Hello all,

Can you guys please tell me any optimum way to detect a low air flow from the gas turbine ventilation fan. It is an Axial fan that we are using.

Presently, what we are using is a funnel in front of the fan blades, that directs the air pressure with the help of the connected tubing to the high side of the differential pressure switch. The low side of the differential pressure switch is kept at the atmospheric pressure. But, every time, we have to adjust the funnel when we change the exhaust fan, and this is quite a tedious procedure. Also, in case of strong winds, opposite to the direction of airflow of vent fan, it trips the turbine detecting the low vent air flow.

Please help me in this regard.

 

I once saw a very ingenious use of a proximity switch and a hinged lock hasp for this very purpose (I'm presuming this is a tube-axial fan blowing into the turbine compartment).

The switch-and-hasp were mounted on a plate that was placed in than discharge air-flow. The longer portion of the hasp was allowed to swing freely, and would be pushed back by the air flowing out of the fan. The proximity switch was located such that when the lock hasp was pushed back by the air flow it would change state (from open to closed), thereby indicating air flow.

This was done to replace an unreliable differential pressure switch set-up. It was a very inexpensive set-up, made using an explosion-proof proximity switch that could be mounted in the turbine compartment and rated to withstand the heat in the compartment. The lock hasp was a general purpose lock hasp, hinged, with a short section (about 7-8 cm) that had bolt holes for mounting the hasp and the longer section (about 15-20 cm) had the slot for the lock ring to pass through.

It was very simple and easy to make. And, according to the instrument technicians on site, it was much more reliable than the differential pressure switch ever was, which would get dirt and oil vapors blown into the high-pressure side of the tubing and bugs would build nests in the
low-pressure side of the tubing.

I wish I'd taken a picture of the set-up. There was a slot in the bracket for the proximity switch to be slid back and forth in, allowing for some adjustability. I believe there was also a stop which the movable portion of the hasp would be pushed up against when air was flowing to keep it stationary in front of the proximity switch.

The proximity switch was the most expensive part of the set-up. It wasn't very large, and could be placed at the outside edge of the fan discharge so as not to disrupt the air flow very much at all.

Essentially, it's just a "sail" hanging in the fan discharge air flow, that's pushed back to a position in front of the proximity switch when air is flowing.

Hope this helps!

 

There's also another option, using a current-sensing relay on one or more of the motor leads. These relays have a discrete output that could be seriesed with the command to run the motor, so that if the current dropped below a minimum value while the motor was being commanded to run a logic signal could be generated for whatever purpose (alarm; trip after a time delay; etc.).

These relays are fairly inexpensive and pretty reliable, and don't involve any device in the turbine compartment or outside of it that can get damaged or require adjustment.

So, there are two options.