Joukowsky Effect in Positive Pressure Vent Systems
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What is the Joukowsky Effect?
The Joukowsky Effect is a low pressure phenomenon which may occur in the exhaust system of large commercial gas fired boiler installations in single or common vented systems when certain conditions occur. When the flow of flue gases is abruptly stopped, for example through a power failure, the momentum of the flue gas pulls the remaining flue gas out of the exhaust vent system. With the burner draft fan off, not enough combustion air is pushed in to replace the vanishing flue gases resulting in low pressure, close to a vacuum, in the exhaust vent system. The exact conditions that are necessary for the Joukowsky Effect to occur are not entirely understood, however evidence suggests that it mostly occurs in the exhaust vent system of large boilers operating at full fire with an abrupt shutdown of combustion air, either caused by a power failure or a BMS system overwriting the boiler’s normal operating sequence.
Why should I be concerned?
Positive pressure exhaust vent systems are designed to operate with a higher pressure inside the vent pipe than outside. When the Joukowsky Effect occurs, a near vacuum is created which results in the pressure inside of the vent pipe becoming lower than the pressure outside of the pipe. In extreme cases, the vacuum becomes strong enough that the structural integrity of the vent pipe may be compromised. This not only could result in costly damage to the exhaust vent system, but also could take the heating system offline until repairs can be made.
What can I do to protect my system?
While the exact conditions needed for the Joukowsky Effect to occur are not fully known, the physics of what happens is known. With this understanding, there are several precautions that can be used to protect the exhaust vent system. When selecting exhaust venting material, a vent material with thicker walls would be stronger and may resist the effect better. In addition, a vacuum relief damper can be installed in new exhaust vent systems, or retrofitted into existing exhaust vent systems to prevent the negative side effects of the Joukowsky Effect from occurring. This vacuum relief damper allows surrounding air to relieve the low pressure in the exhaust vent system if the Joukowsky Effect were to occur.
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