This is a heavy subject to write about and there isn’t a super easy answer unless you really paid attention during that physics minor from your college days.
The beginning of refrigeration starts when a liquid evaporates. The easiest way to explain how something is cooled is to look at sweat on your skin. Sweat is the body’s way of naturally cooling off your body. That sweat comes out of your pores onto your skin and evaporates. As it evaporates it cools off your skin keeping your body temperature constant and in the safety zone. The process of turning a liquid into a gas takes heat away from your skin. This process is what happens in the evaporator on your A/C, refrigerator or freezer.
Now imagine if that evaporated sweat could be recovered and turned back into a liquid to use it again to cool you off later? What turns steam into water? Something cooler than the steam. Something like the soda cup that you are drinking out of right now. The soda is colder than the surrounding air and the outside of the cup is touching that air and condensing that moisture in the air into water droplets. This process of condensing the gas into a liquid again is what happens in an A/C or refrigerator condenser.
Wait, how does that cool my refrigerator to 35°F?
This is where we turn the tables on the evaporation and condensing process. We change the pressure that the liquid and gas are in. How can we do this? We put it in an isolated system and control the pressure with magic dwarves. No. Science. Did you know that water will boil at room temperature? It certainly can but to do that it needs to be at a lower pressure. In fact that pressure is so low that it is almost an absolute vacuum. At just .393 PSI water will boil at 72°F. That is a lot lower pressure than the 14.7 PSI that we are used to living at sea level and the 212°F water boils at there. How do we do this? We do this by isolating the water in a place where we can lower the pressure almost to a vacuum. Here’s a science guy doing science things to demonstrate this scientific phenomena with science.
That’s cool and all, but how does that make my fridge cold?
Water is not a very good medium to use in an evaporative-condensing cycle. Chemists with degrees from somewhere smart sounding spent years developing liquids that evaporate and condense at temperatures that work better for a refrigerator or freezer. The well known refrigerant Freon (aka R22) was an ideal refrigerant for air conditioning, refrigerators and freezers for many years because it was engineered to provide the desired temperatures for all those applications and be surprisingly efficient at doing it.
How can one refrigerant be used for so many temperatures? A freezer is colder than my A/C, right?
In the isolated system we are able to control the pressure. Like we mentioned before with the vacuum where water will boil at room temperature, refrigerants are used at different pressures to reach different temperatures. Below is a PT chart. It shows the boiling point for multiple refrigerants at multiple pressures.
As you can see on the chart, R22 boils at -20°F at 10 PSI. If you go down a little further on the chart you’ll see that R22 boils at 40°F at 68 PSI. Your home air conditioner has an evaporator that is about 40°. As warm air passes through the evaporator coil two things happen. The first is that the air that goes through it is cooled. The second is that the 40°F refrigerant in the evaporator absorbs the heat and evaporates turning the liquid into a gas. Neat huh? This is the evaporative process that makes things cool. What about the other side of the process? The condensing part? How does it turn that gas back into a liquid?
I was hoping you wouldn’t ask and have gotten bored reading this a while back and just called us to come and fix your refrigerator. But since you are still hanging in there we can delve into the condensing part of the cycle and add some more parts into the system that we haven’t really talked about yet. The whole system is driven by the compressor. It sucks refrigerant gas from the evaporator, raises the pressure of it and pushes it into the condensing coil. Remember, pressure is the name of the game here and when we raise the pressure we also raise the boiling point as well as that temperature where it turns back into a liquid called the dew point. If it is 80°F outside today the temperature of the refrigerant in the condenser will have to be hotter than the outside air temperature to condense the refrigerant. The evaporator takes in warm air and evaporates the refrigerant into a gas. The condenser works in reverse compared to the evaporator. The condensing coil takes in air cooler than the refrigerant and cools the refrigerant changing it back to a liquid. The compressor keeps it moving through the condensing coil and on back towards the evaporator to start the process again.
The evaporator and condenser are different pressures. What are you trying to pull here?
Nothing. There is one more section of the cycle that makes this unique system work. You need something to control the pressure between the condensing side of the system and the evaporator. You need a pressure regulator. Or as they call it in refrigeration: the metering device. The metering device maintains the pressure on both sides of the system keeping the low pressure side low and the high pressure side high. Crazy! Below you’ll find a diagram of the refrigeration cycle that will make you wonder why you suffered through a thousand words of explanation.
Did I forget anything? Oh yes. I avoided talking about pressure switches, all of the different metering devices, thermostats, receivers, accumulators, liquid injection, transubstantiation, and latitudinariation. I might have made up a couple of the last ones but it is still true that refrigeration happens because of a lot of moving parts working together in a strange harmony.
If you need help with problems with your commercial refrigeration give us a call. Call 512-651-4565 in Austin and Lubbock, Texas (806) 787-4985 Or Visit our website at www.tripointrefrigeration.com
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