I was relaxing with a friend discussing the week we had, and I mentioned my examination of a facility with a pretty sophisticated refrigeration system that had some issues. My friend said he didn’t know anything about refrigeration and did not want to know, but being a good friend, he listened anyway. A short time later, my friend was glad we had discussed it and, as it turns out, he really did want to know.
The issue is heat. Heat likes to move from hot places to cold places and nothing can stop it. Sure, we can slow down the movement of heat with insulation, but it cannot be stopped. Refrigeration is the answer. Refrigeration is controlling the temperature of an enclosed space that requires a lower temperature than its surroundings. We need our freezers to stay cold enough so that the things inside don’t thaw. We need our refrigerators to keep our food cool too. When it’s hot outside, we like to keep the air inside cool so we can escape the heat. Okay, so we can all agree that we need refrigeration, but do those who do not design, maintain, or troubleshoot refrigeration systems need or want, to know how refrigeration works? I’ll let you decide.
Have you ever been warming up the gas grill and the tank ran out of propane? Then, when you grabbed the tank to change it, it felt cold? This happened because when the pressure of a gas decreases, it gets cold. When all of the propane in the tank was used, the pressure dropped quickly, making the tank cold. The reverse happens when you pressurize a gas. If you have ever touched the tank of your air compressor after it finished building up the desired pressure, you might have noticed it was warm. We learned a long time ago that we can use this phenomenon in many ways.
Compressing a gas is straight-forward and the simplest compressor is a piston. The trick is how to drop the pressure back to where we started. There is a device in refrigeration piping called an expansion valve. It acts like the nozzle on your garden hose when spraying with a fine mist. The high pressure stays behind the nozzle but allows the fluid to pass through to a low-pressure area.
The air inside a refrigerator is about 35 degrees F. In a refrigerator, there are pipes with cold fluid inside them. We call this fluid ‘refrigerant’. When a fan inside the refrigerator blows air across the pipes, the air gets cold. Remember, the thing that is moving is heat. Heat from the air inside the refrigerator moved into the cold refrigerant in the pipes. The refrigerant in the pipes would probably be about 30 degrees F when leaving the inside of the refrigerator. Now, refrigerators don’t use air as a refrigerant but let’s pretend we have one that does. The refrigerant pressure is a little bit higher than the air around you now. After we compress it to about ten times that amount, the refrigerant would be over 300 degrees! That hot compressed refrigerant passes through a section of piping that has fins on the outside, and a fan is blowing room air across those fins. On refrigerators, this section of piping is often on the bottom and/or back. We know that heat cannot be stopped from moving from hot to cold, and we are making it easy with the fins and fan. It’s not difficult to drop the temperature of the refrigerant over 100 degrees.
You have probably guessed already but, dropping the pressure of the refrigerant back to where we started is going to drop the temperature a lot. This happens when the refrigerant goes through the expansion valve. In fact, if the pressure drops back to where we started, the refrigerant will be below zero degrees! The refrigerant is now back in the piping inside our refrigerator. Heat is going to flow from the 35-degree refrigerator air into our refrigerant as it heats-up from below 0 degrees to 30 degrees, which is where we started.
Of course, refrigeration systems are a bit more complicated than this, and air is not a great refrigerant, but the basic principles are the same. Inside actual refrigerators, the refrigerant changes between gas and liquid, which really makes things interesting!
Now you and my friend know the basics of how refrigeration works. He found it interesting, I hope you did as well. If you are curious to learn more about this or other topics, give me a call.
- David S. Williams
About the Author
David S. Williams, P.E., CFEI is a consulting engineer with our Seattle-Tacoma Office. Mr. Williams provides consultation in the areas of mechanical component failure/fracture analysis, combustion systems, boilers, rotating power equipment, fluid flow control and waste gas handling systems, motor control, and sheet metal stamping equipment. You may contact David for your forensic engineering needs at email@example.com or (253) 345-5187.