Engineered Systems - December 2008 - (Page 48)

Basics For Refrigerant Chillers feet per second (fps) to prevent laminar flow, and the maximum chilled water flow velocity is 12 fps to prevent pipe erosion. Evaporators come in one, two, or three passes configurations with the number of passes being dependent upon chilled water temperature difference and manufacturer’s standard product. Typically, a single-pass evaporator heat exchanger is adequate for chilled water entering/leaving temperature differentials up to 6°F, a two-pass evaporator heat exchanger is adequate for chilled water entering/leaving temperature differentials of between 6° and 12°, and a three-pass evaporator heat exchanger is adequate for chilled water entering/leaving temperature differentials of between 12° and 18°. Note: Manufacturers may configure their heat exchangers differently to reduce the number of passes for higher temperature differences. With design chilled water entering/leaving temperature differentials increasing from 10° to 16° or 18° to reduce pipe sizes and pumping energy, newer chiller evaporator heat exchangers may be larger. Condenser. The condenser is where the refrigerant rejects heat (energy) to the condenser water or air, causing refrigerant phase change from gas to liquid. There are three condenser types: water cooled, air cooled, and evaporative cooled. Most chillers utilize water cooled or air cooled condensers, and these two condensers are discussed. • Water cooled condenser. Water cooled condensers typically involve a shell-andtube heat exchanger with condenser water running through the tubes and the refrigerant in the shell space. The condenser water flow must have the same flow velocity range as the evaporator, 3 fps to 12 fps. Water cooled condensers come in one-, two-, or three-pass configurations with similar water entering/leaving temperature differentials as chilled water. A standard water cooled condenser is rated at 85° ambient outdoor air temperature but performance data is usually provided for 65° and 75° ambient outside air temperature. • Air cooled condenser. The air cooled condenser is where the refrigerant rejects heat (energy) directly to the outside air. Air cooled condensers can be an integral chiller component or be a remote condenser with refrigerant piping connecting it to the chiller. The air cooled condenser is a coil(s) having copper tubing with aluminum fins and condenser fan(s). A standard air cooled condenser is rated at 95° ambient outside air temperature, but performance data is usually provided for 105° and 115° ambient outside air temperature. Compressor. The compressor is the chiller system’s heart. It takes the lower-pressure vaporized refrigerant coming out of the evaporator, compresses it to a higher pressure, and discharges it into the condenser. Chiller systems utilize two types of compressors, positive-displacement and dynamic. Positive-displacement compressors physically compress the vaporized refrigerant into a smaller volume and higher pressure, and include reciprocating, rotary, and orbital types. The two most utilized positive-displacement compressors are the scroll and rotary screw. A centrifugal compressor is the most utilized dynamic compressor. Dynamic compressors increase vaporized refrigerant pressure by the kinetic energy imparted on refrigerant by a rotating impeller. ENERGY SAVING CHILLERS Integrated Free Cooling Simultaneous Heat Recovery Packaged Air & Water Cooled to 350 Tons Choice of Refrigerants Optional Tank & Pumps Booth 4762 COOLING SOLUTIONS www.motivaircorp.com Input 222 at www.esmagazine.com/instantproductinfo 48 En g i neer ed Sy stem s December 2008 http://www.ahrexpo.com http://www.motivaircorp.com http://www.motivaircorp.com http://www.esmagazine.com/instantproductinfo

Table of Contents Feed for the Digital Edition of Engineered Systems - December 2008

Engineered Systems - December 2008 Contents Editor's Note HVAC Challenge Back2Basics Case In Point Commissioning Building Automation HVACR Designer Tips Applications Checklist Lakefront Library: Radiant Systems Meet Natural Ventilation Before (And After) The Flood Basics For Refrigerant Chillers Wire-To-Shaft Efficiency For HVAC Pumps Products Glossary Classifieds Advertiser Index Tomorrow’s Environment

Engineered Systems - December 2008

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