Case study: hospital boiler systems
A hospital project in the Dallas-Fort Worth area lowered the heating-water temperatures to allow for innovative ways to recover heat.
A new regional hospital project used the principle of lowering the heating-water temperatures to allow for innovative ways to recover heat. This 250,000-sq-ft regional hospital in Texas’ Dallas-Fort Worth metro area provides a wide range of medical services to the community.
Early in the design process, the use of heat-recovery chillers was evaluated. Using a contractor partnership, preliminary pricing estimates were made comparing a low-temperature hot-water system with a typical heating-water system. Options that were considered included increasing all of the variable volume terminals by one size, but keeping the original size of inlet connection to maintain accuracy of airflow and provide the normal turndown ratio; an increase in distribution piping due to the larger flows a low-temperature system would require; an increase in a heating-water-distribution pumping system; and the added cost of a heat-recovery chiller over a traditional chiller of equal size.
Due to the critical nature of the hospital’s functions, it was decided that neither the cooling towers nor heating-water boilers would be downsized to take advantage of the heat recovery. Instead, the exact opposite was done—the building would be able to function without any issues if the heat-recovery system was disabled.
After the analysis showed significant savings, the system was designed using two 550-ton centrifugal chillers (with one 455-ton chiller equipped with a separate heat-recovery condenser barrel capable of more than 6 million Btuh of heat at 115°F), oversized terminals with larger reheat coils, and two 4-million-Btuh high-efficiency condensing hot-water boilers. The system was configured as shown in Figure 3.
The domestic hot water was preheated with a small plate-and-frame heat exchanger. The system was configured so that the heating water return (HWR) was piped through the chiller’s heat recovery to 115°F and then to the boilers to bring the heating water to a setpoint of 125°F. After system start-up and a few months of operation, the energy bills indicated something was not working as intended, so a complete review of the system, operating conditions, setpoints, and other features was performed.
The analysis uncovered two important factors. One was that the heat-recovery chillers’ hot-water temperature capability was reduced at less-than-peak capacity. It could only produce 90°F supply hot water when operating at less than 50% capacity. The second was that the return temperature was not low enough to take advantage of the heat recovery, owing to the heating-water boilers’ 125°F heating-water supply.
Changes were made that included resetting the boiler discharge temperature setpoint based on the heat-recovery chiller’s current operating capacity of -2°F. This way, the majority of the heat would be provided by the heat-recovery chiller instead of the boilers. The other major change disabled the air-side economizers on all of the air-handling systems to keep the heat-recovery chiller loaded to provide the heating hot water.
The effect of those changes was immediate. In 1 month, electricity consumption increased by 30% due to the increased load on the heat-recovery chiller, but the gas consumption dropped to just 4% of the previous month. In fact, the gas usage remained so low that the gas meter was checked multiple times to verify that it was operating properly. Today, nearly 2 years later, the heating-water boilers are cycled on occasionally to keep them in good condition, but the system rarely requires them. This would not have been possible using noncondensing boilers. The heating hot-water temperatures could only be lowered to such an extent due to the use of condensing boilers.