Subdividing various systems into a primary/secondary (P/S) loop via a hydraulically dependant interconnection long has been a standard solution for central chilled-water plants in the United States and Europe. This achieves, at a relatively low cost, reasonably good hydraulic separation of central-water-plant cooling-generation systems (primary loop) from distribution piping and terminal units (secondary loop). Primary-loop flow is relatively constant, while secondary-loop flow varies based on load demand. Primary- and secondary-loop water flows are interchangeable.

In a primary loop, control is achieved by maintaining a relatively constant flow rate; water temperature may be changed via a reset control. This commonly is referred to as a qualitative control strategy. In a secondary loop, control typically is achieved by varying water-flow rate. This commonly is referred to as a quantitative control strategy.

Figure 1 depicts a system with a constant- or variable-flow primary loop and a variable-flow secondary loop. The dedicated constant-speed Pump 1 maintains practically constant flow in the primary loop (the pump does not have variable-frequency-drive [VFD] control), even if flow in the secondary loop (which has its own pump, variable-speed Pump 2) varies significantly.1,2 Pump 1 is sized to maintain water flow between the chiller evaporator’s minimum and maximum allowable values.

Typical Control Strategy
In the system in Figure 1, the direction of water flow in the decoupling pipe is not controllable and may vary, depending on the ratio of flow in the secondary and primary loops.

Various modes of operation of the system were investigated.3,4 The major parameters in the evaluation of energy efficiency were supply- and return-water temperature and flow rate before and after the decoupling pipe separating the primary and secondary loops.

Mode 1. When water flow in the secondary loop (F2) exceeds water flow in the primary loop (F1) because of a load increase, a portion of the water returning from the secondary loop recirculates into the supply-distribution system (A-to-B direction) and mixes with the flow in the primary loop. This mode of operation is represented by the following equations: F2 > F1, t3 > t1, t2 = t4, and (t4 − t3) < (t2 − t1).

Mode 2. When water flow in the secondary loop (F2) is less than water flow in the primary loop (F1) because of a load reduction, flow in the decoupling pipe reverses (B-to-A direction). Thus, the excessive flow exiting the cooling-generation system returns to the primary-loop system and the chiller. This mode of operation is represented by the following equations: F1 > F2, t1 = t3, t2 < t4, and (t4 – t3) > (t2 – t1).

Mode 3: When the flow in the primary loop (F1) equals the flow in the secondary loop (F2), there is no flow in the decoupling line. All water from the secondary loop returns to the primary loop and chiller, while all water exiting the chiller flows through the secondary loop. This mode of operation is represented by the following equations: F1 = F2, t1 = t3, t2 = t4, and (t2 – t1) = (t4 – t3). Obviously, this mode of operation is the most beneficial from an energy perspective.

Optimized Control Strategy
Figure 1 depicts the optimized control strategy (Mode 3). Unlike a system with an optional constant-speed primary-loop pump, the system has an additional VFD controlling the speed of the primary-loop pump. The rate of water flow in the secondary loop via Pump 2 is dependent on system load. Water-flow rate in the primary loop is a function of water-flow rate in the secondary loop and adjusted to maintain equalized flow.

Following are simplified thermal-balance equations applicable for both primary and secondary loops, assuming the specific heat of water does not change appreciably:

where:
QPR = Primary-loop cooling load, British thermal units per hour
QSEC = Secondary-loop cooling load, British thermal units per hour
∆tPR = Primary-loop temperature differential, degrees Fahrenheit
∆tSEC = Secondary-loop temperature differential, degrees Fahrenheit

Equations 5 and 6 essentially represent the algorithm for the control of chilled-water plants. For the system in Figure 1, control is accomplished by varying the speed of Pump 1. The pump’s speed should not be reduced to the extent water-flow rate falls below the allowable low limit or increased to the extent water-flow rate exceeds the allowable high limit.

The building-automation system would have to limit VFD-turndown and turn-up ratios to stay within the range of allowable current frequencies correlated to the range of allowable primary-loop water-flow rates. To better match primary- and secondary-loop water-flow rates, two-phase control is suggested. The first phase could consist of quantitative control in both the primary and secondary loops (Equation 5) while the chilled-water-supply temperature remained at a given constant value. The second phase could consist of qualitative control in the primary loop and quantitative control in the secondary loop. (The order in which the control actions are implemented may vary.) Reset water-temperature control (Equation 6) could be realized by varying water temperature (t1) at a given fixed (limited) magnitude of water-flow rate in the primary loop. The change in water temperature would impact flow rate in the secondary loop indirectly; flow via the primary-loop pump could not be changed further because of the aforementioned evaporator-flow limitations. The P/S-loop system with variable flow and temperature control in both loops in Figure 1 is very versatile, allowing the establishment of flow-limiting parameters and temperature set points (t1) over a given time period.