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The Elements Of An Efficient HVAC System
Today’s systems are designed to meet more stringent environmental, indoor air quality and user requirements. Many of the gains in HVAC system efficiency have come as a result of improving the operating efficiency of key system components. Other gains result from the use of technologies that are either new or emerging in the HVAC field. Even the use of computer-aided design tools has helped system engineers design HVAC systems that operate more efficiently.
While there are many individual improvements that have helped improve the operating efficiency of HVAC systems, much of the overall improvement can be attributed to five key factors:
– Development of low kW/ton cooling devices;
– Use of highly efficient boiler management systems;
– Application of the direct digital control system (DDC);
– Use of energy-efficient engines; and,
– Synchronization of variable frequency drives with pump, fan and cooler motors.
For years, building owners have been satisfied with the performance and efficiency of chillers that ran in the 0.8 to 0.9 kW/ton range when new. As they age, the actual operating efficiency drops to more than 1.0 kW/ton at full load.
Today, new chillers are being installed with a full-load efficiency of 0.50 kW/ton, an increase of almost 50 percent. Equally impressive are the partial load efficiencies of the new generation of cooling devices. Although the operating efficiency of almost all older chillers declines rapidly with reduced load, the operating efficiency of new chillers does not decline nearly as rapidly.
Changes in the design of the refrigerator
Several design and operational changes have helped improve chiller performance. To improve the heat transfer characteristics of cooling devices, manufacturers have increased the size of the heat exchanger units. Electromechanical control systems have been replaced by microprocessor-based electronic controls that provide greater precision, reliability and flexibility. Variable frequency drives control compressor speed, resulting in increased part load performance.
Increased energy efficiency is not the only advantage of the new generation of cooling devices for buildings; these chillers offer better refrigerant retention. Although older chillers regularly lost 10 to 15 percent of refrigerant per year, new chillers can limit losses to less than 0.5 percent. Lower leak rates and better scavenging systems reduce the amount of non-condensable gases found in the refrigerant system — a key factor in maintaining chiller performance over time.
Another significant improvement is in boiler operation: replacing pneumatic and manual controls with systems based on microprocessors. As a rule of thumb, the systems can be expected to achieve energy savings of 5 to 7 percent compared to conventional pneumatic systems.
Regulation systems based on microprocessors achieve their savings primarily thanks to the possibility of more accurate modulation of boiler operation than pneumatic systems. By precisely modulating boiler operation, the systems help maintain the proper fuel-to-air ratio and monitor the load placed on the boiler by the HVAC system.
Microprocessor-based systems offer several additional advantages, including remote monitoring and operation capabilities, automated control sequences, steam flow monitoring, and reduced maintenance costs. One way the systems can help reduce maintenance costs is their ability to maintain the proper fuel-to-air ratio. By maintaining the proper ratio, the systems reduce the rate at which soot builds up on the boiler tubes, thereby reducing the frequency of required teardowns and cleaning. Keeping the boiler tubes free of soot also helps improve the boiler’s thermal efficiency.
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 now become the industry standard for control system design. With the ability to provide accurate and precise control of temperature and air and water flow, the systems have largely replaced pneumatic and electric control systems.
DDC systems help building owners save energy in several ways. Their accuracy and precision virtually eliminate the problems of displacement control, overshoot, and tracking commonly found in pneumatic systems, resulting in better system regulation. Their ability to respond to an almost unlimited range of sensors results in better coordinated control activities. This also allows systems to perform more complex control strategies than could be performed with pneumatic control. Ultimately, their simple or automatic calibration ensures that control systems will perform as designed over time with little or no loss of accuracy.
DDC systems offer several other advantages as well. Because management strategies are software-based, systems can be easily modified to match changes in tenant requirements without costly hardware changes. DDC systems are also ideal for applications that benefit from remote monitoring and operation.
Energy efficient engines
Today’s HVAC systems use energy efficient motors. Energy-efficient motors offer a moderate but significant increase in operating efficiency at full load compared to standard motor designs. For example, an energy-efficient 10 hp motor operates at approximately 93 percent efficiency; a standard engine of the same size is usually rated at 88 percent. Similarly, the energy efficient 50 hp motor is rated at approximately 94 percent efficiency as opposed to the 90 percent efficiency of the standard 50 hp motor.
This increase in operating efficiency is accompanied by an increase in the first cost of the engines. How quickly this additional first cost will be recouped depends on two factors: engine load and the number of engine hours per year.
The closer the engine is to rated full load and the more hours the engine runs per year, the faster the first cost difference is recovered. For most applications where the engine runs continuously at or near full load, the payback period for the additional first cost is usually between three and six months.
The combination of constant load and long hours of operation made HVAC applications suitable for the use of energy efficient motors. Energy-efficient motors typically drive centrifugal circulation pumps and system fans. With these loads, a 4 or 5 percent increase in the electrical efficiency of the drive motor means significant energy savings, especially when the systems operate 24 hours a day, year-round.
An additional advantage of an energy-efficient motor design is its higher power factor. Increasing the power factor of the drive motor reduces the current of the electrical system, frees up additional distribution capacity and reduces distribution losses in the system. Although the increase in power factor is not enough of an advantage to justify the price difference of a higher efficiency motor, it is an important consideration, especially for large electricity users where system capacity is limited.
Although the motors have proven to be very cost effective in new applications, their use in existing applications is a bit harder to justify. In most cases, the cost of replacing an existing motor with a higher efficiency motor will not be recouped for five to ten years or longer.
Of the improvements in HVAC systems that have helped increase operating efficiency, variable frequency drives have had the most dramatic results. Applied to system components from fans to chillers, the drives have proven to be highly successful in reducing system energy requirements during part-load operation. And with most systems operating at part-load capacity 90 percent or more of the time, the energy savings produced by variable frequency drives quickly return their investment, typically within one to two years.
In general, the bigger the engine, the bigger the savings. As a general rule, almost any HVAC system motor 20 hp and larger can benefit from installing a variable frequency drive.
Applications of variable frequency drives
Variable frequency drives realize their savings by changing 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 of the drive motor drops rapidly.
For example, a centrifugal fan, when operating at 75 percent flow, consumes only about 40 percent of the power at full load. At 50 percent flow, the required fan power is reduced to less than 15 percent of full load power. While conventional control systems, such as flap or vane control, also reduce energy requirements at partial flow, the savings are much smaller.
Another area where variable frequency drives have improved the operating efficiency of HVAC systems is in centrifugal pumps found in hot and chilled water circulation systems. Typically, these pumps supply a constant flow of water to terminal units. As the need for water for heating or cooling decreases, the control valves on the terminal units are returned. In order for the pressure in the system to be constant, the bypass valve between the supply and return systems is opened. With the flow rate remaining nearly constant, the load on the pump’s electrical drive also remains nearly constant.
Variable frequency drives regulate system pressure in response to varying demands by slowing down the pump. As with centrifugal fans, the power required by the pumps decreases as the load and speed decrease. Again, since most systems operate well below design capacity 90 percent of the time, the savings produced by the reduced operating speed are significant, typically paying back the cost of the unit in one to two years.
A third application of variable frequency drive is centrifugal water chillers. The coolers are sized for peak cooling loads, even though these loads only occur for a few hours a year.
With conventional control systems that close the vanes at the chiller inlet, chiller efficiency drops 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 close to full load operating efficiency over a very wide range of cooling loads. This increase in part-load efficiency translates into a 15 to 20 percent increase in seasonal chiller efficiency.
Energy saving is not the only advantage of variable frequency drives. The electric motor and the mechanical system it drives every time a pump, fan or chiller is started at full voltage are stressed: the motor winding heats up, belts slip, drive chains stretch and high pressure builds up in the circulation systems. Variable frequency drives reduce these loads by running the system at reduced voltages and frequencies in a soft start, resulting in longer motor and equipment life.
Finally, the most important element in an energy efficient HVAC system is how the system works. No matter how sophisticated the system or how extensive its energy-saving features, the system’s performance depends on how it is managed and maintained. Operating personnel must be properly trained to make the best use of the system and its features. Maintenance personnel must be trained and equipped with the proper tools to keep the system functioning as designed. Maintenance cannot be postponed.
Energy efficient HVAC systems offer the facility manager the ability to improve system performance while reducing energy needs. 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 that systems are malfunctioning to the point that they have actually increased energy demands.
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