Spears/Votta & Associates, Inc. received an Outstanding Achievement Award from the Engineering Week Council of the Baltimore Engineering Society for the HVAC renovation of Friendship Annex - FANX 2 and 3.
This project consisted of the total renovation of two neighboring buildings totaling approximately 743,000 square feet. FANX 2 is a low-rise (2-story) building. The existing HVAC system consisted primarily of refrigerant-based (DX) equipment and central station constant volume air handling units. The heating system utilized gas fired hot water boilers. FANX 3 is a high-rise (7-story) building. The existing HVAC system consisted of a conventional chilled water plant, gas fired hot water boilers, and central station multi-zone air handling units. By virtue of age and design, design of both systems were considerably energy wasteful. Both buildings are approximately 25 years old and existing equipment was original and in need of replacement or upgrade.
One of the primary design goals for the renovation of FANX 2 and FANX 3 mechanical systems was to enhance energy efficiency and demand-side reduction through the use of new technology and in conjunction with the new Baltimore Gas and Electric Company (BGE) incentive programs. Given the aggregate size of these buildings, Spears/Votta & Associates (SVA) recognized the potential, both for significant energy savings by the owner, and demand side reduction for BGE with application of the proper systems/technologies. Therefore, thermal ice storage, energy efficient lighting, application of variable frequency drives (VFDs), high efficiency motors, low temperature cooling systems/distribution, and other energy saving features were incorporated into the designs. For each building, the energy efficient system was compared with other, more conventional systems, and proven, through Life Cycle Cost Analyses, to be the most cost effective. The following sections provide more in-depth discussion on the technical and energy conservation aspects of this design.
FANX 2 encompasses approximately 280,000 gross sq. ft. of floor area with occupancies which vary from offices to classrooms to electronics laboratories and computer rooms. It is expected that 2,800 people will occupy the building, each requiring ventilation air. The equipment in the building will emit heat at an average rate of approximately 6.8 watts per square foot. This is equivalent to the heat of 60 watt light bulbs spaced 3'-0" apart throughout the entire building. When considered with heat gains from people, lights and 95º F outside air for ventilation, this results in a peak cooling load of 1,124 tons; or, over the course of a 24-hour day, the cooling required is 14,056 ton-hours. In contrast to the 1,124 ton cooling load described above, the cooling required at night it quite less, only 104 tons.
FANX 3 contains over 463,000 gross sq. ft. of floor area with occupancies similar to those described for FANX 2 above. Average equipment related heat in FANX 3 is approximately 12.1 watts per square foot (60 watt light bulbs spaced 2'-3" apart). The corresponding peak cooling load is 2,067 tons or, over the course of a 24-hour day, 27,669 ton hours. For this building, the cooling required at night is only 550 tons.
There were numerous unique design aspects and constraints including physical space availability and owner/tenant requirements. The most significant of these are described below:
As briefly mentioned before, pre-design studies determined that energy efficient lighting and thermal storage systems were most cost effective. Application of these systems moved this design outside the realm of "standard" practice, thereby negating some of the common "rule-of-thumb" check figures and design methods that this industry has come to rely upon. For example, chillers which are normally selected to generate 44º F water for cooling were required to generate 22º F water to build ice. But isn't 22º F water going to be ice? Not if it is actually a 30% ethylene glycol solution. But such a solution has a higher viscosity than clear water, thus affecting flow rates. Similarly, the use of ice for cooling allowed lower chilled water temperatures to be obtained (36º F). Such lower supply temperatures result in increased heat transfer capabilities of the chilled water system, by about 80%; therefore, flow rates of the chilled water system were drastically reduced. Reducing flow rates decreases energy consumption. Also, lower temperature water made it possible to generate low temperature supply air for air conditioning. Where standard design practices utilize 55º F supply air, the AHUs in FANX 2 and FANX 3 generate 42º F supply air, thus increasing the heat transfer capabilities of the air systems and lowering required air flow rates. These features also reduce energy consumption. For both the air and water-side systems, reduced flow rates translate into reduced fan and pump horsepower. Also, as a byproduct of lower supply air temperatures, lower humidity levels are obtained, resulting in greater occupant comfort.
What a great system! But it is not without drawbacks. Among these are the increased risk of condensation on piping and ductwork due to lower surface temperatures. Insulation performance and maintaining vapor barriers became critical. There was also the matter of delivering "cold" air to conditioned spaces. A greater potential for drafts is created by density and temperature gradients associated with cold air. Drafts could adversely effect occupant comfort. Ensuring air circulation at light loads, as well as maintaining minimum ventilation air for occupants, was another difficult issue. And, finally, a more sophisticated means of system control was required. All of these potential pit falls were addressed and resolved in the design of FANX 2 and FANX 3 Upgrades for systems which are energy efficient, flexible, comfortable, and cost effective.
The heart of each system is the chiller/thermal storage plant. The FANX 2 plant, serving 1,124 ton load, has an installed chiller capacity of 870 tons, and 5,470 ton-hours of thermal storage capacity. The FANX 3 plant serves a load of 2,067 tons with 1,270 tons on-line chiller capacity and 10,350 ton-hours of thermal storage. Basically, the utilization of chillers and ice is optimized based on peak electric rate hours (time-of-day metering), to build ice at night, when demand charges are relaxed and electricity is inexpensive and melt ice for cooling during those hours of the day when electricity is most expensive and demand charges are in effect. The result is an overall plant capacity equal to the cumulative cooling load over the peak electric rate period.
FANX 2 has an average nighttime load of 104 tons served by 870 tons of chiller capacity; therefore, after satisfying the night load, there is still significant chiller capacity left to build ice (456 tons over 12 hours, or 5470 ton hours).
FANX 3 has an average nighttime load of 540 tons which is satisfied by a dedicated nighttime chiller (36º F setpoint, 545 ton capacity), while two other chillers build ice (22º F setpoint, 421 ton capacity each). All three chillers are identical, all are connected to the ice plant and building chilled water system. Through automatic valving, any of these can be designated as the dedicated nighttime chiller. This nighttime chiller is normally deenergized during day time operation and becomes, in effect, a "standby" chiller even though it is not redundant.
For both of these plants, the chillers are York International, helical rotary screw type. Thermal storage units are modular and manufactured by Baltimore Air Coil. FANX 2 utilizes four (4) such units, all mounted on a concrete pad on grade. FANX 3, however, utilizes eleven (11) thermal storage units, four of which are set on concrete at grade. The remaining seven are supported in a steel framework and are stacked as many as three levels (35 ft.) high. This makes for an impressive structure considering each tank when full of water, weighs from 57 to 114 tons and is approximately the size of a tractor trailer. The vertical "stacking" arrangement was selected due to very limited space. This is the first multi-level installation of its kind in the Maryland area and perhaps in the entire east coast region! The presence of this ice plant, adjacent to the 7-story building is of such significance that it has been incorporated into the architectural design as a "signature" feature to be dressed up with special lighting, glass block, etc.
Less dramatic, but no less important are the air distribution systems, which bring the benefits of low temperature air and water to the occupant. As described earlier, central station air handling units in FANX 2 were reused including associated ductwork mains. As these units were originally conventional in design, major modifications were required in the conversion to low temperature systems. Among these were the replacement of the DX evaporator coils with 10-row chilled water cooling coils, the replacement of drain pans, motor replacement, external insulation, and application of VFDs. A task which was necessary prior to finalizing the VFD selection was to verify AHU fan performance at speeds much lower than original design, as might be expected when VFDs are applied. To this end, the Mechanical Project Manager organized and witnessed a mock-up during which a VFD was temporarily installed, connected and used to vary the speed of an AHU fan. Results indicated that VFDs could be applied without difficulty. These modifications turned old conventional, constant volume AHUs into an updated, energy-conserving, variable volume, low-temperature AHUs which will last another 25 years.
Reuse of ductwork mains presented challenges in the form of re-insulating to avoid condensation, and as obstructions to new work installation. However, by virtue of the low temperature air being delivered, the capacity of existing ductwork was increased. In other words, for equivalent flow rates of standard and low temperature air, the low temperature air has greater cooling capacity. To the existing ductwork, new terminal units were connected. Selection of the new terminal unit type was primarily driven by the desired temperature of air delivered to conditioned spaces.
Given that the reconditioned AHUs were generating 42º F air, it would seem most efficient to deliver that air directly to the conditioned space. At the time of the FANX 2 design, there were no commercially available, air diffusers, which were designed to do this. Conventional diffuser design was and remains intended for use with air temperatures at or above 50º F. This meant that the proper terminal unit should elevate the supply air temperature. Since simultaneous cooling and heating (reheat) is frowned upon by code, a fan powered, series flow, terminal unit was selected. This unit is capable of mixing primary (42º F supply) air with induced (78º F) plenum air resulting in 55º F supply air. The only additional energy required to elevate the supply air temperature was that associated with fractional horsepower terminal unit fans, of which there were hundreds in FANX 2. Also, these fans operate continuously for periods in which the building is occupied. The net effect is that energy is being saved by systems delivering 42º F air to the terminal unit. But, energy is then spent in elevating the supply air temperature prior to delivery to the space. All because the diffusers could not handle 42º F air. Thus, the weak link in an otherwise energy efficient system was the diffusers! Despite this, the application of low temperature systems, in conjunction with VFD technology yields an energy efficient HVAC system which will save the owner energy costs for years to come.
In contrast to FANX 2, the neighboring FANX 3 did not allow for reuse of AHUs or ductwork mains. In this building, custom AHUs were designed and applied. They also would deliver 42º F air, and utilize VFD technology. However, one considerable design improvement was implemented: the use of low temperature, induction diffusers. SVA, in conjunction with Titus Products and the local representative, J. K. Mechanical Products, put forth a major effort in studying the application of this brand new technology and product to the FANX 3 design. The study included full scale mockups of various loading conditions including 30 watt/sq. ft. loading and supplemental cooling below the floor; Life Cycle Cost Analyses; data gathering on temperature gradients, room temperatures, relative air motion, etc. All parameters of the study indicated that low temperature induction diffusers would be a benefit. Their application effectively eliminated the "weak link" described above, as 42º F air could then be delivered directly to the conditioned space. No energy was required at the terminal units to elevate supply air temperature.
Consequently, the type of terminal unit required for the FANX 3 air distribution system was redefined. In FANX 3, spaces without perimeter exposures are served by straight through VAV terminal units with no fan. Perimeter spaces still incorporate a fan powered terminal unit; however, the fan runs only intermittently, and only when no cooling air is being supplied. The elimination of hundreds of fractional horsepower motors, running thousands of hours per year, will have noticeable impact on energy costs over the life of the system.
As mentioned above, the high efficiency lighting program sponsored by BGE was incorporated in the design of each building. Under the guidelines of the program, existing building fixtures (four (4) 40 watt T-12 fluorescent lamps with magnetic ballast) were replaced with new three (3) lamp fixtures utilizing 32 watt T-8 Octron lamps and electronic ballast. A one for one replacement reduced the energy consumed by 100 watts a fixtures, and lowered the building lighting electrical load by 54 percent. The lower wattage consumed by each fixture saved not only in lighting electricity usage, but also in air conditioning tonnage required for cooling. Aesthetic comfort is also achieved with the use of Octron lamps and electronic ballast. Because the ballast operates the T-8 lamps at a much higher frequency than standard ballasts, there is no visible nuisance lamp flickering. The solid state components in electronic ballasts also eliminate the audible hum heard in standard coil and core magnetic ballasts.
As stated earlier, Life Cycle Cost Analyses were performed for each of the systems described above to evaluate their feasibility for both FANX 2 and FANX 3. Low temperature, thermal storage systems were compared to a base conventional system which would have reused much of the existing equipment in each building. From these and other analyses, SVA determined the electric peak shaving attributable to the thermal storage and high efficiency lighting programs which were ultimately implemented in design. A sampling of the electric load deferral is presented below. Note that BGE offers a rebate of $200/kW deferred for thermal storage and $400/kW deferred for High Efficiency Lighting.
Spears / Votta & Associates, Inc. - Consulting Engineers
7526 Harford Road - Baltimore, MD - 21234-6944
Phone 410.254.5800 - Fax 410.254.7091
Mechanical Engineers, Electrical Engineers, Plumbing Engineers, Lighting, Alarm & Communications Designers
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