ASHRAE Journal - April 2009 - 36

ASHRAE 19 • Under high load conditions the three- 50 59–2009 YE A R S JOURNAL actual draw on the primary system will increase some 18% over the design values. In both cases, the brake horsepower requirements of the secondary pumps are significant. Fig. 3 shows a similar secondary system A at the end of a large primary system and an adjacent smaller secondary system B. In this case a normal resistance three-way valve and a resistance in the bypass were provided. The first two columns of the tabulation indicate the design values and the last two columns the actual behavior of System A, and its impact on System B through which the flow is reversed. In other words, System B gets the spent water from System A and its air conditioning requirements are not satisfied. This is not a hypothetical case. It has actually happened. An infinite number of variations of the three-way valve interface can be presented. None of them is satisfactory, although to a varying degree. I way valve will lock the primary and secondary systems into a series pumping arrangement which as we will see will upset the flow in the secondary circuit. In extreme cases it has even caused the flow in adjacent secondary systems to reverse. The following examples illustrate diagrammatically the most common misapplications of three-way valves in the primary/secondary system interface. Fig. 2 shows a 44°F secondary system connected to a large 40°F primary system close to the central plant so that the pressure distribution in the mains is not much affected by the behavior of the secondary system. To obtain his 1,000 ton load the designer expected to draw 1,500 gpm from the primary system and return it at 56°F. He sized the pump head of the secondary system circulating pump to match the combined resistance of the branch piping from the primary system, the secondary system piping, and the pressure drop through secondary system control valves and coils. In other words, the pump was designed not as a circulating pump in the secondary loop but as a booster pump. The design values are contained in the first column of the tabulation. The second column shows the actual operating results when the resistance of the three-way valve is small relative to the primary system pressure head. The result is a negative pressure gradient in the bypass. Therefore, no mixing can take place. The system is unworkable until the three-way valve is blocked in the open position for flow from the primary into the secondary system. The impact on the primary system is a 50% increase in the draw by this secondary system. The third column shows the behavior of the same system with a high resistance three-way valve as called for in the quoted master plan. The f i g . 3 Three-way valve interface; constant volume flow secondary chilled water systems connected to large primary system at end of line. 36 ASHRAE Journal a s h r a e. o rg April 2009
ASHRAE Journal - April 2009

Table of Contents for the Digital Edition of ASHRAE Journal - April 2009
