The Elements Of An Efficient HVAC System
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 consequence of improvements in the operating efficiency of meaningful system elements. Other gains are the consequence of the use of technologies that are either new, or new to the HVAC field. already 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 enhance HVAC system operating efficiency, much of the overall improvement can be credited to five meaningful 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 complete load.
Today, new chillers are being installed with complete 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 enhance chiller performance. To enhance the heat move 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 Electromechanical control systems have been replaced by microprocessor-based electronic controls that provide greater accuracyn, reliability and flexibility.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 assistance of the new generation of building chillers; these chillers offer better refrigerant containment. Although older chillers ordinarily 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 meaningful factor in maintaining chiller performance over time.
Another meaningful development is in boiler operation: the substitute of pneumatic and manual controls with microprocessor-based systems. As a rule of thumb, the systems can be expected to unprotected to energy savings of 5 percent to 7 percent over traditional pneumatic-based systems.
Microprocessor-based control systems unprotected to their savings chiefly as the consequence of their ability to modulate the boiler’s operation more precisely than pneumatic-based systems. By modulating the boiler’s operation precisely, the systems help to continue 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 far away 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 by their ability to continue proper fuel-to-air ratio. By maintaining the proper ratio, the systems reduce the rate at which soot collects on boiler tubes, consequently decreasing the frequency of required tear down and cleaning. Keeping the boiler tubes clean of soot also helps to enhance the thermal efficiency of the boiler.
Direct digital controls
A major change in the HVAC field is the extensive 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 accuracyn 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 complicate 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 alternation to match changes in occupant requirements without costly hardware changes. DDC systems also are ideal for applications that assistance from far away monitoring and operation.
Today’s HVAC systems are making use of energy-efficient motors. Energy-efficient motors offer a moderate but meaningful increase in complete-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 complete-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 complete load, the payback period for the additional first cost is typically between three and six months.
The combination of continued 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 excursion motor translates to a meaningful energy savings, particularly when the systems function 24 hours per day, year round.
A side assistance of energy-efficient motor design is its higher strength factor. Increasing the strength factor of a excursion motor reduces the current draw on the electrical system, frees additional dispensing capacity and reduces dispensing losses in the system. Although increasing the strength factor isn’t enough of a assistance 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 emotional results. Applied to system elements 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 retrieve 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 assistance from the installation of a variable frequency excursion.
Variable frequency excursion 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 strength draw of the excursion motor drops off rapidly.
For example, a centrifugal fan, when operated at 75 percent flow, draws only about 40 percent of complete-load strength. At 50 percent flow, the strength requirement for the fan decreases to less than 15 percent of complete-load strength. While traditional control systems, such as damper or vane control, also reduce the energy requirements at uncompletely flow, the savings are considerably 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 continued flow of water to terminal units. As the need for heating or cooling water decreases, the control valves at the terminal units throttle back. To keep the pressure in the system continued, a bypass valve between the supply and return systems opens. With the flow rate remaining nearly continued, the load on the pump’s electric excursion also remains nearly continued.
Variable frequency drives control the pressure in the system in response to varying demands by slowing the pump. As with centrifugal fans, the strength required by the pumps falls off as the load and speed are decreased. Again, because most systems function well below design capacity 90 percent of the time, the savings produced by reduced speed operation are meaningful, typically recovering the cost of the unit in one to two years.
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 traditional control systems that close vanes on the chiller inlet, chiller efficiency falls off considerably during part-load operation. When variable frequency drives are applied to these chillers, they control the operation of the chiller by reducing the speed of the compressor. The consequence is near complete-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 assistance 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 complete-line voltage: Motor winding becomes heated, belts slip, excursion 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 component in an energy-efficient HVAC system is how the system is operated. No matter how complex 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 enhance system performance while reducing energy requirements. But they assistance 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 truly increased the requirement for energy.