The life & times of an HVAC Engineer











I came to the realisation recently that engineers are very good at thinking everyone knows what they’re talking about…and I’m sure that criticism can be applied to me too. For all I write about what I’m thinking and doing as an engineer I don’t always remember that most people outside the process industry have never seen the innards of a process facility, and people outside of engineering generally haven’t had the chance to stick their head into any ductwork. So, for many folk their only experience of what an air-conditioning system looks like on the inside comes from Bruce Willis in Die Hard, Tom Cruise in Mission Impossible, Milla Jovovich in Resident Evil or even Homer & Bart escaping from Willy after stealing grease in the Simpsons. Now those scenes are not entirely accurate, though there was an escape from Alcatraz that utilised the ventilation shafts, but they can still be very helpful in explaining a few fundamental bits of ductwork design. So, without further ado, let me begin the Die Hard School of Ductwork Design:

From an HVAC engineer’s perspective it’s really important when designing ductwork layouts that you ensure air flows are as smooth as possible. The smoother they are, the more energy efficient and quieter the system will be…and the more likely the system is to work properly! The same goes for designing the ductwork from Bruce Willis’ perspective though. After all, whatever gets in the way of air is bound to get in the way of Mr. Willis, no matter how much of an action hero he is! So…if you were clambering around in air-conditioning ductwork, trying to escape from the bad guys, what might get in your way?

1) Corners

Right angles bad, curves good


Obvious as it may seem, it’s still worth a mention. It’s never really possible to lay all the ductwork out in straight lines with no corners, so they are a necessary evil. However, putting yourself in John McClane’s shoes (or lack thereof), how would you like the corners to be designed? Personally I think a nice gentle curve would be alot easier to get around than a sharp right angle, and from the look of this I think Mr. McClane agrees:
It’s certainly the case that airflow is alot smoother around a curve, which means it looses less pressure so less power is needed to get the air to wherever its going.

2) Joints

Internal flanges bad, smooth insides good


Anything that gets in Bruce’s way, and makes his life more difficult when navigating buildings via the ventilation will get in the way of the air. So when joining the lengths of ductwork together it’s best to put the joints on the outside. The same goes for any other obstructions in the duct work – if Mr Willis would have to put in extra effort to squeeze through then so will the air.

3) Access Hatches

Obstacles bad, access good


When trying to sneak up behind the bad guy through cunning use of ductwork the last thing you want is to be stopped by some impassable obstacle. So to make it possible for Bruce Willis/John McClane to out manoeuvre his enemies you should always put in an access hatch nearby. These access hatches are also rather essential for maintenance staff to keep everything in order without having to take down the duct work to access moving parts – in this instance a damper.

You can also help Bruce, Milla, Tom & Homer out by making ducts large with nice smooth inside surfaces. The less of a squeeze it is for Hollywood stars or air then the less energy it takes, and the same is true for keeping the friction low.

So if you’re ever asked to design some ductwork, bear Bruce in mind and think “What would John McClane want?”.

[Artwork created by my fiancé James Agg from my terrible sketches]

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