Problem Statement In order to satisfy production and storage requirements, small and medium-scale industrial facilities commonly occupy spaces with ceilings ranging between twenty and thirty feet in height. While these large spaces are ideal for manufacturing, air handling units are challenged with ensuring the climate remains near a desired set point. Natural convection within these spaces causes warm air to rise towards the ceiling, sometimes creating a significant temperature gradient known as thermal stratification. To overcome this temperature difference, air handling units increase the air circulation by forcing more conditioned air into the space, resulting in higher energy use. One alternative to this practice reduces thermal stratification through the use of large, high efficiency, ceiling fans. In New York State, the climate requires facilities to be heated for roughly 7 months of the year. Often times many facilities will have heating ducts at the top of the building, near the ceiling. Because of this the heat stays at the ceiling, thus, not using the energy produced by HVAC units as effectively as possible. Since the heating season is so long in this area of the country, this concept affects many industrial facilities in the community of Central New York. Project Summary/Background High ceilings make it difficult to effectively distribute conditioned air to the area. The effects of natural convection cause hot air to rise to the ceiling. In order to maintain work space temperatures close to specified set point, air handling units must force more air into the space, increasing air circulation, thus resulting in higher energy use. One solution to this problem is to install ceiling fans to reduce stratification. Industry currently recommends ceiling fan use based on the existence of a significant (>5 F) temperature gradient between the floor and ceiling. Average surface temperatures are found using a thermal camera and are assumed to closely match air temperatures based on steady state 1
conditions. A convective heat transfer coefficient is calculated using the difference in temperatures between the floor and ceiling. That coefficient is later combined with room specifications to determine the current amount of energy being used by the facility. The cost savings associated with the installation of ceiling fans is based on the reduced heating load resulting from better air circulation. This project is aimed towards developing a better understanding of the effects ceiling fans have on temperature profile, in order to optimize the configuration of destratification fan systems. Traditional methods of determining optimal fan placement are dependent upon fan specifications and ceiling areas. In this method, Computational Fluid Dynamics (CFD) models will be generated for various room sizes, air conditions, and fan specs in order to determine the optimal placement for the fans, whereas the previous method was more arbitrary. By improving the methodology of fan placement, the optimal placement of fans will be determined, which will in turn optimize the best use of heating and air conditioning equipment. It is believed that by enhancing the methodology used analyze and reduce thermal stratification, we can increase work zone comfort while simultaneously lowering the energy and emissions associated with climate control. Other ways of optimizing fan placement that were considered were using a small dronelike approach, which would determine a temperature profile before and after the installation of ceiling fans; using a fish-tank to model flow of dye in water when fans are used; and creating a zonal analysis using engineering equations. Using CFD modeling was chosen because it is believed to give the optimal and most accurate depiction of ceiling fans and flow. It also is beneficial because model characteristics can be changed more easily and quickly than with other methods that were considered. Relationship to Sustainability 2
Optimizing the approach taken to decrease thermal stratification in industrial facilities has social, economic and environmental benefits. In the winter, the fans can be used to pull cool air up from the floor, which gently pushes warm air near the ceiling towards the floor. This practice mixes the air while minimizing the velocity of the air near the floor, limiting body heat that would be lost to convective heat transfer. During warmer months, the rotational direction of the fan can be switched to force air down, maximizing air velocity near the floor, and thereby increase the body heat lost to convective heat transfer. Ceiling fans provide a level of comfort that cannot be acquired by air handling units alone. The social benefit comes from increase in employee comfort. When the workers of an industrial facility are more comfortable, productivity will be increased. By placing the ceiling fans in areas that will be of the best use of ceiling space, the productivity of employees can also be increased. Economically, the installation of ceiling fans provides year round cost savings due to their seasonal versatility. Based on information previously explained, ceiling fans can be used for both the heating and cooling season, based on their rotational direction. There is an additional capital cost when purchasing and installing ceiling fans, and an electrical cost to run the fans, but the reduced cost of energy usage by the heating and cooling equipment outweighs the other cost factors. The HVAC equipment will run more efficiently, thus reducing associated costs. Environmentally, there will be a reduction in emissions. Because the heating and cooling loads will be lowered, there will be a corresponding decrease in emissions. If the air handling unit uses a natural gas heating system, the cost savings associated with a reduced heating load are directly related to a decrease in the amount of emissions released into the atmosphere. 3
Some of the tradeoffs associated with this solution involve the increase in electrical energy consumption. When installing ceiling fans, the heating load, which is most often produced using natural gas or fuel oil, will be decreased. Simultaneously, will be an increase in electrical consumption, which will still create emissions. However, there are more environmentally friendly ways of producing electricity that will be purchased from the grid, such as hydro-power, wind, solar, and other renewable energy sources. Materials and Methods Throughout the project there were several tasks to be performed. The biggest task was modeling the various scenarios using CFD modeling software. The following describes the methodology of the CFD process. Production areas in a small and medium-scale industrial facilities typically have ceiling heights in the range of 20 to 30 feet. For the basic scenario the room has fixed height of 30 feet, width of 50 feet and length in the range of 70 to 200 feet. There are two ceiling fans that has placed at height of 25 feet from room s floor. The CFD model for the basic scenario is sketched in Figure 1. Figure 1: CFD model for basic scenario The parameters of ceiling fans are chosen from the commercial fans AirVolution 10 (MacroAir Company). Each fan has volume flow rate of 48,882 CFM and diameter of 10 ft. The mesh of the three dimensional CFD model contains 1.1 million cells and was generated in the 4
software Pointwise. The boundary conditions are presented in Figure 1 (right). The ceiling is set as a no-slip wall with temperature of 80 F and the floor is set as a no-slip wall with temperature of 60 F. The room s walls are set as no-slip wall with isolated temperature. The gravity of -9.8 m/s² is turned on due to the buoyancy effect. The symmetry boundary condition is applied to decrease the number of cells and minimize simulation time. The structure mesh is applied in this study and 3-D RANS simulation is demonstrated in the commercial CFD solver Ansys Fluent. The viscous model is k-e (2 equations) model and operating conditions pressure is 101325 Pa. The coupling between pressure and velocity is solved using the SIMPLE scheme. In control of the convergence of the solution all the continuity, x-velocity, y-velocity, and turbulent viscosity residuals are down to at least 10-4. The Fan Model is chosen to model the ceiling fans. In this model the total pressure rise is specified in order to satisfy volume flow rate. First two members of the team worked to research potential solutions to the problem of thermal stratification. They also researched benefits of employee comfort on productivity, and cost savings associated with heating and cooling energy savings. Once it was established that the team would use CFD to model various fan simulations, two members of the team worked to find various fan sizes of a vendor. The various fan sizes and specifications were tabularized and given to the third team member, who completed the CFD analyses. Once the various models were obtained, the team began to put the report together. Results, Evaluation and Demonstration As mentioned, in this study small and medium-scale industrial facilities typically have ceiling heights in the range 30 feet. Hot air rises to the ceiling, and excessive heat loss through ceiling requires additional energy to heat the floor area to the set point temperature in Figure 2 (left). One potential solution to this problem is to install ceiling fans to reduce stratification. The 5
effects of thermal stratification are investigated using three-dimensional CFD simulations. The pathline of air from two ceiling fans is described in Figure 2 (right). In this configuration, freestream air is ingested into the inlet of the ceiling fans located at height of 25 ft, energized using the fan, and expelled at the outlet of the fan in the direction from ceiling to floor. Figure 2 provides a visual presentation of flow colored velocity magnitude. The ceiling fans force more air into the space and increase air circulation. The strong air circulation reduces the heat loss through ceiling and provides additional heat energy to the floor area to maintain work space temperatures close to the specified set point. Figure 2: Thermal stratification in heating season (left) and ceiling fans reduce stratification. The velocity contour of airflow in room is shown in Error! Reference source not found. (left), shown below. The maximum velocity at the outlet of fans is 5 m/s, and then reduces along vertical direction from ceiling to floor. The velocity is high at the fan tip and low around the fan center because of the distance from fan center. The working space is specified at 6 ft from the room floor. At that working level the velocity is 0.5 1 m/s. This is acceptable comfort level at the working place. The total pressure of air is provided in Error! Reference source not found. (right). Again, the total pressure is highest at the fan tip and reduces to the fan center. Please note that only axial and radial velocities are defined in this fan model and the swirl velocity is not specified. 6
Figure:3 Velocity contour (left) and total pressure contour in basic room The most important parameter to study is temperature distribution in room. The temperature contour is illustrated in Figure 4. Due to thermal stratification the temperature is high near ceiling (80 F) and low at floor (60 F). However, when the ceiling fans are installed, a strong air circulation mixes temperature in the room and thermal stratification significantly reduces. Figure 4 presents temperature difference between floor and ceiling as less than 5 F. At the working place the temperature is remained about 72 F, which is comfortable for working people in the small and medium-scale industrial facilities. Figure 4: Temperature contour in basic room Using CFD we can determine the optimal number of fans for various scenarios of different sized buildings. In this section only the room s dimensions are changed and other factors are kept the same, i.e. there are two previous 10ft-diameter fans, ceiling temperature is 80 F and floor temperature is 60 F. The velocity contour of air in room dimension: 150 ft x 50 ft x 30 ft is 7
presented in Figure 5. In this case these fans still work well and they are able to reduce the thermal stratification in the room. Figure 5: Velocity contour in room dimension: 150 ft x 50 ft x 30 ft However, when the room dimension becomes larger the number of fans needs to be increased. Figure 6 presents the velocity contour for the room dimension: 200 ft x 50 ft x 30 ft. It is clear that the ceiling fans are not able to force more air in the space between these fans. In this case more fans must be added into this room, or they need to be spaced out more effectively. Figure 6: Velocity contour in room dimension: 200 ft x 50 ft x 30 ft The various scenarios of different sized buildings with various sizes and placement of fans are modeled in CFD simulation to determine the optimal configuration for these fans. Table 1 presents the optimum number of fans for various scenarios. AirVolution 10 Q = 48882 CFM AirVolution 14 Q = 90695 CFM Width Length 50 ft Aspect Ratio #Fans Width Length 50 ft Aspect Ratio #Fans 70 ft 1.4:1 2 70 ft 1.4:1 2 100 ft 2.0:1 2 100 ft 2.0:1 2 125 ft 2.5:1 3 125 ft 2.5:1 2 150 ft 3.0:1 3 150 ft 3.0:1 2 175 ft 3.5:1 3 175 ft 3.5:1 3 200 ft 4.0:1 4 200 ft 4.0:1 4 Table 1: Number of fans for various scenarios 8
For this case, the temperature difference was reduced to less than 5ºF from 20ºF. For a typical warehouse space, the average BTU/ft² is 13,400, for a non-refrigerated space 1. According to industry, for a room with 30 ft ceilings, the percentage of energy savings when ceiling fans are introduced is roughly 29% 2. Based on a room that is 100,000 ft², which is a typical size room for a manufacturing facility in New York State, and the energy content per square foot of a typical non-refrigerated warehouse, the potential energy savings is 3,886 CCF Natural gas. The emissions reductions from this natural gas savings are displayed in Table 2. Values for the emission factors of each element in the table were taken from EPA standards 3. Calculations for this can be seen in the Appendix of this report. Reductions in Pollutants and Greenhouse Gas Emissions (lbs) CO2 NOx PM SO2 CH4 TOC VOC 171,474 3.14 10.86 0.86 3.29 15.72 7.86 Table 2: Reduction in Pollutants and Greenhouse Gas Emissions As previously stated, there are social economic and environmental implications associated with the installation and optimization of ceiling fans. By utilizing ceiling fans for heating and cooling, there are many benefits. This project only examined the benefits associated with small and medium scale industrial facilities, but this could be used to determine and quantify the savings associated with installation of ceiling fans. For the presentation the results will be presented in a poster format. Conclusions It is evident from this study that the use of ceiling fans effectively decreases thermal stratification. Through the use of CFD modeling, it is shown that the temperature difference between the ceiling and floor could be reduced from 20 F to less than 5 F. Such reduction in the extent of thermal stratification results in near floor temperature of approximately 72 F, leading to a more comfortable work environment during the heating season. Reducing thermal stratification 9
is also economically beneficial, since much less natural gas will be needed to provide fuel for the heating systems. Reduced natural gas use during winter, in turn, reduces the amount of pollutants and greenhouse gases released to the atmosphere. Future work on this project includes introducing new heat or flow sources into the room for a more accurate depiction of the flow around people and objects. As previously stated, the traditional methods of installing ceiling fans depend upon certain fan specifications and room size. This project provides a methodology on how to best place ceiling fans. This project examined a room with no heat sources, machinery, nor people. Comparing traditional ceiling fan calculations with the CFD modeling, as was done in this project, does not show significant differences between the two approaches. However, when a heat source is present in a room, the CFD modeling will likely be very effective, as the placement of fans would be more important. This CFD modeling approach to ceiling fan placement could be used in other applications as well, aside from warehouse space. On college and university campuses any room, such as large lecture halls and auditoriums, that has a large open floor plan could benefit from this type of advanced CFD modeling, identify fan placement to reduce thermal stratification. Similarly, school buildings, such as gymnasiums, that have high ceilings could also benefit from CFD model based fan placement to reduce the thermal stratification. This project provides an insight into the extent of cost and environmental benefits that could be realized, through the use of CFD modeling tools, by appropriate fan placement to reduce the extent of thermal stratification typically present in large heated open space buildings. The impact of the dual benefits of reduced fuel use and reduced atmospheric emissions could be substantial, when we consider the large number of such heated open space buildings currently in use throughout New York State. 10
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Appendix 13,400 Btu/ft² Energy content per square foot 100,000 ft² Typical building size 1,340,000,000 Btu 100,000 Btu/CCF NG Energy content per CCF Natural Gas 13,400 CCF NG 29% Percent energy savings 3,886 CCF NG Natural gas savings 1,428,949 SCF Based on 10CCF NG = 3677.172051106 SCF End Notes 1 "Managing Energy Costs in Warehouses." Managing Energy Costs in Warehouses. E Source Companies LLC, 21 Mar. 2013. Web. 08 Apr. 2015. 2 "HVAC Energy Savings Destratification Fans by Airius." HVAC Energy Savings Destratification Fans by Airius. Airius LLC, n.d. Web. 08 Apr. 2015. 3 U.S. Environmental Protection Agency, Office Of Air Quality Planning and Standards, Compilation of Air Pollution Emission Factors: Volume 1: Stationary Point And Area Sources, (North Carolina, 1995), 1.4:5-6. 12