Research Of Vairable Load Operation Control Mechanism And Engineering Application For Cold Source System Of Central Air-conditioning

Air-conditioning system energy-saving operation is the key problem to large-scale publicbuilding energy-saving and emission-reduction, its energy consumption depends on thesystem design type, the operation control mode, as well as the load distribution characteristics,variable load operation control mechanism of cold-source system is the theoretical basis for acentral air-conditioning system to optimize the design and operation. Chillers group controland chilled water pumps group control, chilled water system pipe network variabletemperature control and chilled water system variable pressure control have been researcheddetailedly in this paper, and the research have been verified by experiment. On the basis ofapplied research a central air-conditioning intelligent-optimize control system has beendeveloped.Under the condition of uniformly load distribution, based on the different way of bearingpart load, mathematical model which was used to solve EER of single water chilling unit,evaporating temperature, condenser temperature and the model of EER with multiple units ofthe same type under different way of parallel operation are established, the operatingcharacteristics are compared and analyzed. The result shows that centrifugal water chillingunit can maintain higher EER only under terms at70%-100%load rate, EER decreasessharply with the decrease of load rate, the units' running are controlled by single unit's ratedload according to total load reduction, it can keep total EER in high-level on very large loadrange.For large central air-conditioning systems the accurate calculation of pipe networkhydraulic characteristics is the necessary condition to research its optimization design andoperation. Simplified Model can't be applied to hydraulic calculation of large pipe networktopological structure because of large calculation error. In this paper a reverse-return chilledwater system and a direct-return chilled water system are used as the research objects. Basedon fully taking into account the regulating characteristics of terminal branch temperatureregulating valve, an accurate mathematical model of pipe network hydraulic characteristicshas been established and a computer logic algorithm with virtual flow has been put forward.This calculation method can ensure calculation convergence, it not only realizes using onelogical program to calculate various kinds of pipe network hydraulic characteristics, but alsoprovides a very good reference to the research of pipe network hydraulic characteristics forreverse-return chilled water system and direct-return chilled water system.Terminal load distribution and pipe network structure are the key factors that affect the energy-saving potential of central air-conditioning chilled water systems, nonlinearthermodynamic performance of an air-conditioning system with large inertia will mainly exertinfluence on the stability and reliability of energy-saving operation control. With adirect-return chilled water system and a reverse-return chilled water system as study object,this paper built a hydraulic calculation model of pipe network topology, bypass loop hydrauliccalculation model, AHU thermodynamic model, and water pump variable frequency operationmodel. Operating frequency of a water pump for different flow ratio, pump power,temperature difference of pipe network supply and return water, pressure difference of pipenetwork supply and return water, bypass control valve characteristics, system adjustabilitycoefficient, and pipe network resistance characteristics of a chilled water system are studied inthis research work, and energy consumption characteristics of constant temperature differencecontrol and variable temperature difference control are also analyzed with comparison. Thecontrol mechanism for variable load operation has been researched detailed based on twodifferent control models of temperature control and pressure control in this paper. The resultscan provide theoretical guidance for the stable and reliable energy-saving operation of achilled water system.Pumps parallel operation is the common structure form for large central air-conditioning,in part load conditions. In actual engineering, the more common viewpoint is that the energyconsumption of the pump frequency operation is proportional to the cube of frequency, but itignores that the bypass loop regulation characteristics and pipe network hydrauliccharacteristics have influence on the energy consumption of parallel pump group. In thispaper, the analysis model of chilled water pump group parallel operation characteristics havebeen established based on the bypass loop regulation characteristics, and a pump groupconsisting of four chilled water pumps is for the study. In conditions of different supply andreturn water pressure difference for pipe network, the operating characteristics are analyzed indetail for four control strategies including: power frequency pump quantity control, singlepump variable frequency operation with power frequency pump quantity control, two pumpssynchronous variable frequency control, three pumps synchronous variable frequency controland four pumps synchronous variable frequency control. The paper thinks that pumpssynchronization variable frequency operation with variable frequency pump quantity controlis the most energy-efficient control strategy, and the viewpoint that single pump variablefrequency operation with power frequency pump quantity control is more suitable for chilledwater system variable flow operation in conditions of low flow-ratio and high supply-returnwater pressure difference has been provided. A large number of experimental researches have been completed on the experimentalplatform for a real run central air-conditioning chilled water system, and the experimentalresults verify the feasibility and correctness for the theoretical research. On the basis oftheoretical and experimental studies, a central air-conditioning optimize control system isdesigned and developed based on LabVIEW graphical programming software developmentplatform, and chilled water temperature, chilled water flow and cooling water flow can beoptimization coordinated controlled in order to reduce energy consumption, furthermorechillers group control, chilled water pumps group control, cooling water pumps group controland cooling towers group control can be achieved for energy-saving operation, which hasbeen applied in more than one office building. The results show that the optimize controlmanagement system of central air-conditioning can achieve stable and reliable operation, atthe same time it achieves better energy saving effect.