To realize the optimal control of variable pressure difference in fan-coil unit systems

a phase plane fuzzy identification method was proposed and verified

which takes the terminal branch pressure difference of the most unfavorable thermal loop as the control variable. For fan-coil variable flow air conditioning water system

based on the heat balance equation of air conditioning room

the mathematical relations among the terminal energy difference between supply and demand

the deviation (e) between room temperature and its setpoint

and the deviation change (ec) were established. The most unfavorable thermal loop is defined as the one with the largest terminal energy difference. In the phase plane composed of e and ec

the terminal energy difference of the phase plane could be reflected by the trajectory characteristics of the phase plane connected by phase points S(e

ec). According to the fuzzy pattern recognition process

by taking the real-time tracer direction vector angle (βi) and the room temperature deviation (ei) as characteristic indexes

triangle function as subordinate function

and maximum subordinate degree principle as recognition criterion

the table of fuzzy recognition rules was formulated. The phase plane fuzzy identification method for the most unfavorable thermal loop was then established

and a test platform for variable flow air conditioning water system was built. The water flow of the branches was increased synchronously

and the branches that reached the balance between energy supply and demand are the most unfavorable thermal loops identified by the test. Experimental results show that the proposed method is accurate and reliable

which can be applied to identify the most unfavorable thermal loop in fan-coil unit systems.