Selecting chillers, chilled water systems

09/16/2013


Chiller operating costs

Once a chiller is procured, the cost of ownership continues with operation and maintenance costs for the remainder of the chiller’s life. The typical life span of an air-cooled chiller is generally 15 to 20 years based on the chiller’s location in varying exterior conditions. A water-cooled chiller has a life expectancy of around 20 to 30 years, with some lasting much longer with proper maintenance.

Many items affect the operating cost of a chiller, including building loads, chiller run time, control system integration, compressor unloading, evaporator pressure drop, and condenser pressure drop. Similarly, many programs and methods exist for determining annual operating costs. One commonly used method in chiller plant analysis is to use the integrated part-load value (IPLV) rating from AHRI 550/590 – Performance Rating of Water Chilling Packages Using the Vapor Compression Cycle to calculate costs quickly from a single value. IPLV is a blended kW/ton based on a predefined percentage of operating hours at respective percentages based on pre-defined operating conditions, such as 44 F chilled water supply at 2.4 gpm/ton and entering condenser water at predefined temperatures at 3 gpm/ton. A variation of IPLV is NPLV, which uses nonstandard temperatures based on actual operating conditions instead of the AHRI defined temperatures in the standard. In either case, the kW/ton value is calculated based on the following equation:


Where:

  • A = kW/ton at 100%
  • B = kW/ton at 75%
  • C = kW/ton at 50%
  • D = kW/ton at 25% 

AHRI 550/590 Appendix D specifically states in the scope that “The IPLV equations and procedures are intended to provide a consistent method for calculating a single number part-load performance figure of merit … and was derived to provide a representation of the average part-load efficiency for a single chiller only.” The standard further states that “it is best to use a comprehensive analysis that reflects the actual weather data, building load characteristics, operational hours, economizer capabilities and energy drawn by auxiliaries such as pumps and cooling towers, when calculating the chiller and system efficiency.” 

It is important to note that AHRI 550/590 was created to compare the unloading characteristics of similar chillers and not to be used as a basis to determine operational costs for chillers. Also, most chiller systems are operated in multiple chiller plant systems, not individual operation per the rating method. The whole system must be analyzed as an entire package including pumps, chillers, towers, weather, and building loads to determine the most cost-effective solution and optimize the entire plant in lieu of individual components. A single number IPLV provides a predetermined unloading of your chiller, which may or may not be accurate for your building and assumes that the chiller receives condenser relief, and therefore a reduction in power because lowering the condenser water temperature greatly affects chiller power consumption.

In lieu of using a single kW/ton value to determine plant operating costs, a more detailed review of the system must be made because kW/ton is highly dependent on the actual operating conditions and the chiller type selected. The weather is continuously changing in all locations, and outside ambient dry bulb and wet bulb have a significant impact on condenser operating conditions (air- and water-cooled). Similarly, a building and chiller load profile also shall be used in conjunction with weather data to determine chiller operating costs. A load profile will be different for every building depending on building type, building schedule, and other outside factors. With this information, one can analyze if the chiller selected can turn down to part-load capacities and what the efficiencies are expected to be at these part-load conditions, as most chillers will operate the majority of their hours at part load.

Finally, energy costs should be calculated using a load program. Some programs do not require entering the entire project to do simple chiller plant analysis. Also, load programs can quickly and easily apply actual building rate structures to an analysis, including any ratchet charges, consumptions charges, and demand charges. To properly analyze each option, chiller selections should be provided with an unloading curve in at least 10% load increments. This is sufficient for an analysis and will yield relatively accurate results without much effort.



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