Engineered Systems - December 2008 - (Page 46)

That pretty much covers it. You know, design options, performance standards, part-load values, operating factors, and more. Use it as a quick brush-up, or pass it along as a handy reference. BY VINCENT A. SAKRAIDA, P.E., LEED® AP ith over 80,000 operating units in North America, chillers are the cooling workhorses for many commercial and industrial facilities. There are two primary chiller types: absorption and refrigerant, which operate very differently. This article will focus on refrigerant chillers. There are two primary types of refrigerant chillers, either air cooled and water cooled condenser with either scroll, screw, or centrifugal compressors. Also, a number of design variables affect chiller design, selection, and performance, including chilled water supply and return temperatures, percentage of chilled water glycol, condenser water supply and return temperatures, refrigerant-type used, chilled water system part-load requirements, chiller installation altitude, sequence of operation requirements, and available electrical service. These and other factors make refrigerant chillers the most complex piece of HVAC equipment. This article’s goal is to provide the reader with a basic refrigerant chiller description, applicable chiller performance standard, normal operating parameters, and information on chiller operation outside of normal operating parameters. REFRIGERANT CHILLER DESCRIPTION Refrigerant chillers follow the Carnot Cycle, which has four main steps (Figure 1): compression (compressor), condensing (condenser), expansion (expansion valve), and evaporation (evaporator). Note the chiller has a high-pressure side (from compressor through condenser to expansion valve) and low-pressure side (from expansion valve through evaporator to compressor). Most scroll and screw chillers have available options but are not considered custom-made machines, while centrifugal chillers are typically custom-designed and built, giving the designer substantial flexibility. The following component descriptions will include some available options and standard operating parameters: 46 En g i neer ed Sy stem s December 2008 W Evaporator. The evaporator is where the refrigerant removes heat (energy) from the chilled water. The evaporator can be a brazedplate-frame heat exchanger (smaller scroll and screw chillers) or a shell-and-tube heat exchanger. The two shell-and-tube evaporator types include DX barrel (refrigerant runs through tubes and water runs through shell) and flooded (water runs through tubes and refrigerant runs through shells). To ensure good heat transfer between the chilled water and refrigerant, it is important to maintain turbulent chilled water flow. The recommended minimum chilled water flow velocity is 3 Condenser water supply Condenser water return Condenser High pressure side Expansion valve Compressor Low pressure side Evaporator Chilled water supply FIGURE 1. Water cooled chiller diagram. Chilled water return

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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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