Engineering Manual o.. - HVAC.Amickracing

AIR HANDLING SYSTEM CONTROL APPLICATIONS17. PLACE CONTROL VALVES ON THE LEAVING SIDEOF WATER COILS.Control valves on the leaving side of water coilsleaves pressure in the coil when the valve is closed,thus aiding in eliminating air through the air venton the leaving side of the coil, and also preventsthe possibility of air being induced into the systemthrough the vent if the pump produces a negativepressure in the coil return line.18. CONSIDER THE ABILITY OF THE HVAC SYSTEMOPERATOR TO UNDERSTAND THE SYSTEMWHEN DESIGNING GRAPHICS FOR THEOPERATOR INTERFACE.APPLICATIONS-GENERALThe following applications are presented in a DDC formatusing notation from the Symbols in this section. In some casesthe degree and complexity of control shown is not practical inother forms of control.Suggested microprocessor data points are shown as they couldappear on a color graphic display of a PC operator workstation.In some cases data points, other than those required for controland operation, are shown to help an operator understand theloading and performance of the HVAC system and relatedcontrol loops. If a PC station is not required, the data pointsrequired for control and operation should still be specified forthe operator by listing the points or including a graphic sketch.Values, setpoints, gains, timings, etc. shown in these examplesare general, and actual values should be determined on a projectto-projectbasis.The following applications were selected for this section onAir Handling System Control Applications. Caution should beused in simply combining any of these applications together as acontrol solution for a more complex system. Application variationsmay be required depending on the heating, cooling, and pumpingconfigurations, the building use and occupants, the ability ofcontrol vendors and related control systems, the ability of localoperating and maintenance persons, codes, and weather.Lines connecting inputs, outputs, values, and controlfunctions have been added to aid in understanding. In manycases these lines would create unacceptable clutter on an actualsystem graphic display. Graphic display and managementfunction (alarms, records, etc.) concepts are discussed furtherin the Building Management System Fundamentals section.Although the control solutions presented are good generalrecommendations, other solutions are acceptable, and in somecases, may better depending on the job objectives.206ENGINEERING MANUAL OF AUTOMATIC CONTROL