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 Electromechanical control systems have been replaced by microprocessor-based electronic controls that provide greater precision, reliability and flexibility.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 design today. With the ability to provide accurate and precise control of temperature and air and water flows, the systems have widely replaced pneumatic and electric control systems.
DDC systems help building owners save energy in several ways. Their accuracy and precision nearly eliminate the control problems of offset, overshoot, and hunting commonly found in pneumatic systems, resulting in better regulation of the system. Their ability to respond to a nearly unlimited range of sensors results in better coordinated control activities. This also allows the systems to perform more complex control strategies than could be performed with pneumatic controls. Finally, their simple or automatic calibration ensures that the control systems will perform as designed over time, with little or no loss of accuracy.
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 without costly hardware changes. DDC systems also are ideal for applications that benefit from remote monitoring and operation.
Energy-efficient motors
Today’s HVAC systems are making use of energy-efficient motors. Energy-efficient motors offer a 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 standard motor of the same size is typically rated at 88 percent. Similarly, a 50 hp energy-efficient motor is rated at approximately 94 percent efficiency in contrast to the 90 percent efficiency rating of a 50 hp standard motor.
This increase in operating efficiency accompanies a first-cost increase for the motors. How rapidly 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 per year the motor is operated, the quicker the first-cost differential is recovered. For most applications where the motor is run continuously at or near full load, the payback period for the additional first cost is typically between three and six months.
The combination of constant loading and long hours of operation have made HVAC applications well-suited for the use of energy-efficient motors. Energy-efficient motors commonly are found driving centrifugal circulation pumps and system fans. With these loads, the 4 percent or 5 percent increase in the electrical efficiency of the drive motor translates to a significant energy savings, 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 power factor of a drive motor reduces the current draw on the electrical system, frees additional distribution capacity and reduces distribution losses in the system. Although increasing the power factor isn’t enough of a benefit to justify the cost differential of the higher efficiency motor, it’s an important consideration, particularly for large users of electricity where system capacity 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 to replace an existing, operating motor with one of higher efficiency will not be recovered for five to 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 components ranging from fans to chillers, the drives have demonstrated themselves to be very successful in reducing system energy requirements during part-load operation. And with most systems operating at part-load capacities 90 percent or more of the time, the energy savings produced by variable 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 HVAC system motor 20 hp and larger can benefit from the installation of a variable frequency drive.
Variable frequency drive applications
Variable frequency drives produce their savings by varying the frequency and voltage of the motor’s electrical supply. This variation is used to reduce the operating speed of the equipment it controls to match the load requirements. At reduced operating speed, the power draw of the drive motor drops off rapidly.
For example, a centrifugal fan, when operated at 75 percent flow, draws only about 40 percent of full-load power. At 50 percent flow, the power requirement for the fan decreases to less than 15 percent 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 an HVAC 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 for heating or cooling water decreases, the control valves at the terminal units throttle back. To keep the 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 pump’s electric drive also remains nearly constant.
Variable frequency drives regulate the pressure in the system in response to varying demands by slowing the pump. As with centrifugal fans, the power required by the pumps falls off as the load and speed are decreased. Again, because most systems operate well below design capacity 90 percent of the time, the savings produced by reduced speed operation are significant, typically recovering the cost of the unit in one to two years.
Chiller loads
A third application for variable frequency drives is centrifugal chillers. Chillers are sized for peak 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 off significantly during part-load operation. When variable frequency drives are applied to these chillers, they regulate the operation of the chiller by reducing the speed of the compressor. The result is near full-load operating efficiency over a very wide range of cooling loads. This increase in part-load efficiency translates into a 15 percent to 20 percent increase in the chiller’s seasonal efficiency.
Energy conservation isn’t the only benefit of variable frequency drives. A strain is placed on an electric motor and the mechanical system it drives every time a pump, fan or chiller is started at full-line voltage: Motor winding becomes heated, belts slip, drive chains stretch and high-pressure is developed in circulation systems. Variable frequency drives reduce these stresses by starting systems at reduced voltages and frequencies in a soft start, resulting in increased motor and equipment life.
Finally, the most important element in an energy-efficient HVAC system is how the system is operated. No matter how sophisticated the system, or how extensive its energy-conserving features, the system’s performance depends upon the way in which it’s 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 system operating in the way it was designed. Maintenance cannot be deferred.
Energy-efficient HVAC systems offer the facility manager the ability to improve system performance while reducing energy requirements. But they benefit building owners only as long as they are taken care of. If facility managers choose to ignore maintenance requirements, they may soon find systems malfunctioning to the point where they have actually increased the requirement for energy.
