Author: Kevin Jensen

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!

While most conversations about building comfort deal with the temperature of the space, humidity plays a larger role than you may realize in building comfort and health.

Humidity is the amount of water in the air.  The total amount of water the air can hold is relative to the temperature of the air and the percent that the air is “full” of water is called the relative humidity.  For example, if we warm 58 °F air at 90% humidity up to 75 °F, the humidity will now be 50% but the actual amount of water hasn’t changed.

For comfort, the less the humidity in the air the faster sweat will evaporate off the human body which makes the air feel cooler.  If you live somewhere that has high humidity in the summer, you know how uncomfortable and hot it can feel.  Perhaps more importantly, humidity can also affect the amount of bad stuff in the air (bacteria, viruses, allergens, etc).  These pathogens survive better in humidity extremes but tend to die in mid-range humidities.  This is partly why people get the flu in winter (cold with low humidity) and in the tropics (warm with high humidity).  The image below was taken from a study published in Environmental Health Perspectives that shows how both high and low relative humidities pose a risk to air health.  The same study concludes by recommended that humidity is kept in the 40-60% range.

To control the comfort and health, engineers try to follow these guidelines and keep the humidity within the desired range.  Most residential HVAC systems do not have dedicated humidity controls, but the air conditioner will remove humidity from the air when it is running.  Some commercial systems are designed to measure the humidity level and run the system in a way that can control the humidity while not over-cooling the space.  During heating mode, the air will decrease in relative humidity when heated (like in our example above), so a humidifier is sometimes included with a system to maintain the humidity setpoint.

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!

It is not always easy to come up with a topic to write about each week.  For this article, I asked a couple friends what is a topic they would be interested in me writing about.  Amidst a few different responses one friend asked, “How much coal does it take to get from here to Maine and back?”  At first this didn’t seem to really fit what I normally write about, but since I’ve tackled heating a building with firecrackers and LED Christmas lights I decided to give it a shot.

The first job is to unpack the question.  We know that our destination is Maine, but where in Maine?  I decided to use the oh-so scientific method of slowly zooming in on Maine in Google maps to see what city was the first to show.  In this case, is was Portland.  So we are going to Portland.  Where are we coming from?  Forward Engineers is based in Rogers, AR so that’s where we are starting.  The distance between these two cities by rail is approximately 1,700 miles.  Our starting elevation is 1,360 ft and final elevation is 53 feet.  For simplicity sake, we are going to ignore the hills and mountains between the two cities (that was easy!).

What are we traveling in?  I don’t know many modes of transportation that use coal besides a steam locomotive so let’s go in that.  Let’s assume our train consists of an engine, 6 passenger cars and a caboose all weighing 800,000 pounds in total.  Once our train is up to a full speed of 70 miles per hour, it will require 1,500 horsepower from the engine to keep going.

Now lets do the energy calculation.  To travel the horizontal distance, we need to convert the horsepower requirement to Btu/hr by multiplying by 2,544.  This conversion gives us an energy usage of approximately 3,800,000 Btu/hr.  Traveling a distance of 1,700 miles at 70 miles per hour, the trip will take about 24 hours to complete.  Our total energy usage so far will be around 91,000,000 BTUs.  However, overall we are traveling downhill so we will be able to use less coal because of our potential energy.  Dropping 800,000 lbs from 1,360 feet to 53 feet saves us about 1,300,000 BTUs.  Therefore, our total required energy output is around 89,700,000 BTUs.  I’m choosing to ignore the extra energy required to get the train moving and any stops along the way.

Unfortunately, piston steam engines are not very efficient.  Towards the final years of steam train travel, the typical locomotive had an efficiency of about six percent.  This means that to produce our required 89.7 million BTUs of energy, we need to burn 1.5 billion BTUs of coal!  One ton of coal contains around 28 million BTUs of coal.  Dividing our energy requirement by the energy content of coal tells us that we would go through about 53 tons of coal on our little trip.  Now, I’m no train engineer (despite what my daughter thinks I do for a living) so I am confident that I have missed enough important factors that you would probably end up on the side of the tracks out of coal somewhere between here and Maine so don’t go using this article to plan your next trip.

The point of all this was to simply show that just about anything can be broken down by its energy use and analyzed.  Forward Engineers often does energy usage calculations for our client’s buildings letting the owner or manager know where their utility money is going and where improvements can best be made.  Most of our calculations are quite different from trains and coal usage, but we still work through the numbers, making assumptions and running the numbers at every turn.  If you have a building or project that could use some extra energy analysis, consider contacting us to analyze the energy usage and to make recommendations that not only save the building energy, but you money.