A flue gas condenser can reduce exhaust temperatures to well below 100 degrees F. Doing so creates a significant amount of condensate from the exhaust gas. The volume depends on the size of the natural gas appliance and the heat recovery process, but it can amount to as much as thousands of gallons per hour. To ensure that the water does not end up getting back inside the boiler, flue gas condenser units are installed alongside and below the original flue stack.
A neat aspect of this design is that most of the condensing action takes place where the cold water enters the heat recovery unit through the top rows of tubes. The condensate from the exhaust gas falls down from the top, washing the heat recovery surfaces of any residue. The condensate then falls into the base section where it drains away.
This recovered condensate, most of which is distilled water, is fairly clean and usable. With minimal treatment, the water can be used as boiler makeup or added to an industrial plant’s wash-down and/or used for make-up water in the cooling towers. In a commercial building, the water can be put into the sanitation water lines. If you don’t want to treat the water, you can store it in an underground container and use it for landscape irrigation. Note that sewer rates are typically 1.5 times more expensive than incoming water rates, and sewer volume is calculated based on your incoming water volume, so by using the water in the flue gas, you will reduce your water and sewer bill.
Every condensing recovery application is different due to differences in heating and cooling requirements at each facility. Some are “closed loop” (with respect to the fluid within the tubes of the heat exchanger), while others are “direct contact” designs where the exhaust gas actually touches the water that is being heated in the waste heat exchanger.
Not everyone can attain 98% energy efficiency, but it is worth reviewing your options because even a small improvement may have a good return on investment. In my opinion, condensing recovery units should be installed on most boilers as the “default” design. Our country would reduce its energy, water, and carbon expenses in a very cost-effective manner.
I would like to thank Mr. Sid Abma of Sidel Systems for contributing to this article. For more information, feel free to contact Mr. Abma at: