The life & times of an HVAC Engineer

Life has been a little busy of late, I’ve moved house, changed job role (I’m currently on a secondment as a ‘Project Engineer’ on some BP projects) and had a few other things on my mind. I hadn’t realised quite how long I’d been away from the blogging though until a colleague from another office commented on how they were missing my blog. So, this one is for you Stuart.

One of the challenges on my previous project was to ensure that no air could pass from room A to room B or vice versa. Not especially difficult you might think, considering there was a wall between the rooms without any doors or windows in. What there was though was a large hole in the wall between these rooms for a conveyor belt to pass through. This was made into even more of an interesting challenge given that the objects on the conveyor belt were very lightweight, meaning that they could easily be sucked up if you had very low pressure air.

Here’s a little sketch to explain:

Don't let the air move between A & B

To make sure that no air could ever go from room B to room A we just made sure that room B was at a lower air pressure. You can see this on the sketch above, room A is at 15 Pascals and room B is at 0 Pascals.

But how do you make sure that air isn’t going from room A to room B? Especially now that you have higher pressure in room A…this means that the air is pushing to get into room A. Normally you would deal with this by having a small airlock room between the two, but here we have a conveyor belt that is constantly running so you can’t do that. What we did instead was to install an extract duct within the wall so air was being pulled into the duct from both rooms, like this:

Use an extract duct to capture air in the opening

It’s a simple idea, but one that is difficult to get right. If suction airflow is too small then you will still have air flowing between the two rooms, if it is too high then you will suck all of the lightweight items on the conveyor belt into your duct. And of course it’s not just about the speed and volume of the airflow, it’s also about the shape of the opening and what airflow patterns that makes – much like the aerodynamics of a formula one car. And, much like the aerodynamics of a formula one car we had to model our designs using ‘Computational Fluid Dynamics’ (or CFD).

Doing this modelling allows us to see what the patterns of the airflows around our design will be, as well as how fast they will be. Usually the results are displayed using colours to indicate direction, pressure, speed or temperature depending on what you’re trying to find out. As it turned out, it was a very good job we did do the modelling, because the first design gave us this result:

The direction of the arrow shows the direction the air is flowing in, and the colour shows you how fast it is. So the red arrows going straight across the middle of the picture from left to right show air flows going straight through the opening at nearly 10 metres per second! This obviously isn’t what we were trying to achieve. We tweaked the design of the opening, and the volume of the air being extracted, and ran the model again. The final version shows flows like these:

Final airflows - x cross section view

Final airflows - y cross section view














So now we can see that the design should work. The arrows/airflows come into the opening from the sides and upwards into the extract duct as we want them to. Though of course a model is only as good as the information you put into it, so we need to make sure that the construction, installation and commissioning makes the reality as close to the modelled design as possible.

With thanks to Richard Ozaki at Mentor Graphics for the modelling and the images


My husband and I are currently trying to buy our first house together, so we spent this weekend looking at houses. By the end of the weekend we’d chosen the house we’ll hopefully manage to buy and we’d discovered that we truly are construction & energy geeks.

Whilst the estate agent showed us around, pointing out the ‘delightful neutral décor’ or the ‘brand new carpets’ or occasionally, at their peril, a room that ‘would be perfect as a nursery’ James and I were ferreting around checking the energy performance and pondering what energy-efficient upgrades we could make.

Of course the compulsory energy performance certificates (EPCs) are a big help these days, they let you know all the basic details about standards of construction, insulation and heating. They also tell you a few areas you could make improvements in, for example a fairly standard EPC comment is ‘install energy efficient light bulbs’. But EPCs are intended for the general public to understand and be able to act on…they wont give you the full potential of what could be achieved.

So, with heads stuck in loft space, peering through ventilation bricks and stomping over gardens we discussed a variety of ‘new’ technologies we’d consider installing in any of the 1930-1950’s houses we were looking at. A couple of our favourites were:

– A ground source heat pump coupled with underfloor heating. This would reduce the energy bills, and by using this heat we could actually gain money from the Renewable Heat Incentive. It would also mean we wouldn’t need any radiators so we’d gain floor/wall space, and they’re something rather lovely about pottering around barefoot on a heated floor in my opinion.

– A mechanical heat recovery ventilation unit, these draw hot, humid air from areas like kitchens and bathrooms and use it to heat fresh air from outside before delivering it to other rooms like bedrooms and lounges. Most houses lose heat through bathroom fans and kitchen extract hoods, and many houses have no fresh supply except through leakage which reduces as we install better doors and windows. People need oxygen to live and generally 8-12 litres of fresh air per second is recommended to be supplied to occupied rooms in order to keep people feeling awake and comfortable. So, instead of losing the heat from the kitchen, we would be re-using that heat whilst still getting nice fresh air into the building, like this:

With so many ideas bouncing round in our heads we can hardly wait to get into a new house and start saving energy!

When I’m asked what I do, and I say “I’m a building services engineer”, people often look at me blankly – it’s not a role they’re familiar with. So I explain it in brief by saying I design air-conditioning…this usually illicits a response akin to being told they’ve got to do the washing up – bored acceptance. People are aware that air-conditioning is sometimes necessary, but they assume it’s a dull job to be designing it.

That wasn’t always the case though, air-conditioning, cooling and ventilation designers were once so respected that their names, or at least their designs, live on hundreds of years later. These are a few of their stories;

The first air-conditioning design on record is a rotary fan intended for cooling. It had 7 wheels, each with a 3m diameter, and was manually powered. This was designed by a Chinese inventor named Ding Huane…right back in the 2nd Century, yet we still know his name now. Cooling designs, such as water powered fans and fountains were then used in Chinese palace design…one particular example being the ‘Cool Hall’ of Emperor Xuan Zang’s court in 747.

Of course the Chinese were not the only culture to have incorporated cooling into their architecture. The Ancient Romans’ ran aqueduct water through the walls of some buildings and the Medieval Persians used wind catcher towers, cisterns and water towers to provide cooling in their buildings – as can be seen in the diagram:

Persian Air-Conditioning - image credit: 'Cyrus' from

Getting gradually closer to our time, one of the first working air-conditioning systems was developed by Cornelius Drebbel and was demonstrated at Westminster in the 17th century. At the time Drebbel and his inventions were considered so exciting that he was employed by King James I & given rooms at one of the palaces so that he could entertain and astonish the court! His inventions also earned him an invitation to the court of the Holy Roman Emperor Rudolph II in Prague.

Last, but by no means least, the inventor of modern air-conditioning was Willis Haviland Carrier. He designed a machine using the same kind of refrigerant circuits as every air conditioning system uses today. His contribution was just over 100 years ago (his invention was in 1902), and his name is still known and for his work he has been inducted into America’s National Inventors Hall of Fame, received an honorary doctorate and been awarded the Frank P. Brown medal.

For me, being part of an area of engineering that has been going for hundreds of years and yet still keeps pushing technology further to remain on the cutting edge, makes my job really exciting. I also love to know that my predecessors made emperors comfortable and entertained kings, that tells me that whilst it may be a hidden art these days there was a time when being a building services engineer was a truly glorious role. Perhaps I can make it a little more glorious again, and my blog is where I shall start…

{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

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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