White Paper Series Volume 0412 Issue 001 April 2012 Rev. 0 Copyright Energy Labs Inc. All rights reserved. Energy Labs Inc. and EnergyLabs Inc. logo are registered trademarks of Energy Labs Inc. in the US and other countries.
What is a fan array? A fan array is defined as a bank of fans in a parallel airflow arrangement. The most commonly used fans in fan arrays are direct drive plenum fans. When should fan arrays be considered for a particular application? There are three main criteria that justify using fan arrays in lieu of a single fan - Redundancy, Sound and Unit Dimensions. There are other reasons for using fan arrays smaller motors, better airflow across components and the ability to adapt to unusual airflow requirements, but these reasons, in themselves, do not justify a fan array. The importance of each of the main criteria will vary from application to application. If an application needs a fan array the first step is to select the proper number and size of the fans and motors and the second step is to select the most efficient and cost effective method of controlling the performance of the fan array. Below are four different options of fans and control methods for an air handling unit that requires 60,000 CFM at 7.7 TSP with a control set point of 1.0" w.g. and requires 100% redundancy when one fan fails. Twin City Model EPFN direct drive plenum fans were used as the basis for these four options. Option A: Option B: Option C: Option D: 12 18.25 diameter direct drive plenum fans each with a 10 HP 3600 RPM motor controlled by variable frequency drives to maintain a 1.0 w.g. set point. 6 24.5 diameter direct drive plenum fans each with a 20 HP 1800 RPM motor controlled by variable frequency drives to maintain a 1.0 w.g. set point. 12 18.25 diameter direct drive plenum fans each with a 10 HP 3600 RPM motor controlled by staging and variable frequency drives to maintain a 1.0 w.g. set point. The number of active fans is based on selecting the lowest fan array brake horsepower. 20 16.5 diameter direct drive plenum fans each with a 7.5 HP 3600 RPM motor controlled by staging only to maintain a 1.0 w.g. set point. Efficiency Efficiency is not a criterion for using a fan array, but if a fan array is the best option for a particular application then the efficiency of the fan array is important. The following table shows the fan array brake horsepower s for the above four options from 60,000 CFM down to 24,000 CFM. The system curve for the four options takes into account the 1.0" w.g. control set point. Summary of Fan Brake Horsepowers Design Rating 60,000 CFM @ 7.70 TSP / Control Set Point of 1.0 WG Control Method Option A Option B Option C Option D CFM @ SP Speed Only Speed Only Staging & Speed Staged Only 60,000@7.70 104.52 97.08 12-104.52 20-120.19@60,000 CFM 57,600@7.17 93.36 86.82 12-93.36 19-113.81@59,660 CFM 55,200@6.67 83.28 77.34 12-83.28 18-107.10@58,464 CFM 52,800@6.19 73.92 68.58 12-73.92 17-100.13@56,950 CFM 50,400@5.73 65.16 60.66 12-65.16 16-92.96@55,248 CFM 48,000@5.29 57.36 53.34 12-57.36 15-85.80@53,340 CFM 45,600@4.87 50.16 46.68 12-50.16 14-78.68@51,324 CFM 43,200@4.47 43.56 40.56 12-43.56 13-71.63@49,192 CFM 40,800@4.10 37.80 35.16 12-37.80 12-64.92@46,932 CFM 38,400@3.74 32.52 30.24 11-32.45 11-58.19@44,495 CFM 36,000@3.41 27.84 25.86 11-27.72 10-51.55@41,750 CFM 33,600@3.10 23.64 21.96 11-23.54 9-44.82@38,745 CFM 31,200@2.81 19.91 18.54 11-19.80 8-38.08@35,384 CFM 28,800@2.54 16.80 15.48 10-16.50 7-31.64@31,787 CFM 26,400@2.30 14.04 12.96 10-13.70 6-25.80@27,888 CFM 24,000@2.07 11.76 10.62 10-11.80 N/A
Notes: 1-2 - 3 - All of the above options follow the system curve from 60,000 CFM @ 7.7 TSP with a set point of 1.0 w.g. except for Option D which will deliver more static pressure than the control point. On Options C & D the data shows the number of fans active and the total BHP for the fan array. When fans are controlled by staging only Option D - they need to be selected as close to the motor speed as possible. In option D this dictated 20 16.5 diameter fans. The graph below shows the normal operating range of VAV systems - 100% to 60% airflow. 132 - Fan Array Brake Horsepower 116-60 Hz Fan Array Brake Horspower 100-84 - 68-52 - 36 - Option D Option A Option B 50 Hz 75 Hz Note: Option C plots on top of Option A. 20-36,000 38,800 41,600 44,400 47,200 50,000 52,800 55,600 58,400 61,200 Looking at the above four options a few facts are clear. Fan Array CFM Option B Option C Option D Is the most efficient fan array option. Has almost the same fan brake horsepower as Option A. The staging with speed control is only slightly more efficient than speed control alone and then only when the unit is operating below 66% of design airflow. Is not a very efficient method of controlling airflow in a fan array. Redundancy if One Fan Fails Option A: 100% Option C: 100% Option B: 100% Option D: 99.4% Applications that do not require 100% redundancy may be able to use more efficient fans and motors, lower connected motor horsepower and reduce cost.
Fan Sound Power Levels The table below shows the inlet and outlet sound power levels for all four options at 60,000 CFM @ 7.7 TSP. Option A: Octave Bands - 1 2 3 4 5 6 7 8 Inlet 97 97 105 108 95 91 90 86 Outlet 98 98 105 108 100 98 94 88 Option B: Octave Bands - 1 2 3 4 5 6 7 8 Inlet 93 93 109 102 91 90 89 84 Outlet 100 100 104 103 99 95 93 88 Option C: Octave Bands - 1 2 3 4 5 6 7 8 Inlet 97 97 105 108 95 91 90 86 Outlet 98 98 105 108 100 98 94 88 Option D: Octave Bands - 1 2 3 4 5 6 7 8 Inlet 96 97 101 104 95 95 95 91 Outlet 100 99 103 109 102 99 99 94 Note: These sound power levels reflect the number of fans in each array. The above sound levels show the sound power of each fan array, but do not take into account the attenuation due to the perforated panels or the attenuation by the air handling unit. Bare fan sound power levels are useful in picking the quietest fans, but if sound is truly critical then the only way to get accurate sound data is to have the unit tested in an AMCA 300 reverberant room. When you compare the sound power levels of a single housed centrifugal fan and a single direct driven plenum fan the direct driven plenum fan is much quieter in the lower octave bands which are the octave bands most difficult to attenuate. The specific application requirements will determine the optimum number of direct drive plenum fans. More fans are not necessarily the quietest option. Unit Dimensions Fan arrays can modify the unit length, width and height. The table below compares the dimensions of the fan array section for the four options and a single housed centrifugal fan. Options Quantity Fan Diameter Qty. Wide Qty. High Length Width Height Tunnel Area A & C 12 18.25 4 3 61 137 103 98 SF B 6 24.50 3 2 73 134 89 83 SF D 20 16.50 5 4 58 157 126 138 SF DWDI 1 44.50 1 1 134 136 95 90 SF Notes: 1-2- 3- The length of the direct drive plenum fan sections include - fan bulkhead, flexible connection, fan length, motor length and an access section with a door sized for motor removal. The fan section length of the single housed centrifugal fan includes additional upstream access, fan length, flexible connection, fan bulkhead, airflow diffuser and an access section for downstream access. The unit widths and heights of the direct drive plenum fan arrays assume 33% of the wheel diameter for clearance on all four sides of each fan wheel. 4- The DWDI fan assumes a clearance of 75% of the wheel diameter on both sides of the fan housing for the width and 16 more that the fan housing height for the height.
In most cases the face velocity across the coils or filters will determine the width and height of the unit, but the fan array dimensional requirements need to be taken into account. Larger Motors versus Smaller Motors The issue of fan arrays with smaller motors versus larger motors is important. Notes: 1. The more motors in a fan array the higher the possibility of a failure. 2. Larger motors have a longer average life than smaller motors. 3. Larger motors are more efficient than smaller motors. 1. TEFC motors have the same trend. 2. If motor replacement is a concern then air handling units should be provided with a lifting aid such as a monorail. Observations Motor Horsepower Nominal Full-Load Efficiency Open Motors Enclosed Motors 2 Pole 4 Pole 6 Pole 2 Pole 4 Pole 6 Pole 5 86.5 89.5 89.5 88.5 89.5 89.5 7.5 88.5 91.0 90.2 89.5 91.7 91.0 10 89.5 91.7 91.7 90.2 91.7 91.0 15 90.2 93.0 91.7 91.0 92.4 91.7 20 91.0 93.0 92.4 91.0 93.0 91.7 30 91.7 94.1 93.6 91.7 93.6 93.0 Looking at the four options for the 60,000 CFM @ 7.7 TSP example the following can be observed. 1. A fan array with larger fans and motors with varying speed control will provide the lowest brake horsepower. 2. Staging with speed control provides no noticeable improvement in fan brake horsepower over speed control alone, and is more complicated and costly. 3. Staging only is not a viable control option due to the inefficiency. 4. Redundancy will vary with the number of fans and the difference between the BHP and the MHP at design CFM & TSP. 5. Direct drive plenum fans are quieter than housed centrifugal fans in the lower octave bands. 6. Fan arrays are a good option when you need to save unit length. 7. Even airflow across components is best when all fans stay active. Comments Unit first cost is always an important consideration. The cost of all air handling systems tends to be lower as the number of fans decrease and the fan sizes increase. This is not only a function of fan and motor cost, but is greatly affected by the quantity and cost of peripheral devices needed such as airflow measuring, isolation dampers, motor overloads, disconnects and variable frequency drives. The issue of fan/motor redundancy has been addressed, but the redundancy of the variable frequency drives needs to be considered. A system where one large VFD controls the entire fan array provides no VFD redundancy and would require another large VFD to provide 100% redundancy. A system where every fan has its own VFD would provide the same VFD redundancy as the fans selected, and would eliminate the need for
motor overloads. There are also options that might be acceptable between the above two depending on the system redundancy requirements. No matter which option is selected for VFD redundancy the air handling unit manufacturer should provide a unit with a single point of electrical connection for each fan array. The issue of fan isolation dampers needs to be addressed. If the system requires 100% airflow be available instantaneously when a fan or VFD fails then each fan in the fan array needs a fan isolation damper. If the system can be operated at less than the minimum airflow levels required for 30 to 60 minutes then the fan array does not need dampers because the maintenance personnel can physically block off the failed fan. When you address the issue of KW for a fan array it gets very difficult to tie down the difference between fan brake horsepower and the actual KW consumption. The following issues need to be taken into account: Motor Efficiency, Power Factor and Variable Frequency Drive Efficiency. There is very little data on the efficiency of motors operating at varying speeds and varying loads. What data there is would suggest the following: Select the most Efficient Fans, Use Premium Efficiency Motors, Use PWM Variable Frequency Drives and Select Fan/Motor combinations that require fan speeds between 60 and 90 Hertz at design airflow and static pressure. Summary: The option that provides the required redundancy, sound levels and unit dimensions with the fewest number of fans and the most efficient and cost effective control scheme will be the best option. Six Fans with Speed Control Holcombe Kelley Air Handling Solutions LLC Holcombe Kelly Holcombe Kelly began his career as a representative for Buffalo Forge 1968 to 1978 before forming his own firm where he actively applied and designed and specified custom fan systems utilizing components manufactured by companies such as Twin City, Howden, American Fan, Industrial Sheet Metal, Miller-Picking, Marlo, Cambridge, Arrow, Vibre Acoustics, Parametrics (ABB), and Niagara Blower. In 1988 he returned to the corporate world and spent the next 22 years in various management roles with Buffalo Forge, Niagara Blower, Buffalo Air and The Trane Company. Holcombe s specialty throughout his career has been in applying and designing custom air handling systems where his knowledge of fans, coils, filters and acoustics is well respected in the industry.