Engineered Systems - December 2008 - (Page 49)

Each compressor type’s method of operation, advantages, available chiller capacities, and energy usage range are as follows: • Scroll compressor. The scroll compressor is a positive displacement machine that consists of two interleaved scrolls. One scroll is stationary while the other scroll orbits eccentrically, compressing the refrigerant. The scroll compressor’s advantages include fewer moving parts, quieter operation, and compact size. The scroll compressor’s hightolerance machining requirement limits the scroll cooling capacity to 30 tons or less. Air cooled scroll chillers are available with cooling capacities between 10 tons and 190 tons and energy usage between 1.1 and 1.5 kW/ton. Water cooled scroll chillers are available with cooling capacities between 30 tons and 120 tons and energy usage of 0.7 and 0.9 kW/ton. • Rotary screw compressor. The rotary screw compressor is a positive displacement machine that consists of either a single or twin screw(s). The screw(s) rotate to compress the refrigerant into a smaller volume, raising its pressure. Screw compressor advantages include higher reliability, compact size, and tighter chilled water temperature control (+0.5°F). An individual screw compressor has a cooling capacity of 200 tons or less. Air cooled screw chillers are available with cooling capacities between 80 tons and 500 tons and energy usage between 1.1 and 1.5 kW/ton. Water cooled screw chillers are available with cooling capacities between 70 tons and 265 tons and energy usage between 0.65 and 0.9 kW/ton. • Centrifugal compressor. The centrifugal compressor is a dynamic machine that includes an impeller. The spinning impeller transfers its kinetic energy to the refrigerant, raising the vaporized refrigerant’s pressure. The critical variable for centrifugal compressors is the impeller tip speed, which must be maintained regardless of chiller loading. Centrifugal compressor advantages include larger capacity chillers, lower energy usage, multi-stage compression, and higher reliability. An individual centrifugal compressor has a cooling capacity of 2,000 tons or less. Centrifugal chillers are water cooled and are available with cooling capacities of 200 tons to 2,000 tons for a single-compressor centrifugal chiller, and 400 tons to 4,000 tons for a two-compressor centrifugal chiller. Centrifugal compressor chillers are the most energy efficient chillers with energy usage between 0.5 and 0.6 kW/ton. • Expansion valves. High-pressure liquid refrigerant passes through expansion valves, reducing pressure and flashing to a gas within the evaporator, absorbing energy from the chilled water. Expansion valves range from a simple orifice opening in the pipe to a complex modulating valve that provides dynamic load control. • Refrigerants. Refrigerant technology is changing because production of several commonly used refrigerants will soon end, including R-22 in 2010 and R-123 in 2020. However, refrigerants should be available for the life of R-22 and R-123 chillers purchased today. Refrigerants come in three classifications: high pressure (R-22, R-407C, and R-410A), medium pressure (R-134A), and low pressure (R-123). The high-pressure and medium-pressure refrigerants are utilized in positive-displacement chillers, and the medium- and low-pressure refrigerants are used for dynamic chillers. Scroll chillers typically use R-22, R-407C, or R-410A refrigerants. Screw chillers typically use R-134A or R-407C refrigerants. Centrifugal chillers typically use R-123 or R-134A refrigerants. CHILLER PERFORMANCE STANDARD ARI Standard 550/590 is the primary performance standard for screw and centrifugal chillers up to 2,000 tons, though many manufacturers interviewed for this article utilized ARI Standard 550/590 for rating their water cooled scroll chillers, air cooled scroll chillers, and air cooled screw chillers. This standard provides a testing protocol for determining the chiller capacity (tons), energy usage (kW/ton), fluid pressure drop (ft of water), and integrated part load value (IPLV) or non-standard part load value (NPLV). The standard has established testing criteria for performing the testing, including: • Leaving chilled water temperature: 44° • Evaporator waterside fouling allowance: 0.0001 • Chilled water flow rate: 2.4 gpm/ton • Entering condenser water temperature: 85° • Condenser waterside fouling allowance: 0.00025 • Condenser water flow rate: 3.0 gpm/ton It is very important to understand that chillers rarely operate at their maximum capacity. 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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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