Fan-Tastic: Cutting Energy Cost Out of Thin Air by Thomas Godbey, Donaldson Company, Inc. Industry consumes 27% of the retail electrical energy produced in the U.S., and fans and pumps account for 40% of that 27% -- with fans being the majority of the 40%. (Ref 1, 2) Yet, when is the last time your company conducted an air audit to see how much air you move each day and the power used to move that air? I dare say most plants move more pounds of air than they produce product and significant energy is used in moving that air. Energy is money, and if we can save energy used for moving air, then we can save money and drive down the cost of the product we produce. Air has weight and because it has weight, we need to exercise good engineering judgment when dealing with exhaust systems. Standard Air is defined as air that consists of 78.1% nitrogen 21% oxygen 0.9% argon That air at standard conditions of sea level, dry, and 70 F weighs 0.75 lbs. per cubic foot. A typical dust exhaust system with a 30 -diameter inlet duct to the fan handles about 17,000 cubic foot of air per minute. Or, expressed in weight, the fan handles over 335,000 tons of air each year based on a 24-hour-per-day operation. Many facilities have multiple systems like this. How much money do we spend moving air in a plant? Energy is money. Implementing changes that save energy is like money in the bank. OVERVIEW 1. What constitutes air energy cost? 2. What constituents can you control? 3. How to change these constituents to reduce energy 4. Examples and case history 1. WHAT CONSTITUTES AIR ENERGY COST? Energy cost for industrial ventilation systems has two primary aspects: 1. The cost to move the air through the system that is, the cost to run the fan, and 2. The cost to heat, cool, and humidify air to replace the air being exhausted from the plant. Most grain elevators do not spend significant dollars to heat, cool or humidify the air in the workspace; therefore, this paper will concentrate on the cost to move the air through the system basically, the cost to run the fan. The power needed on the shaft of the fan is commonly and correctly referred to as break horsepower or Bhp. (Ref 3) Bhp can be calculated using this formula: Whereas: bhp = (Q x P / 6356 x N f ) x df bhp = fan shaft horsepower Q = airflow in cubic feet per minute (cfm) P = pressure drop/rise across the fan in inch water gauge ( wg) 6356 = constant N f = fan efficiency expressed as a decimal df = density factor defined as the actual density/density of standard air For purposes of this paper, the air is assumed to be standard air and the df = 1. Typical fan efficiencies are 60 to 68% for radial bladed fans and 70 to 80% for backward inclined fan designs.
DONALDSON TORIT 2 The type of fan used in an industrial ventilation system should be dictated by its use and performance requirements. Radial bladed fan designs are typically used in dirty air streams, and backward inclined fan designs are used in relatively clean air streams, like on the clean side of a baghouse. Unfortunately, we find many of the older fans in grain facilities are the more inefficient radial bladed design selected because of their rugged performance characteristics with little consideration given to energy use, because energy was cheap at that time. Energy is purchased based on kilowatt (kw) not bhp; thus, the bhp needs to be converted to kw. To convert bhp to kw, simply multiply bhp by 0.746. But to get the energy into the motor, this shaft kw needs to be divided by the motor efficiency, N m. Motor efficiency depends on the motor design but is usually about 0.9. There are electrical losses in the starters and transmission losses in the lines between the electrical meter and motor, but these are small and for purposes of looking at relative savings, these are insignificant and can be lumped into the motor efficiency of 0.9. Now, simply multiply the kw by the hours of operation and the cost of electricity per kw hour. Whereas: Yearly cost = (Q x P x 0.746 x H x 52 x C) 6356 x N f x N m 52 = weeks operation per year H = operating hours per week C = cost per kw hour Yes, the 6356 and the.746 could be combined into a single constant but then the logic string would be broken. This answers the first question: What constitutes air energy cost? 4. EXAMPLES OF HOW TO SAVE ENERGY 4A. GOOD DESIGN PRACTICES One way to save energy is to use good design practices. Design is too broad and extensive a subject to be fully covered in this paper, but there are many good resources for design practices such as the Industrial Ventilation Conferences (http//www. michiganivc.org) and others. These conferences provide training on how to design exhaust systems that work with minimum resistance and maximum fan performance. Many plants have installation similar to the one illustrated in Figure 1. On external appearance, this is a very nice installation; the problem is the elbow on the inlet. For a fan to perform at maximum efficiency it needs about three to four diameters of straight duct on the inlet between the elbow and the fan inlet. (Ref 4) Without that straight run, the air doesn t uniformly fill the fan inlet and the fan performs at less than published rate. It s not delivering the design airflow. In fact, that arrangement is equivalent to 0.9 wg pressure loss. Assuming this system is 17,000 cfm, operating 24 hours per day with an 80% efficient fan at $0.09 per kw hour, this installation consumes an unnecessary $1960/year in energy. That is a cost each and every year. Because of equipment layout and cost, moving the fan a length equivalent to 4 diameters from the elbow may not be practical. But, unbolting the elbow and replacing it with an identical elbow with three turning vanes inside reduces that annual penalty to $435 a $1525 savings and it provides increase airflow with no additional energy cost. 2. WHAT CONSTITUENTS CAN WE CONTROL? Of the items in this formula, the only two that designers and operators of industrial ventilation systems can affect are the airflow and pressure drop. 3. HOW DO WE CONTROL THESE VARIABLES? If the objective is to minimize energy, then flow should be minimized and pressure drop (or resistance to flow) must be reduced. Figure 1: Elbow on fan inlet to baghouse
DONALDSON TORIT 3 According to a national supplier of ducting components, the added cost for installing the turning vanes inside the 30 diameter, 5-piece elbow on initial installation is $300. A bad design decision. A new elbow with turning vanes would cost $1100 and require two maintenance men about half a day to replace the (Ref 5) old elbow. Good design is important. Less than good design costs extra as long as the system is in operation. 4A-1. CENTRAL SYSTEMS Conventional dust control systems consist of hoods, ducting, collector with hopper, dust disposal and a fan. Layouts similar to the one in Figure 2 are common. The change in flow is directly proportional to the change in speed. If a 20% increase in flow is desired, the fan speed must be increased by 20%. SP (new) SP (old) [ rpm (new) ] 2 = a rpm (old) A 20% increase in fan speed results in a 44% increase in Static Pressure (SP). If 20% more air flows through a fixed duct system, the resistance through the system increases by 44%. HP (new) HP (old) [ rpm (new) ] 3 = a rpm (old) The power requirement, and thus the energy requirement, is a cube function of the change in speed. That 20% increase in flow, and therefore speed, increases the power requirement by 74% a lot of money for just 20% increase in flow. 4A-3. DEDICATED AND INTEGRATED COLLECTORS Short of ripping out the entire system and replacing it at major cost, what is a plant engineer to do? Figure 2: Central dust collection system 4A-2. FAN LAWS OR SYSTEM LAWS There is always pressure on the facility for more tons of throughput and, over time, belt speeds are increased, bucket elevator drives are upgraded, etc. Soon the exhaust system is no longer adequate because the airflows were never increased to accommodate the production changes. What is the answer? The classic response is to speed up the fan, and that will improve flow. But, you pay a penalty a big penalty. Physical science laws govern what happens. These laws are referred to as Fan Laws or (Ref 3) System Laws and are shown in these equations. Consider taking one or more of the pickup points off the ducted system and fit these with individual dedicated collectors. The air will redistribute itself through the remaining pickups, increasing the flow through the remaining hoods. This new dedicated collector can be a smaller version of the free-standing central collector with fan and dust discharge device. Or if the application permits, it can be a collector integrated into the hood enclosure as shown in Figures 3 and 4. cfm (new) cfm (old) = rpm (new) rpm (old) Figure 3: Dust collector on belt transfer
DONALDSON TORIT 4 Filter media surface treatments Surface treatments can perform many functions: increased efficiency, decreased filter pressure drop, increased resistance to moisture and chemistry, better dust cake release, reduced bridging, etc. Many surface treatments exist. The treatments can be mechanical, chemical or a combination. The primary goal of most finishes is to retain and release the majority of the particulate on and from the surface of the media but can also be used to increase resistance to moisture and chemistry and reduce bridging. Some of the more common filter media surfaces include: Figure 4: Dust collector on bucket elevator These small dedicated collectors and integral collectors are just a reliable as the larger central baghouse and require much less energy. In Figure 3, a dedicated collector located near the source will require less than 20% additional energy compared to 74% for a collector with increased fan speed. The integral collector has the added advantage of no ducting, no dust discharge device, and lower power cost since all ducting losses are eliminated. To maximize the impact, locate these units: In points farthest from the central collector, or Points where the exhaust equipment is not always in use. Then, the collectors can be shut off when the equipment being exhausted is not in use and not producing dust. (Nothing saves energy like a properly pushed OFF button.) 4B. PERFORMANCE FILTER MEDIA Recall that energy savings result from minimizing airflow and pressure drop while satisfying the application requirements. One of the ways to reduce pressure drop is to upgrade the filter media in fabric collectors to the newer performance media. Performance media fall into three general categories. 1. Filter media surface finishes 2. Pleated bags 3. New technology media Plain standard filter bags have a plain felt finish with a natural softness attributed to the open fibers. These fibers aid in the capture of fine particulate and hold the dust cake, a particular problem for dust like protein and starch that agglomerate easily and form a hard cake in the presence of high humidity. Singed finish is provided by melting the surface fibers with a gas flame to reduce the tendency of dust particles to stick to the surface. A singed finish usually provides better dust cake release and thus a lower pressure drop. Glazed finish, also known as an eggshell finish, is the melting and smearing of a microscopic layer of the media fibers to form a slick surface for better dust cake release. It does provide improvement for dust cake release and lower operating pressure drop for some very tenacious dust. Silicone treatments aid the initial build up of the dust cake and reduce moisture absorption into the fiber, allowing better release of moisture sensitive dust and lower operating pressure drop. Oleophobic, hydrophobic and universal chemical finish are all terms describing felt that has been immersed in a fluorocarbon bath, squeezed and then heat set into the felt. The fluorocarbon reduces the absorption of moisture and acids into the fibers and provides a slick surface for dust cake release. It can increase fiber resistance to hydrolysis from moisture and heat and increase resistance to acid attack. Expanded PTFE, a membrane, can be thermobonded to the surface of conventional needled felts or woven fabrics. This membrane provides high efficiencies, superior dust cake release and improved airflow all at a cost.
DONALDSON TORIT 5 Many other surface treatments such as acrylic foam are available and have been developed to fit the unique requirements of specific industries and applications. Pleated bags: The need to fit more media into a baghouse to provide increased airflow, plus the desire to enhance the performance of existing conventional round tube collectors, has spawned a new type of pleated filter. The filter is round in shape like a conventional 6 or 6.25-inch diameter round bag except the media is pleated to provide more filtering surface area. The pleated bag can double or triple the filter area of an existing unit. This does not mean the airflow is tripled. These pleated bags normally operate at lower filtration velocities than the fabric bags they replace. How much lower depends on the dust characteristics and housing design, but retrofitting existing round bag collectors with pleated bags does offer increased airflow or lower pressure drop and can be a cost effective answer to increased airflow or to reduced pressure drop on existing collectors. The media is usually spunbond polyester although rigidized felt is also used. The latest development in the pleated bag media is spunbond with a surface treatment of high efficiency nanofibers, Ultra-Web. The nanofiber technology keeps the dust on the surface of the media where it is easily pulsed off during the collector cleaning cycle. The results are lower pressure drop and lower emissions than conventional spunbond. This media is currently available for both pleated round bags and conventional cartridge filters marketed as Ultra-Web SB. New technology media: These include graduated denier media, composite media, hydroentangled media and others. Most of the new technology media did not exist 10 years ago. They were introduced long after many existing collectors were purchased, installed and the filter media specified. Upgrading these collectors by installing a higher performance media at the next scheduled bag change can significantly decrease the operating pressure drop, increase efficiency and increase filter life. Graduated denier medias are constructed with a layer of larger diameter fibers on the clean air side and a layer of finer fibers on the dirty air side. This improves surface filtration while allowing for a lower pressure drop and better pulse cleaning than a felt of all fine fiber. Composite media is constructed on two different fiber types to take advantage of the inherent characteristics of each. One of the more common composites is a thin layer of P84 fibers on the surface of a less expensive felt like polyester. P84 has excellent efficiency and dust cake release characteristics (read that as lower pressure drop) but is expensive. Polyester is an inexpensive yet rugged fiber. A fabric of all P84 could be cost prohibitive when purchased for only reduced pressure loss but combining it with a base of polyester can provide a cost effective answer for some troublesome pressure loss problems. Hydroentangled felt media is the latest technology for making felt and uses computercontrolled, high pressure water jets instead of needles in the manufacturing process. Typical manufacturing processes for fabric incorporate a needling process that pulls, weaves and entangles the fibers together to form a thick felt. The drawback to this mechanical needling process is the inconsistency of fiber pore spacing and size. This inconsistency affects filtration efficiency and pressure loss by allowing dust to migrate through the pore structure and allowing dust particles to become lodged within the depth of the felt, blocking airflow causing excess pressure loss across the filters. The hydroentanglement process, shown in Figure 5, creates a filter material that has a higher proportion of fine fibers and smaller and more consistent pore size as shown in Figure 6. This consistency and uniformity create a filter bag material that more effectively surface-loads dust, allowing for more efficient pulse cleaning and lower pressure drop. The smaller pore size also: Retards depth-loading to promote more efficient filter cleaning, and Lowers pressure losses allowing higher airflows, increased filter life, and lower emissions. The new Dura-life felt is available in different fiber materials such as polyester, Nomex * and others. * Nomex is a registered trademark of E.I. Dupont de Nemours & Co., Inc.
DONALDSON TORIT 6 Figure 5: Needling vs. hydroentangling 10.5 oz. Dura-Life 16 oz. Standard Polyester Figure 6: Properties of hydroentangled felt Hydro-entangling and using smaller fibers result in: More uniform material than standard felt with a 19% reduction in maximum pore size Better uniformity and smaller pore size result in: Better surface loading and less depth loading Better pulse cleaning = lower pressure drop = longer life
DONALDSON TORIT 7 How to realize the savings Just fitting the collector with lower pressure drop filter media will not cause an energy savings to be realized. Why? Because the lower pressure drop across the media causes a lower pressure at the inlet of the fan, and the fan moves more air and more horsepower is required (not less), an energy increase occurs (not an energy savings). To realize the energy savings, the fan speed must be reduced such that the airflow originally designed for the system is achieved. Figure 7 is a fan performance curve showing the Static Pressure (SP) curve and the brake horsepower (bhp) curve. Imposed over these performance curves is the system curve, which represents the resistance of all components in the collection system (hoods, ducting, etc.) less the pressure drop across the filter media. If the design airflow is Q1 with a design P1 and highperformance filter bags are installed that operate at a reduced pressure drop P2, then the SP at the fan inlet is reduced to SP1, resulting in an increased flow Q2 and an increase in horsepower bhp2. A change is required to return the flow to the design flow Q1. One way to reduce the airflow to the original Q2 value, is to simply close a damper to increase the resistance as illustrated in Figure 9. That will reduce the airflow to the original level, but it also returns the bhp to the original level. In effect, the damper is creating a new system curve as shown in Figure 10. This does not accomplish the objective of reducing energy. Figure 7: The difference between the fan SP curve and the system curve = the pressure available for resistance across the filter bags, P. Figure 8: How high-performance filter bags can affect system performance Figure 9: Effect on damper resistance Figure 10: Damper creates new system curve
DONALDSON TORIT 8 To realize an energy savings, the bhp requirement must be reduced and this requires a changed in speed. The two ways of changing speed are variable speed motor drives and simple belt and sheave changes. If the system has not been measured, or a fan curve is not available, or if the system flow is variable, the most efficient solution is a Variable Frequency Drive (VFD). A VFD provides an infinite number of fan SP curves and corresponding bhp curves as shown in Figure 11. Simply dial in the speed that gives the desired flow and reap the benefits of the change in power. Remembering the third Fan Law: For a fixed system, the bhp decreases as the cube of the change in speed. A 5% decrease in speed results in a 15% energy savings. 0.95 3 = 0.85 A 10% decrease in speed results in a 28% energy savings. 0.9 3 = 0.72 The downside is that VFDs are more costly than a simple belt and sheave change. For a 40hp system, a free standing VFD will cost between $5000 and $5500 and require two electricians a half day to install an amount that can still achieve satisfactory payback in many situations. For a fixed system without requirements for variation in airflow, a simple belt and sheave change on the fan drive is all that is required. The change in speed returns the flow to the original Q1 value and reduces the power requirement from bhp1 to bhp2 as shown in Figure 12. The fixed speed change provides 100% energy recovery and is inexpensive. For the example system with a 40hp motor, the cost is about $300 and will require two maintenance men about half day to install. 4C. CASE HISTORY A large grain handling and oil seed processing facility in southern USA has 35 baghouse collectors with fabric filters in the facility. Under pressure to reduce energy cost, and after much research into performance filter media, a decision was made to fit one collector with the Dura-Life hydroentangled needle felt, Dura-Life. If the new media could reduce the pressure loss across the collector by 2 wg, as expected, the resulting energy savings with all 35 collectors fitted with this media would be significant. Figure 13 shows the data for the analysis. The airflow is set at 12,900 cfm. The needle felt pressure drop was 5 wg and the Dura-Life bags stabilized at 3 wg. The fan was a radial blade fan with an efficiency of 63%. An electric transmission/motor efficiency of 90% was used. The facility operates 24 hours per day, 7 days a week, 51 weeks each year. Their electrical cost was $0.09 per kw hour. In addition to stabilizing at lower pressure drop, the hydroentangled filter elements also had twice the filter life. When the plant manager calculated everything (labor savings for fewer filter changeouts, the incremental cost of the drive change and the increased cost of the filter bags) the payback time was 3.8 months. Simply by purchasing and installing Dura-Life filter bags and re-sheaving for a lower speed, the facility determined they could potentially save over $77,000 a year if all 35 collectors were retrofitted. Figure 11: The effect of a variable frequency drive (VFD) on the fan Figure 12: Effect of belt & sheave change
DONALDSON TORIT 9 TRIAL INSTALLATION IN SOUTHERN US GRAIN FACILITY NEEDLED FELT FILTER MEDIA DURA-LIFE HYDROENTANGLED FILTER MEDIA Airflow 12,900 12,900 P 5 3 Fan Efficiency 63% 63% Electrical Efficiency 90% 90% Hours per Week 168 168 Weeks per Year 51 51 Cost per KW $0.09 $0.09 Yearly Cost for Media Loss $10,295 $6,178 Savings $4,117 Figure 13: Data at Dura-Life test site SUMMARY Air has weight and considerable energy is used in moving air within industrial ventilation systems. In order to minimize the energy cost and thus reduce the cost of the product, good engineering judgment must be exercised when designing new systems and alternative systems must be considered. For existing systems, look at alternative products like bags with enhanced filter finishes and new higher performance media with the goal of reducing pressure loss through the system. Both may represent a higher initial cost but when the energy savings is considered, either can provide long term savings. 1 Energy Information Administration, DOE/EIA-0384, June 2007, The National Energy to Electricity Balance for 2006 2 Speaker, Bart van de Velde, Rockwell Automation, Presentation at Donaldson International Technical Conference, Nov. 2007, Leuven, Belgium. 3 Buffalo Forge Co., Fan Engineering, Eighth Edition, 1983 4 Air Movement and Controls Association, Inc., AMCA Publication 201-90, Fans and Systems. 5 Duct Cost, Brian McAlpine, Nordfab Co Inc. Donaldson Company, Inc. Torit P.O. Box 1299 Minneapolis, MN 55440-1299 U.S.A. Tel 800-365-1331 (USA) Tel 800-343-3639 (within Mexico) donaldsontorit@donaldson.com donaldsontorit.com Cutting Energy Cost out of Thin Air? 2011 Donaldson Company, Inc. All Rights Reserved. Information in the document is subject to change without notice.