at the four specified load levels, the standard conditions remain constant, with the exception of condensing temperatures. Fluid or air temperatures entering the condenser are varied linearly between designated values at 0 and 100% load. ARI 550/590-1998 standards for rating performance at varying condensing temperatures associated with part-load conditions are summarized in Table 33-2.

An accurate evaluation of system performance under varying load conditions serves two purposes: sizing equipment and performing economic analyses of operating costs. The IPLV standard is useful because it provides a uniform set of conditions for system comparisons. Still, it is necessary to appreciate its limitations and understand how to predict actual application performance more precisely. The ARI standards are based on a weighted


WC = water-cooled ECWT = entering condenser water temperature

AC = air-cooled EDB = entering air dry bulb temperature

EC = evaporative cooled EWB = entering air wet bulb temperature

In computing COP or thermodynamic performance

Table 33-2 Condensing Temperatures used for Part Load Ratings from ARI Standard 550/590-1988. Source: ARI

average of building types and average weather data from 29 cities across the United States using the ASHRAE temperature bin method. Operating hours are based on the weighted average of various operations with and without the use of an economizer.

It is important to note that the latest ARI standard reflects significant changes from its previous 1992 standard. Perhaps the most significant change is the adjustment of the weighting of part-load points used in calculation of IPLV and NPLV. For example, whereas the previous standard used 17% weighting for full-load operation, the current standard uses only 1% and a greater distribution at the part-load levels. As another example, whereas the previous standard was based on the profile of a single building located in Atlanta, Georgia, the current standard is based on weighted national averages for load and weather conditions. While the new standard may be more representative of conditions nationally, it is still based on the assumption of single-chiller operation and assumed load and weather conditions.

To generate more application-specific representative data, the time-weighing values of the NPLV can be adjusted by the design engineer to reflect the impact of local weather conditions, multiple chiller installations, or internal cooling loads. The NPLV rating method also allows the design engineer to adjust water temperature, flow rates, cooling tower performance, and fouling factors for a particular application.

In a multiple-chiller system, for example, it is possible for one chiller to be fully loaded and one only partially loaded with a common condenser water supply. The assumption that the part-load chiller is operating under a lower condenser temperature than the fully loaded unit would be inaccurate in this case. Analysis should also include energy usage requirements for auxiliary equipment.


Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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