The life & times of an HVAC Engineer

{June 13, 2011}   True colours are beautiful like a rainbow

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


pete says:

Interesting post 🙂

2 questions:
Why do you need to have the two rooms at different pressures, when you have the extraction vent inbetween?

And would it work if you ran it in reverse, i.e. have the vent as a source instead of a sink?

geekchloe says:

Thanks Pete!

The reason we have both is because they are there for different reasons and they are provided by different methods:
– The low pressure room is a ‘dirty’ room full of waste from the process, this room is at a low pressure so that air can never get out of this room except via the filtered extract system as it would contaminate all the other rooms if it did. This could only happen if all the AHUs stopped working – we have lots of systems in place to prevent this because if the pressure regime stops working (i.e. if the AHUs stop working) then the entire factory has to stop, throw away whatever it is currently making, and clean *everything*
– The extract preventing flow between these two rooms is strictly speaking only needed on rare occasions when the factory is producing pandemic vaccines, but we have it in place all the time so that air flows don’t have to be rebalanced during this manufacturing, and to provide more ‘containment’. The extract prevents airbourne live pandemic virus from going into the waste room. In the manufacturing room the operators wear breathing apparatus during pandemic manufacture but in the waste room this is not needed so long as we provide containment using the extract duct.

Notionally yes, you could do this in reverse. However, you have a larger sphere of influence, or at least a more predictable one, using extract than supply (suck rather than blow), this is especially true in this instance as the pressure difference between the rooms means that the air is travelling through the hole at quite a speed. We would need to match or better this with the blowing air, which given the nature of the lightweight but large surface area objects passing through the hole could cause quite a mess!

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