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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 Electromechanical control systems have been replaced by microprocessor-based electronic controls that provide greater precision, reliability and flexibility.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