Optimal Control Of Central Air Conditioning In Double Chilled Water System

With the vigorous development of Chinese economic construction,the urbanization rate has gradually increased,and the total amount of public buildings has gradually increased.More and more attention is being paid to building energy conservation.The energy consumption of air-conditioning systems accounts for a large proportion of building energy consumption.At present,there are still many problems in the air-conditioning systems in most public buildings,resulting in a lot of waste of energy.Therefore,the energy-saving potential of the air-conditioning system in public buildings is huge.This article first takes a variable flow dual chilled water central air-conditioning system in Guangzhou as the research object.Through investigation of users and analysis of the operating data,the following problems are found: the expansion tank of the small system overflows seriously,water flows between the large and small air-conditioning systems;equipments of the system are over-selection in the design phase;the operation and maintenance management of the air-conditioning system are not good;the number of equipment opened does not meet the demand.Subsequently,in view of the engineering pain point--the continuous overflow of the expansion tank of the small system,which is the most concerned by users,the water pressure diagram is used to analyze the pressure distribution of the water system pipe network under different flow ratios during the operation of the two systems.It is found that the pressure of the two systems is different.Pressure imbalance is an important reason for the overflow of the expansion tank of the small system: under different load ratios,the water pressure at the connection of the large system is greater than that of the small system;when the flow ratios of system I and system II are 4:1,3:1,and 2:1respectively,the pressure imbalance rate at the inlet of the water pump at the connection is 2.0%,3.8%,4.4%,and the pressure imbalance rate at the outlet of the chiller at the connection is 3.7%,6.8%,7.2%.To further explore the coupled operating characteristics of the dual chilled water system,the operation characteristics of the water system are simulated and analyzed when the pumps of the two systems are switched simultaneously and the terminal load changes at the same time.The analysis found that:when the two systems adopt the fixed-pipe differential pressure water pump frequency conversion strategy,the pressure at the connection increases with the decrease of the system flow;when the two systems adopt constant-tip pressure difference control,the pressure at the inlet of the connection pump increases with the decreases of the system flow rate,and the pressure at the outlet of the chiller at the connection decreases with the decrease of the system flow rate.Then,in order to reduce the overflow of the expansion tank caused by the water convection,the relationship between the water pressure at the connection of the two systems and the system flow,the pressure difference at the connection and the height difference between the two expansion tanks was analyzed.A coupled pump optimization control strategy with the outlet pressure of the chiller at the connection of the two systems as the control object and an expansion tank height difference optimization model are proposed.The study found that when the most unfavorable constant pressure difference control is used,the water pressure at the connection point is positively correlated with the flow rate,and when the constant pressure difference control is used,the connection point water pressure and the flow rate are negatively correlated;Compared with using a coupled pump optimization control strategy alone,using both of the above optimization model and strategy in the case building has significant energy saving effect: when the height difference of the water tank is set to11.7m,the power consumption and energy saving rate of the chilled water pump reaches85%～95%;when the height difference of the water tank is set to 2.5m,the power consumption and energy saving rate of the chilled water pump reaches 17%.Finally,the energy-saving diagnosis of other problems during the operation of the dual chilled water system central air conditioning of the case project was carried out,and it was found that the COP of the chillers WCC-01,WCC-03 and WCC-04 were lower than the COP limit of 3.The actual value of delivery coefficient of the cooling water pump 1# and 6# is 33.1 and 19.6,which are respectively 5.4% and 44% smaller than the limit of the delivery coefficient.In the case project,the chiller and the cooling water pump have great energy-saving potential.Therefore,suggestions for optimization and transformation from two aspects of optimization and operation and maintenance management are given in this article.