Chw

CHILLED WATER SYSTEMS ASHRAE SYSTEMS 2012          

Diversity factor – difference between sum of space design loads and system peak load For central chiller plants, consider using VFD’s pp. 3.2 Server rooms – may best be served by independent systems pp 3.2 For part load conditions and energy efficiency, variable flow is seen as the best economical choice p 3.2 Typical water velocity of chillers is 3 - 11 fps; trane chilled water design and control pp. 58 Plant expansion can be made easily for primary secondary and vpf ; 83 trane chilled water design and control Air cooled chillers – 7.5 – 500 tons; 80 trane chilled water vav systems; Packaged water cooled chillers – 10 – 3800 tons; 80 In air cooled chillers, condensing temperature of the refrigerant is dependent on dry bulb temp of ambient, while in water cooled chillers are dependent on ambient wet bulb temp

2 types of energy efficient designs: 1. Primary variable flow a. 2 way automatic control valves at terminal equipment and VFD at pump b. 2 way automatic control valves at terminal equipment and distribution pressure control with bypass valve 2. Primary / secondary variable flow – primary loop is constant flow, vfd at secondary pumps

CONTROLS

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Direct digital control is recommended pp. 3.5 3 way valves- variable temperature constant flow 2 way valves – variable flow contstant temp

McQuay Chiller design  

Advantage of 2 way control valves are lower firs cost and operating savings Difficulty is that chiller and control system must be designed for variable flow

Water and Air Temperature and Ranges Mcquay 

Limiting temperatures are the required supply air temperature and either the ambient wetbulb for water or evaporatively cooled chillers or ambient dry bulb for air cooled chillers

Trane Chiller System Design and Control  Chilled water temperature difference of 7 - 11 C (27, trane chiller system design and control)  Condenser water temperature difference - 7 - 10 C ; pp. 28  As per ARI 550/590 , evaporator leaving water temp - 6.7C ; 28  Water cooled condenser entering water temp - 29.4C; 28  Air cooled condenser entering air dry bulb - 35 C; 28  Chilled water supply temperature of 6.7 C for comfort cooling; ‘  2.4 gpm/ton for evaporator for 5.6 Ctemp difference  3 gpm/ton for condenser for 5.1 to 5.6C; 29 Trane Chilled Water VAV  Typical leaving water temp at FCU/coils – 12.2C; 82 

Types of chilled water systems: Primary systems Trane Chilled Water System Design and Control   

Install isolation valves after chillers to shut off chiller flow Header pump recommended Bypass control valve and flow meter (preferrably, magnetic flow meter type) needed to maintain minimum flow

Primary Secondary Systems Trane Chilled Water System Design and Control   

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Installing check valve at the decoupler not recommended for variable speed chillers (recommendd for fixed speed chillers) Recommended for fail safe operation and when there are no experienced staff System operation is simplest if all chillers with the same temperature of chilled water leaving temp and thru same system temperature difference across the chiller (chillers are loaded to equal percentages) use two way modulating valves for variable to chilled water distribution

Constant flow systems Trane Chilled Water System Design and Control  

Recommended for small systems up to 200 tons (trane : Chiller system design and control, pp.19) Controls are either on / off pumps

Variable Primary Flow Systems Trane Chilled Water System Design and Control  Water flow varies throughout the entire system through the evaporator of each operating chiller as well as thru the cooling coils  Needs two way control valves on coils, check valves on chillers, bypass pipe with control valve ; 55  Eliminates need for separate distribution pump; 55  Select min. Evaporator flow limit that is less than or equal to 60% of chiller design flow rate  Most of potential savings are realized by the time system flow rate decreases to 50% of design; 57

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Experience with vpf systems indicates min. Evaporator flow limit of 60% for packaged chillers and 40% for configured chillers Select chillers with equal pressure drops at their design flow rates ; 61 Value added option to reduce initial and operating cost Pressure independent bypass control valve may work; 63 Select slow acting valves to control chilled water coils; 69 Pump control most often performed by maintaiing a pressure differential at selected points in the system; (ex. VSD will increase pump speed of pressure differential is low) ;87