Consulting-Specifying Engineer - March 2009 - (Page 48) Table 1: AHRI energy efficiency standards for RTUs Methodology Energy efficiency ratio Symbol EER Description Ratio of Btu/hr to wattage of all power input An expression of efficiency at part-load Not an efficiency ratio; referred to as a “figure of merit” Notes At full-load only Integrated energy IEER efficiency ratio Integrated partload value IPLV See the standard for full details See the standard for full details Source: Air-Conditioning, Heating, and Refrigeration Institute age occurs? What activities and processes occur in the building? If you’re building a convenience store, people will constantly be opening the door. “For example, in a restaurant, cooking loads and ventilation loads need to be removed,” said Hourahan. “Museums and paper mills have stringent humidity requirements. Once you understand how the building is to be operated, you have to define the desired operating conditions: What temperature and humidity are you trying to maintain? What are the outside conditions? Are there special IAQ concerns that need attention? Generally, you design for the worst-case sensible and latent loads, but you also need to look for equipment that can handle the part-load conditions that will exist most of the time.” A common result of oversizing an RTU, said Hourahan, is a humidity problem. A system may be able to cool a space so rapidly that the latent load is not being managed. In his article, Hourahan states, “The greater the difference between the indoor evaporator coil temperature and the return-air wet bulb temperature, the greater the ability the coil has to remove excess moisture.” Therefore, the solution to excess humidity is to reduce air flow across the coil. Enthalpy recovery units are another option to explore, but be sure to downsize the vaporcompression equipment in an RTU accordingly to account for the moisture removal. It should be added that, quite often, a consulting engineer cannot know what activity will occur in a space. What begins as a strip mall bookstore with a relatively low load might later be pressed into service as a restaurant with a very high load. Given this all-too-common uncertainty, an engineer can be forgiven for erring on the conservative side and oversizing the equipment. Furthermore, today an engineer can address the sensible/latent heat issue by choosing from a wide range of options, such as variable-speed fans, modulating and staged compressors, energy recovery, demand ventilation, and reheat. Pat Banse, PE, a senior mechanical engineer with design firm Smith Seckman Reid in Houston, agrees that oversizing is the No. 1 no-no, but Banse adds an additional warning: “One downside of oversizing is that rapid cycling on the refrigeration side can lead to premature failure—you burn out the compressor or a relay.” He said that, in larger systems, the best stratTable 2: U.S. federal standards egy is often to specify multiple compressors—and depend on Minimum energy efficiency for RTUs more sophisticated controls to Unit capacity Current minimum Minimum EER as manage the situation. (Btu/hr) EER requirement of Jan. 1, 2010 65,000135,000 135,000240,000 10.0 9.7 11.2 11.0 How do you spell energy efficiency? As shown in Table 1, efficiency ratings for RTUs are expressed in EER, IEER, and Source: U.S. Code Title 42, section 6313: Standards. Accessed online at http://tinyurl.com/uscodeRTU on Feb. 26, 2009. IPLV. Minimum efficiency ratings for RTUs first appeared in ASHRAE standards in 1975. Since then the numbers have increased steadily—and also have been the subject of much debate. Table 1 lists the three methods for stating RTU energy efficiency that appear in the most recently published version of the Air-Conditioning, Heating, and Refrigeration Institute’s (AHRI) 340/360– 2007 Standard for Performance Rating of Commercial and Industrial Unitary AirConditioning and Heat Pump Equipment. Beginning Jan. 1, 2010, IEER will replace IPLV in ASHRAE 90.1 as the efficiency metric for RTUs. It will take into account the various add-ons that allow shifting gears from part load to full load, such as variable-speed fans and staged compressors. The new standard will take into account that, on commercial units, the indoor fan is always on, which was not considered in the IPLV calculation. Also on Jan. 1, 2010, AHRI’s certification program for RTUs will be expanded to cover machines up to 760 Btu/hr. How efficient does efficient have to be? Table 2 presents the federal government’s current minimum EERs. Having stated all of the above, manufacturers offer many units that far exceed the government’s minimums, however, the highest efficiencies may have costs outside the boundaries of most budgets. There is ground between the minimum and maximum, such as the voluntary minimum performance levels specified by the Consortium for Energy Efficiency (CEE). CEE provides voluntary minimum efficiency levels for RTUs for utilities that manage rebate programs that help buy down the costs for higher-efficiency units. Afroz Khan, CEE commercial HVAC program manager said, “That while we recognize that efficiency at the box only addresses one aspect of a broader issue, it’s still very much to your advantage to find out what incentives or programs your local utility is able to offer to encourage the installation of higher efficiency rated equipment.” A table of RTU efficiency specifications by size and type are in the “commercial” section of the CEE website at www.cee1.org. 48 Consulting-Specifying Engineer • MARCH 2009 http://www.cee1.org http://tinyurl.com/uscodeRTU
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