National Institutes of Health Building 37 Modernization Bethesda, Maryland MECHANICAL DEPTH EXISTING MECHANICAL SYSTEM

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1 MECHANICAL DEPTH EXISTING MECHANICAL SYSTEM The need to maintain occupancy during the renovation and the strict NIH Design Guidelines were the main driving forces behind the design. The mechanical engineering group, Affiliated Engineers, Inc., worked closely with NIH beginning with schematic design and continuing through construction to develop the following design objectives and requirements, ensuring NIH Design Guidelines were followed. Design Objectives & Requirements The driving design objectives and requirements needed to ensure operability a state-of-the-art laboratory facility include: 1 Provide the facility with sufficient indoor conditions (thermal comfort, indoor air quality IAQ and safety so to maximize efficiency and productivity of the NIH-NCI employees. 2 Provide the facility with a system that integrates and successfully phases out the existing mechanical system with the new design in conjunction with the district heating and cooling systems. 3 Provide the facility with a system that strictly adheres to the NIH Design Guidelines. Table 1 Outdoor and Indoor Design Conditions NIH Building 37 is located on the Bethesda, Maryland campus, which is not listed in ASHRAE Fundamentals Climatic Design Information chapter, so the design conditions for Camp Springs, Maryland Andrews AFB were used. The cooling load for NIH Building 37 was calculated 11

2 using outdoor design conditions of 35ºC dry-bulb temperature and 26ºC wetbulb temperature, corresponding to ASHRAE 0.4% design conditions. Similarly, the heating load was calculated using outdoor design conditions of -23ºC, which is well below ASHRAE design conditions for the area. Based on design documents, indoor design conditions are set at 23ºC and 50% relative humidity. The outdoor design conditions are summarized in Table 1. The requirements for laboratory spaces and perimeter research offices do not differ from one another. Design Heating and Cooling Loads All of the heating and cooling in the building is being supplied by central plants on the NIH campus. The total heating load = MBH and the total cooling load = tons. There is also a cogeneration plant servicing the building through a turbine powered by natural gas that is coupled to an electric generator. The exhaust from the turbine flows through a boiler and produces steam at 100,000 lbs/hr which services all the buildings on the NIH campus. Mechanical System - Airside The airside mechanical system for NIH consists of eight packaged air-handling units supplying once-through 100% outdoor air through a zoned variable-air-volume system, to the occupied spaces at 23ºC. The eight AHUs range anywhere from 28,314 L/s to 29,258 L/s and are located in the mechanical penthouse, directly above the sixth floor. In the basement, where existing mechanical equipment is located, there are two packaged airhandling units; one supplying variable outdoor air with an economizer and one supplying constant volume variable outdoor air for cooling only to the transformer room. ariable-air-volume (A boxes with heating coils distribute air from the air-handling units to all of the occupied zones. Factory 12

3 assembled, horizontal, draw-through type fan coil units were used in some zones not being supplied to by the air-handling system. There are eight fume exhaust fans installed in parallel, all connected to a common fume exhaust plenum. Each fan has a capacity of 32,089 L/s and a schematic of the central fume exhaust flow can be viewed in Technical Assignment #3. In the analysis of the airside mechanical system, only the eight AHUs in the mechanical penthouse will be considered. Mechanical Systems Waterside Chilled water is provided by the NIH central chilled-water distribution system, and is supplied to the building at 6ºC and leaves at 16ºC. Roughly 2500 tons of water is distributed to Building 37 by three (constant speed tertiary pumps. All three pumps are sized at 50% capacity and are piped in parallel, with two pumps operating at any one time and the third acts as standby. Chilled water serves fan coil units, as well as cools process equipment located throughout the building. Steam is provided by large boilers at 165 psi, and then flows from the NIH central heating plant in Building 11 to various buildings on the campus. Approximately 20,000 lbs/hr of steam enters Building 37 to serve the heating coils in the AHUs. An underground tunnel system provides piping from the central plants to the various buildings on the NIH campus. 13

4 MECHANICAL DEPTH - MECHANICAL SYSTEM REDESIGN PROPOSAL & JUSTIFICATION Critique of Mechanical System After performing analyses, calculations and finally making assessments based on ASHRAE Standard Addendum n and ASHRAE Standard in Technical Assignments #1, #2 and #3, key features of the design and features not addressed in the design showed possible areas of improvement. Careful considerations and precautions were made during the design of the modernization of Building 37 to follow the strict design guidelines set forth by NIH. Laboratory/research facilities are inherent consumers of natural resources, with little or no heat/energy recovery, as is with Building 37. Based on Technical Assignment #2, it was estimated that the HAC energy usage per year for Building 37 = 75,673, kwh. Because of the high energy usage per year, the main goal in mind was to implement a low-energy design. Since this facility was designed for 100% OA with a minimum of six air changes per hour (ACH, the large volume of ventilation air required poses the opportunity to reduce the amount of energy required to condition ventilation air. The engineers for Building 37 did just this, as they designed for variableair-volume (A fume hoods, A supply with terminal reheat devices and hood exhaust systems. Also, in regards to the building envelope and its influence on the energy efficiency, existing windows were replaced with inoperable, low-emissive (low-e insulating glass, so not to compromise the mechanical system design. Occupancy controls are utilized in individual research offices, public areas where feasible, such as service corridors, large rooms and lavatories, using ultrasonic-type dual technology with passive infrared sensors. 14

5 Since Building 37 also supports other spaces such as conference rooms and office suites, which require significantly less stringent HAC requirements, as well as laboratory spaces, the integration of these dissimilar types of occupancies increases the potential for wasted energy. While the engineers and designers of Building 37 have implemented several design features to improve building energy performance, the modernization of Building 37 did not include any design for heat/energy recovery. With 100% OA being supplied, heat/energy recovery from exhaust air looks to be a viable option. Information on the waterside mechanical system of NIH Building 37 is unavailable. The National Institutes of Health will not disclose any information concerning the central chiller, central steam and cogeneration plants servicing the buildings on the NIH campus. Alternatives Considered Any proposal to redesign components of the central chiller, central steam or cogeneration plants were disregarded due to the inability to gain access to vital information. NIH has disclosed this sensitive information due to security issues. Other than any redesign ideas for the central plants, the only design alternatives that could be considered involved just the airside mechanical system in Building 37. Focus was turned to investigating several different energy recovery alternatives. The different alternative considered include a heat pipe, run-around coils and a total enthalpy wheel (taking into account both sensible and latent loads. The options are briefly outlined below. 15

6 Heat Pipe In a heat pipe system, supply and exhaust airstreams should be located next to each other to ensure proper operation. The basic mechanics of the heat pipe system uses the movement of boiling and condensing refrigerant inside a sealed pipe to act as the heat transfer medium. There is no mixing of the airstreams, and no power source is needed to drive the heat transfer process. According to ASHRAE Laboratory Design Guide, the efficiency of sensible heat transfer ranges between 45% and 65%. Run-around coils A run-around coil heat recovery system consists of a matched pair of coil heat exchangers, in which the two coils are piped together in a continuous loop for the heat transfer medium to flow. Since the unit is self-contained, there is no mixing of the airstreams Total enthalpy wheel Passive Desiccant Dehumidification Wheel In a desiccant dehumidification unit, it is necessary for the supply air and exhaust air streams to be located next to each other to ensure proper operation of the system. This close proximity to contaminated exhaust air Table 2 smallest virus 1000 angstroms > 3 angrstrom sieve water molecule 265 angstroms < 3 angstrom sieve raises serious questions of crosscontamination in laboratory facilities. In response to this concern, SEMCO manufactures a wheel (EXCLU- SIEE with a sieve of 3 angstroms (Å, prohibiting anything larger than this to absorb/transfer from the wheel. To gain a perspective on size, refer to Table 2. The EXCLU-SIEE wheel utilizes a 3Å molecular sieve desiccant coating to limit the risk of desiccant cross-contamination between the exhaust air stream and outdoor air stream. Molecular sieves are structurally stable, chemically inert and have a strong 16

7 affinity for water vapor and it is this strong affinity for water vapor which produces the high rate of adsorption resulting in superior latent transfer performance. In this type of system, typical heat transfer efficiency ranges between 50% and 85%, according to the AHSRAE Laboratory Design Guide. Redesign Proposal & Justification The driving factors behind the final redesign proposal for NIH Building 37 Modernization were finding a cost effective solution to the issue of heat recovery for the airside mechanical system. After contemplating the pros and cons of the other design alternatives, the final solution will incorporate a passive desiccant dehumidification system in the airside mechanical system to account for heat recovery. Due to the humid climate during the summer months of the facility, the need to dehumidify is necessary in any redesign proposal. The two primary loads in conventional air-conditioning are sensible and latent loads the sensible load taking into consideration the temperature component and the humidity portion being taken care of by the latent load. The only energy used by the desiccant dehumidification units is the fan energy required to move the air. Desiccant dehumidification is beneficial in all types of buildings by improving indoor air quality (IAQ, reducing the latent portion of the cooling load, reducing odor and decreasing operational costs of facilities. The process involves the removal of moisture from humid air with the aid of a desiccant material that absorbs the water vapor as opposed to condensing it. Although the desiccant system s first cost cannot compete with conventional air-conditioning, the conventional system can be reduced in size when configured to work with desiccant dehumidification units. 17

8 MECHANICAL DEPTH PASSIE DESICCANT DEHUMIDIFICATION Redesign Introduction The existing mechanical system in NIH Building 37 does not account for any heat or energy recovery. In research and laboratory facilities, where fume hoods are in place, the concern of cross contamination in air-to-air heat recovery is heightened. In NIH Building 37, the exhaust airstream ducts are located relatively close to the outdoor airstreams, so the proximity of the airstreams is not an issue for the redesign. This section goes into the calculations and simulation results from applying a passive desiccant dehumidification system to the existing mechanical system for NIH Building 37. Existing Loads Through Carrier s Hourly Analysis Program (HAP it was found that the total cooling coil load = tons and the total heating load = MBH. The loads contributed to each air-handling unit are summarized in Table 3. The annual cost to operate Building 37 is $858,756/year. Table 3 Total Cooling Coil Load (tons Total Heating Load (MBH AHU AHU AHU AHU AHU AHU AHU AHU Total Appendix I includes a complete printout of the HAP output from the simulation. SEMCO Model The SEMCO Energy Recovery Wheel Technical Guide and SEMCO TE Wheel Modeling Program were used to size and select an appropriate passive desiccant wheel. Both references were supplied by SEMCO Representative, Rick Caldwell. The design guide presents the SEMCO EXCLU-SIEE total energy (TE3 recovery wheel. The selection procedure in the technical guide was 18

9 followed to select the appropriate total energy recovery wheel, and then the modeling program was used to calculate an energy analysis and cost analysis. SEMCO ELCLU-SIEE 3Ǻ Molecular Sieve SEMCO s EXCLU-SIEE total energy recovery wheel provides features that optimize the sensible (temperature recovery of performance, and also provides latent (moisture recovery efficiencies that match the improved sensible values. This is accomplished through EXCLU-SIEE s 3Ǻ molecular sieve desiccant coating. The high rate of absorption allows for effective moisture transfer between the outdoor and exhaust airstreams. Picture 3 Molecular sieves are crystalline metal alumino-silicates, and a close up view can be seen in Picture 3. When it is combined with oxygen atoms, the threedimensional interconnecting network expands its internal surface area where passing liquids and gases in the airstreams are adsorbed. The 3Ǻ molecular sieve has the unique capability of limiting adsorption to materials that are smaller than approximately 3 angstroms. 19

10 SEMCO Technical Guide Calculations An excel worksheet was created following the design procedure. The complete printout of calculations for the unit selection can be found in Appendix II. The first step involved was selecting the wheel, based on airflow. Using Figure 2, EXCLU-SIEE TE3-70 model was determined to be suitable given all the airflows. Step 2 involved determining the unit effectiveness and the following equations were used. Figure 2 ε s = s min ( X 1 X 2 ( X X 1 3 ε r = r min ( X X 4 ( X X 1 3 X 2 = X 1 εs min s ( X X 1 3 X 4 = X 3 + εs min r ( X X 1 3 Step 3 involved the calculations of unit performance and these calculations can be found in Appendix II. 20

11 The purge volume was calculated in step 4 of the design procedure. A purge section is utilized to avoid carry-over of exhaust air into the supply air-stream. or all AHUs, the purge volume = 3800 cfm with a purge index setting = 4. A schematic for purge operation is shown in Figure 3. Figure 3 Step 5 calculated the reduction in required chiller and/or boiler capacity, and was estimated through the following equations. Chiller capacity = scfmh Boiler capacity = scfmh ( h IN ( h IN OUT OUT , ,000 Table 4 summarizes the wheel selection results given the required airflows for each AHU. Table 4 Face elocity Pressure Loss Effectiveness Unit Effectiveness Cooling Unit Effectivenss Heating Purge olume Chiller Reduction Capacity Boiler Reduction Capacity fpm in. wg % Supply Return Supply Return cfm tons boiler hp , ,04 21

12 Based on SEMCO Energy Recovery Wheel Technical Guide design procedure, SEMCO TE3-70 energy recovery wheel was selected, and Table 5 summarizes the performance data and unit dimensions, which are common for all eight AHUS. Figure 4 shows the dimensions of the wheel. The TE3-70 wheels add an additional 37,440 lbs of load in the mechanical penthouse. Table 5 Figure 4 Performance Data for TE3 Wheels elocity 900 fpm Wheel Efficiency 76 % Pressure Drop 0.94 in. wg Wheel Model Size 70 Airflow Rate 63,360 cfm Unit Dimensions A in. B 79.1 in. C 89.4 in. D 84.3 in. W 23.0 in. Net Wt lbs Flow Area/Side 70.4 ft 2 Nominal cfm cfm SEMCO TE Wheel Modeling Program This program was used to check the results of the excel worksheet derived results and also to compute cost analysis information. The complete printout of results from the simulation can be found in Appendix III. Table 6 summarizes the costs and also compares the cooling and heating loads for the new input capacities to the existing loads. Each unit is approximately $45,000 with a $22,500 installation cost. The first cost savings range between approximately $60,000 and $65,000 depending on the AHU. There is an immediate payback with positive present cash flow values ranging between $234,000 and $241,

13 Table 6 Equipment Installation First Cost Immediate Payback w/ Cooling Capacity Cooling Load from First Cost ($ Cost ($ Savings ($ Positive Present Cash alue of Input Required (tons Existing System (tons Heating Capacity Required (MBH Heating Load from Existing System (MBH AHU 1 45,000 22,500 60, , AHU 2 45,000 22,500 64, , AHU 3 45,000 22,500 64, , AHU 4 45,000 22,500 60, , AHU 5 45,000 22,500 60, , AHU 6 45,000 22,500 60, , AHU 7 45,000 22,500 64, , AHU 8 45,000 22,500 64, , Total 360, , ,568 1,900,