Consulting-Specifying Engineer - August 2007 - (Page 42)

How To B E AT T H E C O D E S Ice-storage systems BY PAUL VALENTA, North American sales manager, CALMAC Manufacturing, Fair Lawn, N.J. C onsulting engineers must provide cooling designs that will work in a variety of conditions from low loads to unexpected heavy loads on peak design days. Therefore, safety factors become a consideration of risk, first cost and operating cost. Is there a better choice than to install a 20% larger chiller for a facility to occasionally meet unusually high or unexpected loads? That extra chiller capacity with its larger support equipment can result in lower energy efficiency and increased maintenance costs with poor return-on-investment (ROI) because oversized cooling systems that quickly cycle on and Instead of adding 20% chiller capacity to a design day building peak load, reduce chiller capacity by at least 20% and add ice storage for safety factor. off to meet set points might not dehumidify the air properly. In addition to operational inefficiency, poor indoor air quality can be a side effect of oversizing. Is there a better choice? Consider a building that has a peak cooling requirement of 1,000 tons on a design day. One 1,000-ton chiller would do the job on most days, but this choice provides no safety factor for days warmer than ASHRAE design—days with unexpected loads—or for future growth. Additionally, should the chiller fail, no cooling would be available. Designing the system instead with two 500-ton chillers would not provide extra safety factor capacity, but would provide half capacity should one chiller fail. The U.S. General Services Administration directs engineers to design three equal-size chillers that total 120% of the buildings peak load. In this case, three 400-ton chillers would be required to provide 20% safety factor and 80% capacity should one chiller be unavailable. A cooling load profile from a popular HVAC modeling program (see Table 1) for a 1,000-ton building in Chicago shows that 87% of the time the load is less than 800 tons, or 66% of the recommended GSA installed chiller capacity. Seventy-six percent of the time the cooling load is less than 700 tons, or just under 60% of the GSA chiller installed capacity. The investment in 200 tons of safety factor capacity, including the larger chillers, cooling towers, fans, pumps and electrical equipment, results in a higher cost and connected electrical load for capacity that is seldom used and has minimal ROI. Should one chiller become unavailable, the conventional selection of three 400-ton chillers provides 80% capacity. This is a reasonable operational risk because most operating hours have capacities less than 80% design-day load. Figure 1 shows a cooling load profile for a 1,000-ton design day load profile for an office building. The conventional scenario would be to install three 400-ton chillers to supply safety factor and redundancy. Figure 1 also shows the chiller potential over the required cooling load profile the building requires. Now compare the conventional-chiller-only system with a partial-ice-storage system, keeping in mind that safety factor and redundancy are requirements of the ice storage system as well. For this example, the third chiller or “safety factor chiller” is replaced with ice storage. Two 400-ton ice making chillers and 3,900-ton hours of storage are selected for the system. The ice storage provides the safety factor and redundancy just as the third chiller did in the conventional design. Cooling towers and all chiller support equipment are now sized for 800 tons. The potential load profiles are compared in Figure 1. Notice that the ice storage system is capable of a larger peak capacity than the conventional system if needed. In this case the safety factor is comparable. 42 Consulting-Specifying Engineer • AUGUST, 2007

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - August 2007

Contents M/E Roundtable Giants 100 Pullout Poster Giants Stand Tall: CSE Giants 100 Report Electrical Design for Tall Buildings HVAC and Fire Safety for Elevator Systems How to Beat the Codes GAS Technology Supplement Recovering Waste Heat from Boilers New Applications for Tankless Water Heating Systmes Techniques Improve for Powder Coating MDF New Tools Available for Heat Treating Steel Direct Drive Engines

Consulting-Specifying Engineer - August 2007

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