Thermal Shock-Ready Heat Pumps on Georgia's Mind

When thermal shock rendered boilers at North Springs High School inefficient and ineffective, Joseph Clements, coordinator of utilities services for Fulton County schools in Georgia, knew it was time for a change. Low-temperature water entering the gas-fired boilers had caused sooting in the combustion chamber and serious cracks in the refractories.

"The original boilers had been poorly applied," says Clements, referring to the fact that these boilers were not designed for low-temperature operation. Technically, there should have been a three-way valve redirect ringing the cooler water to prevent thermal shock on the boilers. However, by the time the school sought help, the boilers were beyond repair.

In the case of North Springs High, low return-water temperature was a product of the existing water-source heat pumps, which provide cooling in the summer and heat in the winter.

The boilers were connected to the heat pump loop, and water was drawn from the main building loop through the boiler using recirculation pumps. Heat pumps extract heat from the loop water, returning water at very low temperatures and causing thermal shock and condensing in the existing noncondensing boilers.

The goal of a retrofit was simple: employ a reliable and long-lasting replacement while avoiding expensive changes to the surrounding heating plant infrastructure.

Two high-efficiency 2-MBh boiler units were selected for the 250,000-sq.-ft. school. The stainless-steel, fire-tube design of the new boilers can withstand low return-water temperatures without degrading the heat exchanger over time.

And there were other benefits, as well. No changes or re-piping were required in the existing infrastructure. Also, since the new installation, gas usage has dropped. In fact, a comparison of energy consumption from 1998 to 2002—the year after the boilers were installed—shows that gas usage was almost cut in half during the major heating months.

With seamless 20:1 turndown, the equipment eliminates cycling losses in the two-unit installation. Offering up to 97% efficiency at part-load firing rates, the units are configured to operate at low-fire in parallel with each other.

One final attraction of the new equipment is that if one boiler is shut down, the other is able to maintain the typical heating load for the school's more than 1,600 students.