IN-91-21-2 -- Modeling and Optimization of Commercial Building Chiller/Cooling Tower Systems PDF

Guidelines for specifying condenser water flow rate for commercial building chiller/cooling tower systems have not changed significantly during the past 30 years, while chiller efficiencies have increased and the performance of cooling towers has improved. The present specified value, at design operating conditions, for the condenser water flow rate is typically 2.8 to 3.9 gallons per minute (gpm) per ton of cooling. The purpose of this study is to investigate the effect of using lower condenser water and cooling tower air flow rates on peak power demand, annual energy use, and capital costs for the overall system. In order to do this, mathematical models were developed that simulated the system performance based on manufacturers' component performance data. The results point to using a condenser water flow rate at a design operating condition of 2.0 gpm/ton and across-flow cooling tower air flow rate of 240 cfm/ton (using the present cooling tower sizing criteria). When operating the system at the lower fixed flow rates, savings of 3.5% in system peak demand and 10.5% in annual system energy use were found compared to using flow rates determined by the present guidelines. With the use of optimum variable flow rates, savings of 15.6% in annual energy use charges were found. Reduced equipment costs would also result from the lower pumping requirement.

KEYWORDS: calculating, optimisation, commercial, buildings, cooling towers, chillers, performance, regulations, specifications, condensers, water flow, cooling, air flow rate, energy consumption, peak load, costs, components, energy conservation