The elements of an efficient hvac system

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Today's systems are designed to meet stricter environmental, indoor air quality and user requirements. Many of the gains in HVAC system efficiency have come as the result of improvements in the operating efficiency of key system components. Other gains are the result of the use of technologies that are either new, or new to the HVAC field. Even the use of computer-aided design tools have helped system engineers design HVAC systems that perform more efficiently.

Although there are many individual advances that have helped to improve HVAC system operating efficiency, much of the overall improvement can be attributed to five key factors:

- The development of low kW/ton chillers;
- The use of high-efficiency boiler control systems;
- The application of direct digital control (DDC) systems;
- The use of energy-efficient motors; and,
- The matching of variable frequency drives to pump, fan and chiller motors.

For years, building owners were satisfied with the performance and efficiencies of chillers that operated in the range of 0.8 to 0.9 kW/ton when new. As they age, actual operating efficiencies fall to more than 1.0 kW/ton at full load.

Today, new chillers are being installed with full load-rated efficiencies of 0.50 kW/ton, a near 50 percent increase. Equally impressive are the part-load efficiencies of the new generation of chillers. Although the operating efficiency of nearly all older chillers rapidly falls off with decreased load, the operating efficiency of new chillers does not drop off nearly as quickly.

Chiller design changes

Several design and operation changes have helped improve chiller performance. To improve the heat transfer characteristics of the chillers, manufacturers have increased the size of the units' heat exchangers. Electromechanical control systems have been replaced by microprocessor-based electronic controls that provide greater precision, reliability and flexibility. Variable frequency drives control the speed of the compressor, resulting in an increase in part-load performance.

Increased energy efficiency is not the only benefit of the new generation of building chillers; these chillers offer better refrigerant containment. Although older chillers routinely may have lost 10 percent to 15 percent of the refrigerant charge per year, new chillers can limit losses to less than 0.5 percent. Lower leak rates and better purge systems reduce the quantity of non-condensable gasses found in the refrigerant system -- a key factor in maintaining chiller performance over time.

Another significant development is in boiler operation: the replacement of pneumatic and manual controls with microprocessor-based systems. As a rule of thumb, the systems can be expected to achieve energy savings of 5 percent to 7 percent over conventional pneumatic-based systems.

Microprocessor-based control systems achieve their savings primarily as the result of their ability to modulate the boiler's operation more accurately than pneumatic-based systems. By modulating the boiler's operation accurately, the systems help to maintain the proper fuel-to-air ratio and track the load placed on the boiler by the HVAC system.

Microprocessor-based systems offer several additional advantages, including remote monitoring and operating capabilities, automated control sequences, monitoring of steam flow, and reduced maintenance costs. One way the systems can help reduce maintenance costs is through their ability to maintain proper fuel-to-air ratio. By maintaining the proper ratio, the systems reduce the rate at which soot collects on boiler tubes, thus decreasing the frequency of required tear down and cleaning. Keeping the boiler tubes clean of soot also helps to improve the thermal efficiency of the boiler.

Direct digital controls

A major change in the HVAC field is the widespread implementation of direct digital controls (DDC). Introduced more than 15 years ago, DDC systems have become the industry standard for control systems desig

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The elements of an efficient hvac system

  1. 1. The Elements Of An Efficient HVAC SystemThe typical HVAC system installed in todays new construction offers building owners operatingefficiencies that are unparalleled in previous construction. Owners can expect a new, energy-efficientsystem designed to meet the needs of occupants better and more reliably than older systems, and todo it using one-third to one-half the energy required by older systems.Todays systems are designed to meet stricter environmental, indoor air quality and userrequirements. Many of the gains in HVAC system efficiency have come as the result ofimprovements in the operating efficiency of key system components. Other gains are the resultof the use of technologies that are either new, or new to the HVAC field. Even the use ofcomputer-aided design tools have helped system engineers design HVAC systems that performmore efficiently.Although there are many individual advances that have helped to improve HVAC systemoperating efficiency, much of the overall improvement can be attributed to five key factors:- The development of low kW/ton chillers;- The use of high-efficiency boiler control systems;- The application of direct digital control (DDC) systems;- The use of energy-efficient motors; and,- The matching of variable frequency drives to pump, fan and chiller motors.For years, building owners were satisfied with the performance and efficiencies of chillers thatoperated in the range of 0.8 to 0.9 kW/ton when new. As they age, actual operating efficienciesfall to more than 1.0 kW/ton at full load.Today, new chillers are being installed with full load-rated efficiencies of 0.50 kW/ton, a near 50percent increase. Equally impressive are the part-load efficiencies of the new generation ofchillers. Although the operating efficiency of nearly all older chillers rapidly falls off withdecreased load, the operating efficiency of new chillers does not drop off nearly as quickly.Chiller design changes
  2. 2. Several design and operation changes have helped improve chiller performance. To improve theheat transfer characteristics of the chillers, manufacturers have increased the size of the unitsheat exchangers. Electromechanical control systems have been replaced by microprocessor-based electronic controls that provide greater precision, reliability and flexibility. Variablefrequency drives control the speed of the compressor, resulting in an increase in part-loadperformance.Increased energy efficiency is not the only benefit of the new generation of building chillers;these chillers offer better refrigerant containment. Although older chillers routinely may havelost 10 percent to 15 percent of the refrigerant charge per year, new chillers can limit losses toless than 0.5 percent. Lower leak rates and better purge systems reduce the quantity of non-condensable gasses found in the refrigerant system -- a key factor in maintaining chillerperformance over time.Another significant development is in boiler operation: the replacement of pneumatic and manualcontrols with microprocessor-based systems. As a rule of thumb, the systems can be expected toachieve energy savings of 5 percent to 7 percent over conventional pneumatic-based systems.Microprocessor-based control systems achieve their savings primarily as the result of their abilityto modulate the boilers operation more accurately than pneumatic-based systems. By modulatingthe boilers operation accurately, the systems help to maintain the proper fuel-to-air ratio andtrack the load placed on the boiler by the HVAC system.Microprocessor-based systems offer several additional advantages, including remote monitoringand operating capabilities, automated control sequences, monitoring of steam flow, and reducedmaintenance costs. One way the systems can help reduce maintenance costs is through theirability to maintain proper fuel-to-air ratio. By maintaining the proper ratio, the systems reducethe rate at which soot collects on boiler tubes, thus decreasing the frequency of required teardown and cleaning. Keeping the boiler tubes clean of soot also helps to improve the thermalefficiency of the boiler.Direct digital controlsA major change in the HVAC field is the widespread implementation of direct digital controls(DDC). Introduced more than 15 years ago, DDC systems have become the industry standard forcontrol systems design today. With the ability to provide accurate and precise control oftemperature and air and water flows, the systems have widely replaced pneumatic and electriccontrol systems.DDC systems help building owners save energy in several ways. Their accuracy and precisionnearly eliminate the control problems of offset, overshoot, and hunting commonly found inpneumatic systems, resulting in better regulation of the system. Their ability to respond to anearly unlimited range of sensors results in better coordinated control activities. This also allowsthe systems to perform more complex control strategies than could be performed with pneumaticcontrols. Finally, their simple or automatic calibration ensures that the control systems willperform as designed over time, with little or no loss of accuracy.
  3. 3. DDC systems also offer several other advantages. Because the control strategies are software-based, the systems can be easily modified to match changes in occupant requirements withoutcostly hardware changes. DDC systems also are ideal for applications that benefit from remotemonitoring and operation.Energy-efficient motorsTodays HVAC systems are making use of energy-efficient motors. Energy-efficient motors offera moderate but significant increase in full-load operating efficiency over standard motor designs.For example, an energy-efficient 10 hp motor operates at about 93 percent efficiency; a standardmotor of the same size is typically rated at 88 percent. Similarly, a 50 hp energy-efficient motoris rated at approximately 94 percent efficiency in contrast to the 90 percent efficiency rating of a50 hp standard motor.This increase in operating efficiency accompanies a first-cost increase for the motors. Howrapidly this additional first cost is recovered depends on two factors: the loading of the motor,and the number of hours the motor is operated per year.The closer the motor is operated to its full-load rating and the greater the number of hours peryear the motor is operated, the quicker the first-cost differential is recovered. For mostapplications where the motor is run continuously at or near full load, the payback period for theadditional first cost is typically between three and six months.The combination of constant loading and long hours of operation have made HVAC applicationswell-suited for the use of energy-efficient motors. Energy-efficient motors commonly are founddriving centrifugal circulation pumps and system fans. With these loads, the 4 percent or 5percent increase in the electrical efficiency of the drive motor translates to a significant energysavings, particularly when the systems operate 24 hours per day, year round.A side benefit of energy-efficient motor design is its higher power factor. Increasing the powerfactor of a drive motor reduces the current draw on the electrical system, frees additionaldistribution capacity and reduces distribution losses in the system. Although increasing thepower factor isnt enough of a benefit to justify the cost differential of the higher efficiencymotor, its an important consideration, particularly for large users of electricity where systemcapacity is limited.Although the motors have demonstrated themselves to be very cost-effective in new applications,their use in existing applications is a little more difficult to justify. In most instances, the cost toreplace an existing, operating motor with one of higher efficiency will not be recovered for fiveto 10 years or longer.Of the improvements in HVAC systems that have helped to increase operating efficiency,variable frequency drives have had the most dramatic results. Applied to system componentsranging from fans to chillers, the drives have demonstrated themselves to be very successful inreducing system energy requirements during part-load operation. And with most systems
  4. 4. operating at part-load capacities 90 percent or more of the time, the energy savings produced byvariable frequency drives rapidly recover their investment, typically within one to two years.In general, the larger the motor, the greater the savings. As a rule of thumb, nearly any HVACsystem motor 20 hp and larger can benefit from the installation of a variable frequency drive.Variable frequency drive applicationsVariable frequency drives produce their savings by varying the frequency and voltage of themotors electrical supply. This variation is used to reduce the operating speed of the equipment itcontrols to match the load requirements. At reduced operating speed, the power draw of the drivemotor drops off rapidly.For example, a centrifugal fan, when operated at 75 percent flow, draws only about 40 percent offull-load power. At 50 percent flow, the power requirement for the fan decreases to less than 15percent of full-load power. While conventional control systems, such as damper or vane control,also reduce the energy requirements at partial flow, the savings are significantly less.Another area where variable frequency drives have improved the operating efficiency of anHVAC system is with centrifugal pumps found in hot and chilled water circulation systems.Typically, these pumps supply a constant flow of water to terminal units. As the demand forheating or cooling water decreases, the control valves at the terminal units throttle back. To keepthe pressure in the system constant, a bypass valve between the supply and return systems opens.With the flow rate remaining nearly constant, the load on the pumps electric drive also remainsnearly constant.Variable frequency drives regulate the pressure in the system in response to varying demands byslowing the pump. As with centrifugal fans, the power required by the pumps falls off as the loadand speed are decreased. Again, because most systems operate well below design capacity 90percent of the time, the savings produced by reduced speed operation are significant, typicallyrecovering the cost of the unit in one to two years.Chiller loadsA third application for variable frequency drives is centrifugal chillers. Chillers are sized forpeak cooling loads, although these loads occur only a few hours per year.With conventional control systems that close vanes on the chiller inlet, chiller efficiency falls offsignificantly during part-load operation. When variable frequency drives are applied to thesechillers, they regulate the operation of the chiller by reducing the speed of the compressor. Theresult is near full-load operating efficiency over a very wide range of cooling loads. This increasein part-load efficiency translates into a 15 percent to 20 percent increase in the chillers seasonalefficiency.Energy conservation isnt the only benefit of variable frequency drives. A strain is placed on anelectric motor and the mechanical system it drives every time a pump, fan or chiller is started atfull-line voltage: Motor winding becomes heated, belts slip, drive chains stretch and high-
  5. 5. pressure is developed in circulation systems. Variable frequency drives reduce these stresses bystarting systems at reduced voltages and frequencies in a soft start, resulting in increased motorand equipment life.Finally, the most important element in an energy-efficient HVAC system is how the system isoperated. No matter how sophisticated the system, or how extensive its energy-conservingfeatures, the systems performance depends upon the way in which its operated and maintained.Operating personnel must be properly trained in how best to use the system and its features.Maintenance personnel must be trained and equipped with the proper tools to keep the systemoperating in the way it was designed. Maintenance cannot be deferred.Energy-efficient HVAC systems offer the facility manager the ability to improve systemperformance while reducing energy requirements. But they benefit building owners only as longas they are taken care of. If facility managers choose to ignore maintenance requirements, theymay soon find systems malfunctioning to the point where they have actually increased therequirement for energy.Julian Arhire is a Manager with DtiCorp.com - DtiCorp.com carries more than 35,000HVAC products, including industrial, commercial and residential parts and equipmentfrom Honeywell, Johnson Contols, Robertshaw, Jandy, Grundfos, Armstrong and more.

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