Could an innovative control make outdated equipment work like its modern counterparts?
Read the article in Today’s Boiler here.
New York City regulates the size and efficiency of draft fans in most large physical plants and municipal properties, but the NYC Department of Education was struggling to meet those guidelines. The department utilized old, outdated boilers that were equipped with oversized induced draft (ID) fans, which were inefficient and caused many physical plants to be noncompliant.
These kinds of ID fans were commonly used decades ago, primarily to control excess draft from tall chimneys. Typically, the fans run constantly at full speed whenever the boiler is running. This creates high draft, which can cause numerous problems with boiler operation and efficiency.
For one thing, large amounts of draft can cause the flame from a burner to become long or unstable. High draft pulls the flame through the system too fast, so it doesn’t have enough time to transfer the heat being created by the burner to the water wall in the boiler. This poor heat transfer means the boiler isn’t producing the heat that’s required, and because the ID fans are constantly running at high speed, electricity is being wasted too.
Another obstacle in this application was the fact that the equipment was so archaic. The draft dampers on these old boilers were large and sloppy, and most weren’t even supported by their original manufacturer, as those companies had folded years ago. The technology wasn’t just inefficient — it was practically obsolete.
School system administrators needed to rectify this situation to comply with regulations and increase efficiency, but the expense of a complete boiler replacement was out of the question. Even replacing the dampers wasn’t feasible because of how antiquated the design was. They required something that would solve the problem without a major overhaul or tons of new equipment; in short, they needed a solution they could afford. In the end, they found an answer — one that didn’t require a boiler or damper replacement.
How did they do it?
The district’s service provider proposed a novel idea. Working with the school system’s facilities management team, the provider came up with a control for the induced draft fans that would utilize a draft controller and draft transmitter with a variable frequency drive (VFD) that allowed for operation at variable speed. This system measures the draft pressure and uses that input to determine whether the boiler needs more or less draft. It then adjusts the ID fan speed accordingly to meet the predetermined set point. This allowed the old boilers to work similarly to modern ones, but instead of modulating the damper like it would on a modern boiler, the system modulates the ID fan.
“The variable-speed drive was put in place to slow the motor down,” said Joe Wallace, who served as a senior application engineer on the project. “That not only fixed the draft issue, but it also helped with operational costs because they aren’t running their motors at full speed anymore. We fixed the problem and saved them money on the operations side.”
Good draft control is a key component of proper boiler operation that often gets overlooked in many boiler rooms. Proper draft control directly affects burner performance, boiler efficiency, and standby heat losses. A variable-speed drive coupled with a draft control system, like the one implemented in the NYC Department of Education application, can have a direct positive impact on issues like these.
The first school selected for a trial installation was on Staten Island. Once the new control system was in place, the savings could be observed right away. Flue temperatures dropped, steam output rose, and the ID fans could run at half the speed they did before the installation — down from 60 Hz to 30 Hz in some cases. An added bonus associated with these electrical savings: The NYC Department of Education’s electrical utility incentivizes efficiency improvements. On top of the cost savings, they can earn an additional 30 cents per kilowatt hour with this incentive.
Once the successful Staten Island application was in place, the department of education decided to apply the same solution across the entire district. The schools were ranked in terms of need, with the ones with the poorest efficiency (measured in fuel usage per square foot) selected to receive the new control systems first. The initial order was for 135 units, and additional installations are expected in the coming months.
David Bohn is president and CEO of Preferred Utilities Manufacturing Corporation.