Author: Kevin Jensen

One of the most interesting phenomena in fluid dynamics (at least to me) is cavitation.  In short, cavitation occurs when bubbles of liquid vapor are formed in a fast moving liquid where the pressure is temporarily reduced.  Think bubbles coming off of a propeller in the water.  Read on to see exactly how this happens and why engineers care about it.

Water is considered an incompressible fluid.  What this means is that you cannot reduce it’s volume by compression (aka density is constant).  Try to squeeze a full water bottle into a smaller shape.  You won’t be able to do it because the water won’t compress.  Now, if you can’t compress it you also can’t expand it in its liquid form.  This means that you can’t take a half full glass of water, put a vacuum pump on it and somehow get the liquid water to fill the entire glass.  You would only move the water around.  with the pressure reduced, you would eventually cause more of the liquid to become a gas but now you have less liquid than you started with.  It’s this last property, conversion of liquid to vapor when the pressure is low, that is the principle cause of cavitation.

Whenever water is travelling fast enough, usually around a propeller, impeller or a section of pipe where flow is constricted, the pressure in small regions can temporarily get below the vapor pressure.  Below this pressure the liquid can phase change to a gas.  When this happens, a cavity filled with liquid vapor is formed which appears as a bubble.  This is cavitation.  The danger of cavitation comes when the pressure returns to normal and the cavity collapses.  This can generate a shockwave capable of damaging metal over time.  Because of this, engineers design pumps and propellers to avoid cavitation so that they do not become damaged.

I already mentioned that you can often observe cavitation behind a rapidly rotating propeller.  Interestingly enough, you can see it in nature.  Look at rocks in a fast flowing stream where the water is flowing particularly fast (such as right before a waterfall) and look for bubbles.  Those bubbles are caused by natural cavitation.

Cavitation is generally not a concern for most building designs, but engineers do consider velocity in water pipes when selecting sizes in order to avoid excess noise and damaging wear and tear. 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!

I was recently flipping through a brochure published by the Arkansas Public Services Commission on lowering energy usage when I spotted a familiar recommendation about lowering refrigerator energy usage by keeping it full.  You may have heard this tip before (my wife said she heard it from her mom), but it may not be obvious how exactly it saves energy.

Refrigerators work by pumping heat from the inside to the outside which lowers the temperature inside.  Since the walls are insulated, once the inside reaches the desired temperature the refrigerator doesn’t have to work very hard to keep it cold.  Unfortunately, whenever the door is opened a lot of the cold air will rush out to be replaced by warm air from the room.  Once the door is closed, the refrigerator now has to cool all this fresh air down to the cold temperature again.

This is where the tip of keeping the refrigerator full comes into play.  By keeping your refrigerator full, the amount of air that can rush out is reduced.  This lowers the load on the refrigerator once the door is closed.  In short, there is less warm air that must be cooled off after each door opening.  One caveat to this is you want to make sure that there is still enough space between items so that air can still flow throughout the refrigerator for even cooling.

Another factor would be the cost of food you are now using to keep your refrigerator full.  You aren’t saving any money if you buy more food than you need and it ends up spoiling.  In this case, you could always use bottles of water or other containers that will take up space.

Other tips for reducing refrigeration energy usage include letting warm foods cool down before placing them in the refrigerator and keeping the seal around the doors clean (a dirty seal could let warm air in).  Also be sure that there is enough space behind the refrigerator for air to flow around the compressor and condenser.  Be sure to check your manual for the recommended minimum space.

There are a lot of simple things that can be done around a home or business to save energy.  Many of these efforts are low to no cost changes and we recommend them as the first measures to adopt when looking to save money.   Forward Engineers provides energy auditing services for any size of building where we review your current usage, inspect the building and make recommendations that will save you money by saving energy.  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!

Unless a building is quite old, it is required to be insulated.  Just about every surface (walls, windows and doors) all have insulation.  You may have not thought about it in this way, but clothes and blankets are also insulation.  So, how does insulation work?  Read on to find out!

The purpose of insulation is to slow down conductive heat transfer between two points.  In the case of a wall, the exterior may be very cold with the inside nice and warm.  The heat that is inside wants to get to the colder outside.  This is a law of nature similar to water flowing downhill or air moving from high pressure area to low pressure areas.  To slow the movement of heat, engineers can select materials that don’t transmit it as effectively.  Metal is an example of a material that would transmit the heat very quickly while air is a very inexpensive (it’s free) substance that slows down the rate of transfer.  The trick is to make the air stagnant and this is done by trapping it in a lightweight, bulky material.  Another way to put it is to say that the insulation works not because of the material but because of the air trapped in the material.

There are many different types of insulation, and they are rated using a number called the R-value.  The R-value is mathematically the inverse of the U-value which is the overall heat transfer coefficient.  For the U-value, smaller numbers conduct less heat.  Since the R-value is the inverse, the larger the R-value the better the insulation.  Typical R-values are R-13 for walls, R-30 for roofs and R-1 to R-5 for windows.

When a wall is designed, the R-value of the insulation doesn’t tell the whole story.  The total R-value of the wall must be calculated to include additional building layers or to include any thermal bridges.  Additional layers such as drywall, vapor barriers and siding can all slightly increase the R-value.  The wall framing (especially if it is metal), window frames without thermal breaks and any gaps in the insulation can all cause the overall R-value to be lower than the insulation’s R-value.  As you probably guessed, engineers have computer programs that help a lot with these calculations.

The walls, doors and roof of a building (AKA: the envelope) are a critical element in energy efficient design.  It is also a part of the building where more isn’t always better.  At Forward Engineers, we can test design options and provide a client with estimated expenses letting them make an informed decision on how much insulation to install.  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!