We were on the processing floor viewing the freezing tunnel lines when we heard a loud vibration. Nitrogen hissed from the tunnel, and fog formed in the room. Our host hurriedly called his maintenance technician to “fix it.”
This was not a unique occurrence: The company kept the scissors lift parked conveniently next to the wall. The technician climbed in and raised the lift to an access door in the false ceiling, and disappeared up into it. Moments later, the vibration stopped, as did the freezing line conveyors. Several minutes of hammering on metal followed. Eventually the tunnel exhaust system was restarted, and the fog lifted.
The problem was that the impeller blades on the tunnel's inline centrifugal fan had accumulated ice on the wheel, throwing the blower out of balance. The maintenance technician's well-rehearsed job was to quickly get to the roof, shut down the blower before any permanent damage could occur, hammer the ice accumulation off the wheel, and restart the system.
It is reasonable to expect ice accumulation on the wheels of inline fans in applications where temperatures of -250 to -500oF (-157 to -296oC) are the norm. The ice buildup can gradually reduce blower efficiency, producing a dense fog that causes the line(s) to be shut down. In some cases, an out-of-balance blower will experience a catastrophic failure.
Some plants bleed in warmer, but expensive, conditioned room air to reduce the overall temperature and lessen the occurrence of freezing. But why waste money exhausting expensive conditioned room air, shutting down production lines and misusing the valuable time of a maintenance technician when a venturi exhauster can handle this airstream without freezing?
Venturi Exhauster DesignThe blower in a venturi exhauster is outside of the exhaust airflow path and handles only ambient air, thereby eliminating the possibility of ice buildup on the impeller. The blower wheel and motor are directly connected, and the system does not contain V-belt drives to adjust or replace. All of the surfaces in contact with the exhaust airstream are stainless steel and can be caustic flushed.
During operation, ambient air is forced through a peripheral slot in the venturi, where it discharges at a high velocity and pressure. This discharge creates a low-pressure area below the venturi and causes the air (or, in this case, the air and nitrogen gas) to rush toward the low-pressure point. Ducting the tunnel to the inlet of the venturi exhausts the gas and air from the tunnel. The warmer high-velocity air from the venturi nozzle sweeps the sides of the venturi discharge stack duct, effectively preventing icing.
The duct system from the tunnel to the inlet of the venturi exhauster must be adequately insulated to prevent a temperature transfer (resulting in condensation and freezing) between the tunnel exhaust and the room. In addition, the duct system must be straight, as any bends or elbows create eddy currents that produce locations for ice buildup. Where a bend cannot be eliminated, suitable insulated access panels should be included for inspection, removal of ice and sufficient heat tracing.
With a venturi exhauster, tunnel exhaust fan freezing can be a thing of the past.