The life & times of an HVAC Engineer

One thing I have to take into account as an HVAC engineer is how noisy the air distribution is going to be. You can’t usually see or feel air, but if it’s got some force behind it you can sometimes hear it – think about the sound of the wind howling around tall buildings, the squeal you can get out of a deflating balloon, the background noise around the car on the motorway (it’s not all from the tyres & engine!) or even the beautiful noise you can get from flutes, clarinets & the like.

Well since my job is often to make rooms more comfortable, by providing them with fresh air and keeping them at a nice temperature, it would somewhat defeat the object if I then made them really noisy. So to avoid the whoosh & hum of fast moving air it’s important for me to design the ducts to be big enough to keep the velocity low, and to avoid obstructions & corners (much like in my earlier post: Die Hard School of Ductwork Design). The lower the air speed, and the smoother the ductwork, the quieter the air will be…but the ductwork will take up more space – so there is a balance to be had. I also need to make sure there are attenuators before and after the fan to make sure the noise generated by the fan isn’t passed into the rooms. On top of this if there are quiet/private meeting rooms I need to consider installing ‘cross-talk’ attenuators to stop conversations from travelling from one room to another via the ventilation. If there is noisy equipment in the plantroom (the room that contains all the boilers, air handling units & other equipment to keep the facility working) then I might also need to consider acoustic louvres to make sure air can get in, but not much sound can get out.

But where else in engineering is sound critical? There’s some relatively obvious roles like sound engineer or telecommunications engineer…but have you thought about the impact of acoustics on car design? Many automotive engineering companies now employ NVH (noise, vibration & harshness) engineers to make sure cars give good feedback and a safe, enjoyable experience to their drivers and passengers.

But what if your car makes no noise? There is a lot of debate at the moment about what noise electric vehicles ought to make. There are concerns that if cars are silent they’re a far greater risk to pedestrians, especially blind & partially sighted pedestrians, who may not be aware that the vehicle is there. Also, many drivers actively enjoy the sound of their engines, so may not wish to drive silent electric vehicle. Not to worry, Elvin from Warwick University has come to our rescue. “Who is Elvin?”, I hear you ask. Elvin is and ELectric Vehicle with Interactive Noise, and he looks like this:

image credit: Warwick University

He’s a bit of a test project to gauge opinion on the acoustics (whether artificially added, or left ‘silent’) of electric vehicles and he lives on the campus of Warwick University. If you’d like, you can go and listen to Elvin and give Warwick your feedback on what you think he should sound like. As electric vehicles are likely to be the transport of tomorrow I think this is a brilliant opportunity to be involved in a little bit of engineering design. I’ll certainly be taking part!

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]

et cetera
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