As the cost of energy continues to rise, it behooves chiller-plant owners and designers to consider the energy impacts of virtually everything under their control, including chilled-water piping.

This article will discuss the energy consumption of the three basic configurations of chilled-water piping currently in use: constant primary flow (CPF), constant primary flow/variable secondary flow (P/S), and variable primary flow (VPF). Also, it will discuss the attributes of equipment-room space, installed cost, and control complexity.

Constant Primary Flow
Operation. Figure 1 illustrates a CPF arrangement at 100-percent system load. The most basic of the three configurations, this is used in many smaller plants. As the name implies, the chilled-water (CHW) pumps are constant speed, and water flow varies only when a chiller and its pump are cycled off for capacity purposes. Throughout this article, the following formula will be used to analyze how systems operate:

Load = Flow × Delta-T

where:
Delta-T = chilled-water-temperature range

In a CPF configuration, load changes are reflected by changing delta-T. Typically, a three-way valve is placed on the discharge of each coil. The valve’s job is to maintain the set-point temperature of the air leaving the coil. If the temperature of the air ventures above or below its set point, the valve modulates the amount of chilled water flowing through the coil, with excess water bypassing the coil. This modulates the temperature of the chilled water within the coil in accordance with the formula above; coil capacity is adjusted until leaving-air temperature once again is at its set point. Throughout this process, operating chillers experience (roughly) constant water flow.

Attributes. CPF designs have several advantages:

• Equipment-room space is minimized because there is only one set of chilled-water pumps.
• Installed cost is low because constant-speed pumps are used, avoiding the expense of variable-speed drives (VSDs). Also, low-pressure-differential-rated three-way valves, which are less expensive than high-pressure-differential-rated two-way valves, are used.
• Control and operation of the system is relatively simple and easy to understand.

The main disadvantage of CPF designs is high pump energy consumption. Any time a chiller plant is operating, active pumps are running at full flow, regardless of whether the load requires it.

Constant Primary Flow/Variable Secondary Flow
Operation. P/S configurations are divided into two loops:

• Primary, consisting of chillers, primary pumps, and interconnecting piping and using constant-flow pumping.
• Secondary, consisting of coils, secondary pumps, and interconnecting piping and using variable-flow pumping.

A decoupler pipe hydronically separates the two loops, allowing temperature exchange, but independent pressures and flows.

Two-way valves control the temperature of air leaving the cooling coils by throttling water flow. Thus, load in a P/S configuration is controlled by varying flow, while delta-T remains relatively constant.

As the two-way valves throttle down chilled-water flow, pressure in the piping upstream increases. A differential-pressure sensor signals the VSDs on the secondary pumps to reduce their speed, which decreases secondary flow and lowers friction loss in the pipe. In the process, pump energy consumption is reduced.

Figure 2 illustrates a P/S plant at 100-percent system load.

The rate of flow in the primary loop must always equal or exceed the rate of flow in the secondary loop. The decoupler allows excess primary water to flow back to the return side of the chillers, ensuring constant flow.

If the rate of flow in the secondary loop exceeds the rate of flow in the primary loop, some of the water discharged from the coils is bypassing the chillers and flowing back to the secondary pumps, mixing with and increasing the temperature of water supplied to the coils. If that continues, the coils will lose their cooling capability, and system control will be lost.

Figure 3 illustrates a P/S configuration at 75-percent system load.

As cooling load falls, chillers and their respective primary pumps can be cycled off, reducing energy consumption in the primary loop. Water flows rebalance so that the rate of flow in the primary loop equals or exceeds the rate of flow in the secondary loop.

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