Engineered Systems - March 2009 - (Page 36)

If motor-driven vapor compression chillers are much more energy efficient than absorption chillers, then why are we even having this conversation? In a hunt for LEED® points or in applications with certain demands, absorption could be just the ticket to sustainability and/or economy. BY VINCENT A. SAKRAIDA, P.E., LEED® AP oes the idea of using steam, hot water, or direct-fired burners to generate chilled water sound like an oxymoron? Well, absorption chillers use these thermal energy sources to produce chilled water. Beyond the type of thermal energy source, absorption chillers are also classified by whether they are single- or double-effect. The goal of this article is to provide the reader with a basic description of absorption chillers and their advantages, specific applications, performance standards, and energy efficiency, plus how they can be used to gain LEED® certification points. WATER AS A REFRIGERANT How about using water as a refrigerant and lithium bromide as a salt to absorb the water? These are certainly not easily understood concepts. However, water has a very high specific heat and latent heat of vaporization, which makes it a great refrigerant. How is water boiling at 212°F going to create chilled water at 44°? First, the boiling temperature of water is a direct function of pressure and at a pressure of 1 atmosphere (29.92 Hg), water boils at 212°. When the pressure on the water is decreased, the water boiling temperature is lowered. The following table gives the total pressure in inches of mercury and the corresponding approximate water boiling temperature at different pressures: D to take advantage of the lower water boiling temperatures. Absorption chiller internal pressures can range from 0.1 atmosphere (atm) to below 0.01 atm. ABSORPTION CHILLER DESCRIPTION There are a number of absorption chillers available, including singleeffect indirect-fired (steam, hot water); double-effect indirect-fired; and double-effect direct-fired (gas and/or oil burner). Single-effect absorption chillers have a single generator/concentrator and condense all vaporized refrigerant in a single condenser. Double-effect absorption chillers have two generator/concentrators and the vaporized refrigerant from the high temperature generator/concentrator is the thermal source for the low temperature generator/concentrator, reducing the cooling requirement for the vaporized refrigerant. Single-effect indirect-fired chillers are typically available in capacities between 100 and 1,350 tons with one manufacturer providing a unit up to 2,000 tons. Double-effect indirect-fired chillers are typically available in capacities between 100 and 1,500 tons, although one manufacturer provides a unit up to 5,000 tons. Double-effect direct-fired chillers are typically provided in capacities between 100 and 1,500 tons. A description of the various single-effect, indirect-fired absorption chiller components is provided below followed by a description of the double-effect absorption chiller component that is different than the single-effect absorption chiller. Single-effect absorption chiller. The single-effect indirect-fired absorption chiller has five main steps (Figure 1): condensing (condenser), expansion (expansion pipe), evaporation (evaporator), absorption (absorber), and generator/concentrator. See Figure A for schematic chiller diagram and Diagram 1 for the Duhring pressure/ temperature diagram. Like the vapor compression chillers, absorption chillers have a high-pressure side (generator/concentrator, con- Absolute pressure 29.92 Hg (1 atm) 2.99 Hg (0.1 atm) 1.01 Hg 0.30 Hg (0.01 atm) 0.23 Hg Water boiling point (°F) 212° 115° 80° 45° 38° Absorption chillers have substantially reduced internal pressures 36 En g i neer ed Sy stem s March 2009

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Engineered Systems - March 2009 Contents Editor’s Note Back2Basics Case In Point Commissioning Building Automation HVACR Designer Tips Application Checklist Teaching An Old School New Tricks Basics For Absorption Chillers A New Haven For Sustainable Schools Glossary Classifieds Advertiser Index Tomorrow’s Environment

Engineered Systems - March 2009

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