Sensible Energy Recovery System Using run-Around coils.

Effective use of energy recovery devices results in decreased loads on the heating and cooling mechanical equipment. Equipment with reduced capacities can then be purchased. The savings gained from purchasing smaller equipment often exceeds the first cost of the energy recovery devices.

Air-to-air heat exchangers increase the fan power needed to supply the outside air to the building and to discharge the exhaust or relief air from the building. Even though the fan energy increases, the total energy use of the system decreases because the overall heating and cooling system energy use decreases. Including a bypass damper to redirect the air around the recovery device when the outdoor conditions do not warrant energy recovery improves the performance of these energy recovery systems.

Locate the outside air intake riser and the exhaust or relief air riser in close proximity to one another to further improve the performance of energy recovery systems. To do this, it is important to coordinate plans for energy recovery systems early in the design process.
There are two typical types of air-to-air energy recovery: sensible and total. Sensible energy recovery systems transfer only sensible heat. Total energy recovery systems transfer sensible and latent heat. Because of the dry Fort Carson climate, latent heat recovery is typically not important unless the building requires a minimum humidity level.