Lab hood control by heat sensor 1

[DouginMB]

I'm in the process of designing the ventilation systems for a lab. A couple of the lab hoods simply vent heat from heat generating equipment. Is anyone aware of a sensor I could use to operate the hoods by detecting the radiant heat from the equipment? I'm assuming I can't use a temerature sensor because the airflow through the hood would immediately cool the sensor and shut off the fan.

 

[MintJulep]

How about a current sensor in the supply to the equipment. You could turn on the fan when the equipment is operating.

 

[DouginMB]

i thought if that, but I think the equipment they use are bunson burners and plug-in hot plates, not things you can easily meter.

 

[ChasBean1]

DouginMB, check this out:

http://www.mikroninfrared.com/products/process/mi-n500.htm

 

[billygoat22]

Interlock a gas flow sensor in the gas supply line to the exhaust fans.

 

[lilliput1]

Keep it simple. If the lab OA requirements are such that you can make the hood exhaust constant volume then do so.

 

[quark]

Please pardon my ignorance. Can I know what does it mean by I'm assuming I can't use a temerature sensor because the airflow through the hood would immediately cool the sensor and shut off the fan?

In my opinion, the air should first be cooled down to cool any other thing. If your apparatus are continuously adding heat to the air stream, then there should not be any chance that your temperature sensor can be cooled off.

You have two options here.

1. You can reduce the air flow to avoid quick on/off s.
2. You can lower your temperature set point.

ASHRAE Journal December Issue(as I remember) dealt with some good techniques about controlling ventillation hoods of restaurant kitchens. It may be of help to you.

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[DouginMB]

If your apparatus are continuously adding heat to the air stream, then there should not be any chance that your temperature sensor can be cooled off.


Therein lies the rub. The heat sources they use under these hoods isn't always continuous. Also, I will guess there's 24" between the heat source and the inside top of the hood. If I put the sensor at the top of the hood, it will heat-up (say 90[°]F). The ventilation fan would energize and the air temperature around the sensor would immediately drop as 75[°]F air replaces the 90[°]F air. I imagine this cycle would continue and the fan would stop-start. If i can sense a rise in radiant heat I wouldn't have to depend on air temperature for control.

Also, these are existing hoods with fans and I can't go to a variable-speed type.

 

[dpoppeli]

A time delay relay would eliminate short cycling. This could be initiated by the temp sensor, occupancy sensor, or lighting interlock. It all starts to sound sort of overly complex however. I like the gas-valve interlock since you are acting on the primary item, not some secondary effect. Also, seems like an on/off switch would work pretty well since the room is going to heat up and when they become uncomfortable they would remember to turn it on!

 

[ChasBean1]

DouginMB, have you tried the link I posted above? It's IR - it looks like it can be aimed at a surface and measure surface temperature to give an analog output. You wouldn't have to worry about heat addition to an airstream. A rise in milliamp output could be tranlated to a go/no-go fan signal.

 

[DouginMB]

Yes ChasBean I finally had a chance to examine it and it looks exactly like what I'm looking for. Thanks.

 

[MarauderX]

Given that a typical lab hood will be drawing ~80-120 FPM across the opening plane, temperature shouldn't be the most important issue. If interested, you can change your system to react better to what is going on per detection. Contact these guys to get a good perspective and help:

http://www.phoenixcontrols.com/

No, I don't work for them, just had a good presentation by them for all of the lab work I used to do.