Hot-water-temperature control

Some chillers use entering- and leaving-condenser-water temperature to determine the stage of compressor capacity necessary to maintain a hot-water-temperature set point. As lcwt deviates from the heat set point, the chillers adjust compressor capacity to ensure hot-water temperature is maintained.

One exception: During heat mode, chilled-water temperature is allowed to float and no longer is the primary input for capacity control. This could overcool the chilled-water loop during light-cooling-load conditions. However, when leaving-chilled-water temperature falls below the cooling set point, an additional software routine removes a stage of capacity from the chiller. This is known as “low-source protection.” This ensures that the greatest amount of useful heat can be extracted from a heat-reclaim chiller without the stability of the chilled-water system being sacrificed.

Primary/secondary chilled-water system with heat-recovery chiller

A conventional primary/secondary chilled-water system with a small-capacity heat-recovery chiller installed in parallel with the chiller plant (Figure 7) minimizes chiller-plant lift and maximizes energy efficiency while allowing direct control of both hot- and chilled-water temperature. The heat-recovery chiller produces a base load of chilled water while generating hot water controlled to the heat set-point temperature. The heat-reclaim chiller offsets the main chiller-plant load by providing the base load of chilled water before it enters the primary chillers. Under base-load operation, the heat-reclaim chiller provides chilled water to the secondary loop. The low-source-protection feature of the heat-reclaim chiller ensures chilled-water temperature will not fall below cooling-set-point temperature, ensuring stable chiller-plant operation while providing a controlled source of hot water. If the chilled-water temperature cannot be achieved with the heat-reclaim chiller alone, the first chiller in the primary loop can be energized, thus, maintaining secondary-loop water conditions. This configuration provides a stable source of controlled hot water and a base load of chilled water without affecting main-chiller-plant efficiency.

Variable-primary-flow chilled-water system with heat-recovery chiller

Excluding benefits from heat reclamation, a variable-primary-flow system can offer energy savings beyond those of other chiller-plant configurations. One study² found that “variable-flow, primary-only systems reduced total annual plant energy by 3 to 8 percent, first cost by 4 to 8 percent, and life-cycle cost by 3 to 5 percent relative to conventional constant-primary-flow/variable-secondary-flow systems.”

This system works much the same as the primary/secondary system described earlier, with the heat-reclaim chiller providing a base load of chilled water for the primary chiller plant (Figure 8).

Series-counterflow chilled-water system with heat-recovery chiller

Additional energy savings can be realized with a series-counterflow system (Figure 9). Variable-speed screw chillers offer precise capacity control under varying return-water flow and temperature conditions while providing high full- and part-load efficiency. These chillers are well-suited for such use because the instabilities associated with surge in centrifugal chillers are not present. Although screw chillers with variable-frequency-drive speed control respond well to variations in load and head, other chiller types can benefit from the higher overall chiller-plant efficiency offered by the series-counterflow arrangement. This system takes advantage of the lower lift provided by smaller differences between leaving chilled-water temperature and leaving condenser-water temperature. A series-counterflow plant benefits from significantly lower lift and improved overall efficiency. As with the primary/secondary- and primary-variable-flow configurations, the first stage of cooling is provided by the heat-reclaim chiller.

USING CAPTURED HEAT

Some heat-reclaim chillers can produce water as warm as 135°F. Although that is warm enough to satisfy many hot-water applications, some applications require that a source of water warmer than 135°F always be available. In such cases, a system similar to the one in Figure 10 should be considered. This system captures heat from a heat-reclaim chiller and provides water at the temperature required by an application.

To ensure stable hot-water-temperature control, loop volume should be no less than 6.0 gal. per ton of heating capacity within a water heater and preheat storage tanks and connecting piping.

Note the location of tank piping connections. Thermal stratification or improper thermal mixing within tanks can lead to poor hot-water-temperature control. To ensure proper mixing, the heat-reclaim water pump between a preheat storage tank and a heat-reclaim chiller should remain on at all times.

A backup water heater — a conventional water heater, a hot-water boiler, or a steam converter — must be provided to ensure a controlled source of hot water is available when a heat-reclaim chiller is not operating. This ensures that cold makeup water is preheated by the hot water generated by a heat-reclaim chiller and that the temperature of the hot water supplied to a heat load is controlled.

SUMMARY

Understanding how to design a heat-reclaim system to meet the heating requirements of a building or process is important, as is specifying appropriate equipment when a controlled source of hot water is desired. Several chiller-plant configurations are capable of accomplishing these goals. Equipment that can provide a controlled source of recovered heat for hot water must be specified. On-board chiller controls can maintain hot-water-set-point temperatures without sacrificing chilled-water-plant efficiency. The combination of captured heat and optimum chiller-plant efficiency helps to minimize energy consumption and maximize LEED certification points.

REFERENCES

1. ASHRAE. (2004). Standard 90.1-2004 User's Manual. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

2. Bahnfleth, W.P., & Peyer, E. (2004). Variable primary flow chilled water systems: Potential benefits and application issues. Arlington, VA: Air-Conditioning and Refrigeration Technology Institute. Available at http://www.arti-21cr.org/research/completed/finalreports/20070-final2.pdf

For past HPAC Engineering feature articles, visit www.hpac.com.

---

One of Carrier Corp.'s green-building sustainability advocates and a resource for information on high-performance HVAC-system solutions, Brian Key, PE, LEED AP, has more than 20 years of experience in HVAC, including system design, product marketing, business management, software system application, and business development.

Acceptable Use Policy blog comments powered by Disqus