ASHRAE 19 installed in the branch pipe what is the need for the three-way control valve at the interface? 50 59–2009 YE A R S JOURNAL constant Delta T and variable flow in both the primary and secondary systems over the normal operating range by replacing the bypassing control at the coils with throttling control. This system provides the desired compatibility of primary/secondary systems and will permit starting and stopping of machines, towers and pumps in the central plant in proportion to the load. However, it is not yet perfect. When reducing the chilled water flow through air conditioning coils in proportion to the load, a transition zone is reached in which the flow pattern turns from turbulent to laminar, and the coil performance becomes erratic. For the coils it is immaterial Correct Design of Interface Let us examine an interface without the three-way valve. Fig. 4 shows a primary/secondary system interface with a two-way modulating control valve in the branch pipe. The secondary system is an independent loop circuit. The resistance in the main closing the loop is practically nil; it is no longer a bypass. The pump is designed to provide only the circulating head within the secondary system and cannot become a booster pump. The control valve throttles only against the primary system circulating head and can be given the plug characteristics best suited to accomplish this. This interface design can provide the desired system design criteria at 100% demand and will properly control over the entire load range. However, as discussed previously, it cannot improve the primary system Delta T as long as the secondary system flow rate remains constant and its return temperature variable. Improved Designs of Primary/ Secondary System Interface Fig. 5 shows a design of a secondary system which achieves a reasonably f i g . 6 Valveless interface; throttling valve in branch piping; variable volume flow in secondary system; alternate 2. April 2009 ASHRAE Journal 39