Conceptually, the variable-primary-flow (VPF) system resembles the familiar constant primary-variable secondary design more commonly known as the "decoupled" system. A comparison of the schematics in Figure 1 and Figure 2 reveals the similarities between "decoupled" and VPF systems. For example, both require a bypass. Each chiller in the arrangements shown has a dedicated pump whose operation coincides with the chiller it serves. Differences between the two systems become apparent upon closer examination.

The "decoupled" system shown in Figure 1 uses constant water flow through each chiller evaporator and variable water flow through each cooling coil to satisfy space loads. Implementing this design requires:

• a constant-speed, essentially constant-volume pump (and check valve) for each chiller;

• two-way control valves to regulate the amount of chilled water that flows through the cooling coils;

• a variable-flow distribution pump to serve the coils (flow modulation is usually accomplished by providing the pump with a variable-frequency drive);

• a bypass to hydraulically decouple the primary (production) and secondary (distribution) sides of the system.

As each two-way valve adjusts the flow of chilled water through the coil to satisfy the existing load, the distribution pump responds by regulating the amount of chilled water delivered. Water flows through the bypass in either direction as needed to balance the system.

Contrast this with the VPF system in Figure 2, which varies water flow throughout the entire system—that is, through the evaporator of each operating chiller as well as through the cooling coils.

Two-way control valves, check (or isolation) valves, and a bypass are required to implement a VPF system. However:

• Variable-flow chiller pumps eliminate the need for a separate distribution pump.

• The bypass can be positioned either upstream or downstream of the cooling coils.

• A control valve in the bypass ensures that the amount of flow that returns to the operating chiller(s) never falls below the minimum limit.

Paradigm Shift. For many years, chiller manufacturers encouraged cooling-plant designers and operators to maintain a constant flow of water through the chiller evaporator. The overriding concern was one of protection since reducing water flow too quickly (that is, faster than the chiller safeties could respond) could result in nuisance shutdowns; perhaps even freezing temperatures, ruptured evaporator tubes and costly equipment downtime.

What "suddenly" made variable primary flow feasible? Recent advances in control technology improved chiller operating stability. The strategically placed sensors and real-time response of today's control systems let the chiller perform its primary function—producing cold water—even when evaporator flow rates vary.

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