September • 2013
HVAC systems are the greatest energy consumers in most commercial buildings, and most of these air conditioning systems vary according to the application, but they all have similar attributes. Outside of a good control automation system, the chiller, boiler and motors have the greatest potential to capitalize on energy efficiency and offer the best return on investment.
Generally, chillers are the primary consumer of energy in most building systems. So, even small changes in operating efficiency can affect energy costs. Chillers are typically meant to carry the largest cooling load during an ordinary year in the facility, even though that load may only happen for a few hours. For the remainder of the time, the chiller system works at a lesser load, occasionally an even greatly reduced load.
Unfortunately, chillers run at their highest efficiency when they run at or near full-load settings. So, as the load on the chiller declines, the efficiency of the chiller also declines.
If the location has more than one chiller, operating efficiency can be improved by staging the function of the chillers. When the cooling load in the building is moderately low, only one chiller needs to be Online. As the demand increases, other units can come Online to meet the increased need for cooling.
Variable-frequency drives (VFD) can be installed on a chiller motor to reduce the operating speed. The VFD slows the chiller motor to match the cooling-load conditions. The chiller continues to operate at near full load efficiency, even as the cooling load drops to as little as 30 percent of the unit's full-load rating.
Typically, a building's chiller is very dependable and has a long service life, however, it does need regular maintenance to run cost-effectively. Lack of appropriate maintenance will slowly deteriorate the chiller’s performance and will likely go unnoticed without diligently observing energy usage.
Maintenance tasks vary based on the type and size of chiller, but some of the more important procedures include conducting frequent leak testing, chemical water-treatment to reduce scale, and performing annual cleaning of the chiller's heat-transfer tubes.
Combustion efficiency for air and fuel mixtures is vital to proper boiler operation. Too much oxygen reduces the operating efficiency of the boiler and causes undesirable pollutants.
Likewise, too little oxygen will result in fuel not completely combusting and will create a build-up of soot, clogging surfaces and reducing boiler efficiency. Therefore, some excess oxygen is necessary to ensure complete combustion and prevent damage from soot build-up.
Fine-tuning the boiler's combustion controls will limit the excess air introduced into the boiler and, consequently, increase its periodic operating efficiency.
All boilers collect water deposits within the tubes. Automatic blowdown controls remove a portion of the water and replace it with fresh, chemically treated water. This process helps to manage and control the accumulating deposits. If the deposits are not regularly maintained and purged, they will begin to accumulate within the boiler’s tubes, reducing operating efficiency.
For the blowdown process to be effective, the rate at which water is removed and replenished must be properly scheduled. When the blowdown rate is too frequent, energy is lost through the discharged water and when it is not frequent enough, the solids concentration remains too high, causing scale to form.
Commercial buildings often have many more motors running in their HVAC systems than their owners are aware of. Too often these motors have a rated horsepower that is not suited to the load it is driving. This situation is a primary cause of energy inefficiency but also a simple opportunity to notably effect energy consumption.
Most induction motors that are used in air conditioning systems are effective and dependable. However, their efficiency declines when they run under partial load circumstances. Energy savings are realized when a mismatched motor is replaced with a motor that accurately pairs the horsepower to the load needs.
Another energy saving consideration is older motors. Improvements in motor designs have has given rise to a breed of motors that operate 2%-8% more effectively than older, typical motors. Combining new superior efficiency with a horsepower-to-load balance provides a relatively quick return on investment to owners.