Sustainable Chilled Water Systems. By Thomas Hartman, P.E. The Hartman Company Georgetown, Texas www.hartmanco.com. Sustainable Chilled Water Systems. Session Outline Overview: New Directions Required for Sustainability Plant Issues: What Equipment, What Configurations
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Thomas Hartman, P.E.
The Hartman Company
The Equal Marginal Performance Principle
Equal Marginal Performance Principle Based Control
Characteristics of Cooling Loads for Various Climates
Variable vs. Constant Speed Chiller Performance
The Variable Speed Difference
"Natural Curve" Of VS Centrifugal Chiller
The "Natural Curve" of a variable speed centrifugal chiller is shown as the black line. The Natural Curve is the locus of load points at various condenser and evaporator conditions at which the highest operating efficiency is achieved.
To achieve optimum performance, chillers should be sequenced such that the on-line chillers operate as close as possible to their Natural Curve.
Tower Approach Temperature For 3 Tower System
At lower wet bulb temperatures, the approach of cooling towers rises due to reduced moisture capacity of cooler air. By keeping towers on line and slowing fans and pumps, greater air and water volumes pass over larger surface areas per unit energy expended to improve part load approach temperatures, as shown in this chart.
The total power to fans and pumps is the same for the variable speed and constant speed systems at each condition. Note the improved approach by slowing, not shedding towers at low loads.
Chiller Plant with Conventional Controls
Equipment is operated independently with local PID temperature and pressure loops. Chillers and towers are sequenced to keep on-line equipment as fully loaded as possible. Plant optimization, if it is applied at all, requires another level of control that continuously resets the various setpoints, which can reduce overall system stability.
All-Variable Speed Demand Based Control
Equipment is connected via an integrated network and operation is coordinated to maintain lowest overall energy use at all load conditions using demand based control and natural curve sequencing of equipment. Unless incorporated as design criteria, temperature and pressure setpoints are not employed except as operating limits.
Network Enabled Demand Based Control Of All-Variable Speed Chiller Plants
Sustainable Chilled Water Plants
All-Variable Speed/Flow Distribution Systems
Primary/Secondary CHW Distribution System
Variable Primary Flow CHW Distribution System
Variable Speed Pump Operation Comparison
Improving Efficiency of Variable Flow Distribution System for Demand Based Control
Demand Based Control Applied to Distribution System Elements
Multivariable "Intelligent Iterative Control" is used whereby valve adjustment is based on setpoint error, pump speed, chilled water temperature, and current valve position.
"Orifice area" pump speed control adjusts speed based on percent of total valve orifice area currently open and further adjusted by the number of valves nearly wide open. Pump Sequencing is based on "natural curve" principle.
Demand Based Control of Variable Flow Distribution System
Network Enabled Demand Based Control Of All-Variable Speed Distribution Systems
Demand Based Control of All-Variable Flow Primary/Booster Distribution System
Sustainable Chilled Water Distribution Systems
Implementing All-Variable Speed Chilled Water Systems
Achieves approximately 0.5 kW/ton overall annual plant & distribution system performance – about half the energy use of most existing plants.
Simple, stable, ultra-efficient control with reduced system maintenance.
Network control coordinates all-variable speed equipment for optimum operation in response to actual demand on system.
Plant is designed for direct control among components, and distribution system to minimize pressure drops and variations
Success With All-Variable Speed Chiller Plants
Next Steps Toward a Sustainable Chilled Water System
Thomas Hartman, P.E.
The Hartman Company
Questions & Discussion