Two objectives underlie the design of virtually every HVAC system: lower first cost and lower energy (life-cycle) cost. These goals are largely responsible for the growing popularity of “low-flow” chilled water systems. “Low-flow” designs provide required cooling capacity by using less water at colder temperatures, essentially trading an increase in chiller energy consumption for a greater reduction in pumping costs.

How does reduced water flow affect the performance of the cooling coil? An understanding of thermodynamics and the heat-transfer equation, Q = U × A × LMTD, tells us that less water flow through the coil tubes reduces heat-transfer coefficient U (waterside resistance to heat transfer increases). But as the graph illustrates, the log-mean temperature difference (LMTD) increases because the entering water temperature is colder.

The higher LMTD that accompanies low flow offsets the reduced heat-transfer coefficient. In effect, the capacity of the coil remains the same whether the water flow is 127 gal/min or 77 gal/min — without changing surface area A. 

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