Depth: Mechanical Redesign

Transcription

1 Depth: Mechanical Redesign

2 Redesign Objectives The most important objective of the mechanical redesign is to provide Greenbriar East with ventilation that meets the requirements of ASHRAE Standard While adequate ventilation is important in any building, it is especially important in a building whose primary occupants are children. These children are still developing physically and are more likely to suffer the consequences of indoor air pollutants. Providing the correct amount of ventilation air will remove these pollutants from the space. Another objective is to adequately control the humidity within Greenbriar East. By controlling humidity, the potential for microbial growth will be minimized. This will lead to better health among the students and staff and hopefully a decrease in the occurrences of asthma, which is occurring in 49% more children today than it was in (Bayer) The objective that often drives the design of schools is budget. Both the first and operating costs of the redesign will be considered, but the driving force will be the health and safety of Greenbriar East s occupants. The redesign should provide a healthy environment for the occupants by using energy efficient equipment which will lower the operating costs of the building which benefits both Fairfax County Public Schools and the taxpayers. The System A Dedicated Outdoor Air System (DOAS) will be used for the mechanical redesign of Greenbriar East. This system was used for the classrooms in the current design, but will be used for all areas of the building in the redesign, allowing for an ease of maintenance that is found when only one type of system is used. While the type of equipment used for DOAS will change, the basic concept will remain the same. The purpose of DOAS is to meet the ventilation loads of the space that it serves by supplying 100% outdoor air. When this system is sized correctly, it will always meet the ventilation requirements set by ASHRAE Standard because the required and conditioned ventilation air is placed directly into the space without first mixing it with return air from the 11

3 building. At the same time that DOAS is providing adequate ventilation, it is able to remove the latent loads from the space, thus decoupling the space sensible and latent loads. While some of the sensible loads are met with DOAS, the remaining sensible loads are met through the use of a parallel system which can be in the form of radiant panels, unitary equipment, fan coil units or water source heat pumps (Mumma). DOAS and its parallel system can be seen in Figure 8. Vent air generally about 30% less than with an all air system due to eq. 6.1 of STD 62. DOAS meeting all of the latent load and a portion of the sensible Parallel mechanical system meeting the sensible loads not met with the DOAS system: may be VAV, fan coil, WSHP, radiant, etc. Partial sensible and complete latent cooling via properly distributed 100% ventilation air Sensible only cooling Building with sensible and latent control decoupled Figure 8 DOAS/Parallel Arrangement Courtesy of Dedicated OA Systems by Dr. Stanley A. Mumma Figure 8 lists one benefit of using DOAS rather than a conventional variable air volume (VAV) or constant volume system. This benefit is the reduction in the amount of ventilation air, meaning supply air, by approximately 30%. This leads to a reduction in the chiller, boiler and pump sizes as well as duct size. In the case of Greenbriar East, it is predicted that the ducts will actually increase for the classrooms even though they were being supplied with 100% OA in the current design. This is a result of the current design s failure to meet the ventilation requirements of ASHRAE Standard However, the duct size should decrease in the areas of the building that were supplied by the constant volume RTUs as a result of the reduction in the amount of supply air. 12

4 Once again the most important benefit of DOAS, which is related to the primary objective of this redesign, must be stressed. This benefit is the knowledge that ventilation requirements will always be met due to the lack of mixing of outdoor and return air. These proposed benefits will be addressed in the following sections, Ventilation and Heating and Cooling, which detail the design of DOAS and the parallel system. Ventilation Requirements In order to design for DOAS, the ventilation loads must first be found. In order to do so, the Ventilation Rate Procedure (VRP) as specified in ASHRAE Standard was used to determine the required ventilation airflow for each of the spaces in Greenbriar East. The required ventilation airflow was found based on the floor area, occupancy and function of each space. The floor area of each space was taken from the AutoCAD files prepared by the architects and engineers at Hayes, Seay, Mattern and Mattern (HSMM). The occupancy and function were determined from the Trane Trace 700 input values also prepared by HSMM. Once the required ventilation was known for each space, the required ventilation for each group of zones to be served by one DOAS unit could easily be determined. This was calculated according to Section of ANSI/ASHRAE Addendum n to ASHRAE Standard This section states When one air handler supplies only outdoor air to one or more zones, the outdoor air intake flow (V ot ) shall be determined in accordance with Equation 6-4. Equation 6-4 states V ot = ΣV oz where V oz is the ventilation airflow of an individual space (ANSI/ASHRAE). Spreadsheets showing the ventilation rate procedure can be found in Appendix A. In order to verify the proposed benefit of a reduction in the supply airflows when using DOAS, a comparison between the supply airflows of the current system and the DOAS redesign can be seen in Figure 9. For this comparison, the same zone groupings were used for DOAS as for the current system. These groupings are characterized by unit names such as RTU-1 and MAU-1. These names do not describe the pieces of equipment that are to be used in the DOAS redesign. 13

5 Supply Airflow Comparison Between the Current System and DOAS Unit Current System Supply Airflow (cfm) DOAS Supply Airflow (cfm) RTU RTU RTU RTU RTU RTU RTU MAU MAU MAU MAU MAU TOTAL Figure 9 Supply Airflow Comparison As predicted, the amount of air that is being supplied by each DOAS unit for the current RTU groupings decreased due to the use of 100% outdoor air. Also as predicted, the amount of air that is being supplied by each DOAS unit for the current MAU groupings increased due to the current system s failure and the redesigned system s ability to comply with ASHRAE Standard Even with this increase, the total amount of supply air was still reduced by 33%. Equipment As Figure 9 shows, there is a large variety in the amount of supply airflow that is required for each zone grouping. It would be impractical to select equipment for the groupings as they are in the current design because of this variety. Instead, zones were re-evaluated and regrouped so that each grouping will be served by a DOAS unit with an approximate capacity of 3,000 cfm. These groupings are reflected in the ventilation calculations of Appendix A. 14

6 Along with meeting the required ventilation loads, the DOAS equipment should also meet the design objectives of being energy efficient and of being able to provide some degree of humidity control. Semco s packaged energy recovery units (ERUs) meet all of these goals and are available in two models that would fit the needs of Greenbriar East. Both models have added benefits of minimal maintenance and reliable operation. One such model is the EP series which can be applied as a preconditioner to traditional HVAC equipment such as rooftop units. This ERU is primarily composed of Semco s Exclu-sieve energy recovery wheel, otherwise known as an enthalpy wheel. An enthalpy wheel transfers heat and humidity between the exhaust and supply air streams and as a result reduces the load on the heating and cooling system. Semco s Exclu-sieve wheel ensures that cross contamination will not occur by applying their patented 3Å molecular sieve-dessicant coating to the wheel. This coating rejects airborne contaminants while it transfers water vapor, thus providing total energy transfer from the exhaust to the supply airstream. The cross contamination rate is certified to be less than 0.04% when the Exclu-sieve wheel is used. The wheel, along with the supply and exhaust fans, can be seen in Figure 10. The schematic illustrates how the outdoor air is precooled and dehumidified in the cooling season. Figure 10 EP Schematic Courtesy of Semco Incorporated 15

7 The EPHC model builds on the EP model by including full heating and cooling options. The cooling options include either chilled water or DX cooling coils while the heating options include either hot water, steam or electric coils. Semco specifically states in their product catalog that this product can be applied to installations where there is a need for 100% outdoor air. Therefore, this model would be appropriate for the mechanical redesign of Greenbriar East. A schematic of the EPHC model is shown in Figure 11. Once again the schematic illustrates how the outdoor air is precooled and dehumidified in the cooling season, but it also adds the results of the heating and cooling coils. Figure 11 EPHC Schematic Courtesy of Semco Incorporated The costs of both the EP and EPHC units were provided by Semco for the two sizes in each model that would meet the ventilation capacity of approximately 3,000 cfm that is required for each zone grouping. These costs can be seen in Figure 12. Cost Comparison of EP and EPHC Models Size Cost of EP Model Cost of EPHC Model 3 $ 28,038 $ 32,792 5 $ 33,812 $ 39,360 Figure 12 Cost Comparison of EP and EPHC Models 16

8 While the EP model appears to be the least costly option, it must be acknowledged that a makeup air unit would need to be used in conjunction with the EP model. This would lead to an increase in first costs and in the space required for the DOAS equipment on the roof. Knowing this, the decision was made to use the EPHC model with the DX cooling and hot water heating options. The EPHC unit will allow for only one piece of equipment to be used for the humidification and air conditioning needs instead of the two that would be needed if the EP model was used as a preconditioner. This slight increase in cost for the EPHC model is less than the cost of a make-up air unit with heating and cooling options and therefore makes the EPHC model the least costly option. Additional benefits and information relating to the EPHC are detailed in its product sheet which can be found in Appendix G. Once the model was chosen, a detailed selection procedure as prescribed by Semco took place and can be found in Appendix B. The selection procedure was used to determine the specifics of the ERU such as the internal and fan total static pressures. One very important characteristic of the ERU that is determined through the selection procedure is the supply air efficiency of the enthalpy wheel. This efficiency is a result of the supply air to return air ratio and the base effectiveness of the wheel which is determined by the supply air quantity. The supply air to return air ratio was first determined for each zone grouping through an air balance based on the results from the Ventilation Rate Procedure. The air balance is found in Appendix C and a summary of the base effectiveness, supply air to return air ratio, and resulting supply air effectiveness for the enthalpy wheel in each ERU can be found in Figure 13. Enthalpy Wheel Efficiency Unit Base Effectiveness (%) SA/RA Supply Air Efficiency (%) ERU ERU ERU ERU ERU ERU ERU ERU Figure 13 ERU Efficiency 17

9 The majority of the ERUs have an efficiency that is typical of most enthalpy wheels. Those with low efficiencies are a result of large amount of air being exhausted from the zone grouping that the ERU serves. ERUs 4, 5, and 7 serve zones such as the kitchen, cafeteria, art lab, and two large restrooms. While the efficiency of these units may be low, they still cause changes in the supply air temperature which will reduce the cooling and heating loads. These supply air temperatures can be determined based on the supply air efficiency and the indoor and outdoor design temperatures using the following equation from Semco s selection procedure: X SA = X OA E SA (X OA X RA ) where X = the dry bulb temp ( o F) or enthalpy (BTU/lb) and E SA = Supply Air Effectiveness. The outdoor design temperatures for this equation were taken from ASHRAE Fundamentals while the indoor design temperatures are required by Fairfax County School District. A summary of these temperatures, efficiencies and the resulting supply air conditions can be found in Figure 14. Supply Air (SA) Conditions Summer OA Summer SA Winter OA Winter SA Conditions Contitions Conditions Contitions Unit DB DB h (BTU/lb) h (BTU/lb) DB ( F) DB ( F) ( F) ( F) MAU MAU MAU MAU MAU MAU MAU MAU Figure 14 Supply Air Conditions As Figure 14 shows, the supply air temperature and humidity can significantly be altered by the use of enthalpy wheels. Even those ERUs that have small efficiencies (ERUs 4, 5, and 7) have an average reduction in the summer supply air temperature of four degrees and an average increase in the winter supply air temperature of eight degrees. These changes may still seem slight, but as Figures 15 and 16 show, these changes will also alter the heating and cooling loads for the ERU. 18

10 Figure 15 depicts the cooling capacity of the ERUs both with and without an enthalpy wheel. These capacities were determined by using the supply air quantity along with the OA and SA conditions in the following derived equation: Q = 4.5 * cfm * (h OA h SA ) This calculation can be seen in Appendix D and finds that the cooling load is reduced by 29% with the use of an enthalpy wheel in the ERUs. Cooling Load (BTUh) Energy Recovery Unit Cooling Loads Without Enthalpy Wheel With Enthalpy Wheel Energy Recovery Unit Figure 15 Comparison of Cooling Loads with and without the Utilization of Enthalpy Wheels Likewise, Figure 16 depicts the heating capacity of the ERUs both with and without an enthalpy wheel. These capacities were determined once again by using the supply air quantity along with the OA and SA conditions in a similar derived equation as follows: Q = 1.1 * cfm * (T SA T OA ) This calculation can also be seen in Appendix D and finds that the heating load is reduced by 46% with the use of an enthalpy wheel in the ERUs. Heating Load (BTUh) Energy Recovery Unit Heating Loads Without Enthalpy Wheel With Enthalpy Wheel Energy Recovery Unit Figure 16 Comparison of Heating Loads with and without the Utilization of Enthalpy Wheels The selection procedure concluded that five EPHC-3 units and three EPHC-5 units are needed for the DOAS design in Greenbriar East. Based on the costs in Table 12, it is determined that the total first cost for the new DOAS equipment is $282,

11 Heating and Cooling The Parallel System The preceding section has shown that DOAS does in fact meet some of the thermal loads for the spaces that the ERUs serve. As stated previously, the remaining thermal loads will be met by a parallel system. A decision had to be made as what type of system or equipment will be used as the parallel system. Consideration was first given to the use of unit ventilators because this type of unitary equipment was already utilized as the parallel system in the current classroom design. While this selection would lead to an ease of installation and a familiarity of the maintenance staff with the equipment, the use of unit ventilators was disregarded primarily because of a significant negative characteristic of the equipment. Unit ventilators are known to have poor temperature control. The second type of equipment to be considered, and eventually chosen, for the parallel system were geothermal heat pumps. The use of geothermal heat pumps is becoming a more recognized choice for schools today. They were recently used in Walker Upper Elementary School in Charlottesville, Virginia with great success and have now been approved for the renovation of other schools throughout the City of Charlottesville school district. Charlottesville s proximity to allows the locations to share similar characteristics that will enable the use of geothermal heat pumps to be a success in Greenbriar East as well. The location of the two schools, with Greenbriar East marked as Start and Walker marked as End, can be seen in Figure 17. Figure 17 Locations of Greenbriar East Elementary and Walker Upper Elementary Courtesy of Mapquest 20

12 Geothermal heat pumps have many benefits that would be attractive to Greenbriar East. One such benefit is the high level of reliability in geothermal heat pumps when routine maintenance procedures are followed. A study conducted by Washington State University found that these systems perform with high reliability in excess of years (Bloomquist). A high level of reliability is also found in the system s ability to accurately control temperature unlike unit ventilators. A benefit of geothermal heat pumps that enables the redesign objectives to be met is the system s high energy efficiency. This is a result of the constant temperature of the earth and of the groundwater below the surface that is utilized by the system. The constant temperature of the earth, which is approximately 60 o F in Fairfax, enables the liquid in the heat exchanger to also be maintained at a constant temperature. This allows the geothermal system to operate consistently during a wide range of weather conditions, resulting in an increased energy efficiency of the system. (Silberstein) A decision must be made as to whether to utilize the earth or the water in a geothermal system. As shown in Figure 18, an open loop system utilizes the water; therefore, a constant and adequate source of water must be available. The source of water may be in the form of a lake, river, or groundwater. Because large amounts of water are not available at the site of Greenbriar East, an open loop system will not be used for the redesign. well pond Figure 18 Open Loop Configurations Courtesy of John W. Lund 21

13 While an open loop system is dependent on water, a closed loop system is dependent on a sufficient amount of land. In the case of Greenbriar East, the site s size of 10 acres is sufficient for the burial of a piping circuit. The piping circuit is filled with water or antifreeze in areas where the groundwater temperatures are below 60 o F. The liquid flows through the piping circuit and absorbs heat from the ground during the heating mode. The heat is then transferred to a refrigerant circuit. During the cooling mode, the liquid absorbs heat from the refrigerant circuit and transfers the heat to the ground. A schematic of the cooling mode can be seen in Figure 19. Figure 19 Geothermal Heat Pump Schematic Courtesy of Heat Pumps by Eugene Silberstein 22

14 The piping circuit can be arranged in two ways as shown in Figure 20. Vertical ground loops are desirable in areas where ground space is limited. Horizontal ground loops are ideal when a large amount of land is available, as is the case with Greenbriar East. Because the piping is laid closer to the surface of the earth, longer piping runs are needed, but the cost of installation is decreased. The costs of installation for both systems are summarized in Figure 21 and were the final deciding factor as to whether to use a horizontal ground loop. Cost Comparison of Horizontal and Vertical Ground Loops Horizontal Loop Vertical Loop Figure 20 Closed Loop Configurations Courtesy of John W. Lund Cost per ton $ 750 $ 1,050 Figure 21 Cost Comparison of Ground Loops Courtesy of John W. Lund Requirements Before the ground loop is able to be designed and equipment can be selected, the total thermal loads of the spaces within Greenbriar East had to be determined. These loads were determined using Trane Trace 700 based on the building envelope, outdoor conditions, and indoor design conditions. The resulting peak cooling and heating loads can be found in Appendix E. The heating and cooling capacities of the ERUs were then subtracted from the total thermal loads as determined by Trane Trace 700 in order to calculate the amount of heating and cooling that is required by the parallel system. These calculations can be found in Appendix F and determined that 55.4 tons of heating and 52 tons of cooling are required of the heat pumps. 23

15 The Ground Loop The piping circuit length is determined based on the maximum heating or cooling load and several other factors. Such factors include the temperature of the ground, temperature of the fluid in the pipes, and the thermal resistance of the ground. The thermal resistance is determined through a geotechnical evaluation of the site. In the case of Greenbriar East, a geotechnical evaluation was performed in order to determine the strength of the ground, but the thermal resistance was not evaluated. Because of this, the soil at Greenbriar East must be considered typical. For typical soil, the general rule of thumb is that 150 to 200 feet per ton are needed for vertical loops. The length is 30% to 50% longer for horizontal loops. The ground loop for Greenbriar East will be sized for the 55.4 tons of heating based on 175 feet per ton plus an additional 40% for the loop being horizontal. This results in a ground loop of 13,573 feet for a cost of $41,550 including piping and installation. The location of the loop on the site can be seen in Figure 22. Figure 22 Location of the Horizontal Ground Loop As Figure 22 shows, the ground loop doesn t appear to take up much land when compared to the site on which Greenbriar East is located. The location for entry into the building was chosen at this spot because of a large mechanical chase that is found there. 24

16 The Equipment Several configurations of heat pumps will be used throughout Greenbriar in a variety of sizes. Trane s Axiom water source heat pumps, which range in capacity from ½ to 1 ½ tons, will be used in each classroom. These units are described as a high efficiency console water source comfort system. The console configuration was chosen so that the unit ventilator piping can be utilized for the refrigerant circuit. The non-classroom spaces of Greenbriar East will be served by several high efficiency horizontal water source heat pumps, also by Trane. These units will be installed in the plenum to serve one or multiple spaces. The units support six airflow combinations. The combination that will be most effective in Greenbriar East can be found in Figure 24. Figure 23 Horizontal Heat Pump Configuration Courtesy of Trane The number of console and horizontal units, in their various capacities, can be found in Figures 24 and 25. Cut sheets of the equipment can be found in Appendix G. Summary of Horizontal Units Unit Cooling Heating Capacity Capacity Quantity Figure 25 Summary of Horizontal Heat Pumps Summary of Console Units Unit Capacity Quantity 006 1/2 ton ton /2 ton 17 Figure 24 Summary of Console Heat Pumps 25

17 The Savings It was proposed that a reduction in the chiller, boiler, and pump sizes will occur when DOAS is used. The equipment selection and use of a heat pump system in parallel with DOAS has caused significant reductions as can be seen through the following comparisons in Figures 26 and 27. Properties of the current system s equipment were taken from the schedules created by HSMM. These schedules can be found in Appendix H. Chilled Water System Current System Redesigned System Unit Capacity Capacity GPM Unit (MBH) (MBH) GPM Make-up Air Units Fan Coil Units Unit Ventilators TOTAL TOTAL 0 0 Figure 26 Comparison of Chilled Water Systems Figure 26 details the equipment that require chilled water and shows that the chilled water requirement was reduced by 100%. The current system used a large number of various pieces of equipment for the cooling needs of the building. In turn, a large amount of chilled water was needed. The redesigned system, on the other hand, does not require chilled water. This is partly due to the use of a DX cooling coil in the energy recovery units. The DX cooling coil uses a refrigerant instead of chilled water to cool the supply air. The lack of chilled water is also due to the utilization of the ground by the geothermal heat pumps to cool the recirculated air in the spaces. Hot Water System Current System Redesigned System Unit Capacity Capacity GPM Unit (MBH) (MBH) GPM Make-up Air Energy Recovery Units Units Cabinet Heaters Cabinet Heaters Unit Heaters Unit Heaters Fan Coil Units Unit Ventilators Fin Tube Radiators TOTAL TOTAL Figure 27 - Comparison of Hot Water Systems 26

18 Figure 27 details the equipment that require hot water and shows that the hot water requirement was reduced by 42%. Once again, the current system used a large number of various pieces of equipment for the heating needs of the building. In turn, a large amount of hot water was needed. The redesigned system does require hot water, some of which is for the same heating only equipment that is found in the current redesign. These pieces of equipment are not part of DOAS or of the parallel system. The only pieces of equipment for DOAS that use hot water are the energy recovery units. The parallel system utilizes the ground to heat the recirculated air in the spaces. The reduction in the chiller and boiler will lead to energy savings throughout the year. To estimate these savings it was assumed that the building will be occupied with the mechanical system running at full load for twelve hours a day, five days a week, ten months a year. The system will run at part load (50%) for the other two days during those ten months and for the entire remaining two months. The estimate was also based on the electric rate of $7.27/MMBH and gas rate of $9.10/MMBH for Virginia as reported by the Department of Energy. The resulting energy costs for a year can be seen in Figure 28. Mechanical System Energy Cost Estimates Gas Utilization Gas Cost Electricity Utilization Electric Cost Total Cost Current System $ 97, $ 70, $ 167, Redesigned System $ 56, $ - $ 56, Savings $ 40, $ 70, $ 111, Figure 28 Mechanical System Energy Cost Estimates 27

19 Final Recommendations The most important objective of the redesign was to meet the ventilation requirements of ASHRAE Standard to ensure the health of Greenbriar East s occupants. By disregarding the current equipment and ducts and starting the design from scratch, this objective was met. It was further ensured by using a Dedicated Outdoor Air System which will always meet the ventilation requirements set by ASHRAE Standard because the required and conditioned ventilation air is placed directly into the space without first mixing it with return air from the building The second objective of controlling humidity within Greenbriar East was met by the use of Semco s energy recovery units. These units effectively heat and humidify the outdoor air during the heating mode and cool and dehumidify the outdoor air during the cooling mode. Along with reducing humidity levels, the enthalpy wheels significantly reduce the heating and cooling capacities of the energy recovery units, resulting in a decrease in energy costs. The third objective of attempting to keep the first cost of the construction on budget and save in operating costs was also met through the redesign. DOAS, when paralleled with geothermal heat pumps, results in a substantial reduction in the size of or need for certain pieces of equipment such as the chiller. The lack of a chiller will save Fairfax County in both first and energy costs. Energy costs will be reduced further by the use of energy efficient equipment such as the geothermal heat pumps and energy recovery units. Because the Dedicated Outdoor Air System and parallel geothermal heat pump system that were used in this mechanical redesign clearly meet the three objectives of the redesign, this mechanical redesign would be recommended for Greenbriar East. 28