Category: News

While flying on a recent trip, I got to wondering about how the air in the cabin was conditioned.  I assumed it had to be either warmed or cooled in some way and fresh outside air would need to be introduced at some point, but how is the fact that the cabin is pressurized affect all that?  My curiosity led me to do some research and here is what I found.

Similar to commercial HVAC systems, the conditioned air in the cabin is a mixture of recirculated and fresh air.  A normal commercial system is usually designed with about 10-15% of the air coming from outside while an airliner’s system is around 50% outside air.  Commercial building systems are usually limited by the humidity that they have to remove from the air (called the latent load).  This isn’t really a factor for airplane systems as there is very little humidity at high elevations.  In fact, the relative humidity in an airliner cabin while flying can be around 12% which is less than ideal to say the least.  Also, the occupant density of an airplane is a bit higher than your normal building so it makes sense that you would want more fresh air anyway.

The fresh air doesn’t just come through a grate on the side of the plane, though.  It is bled off of from the compressor sections of the engines.  These compressors have pressurized the air and, as a byproduct of the compression, made it very hot.  You may recall from a previous article on humidity, that by heating the air any measure of relative humidity that was present is now even lower.  The hot dry air now goes through the air conditioner’s evaporator and is cooled down to a comfortable temperature.  These air conditioner units are known as “packs” in the aviation world.  From there the air is mixed with recirculated air (which came through a filter from the cabin, just like in a building system) and blown into the cabin.  Excess air, which has to get out somehow or else the cabin pressure would continue to increase, is let out through a device called the pressurization outflow valve.

On the refrigeration side of the system, the refrigerant goes through the refrigeration cycle just like it would in a system on the ground.  The evaporator absorbs the energy from the air into the refrigerant turning it into a vapor.  This produces the needed cooling effect on the air.  It then goes through a compressor (powered by the engines) raising its temperature and pressure.  Next it’s piped to the condenser that has air blowing over it which cools the refrigerant back into a liquid.  The last step is for the liquid pressure to be lowered by using a thermostatic expansion valve which gets the liquid ready to evaporate in the evaporator.

When sitting on the ground with the engines off, the airplane will simply get connected to a mobile air conditioning unit or connected to a central system in the terminal.
While a bit different from your everyday home or business air conditioner, you can see that the components and basic operation is very similar.  One big difference is if your air conditioner stops working at your home or office you can usually just open a window, but I can’t recommend trying that on a plane.

At Forward Engineers, we seek to not only be a design and consulting firm but to also educate our clients about engineering technology. While doing so, we inevitably refresh our own knowledge and sometimes even learn something ourselves. If you are seeking to work with an engineering firm that is client-centered and strives to provides services that are on time, on budget and exceed expectations, please contact us. We would love to work with you on your next project!

As I mentioned in my last newsletter, the 179d federal tax deduction for energy efficient buildings expired on January 1 of this year.  Any project completed before this date can still be certified for a deduction on your 2016 taxes.  Congress continues to discuss extending the tax deduction, but little formal action has been taken.  All of the current extension bills are awaiting action in various committees.  Stay tuned to our website for any updates on the extension efforts.

As you may know, we regularly post links to our articles on our Facebook and LinkedIn pages.  This month we added a Twitter account which you can follow to get updates on new posts.  Here’s what we were talking about this month:

1. Variable Speed Fans on HVAC Units – One of the most common recommendations we make to clients is to consider installed variable speed fans on their HVAC equipment.
2. Why not Take the Train? – A friend asked me how much coal it takes to get to Maine. I’m not that kind of engineer but I took on the challenge.
3. Humidity and Healthy Air – You may already know the effects of humidity on comfort, but did you know it can also influence your health?
4. Cooling off with Sunscreen – Ever notice how a spray can is cold after use? You just experienced a principle of refrigeration.

Be sure to check our website regularly for updates or follow us on Facebook, LinkedIn or Twitter.  We wish you the best this month and if you ever have need of any of our services, please don’t hesitate to contact us.   Have a great day!

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Have you ever used a can of spray suncreen and noticed how the can will get colder as you let the sunscreen out?  If you missed that, perhaps you’ve noticed how cold the sunscreen can feel once it hits your skin?  Both allow you to experience first-hand one of the principles of refrigeration.

A liquefied gas aerosol spray can works by having two liquids inside.  The product you want out, called the payload, and the propellant.  The propellant is a liquid with a low boiling point meaning that it can change from a liquid to a gas at room temperatures.  When you push the button on the can, the propellant forces the product out, lowering the pressure and causing part of it to evaporate.  When the propellant evaporates, it absorbs heat from its surrounding and thus the can cools off slightly.  Similarly, some of the propellant will come out of the can with the product and evaporate almost instantly once it hits the air which cools the product off as well.

Refrigeration systems (kind of) do the same thing, but they also do a lot of other stuff to make it far more efficient.  Refrigerators use a closed system and take advantage of the relatively large amounts of energy required to change a substance from a liquid to a gas and vice versa.  Just like the propellant, refrigerants are selected based on their convenient boiling points.  As you can see in the diagram below, there are 5 main “stops” on any trip around the refrigeration cycle.

It’s always tough to know where to start, so we’ll start with number 1.  At stop 1, or the entrance to the compressor, the refrigerant is what is called a saturated vapor.  This means that every bit of the refrigerant is vapor and any additional energy added at this point would only make it a superheated vapor which is exactly what the compressors do between stops 1 and 2.  At stop 2, we exit the compressor and enter the condenser where the refrigerant starts to cool down.  In the condenser, the refrigerant changes from a superheated vapor back to a saturated vapor (stop 3), then to a mixture (part vapor and part liquid) and finally end up at 100% liquid which is also called a saturated liquid (stop 4).  From there the liquid refrigerant goes through a valve which lowers the pressure allowing the liquid to flash into a gas at a much colder temperature.  Now that we are at stop 5 and the refrigerant is cold, it starts absorbing heat from its surroundings which is what produces the cooling effect (remember that can of suncreen?).  Once it has absorbed all the energy it efficiently can, it’s back to the compressors at stop 1.

There’s a lot more that can be added to a refrigeration system to increase it’s efficiency, but that will have to wait for a future article.  At Forward Engineers, we seek to not only be a design and consulting firm but to also educate our clients about engineering technology. While doing so, we inevitably refresh our own knowledge and sometimes even learn something ourselves. If you are seeking to work with an engineering firm that is client-centered and strives to provides services that are on time, on budget and exceed expectations, please contact us. We would love to work with you on your next project!