The life & times of an HVAC Engineer

{November 8, 2010}   Is this thing switched on?

In the facility I am currently working on, part of the process is to spray the product with a fine mist of 70% Isopropyl Alcohol, ‘IPA’. As you can imagine, that poses something of a hazard. To paint a picture of how much of a hazard, here are a couple of facts:

Lower Explosive Limit of IPA = 2%
[i.e. only 2% of the air volume needs to be IPA for it to still be flammable]

Flash Point of IPA = 12oC
[i.e. the room temperature only needs to be 12oC for the gas to vaporise & be ignitable]

Image credit: bruce7 from istockphoto

So, as it’s critical to spray the product with this hazardous substance, how do you go about making sure the operators don’t get blown up? Well there are a variety of different ways, so to name just a few;

  • Minimise the amount of spray used
  • Ensure all equipment within the hazardous zone created is safe for that environment (i.e. it is non-sparking / intrinsically safe / ATEX rated)
  • Provide extract ventilation to keep the amount of IPA in the room below the lower explosive limit

Well as a building services engineer, and thus a designer of ventilation systems the latter is the most relevant to me. So off I went & designed the ventilation to remove the IPA and protect the operators. Brilliant, Chloe saves the day…just one problem though…how do we know it’s working? And if it’s not working, how do we stop the machine from continuing to spray IPA into the room? Aah. Yes. Well…best do something about that hadn’t we.

So to make sure the machine doing the spraying knows that it’s safe to spray, we’ve included a flow sensor in the extract duct. The machine receives a signal from the sensor to say there is air flow, and then it can safely spray the product with IPA. We can all breath (an IPA free) sigh of relief. But no…what if the sensor is broken?! Okay guys…we’re getting into double jeopardy here, but as it’s for safety then the more the merrier, what do you suggest?

A couple of process engineers later and to ensure we have a double layer of protection to check the ventilation is working we are installing a sensor on the fan motor – that way we know it’s running. If the fan motor isn’t running then you know it’s not safe to spray the IPA.

I can’t help but thinking though, just because the fan motor is running doesn’t mean that there is extract ventilation…the fan or drive shaft could be broken. A little bit of me thinks that a few ribbons (perhaps that’s giving way to my girly side though) around the ventilation intake would be a visible indicator of the extract working that could never give a false signal. It would be reliant on the operators stopping the machine from spraying though, as ribbons can’t give a signal directly to the machine!

Image credit: The Seattle Times


Jemma says:

i love this 🙂

i also love the dumbing down so the likes of me can actually understand what you’re talking about 😛

i also love how pretty your *genius* idea will be 😉

ribbons FTW 😀

now if only there is a way to include some other essentials like chocolate…

hope urok and all is well?
you looked great on your hen do btw 🙂

speak soon, take care


geekchloe says:

Hi Jemma,

Thanks for reading, I’m glad I’m managing to make the technical side of things easy to understand…that’s the aim of the blog after all! Making engineering easier to understand and therefore more exciting/interesting. Or prettier, prettier is good too.

No chocolate this time around, but one of my favourite projects was one I did for Cadbury’s! Hurrah for engineering chocolate making.

I’m very well cheers, hope you’re well too. The hen do was fantastic. Now I have 3 weeks to figure out how to write an engineering blog post about my wedding…hmmm…


Richard C says:

Safety critical stuff is always tricky. You can end up allowing for so many unlikely things happening, and complexity adds cost and reduces reliability.

I used to work in a company that did safety shutdown systems for the oil, gas and chemical industries. We had to document the system was safe enough by showing we met the requirements of a standard, IEC EN 61508 So we had a 2-out-of-3 voted processor with dual redundant everything and a watchdog on top of that.

Do they use standards like that in your industry?

I like your ribbon idea as a backup: it’s simple and people can understand it.


geekchloe says:

Hi Rich,

We follow standard HAZOP (Hazard & Operability Study) procedure to identify hazards, and HSE guidance on Hazardous Area Classification (HAC). But I must confess to not being an expert, it’s usually our process/chemical engineers that get the most involved in selecting standards and ensuring the solutions meet them.

I am a big fan of simple solutions that can’t fail/break without you being able to see it though. Bring on the ribbons!



Speaking of “Is this thing turned on?” . . . A future HVAC topic you might want to address, covered in the Wall Street Journal:

geekchloe says:

Thank-you, that’s a really interesting article. The balance between providing individuals with what they want, whilst not making other individual’s uncomfortably hot/cold and keeping the heating/cooling bills down can be very difficult to manage and this article really highlights that.
Certainly food for thought for a blog post!

Edgar Valentine says:

How about metallic ribbons just inside – so still visible as well – the extract duct which has a metallic bottom surface. If ribbons touch latter the IPA system shuts down / cannot be started. Need to be v. low voltage or the sparks could negate the whole idea!

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