Sciencemadness Discussion Board

fume hood airflow is cooling off hotplate

CouchHatter - 9-8-2019 at 16:09

I have a good hot plate and whenever I use it inside my fume hood, it takes FOREVER to heat up. Whenever I start the blower after my oil bath is heated up, it cools everything down. The face velocity is right around 100ft/min (measured with a Vaneometer) so decreasing it substantially isn't really an option, otherwise it defeats the entire point of a hood.

Is this a common problem? All I can find when searching is 'malfunctioning hot plate'. Should I just accept that I'll have to run it +25°-50° hotter and dial in the 'actual' temperature (like calibrating a thermometer at non-sea level by boiling water) than I would normally, or is there something in my setup that I can change? My hotplate is on a scissor lift about 5 inches off the bottom of the hood.

NZniceguy - 9-8-2019 at 18:54

Maybe put a shroud around the hotplate?

draculic acid69 - 9-8-2019 at 21:08

Definitely try the shroud idea.its the simplest fixed

Ubya - 9-8-2019 at 21:48

100ft/min is around 50cm/s, if i remember correctly tge minimum was 10cm/s and a good value was 30cm/s, so maybe you could dial down a bit your hood blower.
at uni i always worked under hood, and i don't remember to have ever had a problem with a hotplate being cooled too much by the air flow

CouchHatter - 10-8-2019 at 09:57

Valuable information, all of the above. I guess I'll have to retrofit a belt pulley system for my motor to slow it down. It's a 4-speed motor on speed 1. I'll try the shroud first, but was planning a pulley anyways for explosion-proofing the blower.

DavidJR - 10-8-2019 at 10:37

Face velocity is a terrible performance metric for fume hoods. It has very little correlation to the ability of a hood to actually contain internally-generated fumes. In fact, increased turbulence resulting from too high a face velocity very often results in much poorer fume containment than would be achieved with the same hood if the flow rate was decreased.

Dr.Bob - 11-8-2019 at 16:52

I have seen the same issue when running reactions without good temperature control, when the hood is one full, the reaction stay at a one temp, but if it is on standby mode (very low flow), the temp can climb quite higher. That can actually boil reactions dry at night when the hood go to low mode.

Tsjerk - 12-8-2019 at 09:14

I use aluminium foil for high boilers, works great.