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HVAC SYSTEMS 265 building service personnel. For example the controls on a unit ventilator include a room temperature thermostat which controls the valve on the heating or cooling coil, a damper control which adjusts the proportion of fresh air mixed with recirculated room air, and a low-limit thermostat which prevents the temperature of outside air from dropping below a preset temperature (usually 55 to 60°F; 13 to 16°C). A common error of occupants or building custodians in response to a sense that the air supplied by the unit ventilator is too cold is to increase the setpoint on the low-limit thermostat, which prevents free cooling from outside air or, on systems without a cooling coil, prevents cooling altogether. Controls which are subject to misadjustment by building occupants should be placed so that they cannot be tampered with. The energy consumption of thermally heavy buildings is less related to either the inside or outside air temperature. Both the heating and cooling loads in thermally heavy buildings are heavily dependent on the heat generated from internal loads and the thermal energy stored in the building mass which may be dis- Figure 10.13 Wet-side economizer schematic diagram. 266 ENERGY MANAGEMENT HANDBOOK sipated at a later time. In an indirect control system the amount of energy consumed is not a function of human thermal comfort needs, but of other factors such as outdoor temperature, humidity, or enthalpy. Indirect control systems determine the set points for cool air temperature, water temperatures, etc. As a result indirect control systems tend to adjust themselves for peak conditions rather than actual conditions. This leads to overheating or overcooling of spaces with less than peak loads. One of the most serious threats to the effi ciency of any system is the need to heat and cool air or water simultaneously in order to achieve the thermal balance required for adequate conditioning of spaces. Figure 10.14 indicates that 20 percent of the energy consumed in a commercial building might be used to reheat cooled air, offsetting another 6 percent that was used to cool the air which was later reheated. For the example building the energy used to cool reheated air approaches that actually used for space cooling. Following the 1973 oil embargo federal guidelines encouraged everyone to reduce thermostat settings to 68°F (20°C) in winter and to increase thermostat settings in air-conditioned buildings to 78°F (26°C) in summer. [In 1979, the winter guideline was reduced farther to 65°F (18°C).] The effect of raising the air-conditioning thermostat on a reheat, dual-duct, or multizone system is actually to increase energy consumption by increasing the energy required to reheat air which has been mechanically cooled (typically to 55°F; 13°C). To minimize energy consumption on these types of systems it makes more sense to raise the discharge temperature for the cold-deck to that required to cool pe- rimeter areas to 78°F (26°C) under peak conditions. If the system was designed to cool to 75°F (24°C) on a peak day using 55°F (13°C) air, the cold deck discharge could be increased to 58°F (14.5°C) to maintain space temperatures at no more than 78°F (26°C), saving about $5 per cfm per year. Under less-than-peak conditions these systems would operate more effi ciently if room temperatures were allowed to fall below 78°F (26°C) than to utilize reheated air to maintain this temperature. More extensive discussion of energy management control systems may be found in Chapters 12 and 22. 10.5.7 HVAC Equipment The elements which provide heating and cooling to a building can be categorized by their intended function. HVAC equipment is typically classifi ed as heating equipment, including boilers, furnaces and unit heaters; cooling equipment, including chillers, cooling towers and air-conditioning equipment; and air distribution elements, primarily air-handling units (AHUs) and fans. A more lengthy discussion of boilers may be found in Chapter 6, followed by a discussion of steam and condensate systems in Chapter 7. Cooling equipment is discussed in section 10.6, below. What follows here re- lates mostly to air-handling equipment and distribution systems. Figure 10.14 depicts the typical energy cost distribution for a large commercial building which employs an all-air reheat-type HVAC system. Excluding the energy costs associated with lighting, kitchen and miscellaneous loads which are typically 25-30 percent of the total, the remaining energy can be divided into two major categories: the energy associated with heating and cooling and the energy consumed in distribution. The total energy consumed for HVAC systems is therefore dependent on the effi ciency of individual components, the effi ciency of distribution and the ability of the control system to accurately regulate the energy consuming components of the system so that energy is not wasted. The size (and heating, cooling, or air-moving capacity) of HVAC equipment is determined by the mechanical designer based upon a calculation of the peak internal and envelope loads. Since the peak conditions are arbitrary (albeit well-considered and statistically valid) and it is likely that peak loads will not occur simultaneously throughout a large building or complex Figure 10.14 Energy cost distribution for a typical non-residential building using an all-air reheat HVAC system. Space cooling Other ( magnitude uncertain) Kitchen & process Domestic hot water Cooling of reheat Pumps Fans Lighting Reheat Space heating HVAC SYSTEMS 267 requiring all equipment to operate at its rated capacity, it is common to specify equipment which has a total capacity slightly less than the peak requirement. This diversity factor varies with the function of the space. For example, a hospital or classroom building will use a higher diversity multiplier than an offi ce building. In sizing heating equipment however, it is not un- common to provide a total heating capacity from several units which exceeds the design heating load by as much as fi fty percent. In this way it is assured that the heating load can be met at any time, even in the event that one unit fails to operate or is under repair. The selection of several boilers, chillers, or air- handling units whose capacities combine to provide the required heating and cooling capability instead of single large units allows one or more components of the system to be cycled off when loads are less than the maximum. This technique also allows off-hours use of specifi c spaces without conditioning an entire building. Equipment Effi ciency Effi ciency, by defi nition, is the ratio of the energy output of a piece of equipment to its energy input, in like units to produce a dimensionless ratio. Since no equipment known can produce energy, effi ciency will always be a value less than 1.0 (100%). Heating equipment which utilizes electric resistance appears at first glance to come closest to the ideal of 100 percent effi ciency. In fact, every kilowatt of electrical power consumed in a building is ultimately converted to 3413 Btu per hour of heat energy. Since this is a valid unit conversion it can be said that electric resistance heating is 100 percent effi cient. What is missing from the analysis however, is the ineffi ciency of producing electricity, which is most commonly generated using heat energy as a primary energy source. Electricity generation from heat is typically about 30 percent effi cient, meaning that only 30 percent of the heat energy is converted into electricity, the rest being dissipated as heat into the environment. Energy consumed as part of the generation process and energy lost in distribution use up about ten percent of this, leaving only 27 percent of the original energy available for use by the consumer. By comparison, state-of-the-art heating equipment which utilizes natural gas as a fuel is more than eighty percent effi cient. Distribution losses in natural gas pipelines account for another 5 percent, making natural gas approximately three times as effi cient as a heat energy source than electricity. The relative efficiency of cooling equipment is usually expressed as a coeffi cient of performance (COP), which is defi ned as the ratio of the heat energy extracted to the mechanical energy input in like units. Since the heat energy extracted by modem air conditioning far exceeds the mechanical energy input a COP of up to 6 is possible. Air-conditioning equipment is also commonly rated by its energy effi ciency ratio (EER) or seasonal energy effi ciency ratio (SEER). EER is defi ned as the ratio of heat energy extracted (in Btu/hr) to the mechanical energy input in watts. Although it should have dimen- sions of Btu/hr/watt, it is expressed as a dimensionless ratio and is therefore related to COP by the equation EER = 3.41 • COP (10.4) Although neither COP nor EER is the effi ciency of a chiller or air-conditioner, both are measures which allow the comparison of similar units. The term air-conditioning effi ciency is commonly understood to indicate the extent to which a given air-conditioner performs to its maximum capacity. As discussed below, most equipment does not operate at its peak effi ciency all of the time. For this reason, the seasonal energy effi ciency ratio (SEER), which takes varying effi ciency at partial load into account, is a more accurate measure of air-conditioning effi ciency than COP or EER. In general, equipment effi ciency is a function of size. Large equipment has a higher effi ciency than small equipment of similar design. But the rated effi ciency of this equipment does not tell the whole story. Equipment effi ciency varies with the load imposed. All equipment operates at its optimum effi ciency when operated at or near its design full-load condition. Both overloading and under-loading of equipment reduces equipment effi ciency. This fact has its greatest impact on system effi ciency when large systems are designed to air-condition an entire building or a large segment of a major complex. Since air-conditioning loads vary and since the design heating and cooling loads occur only rarely under the most severe weather or occupancy conditions, most of the time the system must operate under-loaded. When selected parts of a building are utilized for off-hours operation this requires that the entire building be conditioned or that the system operate far from its optimum conditions and thus at far less than its optimum effi - ciency. Since most heating and cooling equipment operates at less than its full rated load during most of the year, its part-load effi ciency is of great concern. Because of this, most state-of-the-art equipment operates much closer to its full-load effi ciency than does older equip- 268 ENERGY MANAGEMENT HANDBOOK ment. A knowledge of the actual operating effi ciency of existing equipment is important in recognizing economic opportunities to reduce energy consumption through equipment replacement. Distribution Energy Distribution energy is most commonly electrical energy consumed to operate fans and pumps, with fan energy typically being far greater than pump energy ex- cept in all-water distribution systems. The performance of similar fans is related by three fan laws which relate fan power, airfl ow, pressure and effi ciency to fan size, speed and air density. The reader is referred to the ASHRAE Handbook: HVAC Systems and Equipment for additional information on fans and the application of the fan laws. 3 Fan energy is a function of the quantity of airfl ow moved by the fan, the distance over which it is moved, and the velocity of the moving air (which infl uences the pressure required of the fan). Most HVAC systems, whether central or distributed packaged systems, all- air, all-water, or a combination are typically oversized for the thermal loads that actually occur. Thus the fan is constantly required to move more air than necessary, creating inherent system ineffi ciency. One application of the third fan law describes the relationship between fan horsepower (energy consumed) and the airfl ow produced by the fan: W 1 = W 2 × (Q 1 /Q 2 ) 3 (10.5) where W = fan power required, hp Q = volumetric fl ow rate, cfm Because fan horsepower is proportional to the cube of airfl ow, reducing airfl ow to 75 percent of existing will result in a reduction in the fan horsepower by the cube of 75 percent, or about 42 percent: [(0.75) 3 = 0.422] Even small increases in airfl ow result in disproportional increases in fan energy. A ten percent increase in airfl ow requires 33 percent more horsepower [1.103 = 1.33], which suggests that airfl ow supplied solely for ventila- tion purposes should be kept to a minimum. All-air systems which must move air over great distances likewise require disproportionate increases in energy as the second fan law defi nes the relationship between fan horsepower [W] and pressure [p], which may be considered roughly proportional to the length of ducts connected to the fan: W 1 = W 2 × (P 1 /P 1 ) 3/2 (10.6) The use of supply air at temperatures of less than 55°F (13°C) for primary cooling air permits the use of smaller ducts and fans, reducing space requirements at the same time. This technique requires a complex analysis to determine the economic benefi t and is sel- dom advantageous unless there is an economic benefi t associated with space savings. System Modifi cations In examining HVAC systems for energy conservation opportunities, the less effi cient a system is, the greater is the potential for signifi cant conservation to be achieved. There are therefore several “off-the-shelf” opportunities for improving the energy efficiency of selected systems. All-air Systems—Virtually every type of all-air system can benefi t from the addition of an economizer cycle, particularly one with enthalpy controls. Systems with substantial outside air requirements can also benefi t from heat recovery systems which exchange heat between exhaust air and incoming fresh air. This is a practical retrofi t only when the inlet and exhaust ducts are in close proximity to one another. Single zone systems, which cannot provide suf- ficient control for varying environmental conditions within the area served can be converted to variable air volume (VAV) systems by adding a VAV terminal and thermostat for each new zone. In addition to improving thermal comfort this will normally produce a substantial saving in energy costs. VAV systems which utilize fans with inlet vanes to regulate the amount of air supplied can benefi t from a change to variable speed or variable frequency fan drives. Fan effi ciency drops off rapidly when inlet vanes are used to reduce airfl ow. In terminal reheat systems, all air is cooled to the lowest temperature required to overcome the peak cooling load. Modern “discriminating” control systems which compare the temperature requirements in each zone and cool the main airstream only to the temperature required by the zone with the greatest requirements will reduce the energy consumed by these systems. Reheat systems can also be converted to VAV systems which moderate supply air volume instead of supply air temperature, although this is a more expensive altera- tion than changing controls. Similarly, dual-duct and multizone systems can benefi t from “smart” controls which reduce cooling requirements by increasing supply air temperatures. Hot-deck temperature settings can be controlled so that the temperature of warm supply air is just high enough to meet HVAC SYSTEMS 269 design heating requirements with 100 percent hot-deck supply air and adjusted down for all other conditions until the hot-deck temperature is at room temperature when outside temperatures exceed 75°F (24°C). Dual duct terminal units can be modifi ed for VAV operation. An economizer option for multizone systems is the addition of a third “bypass” deck to the multizone air-handling unit. This is not appropriate as a retrofi t although an economizer can be utilized to provide cold- deck air as a retrofi t. All-water systems—Wet-side economizers are the most attractive common energy conservation measure appropriate to chilled water systems. Hot-water systems benefi t most from the installation of self-contained thermostat valves, to create heating zones in spaces formerly operated as single-zone heating systems. Air-water Induction—Induction systems are sel- dom installed anymore but many still exist in older buildings. The energy-effi ciency of induction systems can be improved by the substitution of fan-powered VAV terminals to replace the induction terminals. 10.6 COOLING EQUIPMENT The most common process for producing cooling is vapor-compression refrigeration, which essentially moves heat from a controlled environment to a warmer, uncontrolled environment through the evaporation of a refrigerant which is driven through the refrigeration cycle by a compressor. Vapor compression refrigeration machines are typically classified according to the method of operation of the compressor. Small air-to-air units most commonly employ a reciprocating or scroll compressor, combined with an air-cooled condenser to form a condensing unit. This is used in conjunction with a direct-expansion (DX) evaporator coil placed within the air-handling unit. Cooling systems for large non-residential buildings typically employ chilled water as the medium which transfers heat from occupied spaces to the outdoors through the use of chillers and cooling towers. 10.6.1 Chillers The most common type of water chiller for large buildings is the centrifugal chiller which employs a centrifugal compressor to compress the refrigerant, which extracts heat from a closed loop of water which is pumped through coils in air-handling or terminal units within the building. Heat is rejected from the condenser into a second water loop and ultimately rejected to the environment by a cooling tower. The operating fl uid used in these chillers may be either a CFC or HCFC type refrigerant. Many existing centrifugal chillers use CFC-11 refrigerants, the manu- facture and use of which is being eliminated under the terms of the Montreal Protocol. New refrigerants HCFC- 123 and HCFC-134a are being used to replace the CFC refrigerants but refrigerant modifications to existing equipment will reduce the overall capacity of this equipment by 15 to 25 percent. Centrifugal chillers can be driven by open or hermetic electric motors or by internal combustion Table 10.1 Summary of HVAC System Modifi cations for Energy Conservation System type Energy Conservation Opportunities All-air systems (general): economizer heat recovery Single zone systems conversion to VAV Variable air volume (VAV) systems replace fan inlet vane control with variable frequency drive fan Reheat systems use of discriminating control systems conversion to VAV Constant volume dual-duct systems use of discriminating control systems conversion to dual duct VAV Multizone systems use of discriminating control systems addition of by-pass deck* All-water systems: hydronic heating systems addition of thermostatic valves chilled water systems wet-side economizer Air-water induction systems replacement with fan-powered VAV terminals *Requires replacement of air-handling unit 270 ENERGY MANAGEMENT HANDBOOK engines or even by steam or gas turbines. Natural gas engine-driven equipment sized from 50 to 800 tons of refrigeration are available and in some cases are used to replace older CFC-refrigerant centrifugal chillers. These engine-driven chillers are viable when natural gas costs are suffi ciently low. Part-load performance modulates both engine speed and compressor speed to match the load profi le, mainta ining close to the peak effi ciency down to 50 percent of rated load. They can also use heat recovery options to take advantage of the engine jacket and exhaust heat. Turbine-driven compressors are typically used on very large equipment with capacities of 1200 tons or more. The turbine may be used as part of a cogeneration process but this is not required. (For a detailed discussion of cogeneration, see Chapter 7.) If excess steam is available, in industry or a large hospital, a steam turbine can be used to drive the chiller. However the higher load on the cooling tower due to the turbine condenser must be considered in the economic analysis. Small water chillers, up to about 200 tons of capacity, may utilize reciprocating or screw compressors and are typically air-cooled instead of using cooling towers. An air-cooled chiller uses a single or multiple compressors to operate a DX liquid cooler. Air-cooled chillers are widely used in commercial and large-scale residential buildings. Other types of refrigeration systems include liquid overfeed systems, fl ooded coil systems and multi-stage systems. These systems are generally used in large industrial or low-temperature applications. 10.6.2 Absorption Chillers An alternative to vapor-compression refrigeration is absorption refrigeration which uses heat energy to drive a refrigerant cycle, extracting heat from a controlled environment and rejecting it to the environment (Figure 10.15). Thirty years ago absorption refrigeration was known for its low coeffi cient of performance and high maintenance requirements. Absorption chillers used more energy than centrifugal chillers and were economical only if driven by a source of waste heat. Today, due primarily to the restriction on the use of CFC and HCFC refrigerants, the absorption chiller is making a comeback. Although new and improved, it still uses heat energy to drive the refrigerant cycle and typically uses aqueous lithium bromide to absorb the refrigerant and water vapor in order to provide a higher coeffi cient of performance. The new absorption chillers can use steam as a heat source or be direct-fi red. They can provide simul- taneous heating and cooling which eliminates the need for a boiler. They do not use CFC or HCFC refrigerants, which may make them even more attractive in years to come. Improved safety and controls and better COP (even at part load) have propelled absorption refrigeration back into the market. In some cases, the most effective use of refrigeration equipment in a large central-plant scenario is to have some of each type, comprising a hybrid plant. From a mixture of centrifugal and absorption equipment the operator can determine what equipment will provide the lowest operating cost under different con- Figure 10.15 Simplifi ed absorption cycle schematic diagram. HVAC SYSTEMS 271 ditions. For example a hospital that utilizes steam year round, but at reduced rates during summer, might use the excess steam to run an absorption chiller or steam- driven turbine centrifugal chiller to reduce its summer- time electrical demand charges. 10.6.3 Chiller Performance Most chillers are designed for peak load and then operate at loads less than the peak most of the time. Many chiller manufacturers provide data that identifi es a chiller’s part-load performance as an aid to evaluat- ing energy costs. Ideally a chiller operates at a desired temperature difference (typically 45-55 degrees F; 25-30 degrees C) at a given fl ow rate to meet a given load. As the load requirement increases or decreases, the chiller will load or unload to meet the need. A reset schedule that allows the chilled water temperature to be adjusted to meet thermal building loads based on enthalpy provides an ideal method of reducing energy consumption. Chillers should not be operated at less than 50 percent of rated load if at all possible. This eliminates both surging and the need for hot-gas bypass as well as the potential that the chiller would operate at low effi ciency. If there is a regular need to operate a large chiller at less than one-half of the rated load it is economical to install a small chiller to accommodate this load. 10.6.4 Thermal Storage Thermal storage can be another effective way of controlling electrical demand by using stored chilled water or ice to offset peak loads during the peak demand time. A good knowledge of the utility consumption and/or load profi le is essential in determining the applicability of thermal storage. See Chapter 19 for a discussion of thermal storage systems. 10.6.5 Cooling Towers Cooling towers use atmospheric air to cool the water from a condenser or coil through evaporation. In general there are three types of cooling tower, named for the relationship between the fan-powered airfl ow and the fl ow of water in the tower: counterfl ow induced draft, crossfl ow induced draft and counterfl ow forced draft. The use of variable-speed, two-speed or three-speed fans is one way to optimize the control of the cooling tower in order to reduce power consumption and provide adequate water cooling capacity. As the required cooling capacity increases or decreases the fans can be sequenced to maintain the approach temperature difference. For most air-conditioning systems this usually varies between 5 and 12 degrees F (3 to 7 degrees C). When operated in the winter, the quantity of air must be carefully controlled to the point where the water spray is not allowed to freeze. In cold climates it may be necessary to provide a heating element within the tower to prevent freeze-ups. Although electric resistance heaters can be used for this purpose it is far more effi cient to utilize hot water or steam as a heat source if available. 10.6.6 Wet-side Economizer The use of “free-cooling” using the cooling tower water to cool supply air or chilled water is referred to as a wet-side economizer. The most common and effective way of interconnecting the cooling tower water to the chilled water loop is through the use of a plate-and- frame heat exchanger which offers a high heat transfer rate and low pressure drop. This method isolates the cooling tower water from the chilled water circuit main- taining the integrity of the closed chilled water loop. Another method is to use a separate circuit and pump that allows cooling tower w ater to be circulated through a coil located within an air-handling unit. The introduction of cooling tower water, into the chilled water system, through a so-called strainer cycle, can create maintenance nightmares and should be avoided. The water treatment program required for chilled water is intensive due to the required cleanness of the water in the chilled water loop. 10.6.7 Water treatment A good water treatment program is essential to the maintenance of an effi cient chilled water system. Filtering the cooling tower water should be evaluated. In some cases, depending on water quality, this can save the user a great deal of money in chemicals. Pretreating new system s prior to initial start-up will also provide longer equipment life and insure proper system performance. Chiller performance is based on given design pa- rameters and listed in literature provided by the chiller manufacturer. The performance will vary with building load, chilled water temperature, condenser water temperature and fouling factor. The fouling factor is the resistance caused by dirt, scale, silt, rust and other deposits on the surface of the tubes in the chiller and signifi cantly affects the overall heat transfer of the chiller. 10.7 DOMESTIC HOT WATER The creation of domestic hot water (DHW) repre- sents about 4 percent of the annual energy consumption 272 ENERGY MANAGEMENT HANDBOOK in typical non-residential buildings. In buildings where sleeping or food preparation occur, including hotels, restaurants, and hospitals, DHW may account for as much as thirty percent of total energy consumption. Some older lavatory faucets provide a fl ow of 4 to 6 gal/min (0.25 to 0.38 l/s). Since hand washing is a function more of time than water use, substantial savings can be achieved by reducing water fl ow. Reduced-fl ow faucets which produce an adequate spray pattern can reduce water consumption to less than 1 gal/min (0.06 l/s). Flow reducing aerator replacements are also available. Reducing DHW temperature has also been shown to save energy in non-residential buildings. Since most building users accept water at the available temperature, regardless of what it is, water temperature can be reduced from the prevailing standard of 140°F (60°C) to a 105°F (40°C) utilization temperature saving up to one-half of the energy used to heat the water. Many large non-commercial buildings employ recirculating DHW distribution systems in order to reduce or eliminate the time required and water wasted in fl ushing cold water from hot water piping. Recirculating distribution is economically attractive only where DHW use is high and/or the cost of water greatly exceeds the cost of water heating. In most cases the energy required to keep water in recirculating DHW systems hot exceeds the energy used to heat the water actually used. To overcome this waste of energy there is a trend to convert recirculating DHW systems to localized point- of-use hot water heating, particularly in buildings where plumbing facilities are widely separated. In either case insulation of DHW piping is essential in reducing the waste of energy in distribution. One-inch of insulation on DHW pipes will result in a 50% reduction in the distribution heat loss. One often-overlooked energy conservation opportunity associated with DHW is the use of solar-heated hot water. Unlike space-heating, the need for DHW is relatively constant throughout the year and peaks during hours of sunshine in non-residential buildings. Year- round use amortizes the cost of initial equipment faster than other active-solar options. Many of the techniques appropriate for reducing energy waste in DHW systems are also appropriate for energy consumption in heated service water systems for industrial buildings or laboratories. 10.8 ESTIMATING HVAC ENERGY CONSUMPTION The methods for estimating building heating and cooling loads and the consumption of energy by HVAC systems are described in Chapter 9. References 1. ASHRAE Handbook: Fundamentals, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, 1993. 2. ASHRAE Handbook: HVAC Applications, American Society of Heat- ing, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, 1995. 3. ASHRAE Handbook: HVAC Systems and Equipment, American So- ciety of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, 1992. 4. ASHRAE Handbook: Refrigeration, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, 1 9 9 4 . K.K. LOBODOVSKY BSEE & BSME Certifi ed Energy Auditor State of California 11.1 INTRODUCTION Effi cient use of electric energy enables commercial, industrial and institutional facilities to minimize operating costs, and increase profi ts to stay competitive. The majority of electrical energy in the United States is used to run electric motor driven systems. Generally, systems consist of several components, the electrical power supply, the electric motor, the motor control, and a mechanical transmission system. There are several ways to improve the systems' effi ciency. The cost effective way is to check each com- ponent of the system for an opportunity to reduce electrical losses. A qualifi ed individual should oversee the electrical system since poor power distribution within a facility is a common cause of energy losses. Technology Update Ch. 18 1 lists 20 items to help facility management staff identify opportunities to improve drive system effi ciency. 1. Maintain Voltage Levels. 2. Minimize Phase Imbalance. 3. Maintain Power Factor. 4. Maintain Good Power Quality. 5. Select Effi cient Transformers. 6. Identify and Fix Distribution System Losses. 7. Minimize Distribution System Resistance. 8. Use Adjustable Speed Drives (ASDs) or 2-Speed Motors Where Appropriate. 9. Consider Load Shedding. 10. Choose Replacement Before a Motor Fails. 11. Choose Energy-Effi cient Motors. 12. Match Motor Operating Speeds. 13. Size Motors for Effi ciency. 14. Choose 200 Volt Motors for 208 Volt Electrical Sys- tems. 15. Minimize Rewind Losses. 16. Optimize Transmission Effi ciency. 17. Perform Periodic Checks. 18. Control Temperatures. 19. Lubricate Correctly. 20. Maintain Motor Records. Some of these steps require the one-time involve- ment of an electrical engineer or technician. Some steps can be implemented when motors fail or major capital changes are made in the facility. Others involve development of a motor monitoring and maintenance program. 11.2 POWER SUPPLY Much of this information consists of standards defi ned by the National Electrical Manufacturers As- sociation (NEMA). The power supply is one of the major factors affect- ing selection, installation, operation, and maintenance of an electrical motor driven system. Usual service conditions, defi ned in NEMA Standard Publication MG1, Motors and Generators, 2 include: • Motors designed for rated voltage, frequency, and number of phases. • The supply voltage must be known to select the proper motor. • Motor nameplate voltage will normally be less then nominal power system voltage. Nominal Motor Utilization Power System (Nameplate) Voltage Voltage (Volts) Volts ——————— —————————— 208 200 240 230 480 460 600 575 2400 2300 4160 4000 6900 6600 13800 13200 • Operation within tolerance of ±10 percent of the rated voltage. CHAPTER 11 E LECTRIC ENERGY MANAGEMENT 273 274 ENERGY MANAGEMENT HANDBOOK • Operation from a sine wave of voltage source (not to exceed 10 percent deviation factor). • Operation within a tolerance of ±5 percent of rated frequency. • Operation within a voltage unbalance of 1 percent or less. Operation at other than usual service conditions may result in the consumption of additional energy. 11.3 EFFECTS OF UNBALANCED VOLTAGES ON THE PERFORMANCE OF POLYPHASE SQUIRREL-CAGE INDUCTION MOTORS (MG 1-20.56) When the line voltages applied to a polyphase induction motor are not equal, unbalanced currents in the stator windings result. A small percentage of voltage unbalance results in a much larger percentage current unbalance. Consequently, the temperature rise of the motor operating at a particular load and percentage voltage unbalance will be greater than for the motor operating under the same conditions with balanced voltages. Voltages should be evenly balanced as closely as they can be read on a voltmeter. If the voltages are unbalanced, the rated horsepower of polyphase squirrel-cage induction motors should be multiplied by the factor shown in Figure 11.1 to reduce the possibility of damage to the motor. Operation of the motor with more than a 5-percent voltage unbalance is not recommended. When the derating curve of Figure 11.1 is applied for operation on balanced voltages, the selection and setting of the overload device should take into account the combination of the derating factor applied to the motor and the increase in current resulting from the unbalanced voltages. This is a complex problem involving the variation in motor current as a function of load and voltage unbalance in the addition to the characteristics of the overload device relative to I MAXIMUM or I AVERAGE . In the absence of specifi c information it is recommended that overload devices be selected and/or adjusted at the minimum value that does not result in tripping for the derating factor and voltage unbalance that applies. When the unbalanced voltages are unanticipated, it is recommended that the overload devices be selected so as to be responsive to I MAXIMUM in preference to overload devices responsive to I AVERAGE . 11.4 EFFECT ON PERFORMANCE— GENERAL (MG 1 20.56.1) The effect of unbalanced voltages on polyphase induction motors is equivalent to the introduction of a “ negative-sequence voltage” having a rotation opposite to that occurring the balanced voltages. This negative-sequence voltage produces an air gap fl ux rotating against the rotation of the rotor, tending to produce high currents. A small negative-sequence voltage may produce current in the windings considerably in excess of those present under balanced voltage conditions. 11.4.1 Unbalanced Defi ned (MG 1 20.56.2) The voltage unbalance in percent may be defi ned as follows: Percent Voltage Unbalance = 100 × Maximum voltage deviation from average voltage average voltage Example—With voltages of 220, 215 and 210, the average is 215, the maximum deviation from the average is 5 PERCENT VOLTAGE UNBALANCE = 100 * 5/215 = 2.3 PERCENT 11.4.2 Torque (MG 1 20.56.3) The locked-rotor torque and breakdown torque are decreased when the voltage is unbalanced. If the voltage unbalance is extremely severe, the torque might not be adequate for the application. 11.4.3 Full-Load Speed (MG 1 20.56.4) The full-load speed is reduced slightly when the motor operates at unbalanced voltages. 11.4.4 Currents (MG 1 20.56.5) The locked-rotor current will be unbalanced but the locked rotor kVA will increase only slightly. Figure 11.1 Polyphase squirrel-cage induction motors derating factor due to unbalanced voltage. [...]... 98.9 98.8 98.7 98.6 98.8 98.7 98.6 98.5 98 .4 98.9 98.8 98.7 98.6 98.5 98.5 98 .4 98.2 98.0 97.8 88.2 98.0 97.8 97.6 97 .4 98 .4 98.2 98.0 97.8 97.6 97.6 97 .4 97.1 96.8 96.5 97.1 96.8 96.5 96.2 95.8 97 .4 97.1 96.8 96.5 96.2 96.2 95.8 95 .4 95.0 94. 5 95 .4 95.0 94. 5 94. 1 93.6 95.8 95 .4 95.0 94. 5 94. 1 94. 1 93.6 93.0 92 .4 91.7 93.0 92 .4 91.7 91.0 90.2 93.6 93.0 92 .4 91.7 91.0 91.0 90.2 89.5 88.5 87.5 89.5 88.5... 87.5 86.5 85.5 90.2 89.5 88.5 87.5 86.5 86.5 85.5 84. 0 82.5 81.5 84. 0 82.5 81.5 80.0 78.5 85.5 84. 0 82.5 81.5 80.0 80.0 78.5 77.0 75.5 74. 0 77.0 75.5 74. 0 72.0 70.0 78.5 77.0 75.5 74. 0 72.0 72.0 70.0 68.0 66.0 64. 0 68.0 66.0 64. 0 62.0 59.5 70.0 68.0 66.0 64. 0 62.0 62.0 59.5 57.5 57.5 55.0 52.5 59.5 57.5 55.0 55.0 50.5 52.5 52.5 48 .0 50.5 50.5 46 .0 48 .0 Column A Nominal Efficiency —————————————————————————... approved as NEMA Standard 3/ 14/ 1991 †Column C approved as Suggested Standard for Future Designs 3/ 14/ 1991 288 ENERGY MANAGEMENT HANDBOOK Table 11.3 (NEMA Table 12.6B) Full-load efficiencies of energy efficient motors ELECTRIC ENERGY MANAGEMENT Table 11 .4 (NEMA Table 12.6C) (Suggested standard for future design) Full-load efficiencies of energy efficient motors 289 290 ENERGY MANAGEMENT HANDBOOK Same horsepower—different... ELECTRIC ENERGY MANAGEMENT kW = 20 × 0. 746 = 14. 92 kW New CFM with new motor = 1790/1750 × 32,000 = 32,731 or 2.3% increase New HP = (1790/1750)3 × 20 × = 21 .4 HP or 7% increase New kW = 21 .4 × 0. 746 = 15.96 kW 7% increase in kW and work performed by motor Replacing a standard motor with an energy efficient motor in centrifugal pump or a fan application can result in increased energy consumption if the energy. .. convert electrical energy into mechanical energy Electric motors are efficient at converting electric energy into mechanical energy If the efficiency of an electric motor is 80%, it means that 80% of electrical energy delivered to the motor is directly converted to mechanical energy The portion used by the motor is the difference between the electrical energy input and mechanical energy output A major... previous components.) 296 ENERGY MANAGEMENT HANDBOOK Figure 11.7 Motor record form ELECTRIC ENERGY MANAGEMENT 297 Figure 11.8 WHAT IF motor comparison form 298 ENERGY MANAGEMENT HANDBOOK HOW TO GET AROUND IN THE ‘WHAT IF’ FORM COLUMN LINE EXPLANATION EXISTING 1-9 Information can be taken from the Motor Record Form if previously generated If not available, generate data EXISTING 1 ENERGY COST $/kWh Self... Amps LLA = 16.5 NLA = 9.3 NPA = 24. 0 (2 × 16.5) – 9.3 23.7 (%Rated HP) = —————— × 100 = —— × 100 = 61.2% (2 × 24. 0) – 9.3 38.7 Approximate load on motor = 20 HP × 0.612 = 12. 24 or slightly over 12 HP 11. 14 ELECTRIC MOTOR EFFICIENCY The efficiency of a motor is the ratio of the mechanical power output to the electrical power input It may be expressed as: ELECTRIC ENERGY MANAGEMENT 285 Output Input – Losses... WITH INVESTMENT OF $ ?????? 2 94 ENERGY MANAGEMENT HANDBOOK Annual cost of Energy hrs x hp x 746 x % energy consumption x $/kW $ = —————————————————————— 1000 x 100 Annual Cost of Energy Summary: Discharge Damper $ 24, 600 Variable Inlet Vane $19,600 Eddy Current Drive $15,900 Adjustable Speed Drive $13,900 11.18 MOTOR EFFICIENCY MANAGEMENT Many think that when one is saying Motor Efficiency the logical... Example: CFM 2 RPM 2 = CFM 1 RPM 1 RPM 2 P2 = P1 RPM 1 2 2 Pressure (P) varies as the square of fan speed (RPM) S = 100 × 0. 746 × 1 × 0.080 × 40 00 × (100/91.7-100/95.0) = $9 04 Law #3 RPM 2 HP2 = HP1 RPM 1 3 3 S = 100 × 0. 746 × 1 × 0.080 × 40 00 × (100/91.7-100/95.0) × (1790/1775)3 = $262 $ 642 reduction in expected savings Relatively minor, 15 RPM, increase in a motor’s rotational speed results in a 2.6 percent... reasonably accurate results when motor load is in the 40 to 100% range and deteriorating results at loads below 40 % Example: • A 20 HP motor driving a pump is operating on 46 0 volts and has a loaded line amperage of 16.5 • When the coupling is disconnected and the motor operated at no load the amperage is 9.3 • The motor nameplate amperage for 46 0 volts is 24. 0 Therefore we have: Loaded Line Amps No Load . 200 240 230 48 0 46 0 600 575 240 0 2300 41 60 40 00 6900 6600 13800 13200 • Operation within tolerance of ±10 percent of the rated voltage. CHAPTER 11 E LECTRIC ENERGY MANAGEMENT 273 2 74 ENERGY. replacement. Distribution Energy Distribution energy is most commonly electrical energy consumed to operate fans and pumps, with fan energy typically being far greater than pump energy ex- cept in. HOT WATER The creation of domestic hot water (DHW) repre- sents about 4 percent of the annual energy consumption 272 ENERGY MANAGEMENT HANDBOOK in typical non-residential buildings. In buildings