• Improper set points or calibration: For example, dropping coil supply air temperature set point by only 2°F can double the flow rate and halve theT. • Use of three-way valves: Three-way valves should pretty much never be used
in variable-flow CHW systems. Some designers use them to ensure that water is “instantly” available at coils, but there is seldom a need: cooling loads change slowly, and, in a typical building, chilled water will reach a coil when its valve is opened after seconds or perhaps minutes, but definitely fast enough to avoid overheating. Three-way valves simply increase flow unnec- essarily and reduce plantT. They also cost more than two-way valves. • No control valve interlock: It is essential that control valves and associated
control loops be disabled when the associated AHU is shut off. Otherwise the valve will naturally open as the supply air or room temperature cannot meet set point. This is often overlooked in DDC system programming. • Coils piped backwards: CHW coils must be piped in an overall counterflow
arrangement entering at the air discharge side of the coil. If piped back- wards, the coil effectiveness is significantly reduced and the supply air tem- perature set point can almost never be reached.
• Uncontrolled process loads: Plants sometimes provide cooling to process
loads such as medical and biological laboratory freezers. These devices must have automatic valves that shut off flow when the equipment does not need it.
• Incorrectly selected control valves: Valves must have sufficient actuator power to shut off against the DP created by the pumps. This is less of a problem now with electric actuators and the common use of ball valves instead of globe valves—small pneumatic globe valves typically have only about 30 psi of shutoff head capability versus 200 ft for an electric ball valve. Oversized valves can also be an issue because they result in hunting; as shown in Figure 4-17, oscillating flow can increase average T. Again, modern control valves mitigate this problem; they have a rangeability of 50:1 up to 100:1, compared to 10:1 up to 15:1 for large pneumatic valves. Two-position (on/off) control valves, such as those used to control small
fan-coil units (FCUs), are often blamed for low T problems. If these
valves are not equipped with flow-limiting valves, or piped in a reverse return arrangement, they may consume more water flow when open than the design calls for. With full flow through the coil at partial loads, theT will invariably be lower than design. However, because the air temperature entering the FCU is fairly constant and is usually not subject to outdoor air conditions, theT will not degrade significantly.
• Incorrectly selected coils: When connecting to a central plant, it is essential for the designer to select coils that meet the minimum designT for which the plant was designed. It is not uncommon for engineers to select coils for a 10°FT simply because it is the basis of chiller AHRI standard ratings, instead of the 20°F or more assumed in the plant design. On campuses where there may be many engineering firms designing buildings over the years, it can be hard to police their coil selections. Consequently, under-
sized coils are often the most common source of degrading T in campus
CHW plants.
• Improper bridge connection and control: Tertiary pump bridge connections (Figure 4-15) should be controlled off of the CHW supply temperature to the building at a set point that is a few degrees above the temperature of the water supplied by the central plant. If the set point is at or lower than the plant temperature, the bridge control valve will be wide open and water will recirculate directly to the return, substantially reducing T. Nonethe- less, a proper set point will not help improveT if the coils downstream are not maintaining a highT. To resolve this problem, some designers move the temperature sensor controlling the two-way valve from the supply line to the return water line. The control valve is then modulated to maintain the return water temperature at design levels. If return water is too cold, it is recirculated back into the building in an attempt to make it absorb more heat. While instinctively it may make sense that recirculating water will increase the return water temperature, the return water temperature is driven primarily by the entering air temperature and the coil effectiveness, not the entering water temperature. Table 4-4 (based on an eight row, 96 fpf coil designed for 77°F entering dry-bulb, 62°F entering wet-bulb, and 55°F leaving dry-bulb temperatures) shows that for a given load, increasing entering CHW temperature results in a lower, not higher, leaving return water temperature. So, recirculating water not only increases the flow in
Table 4-4 Coil Performance with Increasing CHWST Entering CHW Temperature, °F Flow Rate, gpm T, °F Leaving CHW Temperature, °F 42 30 16.7 58.7 44 34.5 14.7 58.7 46 41 12.3 58.3 48 53 9.5 57.5
the building tertiary loop, it also slightly increases flow in the secondary loop. Furthermore, the same death spiral that occurs with primary/second- ary systems can occur here: as water is recirculated the T gets worse so more water is recirculated until the valve is closed and the system is fully recirculating water. Eventually, in this mode, the return water temperature will rise but only because coils are starved. Once return water temperature does rise above set point, the CHW valve will open, but it will soon close as the return water temperature once again drops. Clearly, this is not a good control strategy.