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Strategies for Efficient System Operation
One way to improve the overall efficiency of mechanical systems is to incorporate strategies that use surrounding conditions. For example, the system might use outdoor air that is less costly to heat or cool than the interior conditioned air.
A few examples of these types of strategies are listed below.
Economizer Cycle:
A system with an economizer cycle uses outdoor air to cool the space without operating a cooling coil. This is typically used in the spring or fall when there is still a cooling load because of occupants, lights, and equipment. When the outdoor air is below the temperature of the air leaving the cooling coil, outdoor air can be used. As temperatures continue to drop below the cold air supply temperature, the amount of outdoor air used for cooling is reduced. Indoor air quality is of most concern during mid-heating and cooling seasons, when minimal outside air should be used. In warm, dry climates where the temperature swings are dramatic between day and night, outdoor air can be used to cool the space when the temperature drops in the evening. This is an excellent strategy to help minimize spikes in temperature during the day in buildings that use thermal mass.
Energy Recovery Systems:
These systems use conditioned exhaust air to precondition entering outside air that will need to be heated or cooled. The exhaust air is usually passed through a heat exchanger where the incoming air is preheated or cooled, thereby reducing the amount of conditioning needed.
Return Air Recovery:
This method reuses return air in the system to minimize coil loads and energy consumption. Because the air has already been conditioned, little energy is needed to bring it to supply conditions. Filtering and dehumidification are usually the biggest concerns when using return air. These systems are subject to ventilation requirements.
Blow-Through Systems:
These systems use a technique in which the fan is situated upstream from the cooling coil and air is blown through the coil, as opposed to the typical method of drawing air through the coil from downstream. The blow-through technique increases efficiency because heat from the fan motor is removed by the coil. In a draw-through system, the motor heat is introduced to the already cooled air and then has to be removed by a subcooling system.
Variable Air Volume (VAV) Systems:
The effectiveness of the HVAC system depends on the efficiency and operation at part-load conditions. This is because so much time is spent at less than peak load conditions. Peak loads are used to size equipment, but larger, long-term operating costs of the HVAC depend on the operation at off-design conditions. VAV systems are the most common systems in commercial applications. They supply varying amounts of air to meet varying loads. The most efficient and most commonly used method of reducing air flow is to use a fan with a variable speed motor. It is the only common design that reduces energy consumption significantly as the load is reduced. Zone loads must be determined accurately to minimize part-load penalties, and flow balancing is essential if adequate air is to be supplied to all zones.
Equipment Options for Efficient Systems
Fans:
Movement of air through the HVAC systems in a building is caused by fans. The energy used by fans can be very significant because they are required to run constantly and can represent 30 percent of the electricity used in commercial buildings. It is very important that all feasible means be used to control fan power. One method of reducing fan power would be to design efficient duct systems. Proper design of the ductwork, especially immediately adjacent to the fan is essential for proper fan operation. Consideration should also be given to specifying the most efficient motors economically feasible.
Ducts:
The objective of the ductwork is to deliver the amount of air needed to meet loads in each zone. Common constraints to building efficient duct systems include availability of space, need to meet loads in a variety of zones, economic criteria, and noise control. Efficient duct systems generally employ the following features: curved elbows to improve flow through turns, flow vanes located in elbows and transitions to improve air flow and reduce pressure on the fan, and properly sized ducts to maintain the same pressure gradient throughout the system (otherwise known as the equal friction method). These features produce a well-balanced design that delivers the required air flows without the need for excessive dampering. Heat losses or gains through the ducts should be controlled with duct insulation, especially ducts in unconditioned spaces. Insulation can also help with sound control. Refer to the American Society of Heating, Refrigerating and Air-Conditioning standards for recommended levels for specific climates.
Grills and Diffusers:
The exit point for the air is called a grill or diffuser. The distribution of air into space and the speed at which the air exits depend on the design of the grill. They are usually mounted in the ceiling plane and are either round or square. Performance depends on the design details, which are provided in the manufacturer’s data. Selecting the correct diffusers depends on variables such as the flow rate desired, the ceiling height of the space, and occupant activity. If occupants are uncomfortable because of poor distribution or excessive air speed, temperature settings in the space will not be optimized. This can happen when supply grills are located too near the return grills, which can cause “short cycling” of the conditioned air. Essentially, the conditioned air is returned to the mechanical equipment without first mixing with the room air. Discomfort can also be caused by air velocities that are too high or low depending on the occupants’ activity. For example, sedentary workers will most likely be uncomfortable at air velocities of 50 ft/min or higher.
Pipes:
The objective of pipe design is to deliver water at proper temperatures needed to meet loads in each zone. Constraints are similar to duct systems and include availability of space, need to meet a variety of loads in different zones, and economic criteria. It is essential that these systems are designed at the beginning of the design process to balance flows, minimize lengths of distribution systems, and minimize noise. Here, too, proper insulation of pipes can improve energy efficiency.
Pumps:
Similar to variable speed fans, pumps with variable speed drives are also available.
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