Let’s keep going with the topic of refrigeration since you may have to work on refrigeration equipment on the job or maybe you want to do some repairs at home. For this post we are going to try and provide some answers for yet another one of those questions that they try to trip you up with (which means stuff like this should be standard knowledge for us.) The question is usually “draw and explain the basic 4 step refrigeration cycle.”
You might get asked this during a facility maintenance job interview, a qualification board, or by your spouse when the house is 105F mid-summer.
The basic drawing was created by me, Steven Van Gogh using Microsoft Paint as the medium. If your boss asks you to sketch the basic refrigeration cycle, this is what you should draw. Learn it, copy it, love it.
Basic 4 Step Refrigeration Cycle Drawing
What you should try to take away
Look at the drawing often as you read, try to understand what happens to the refrigerant as it passes through each part of the cycle. Try to really think about:
- why the refrigerant changes from liquid to vapor.
- why it changes from vapor to liquid.
- what happens in terms of heat because of these changes of state.
Remember the last refrigeration post where we talked about those 2 crazy dudes and their laws? Boyle & Charles? Specifically recall the relationships between pressure, temperature, and volume. That is why we went over it first. To refresh your brain, click here. (And, and, this is also why we went over heat and energy and such a couple weeks ago!)
And this is yet another reason why engineering is cool. Somebody wanted to cool something. Somehow they connected these laws to exploiting the properties of a refrigerant gas. They were able to figure out that – okay, this refrigerant behaves way differently than water. Look, it is already boiling at room temperature! They then figured out how to create a pressure drop and the rest is magic. And below.
Explanation of what is going on
We is a gonna track the refrigerant through this entire cycle, beginning with when the compressor discharges the high pressure refrigerant vapor over those sweet, delicious, and beautiful copper condenser tubes.
This type of refrigeration system has two pressure sides. The low pressure side extends from the metering device and including the intake side of the compressor cylinders. The high pressure side extends from the discharge of the compressor to the other side of the metering device. That is what that diagonal line denotes.
The high pressure vapor is discharged (picture a leaf blower in your head blowing stuff down the walk) from the compressor into the condenser. Here the refrigerant condenses, giving up its superheat (sensible heat) and its latent heat of condensation. The refrigerant, still at high pressure, is now a liquid again.
This whole thing just keeps happening over and over again – nothing “tires” or “wears” out. Stuff does mechanically but not the energy conversions. Liquid, to gas, to vapor, to liquid back and forth.
From the condenser, the refrigerant flows into a receiver. I didn’t include this on my Paint masterpiece because often you can’t even see the receiver. It serves as collection and storage for the liquid refrigerant in the system.
From the condenser (receiver,) the refrigerant goes to the metering device and the cycle “begins.” Liquid refrigerant enters the metering device that separates the high side of the system and the low side of the system. This valve regulates the amount of refrigerant that enters the evaporator.
Because of the pressure differential as the refrigerant passes through the metering device, some of the refrigerant flashes to a vapor. See? See?
From the metering device, the refrigerant passes into the evaporator (or cooling coil.) The boiling point of the refrigerant under the low pressure in the evaporator is lower than the temperature of the space in which it is picking up heat from (in buildings this would be where the chilled water system ties in.) As the liquid boils and vaporizes, it picks up latent heat of vaporization from whatever it is absorbing heat from.
The refrigerant continues to absorb latent heat of vaporization until all the liquid has been vaporized. By the time the refrigerant exits the evaporator, it has not only absorbed this latent heat of vaporization. It has also picked up some additional heat. The vapor has become superheated.
The refrigerant leaves the evaporator as a low pressure superheated vapor. The remainder of this cycle is used to dispose of this heat and convert the refrigerant back into a liquid state so that it can again vaporize in the evaporator and absorb the heat again.
The low pressure superheated vapor is drawn out of the evaporator by the compressor, which also keeps the refrigerant circulating through the system. In the compressor cylinders, the refrigerant is compressed from a low pressure, low temperature vapor to a high pressure vapor, and its temperature increases.
That’s it. Then it all starts over.
Basic Refrigeration Cycle Notes
Compressors are basically like pumps for gases. They no likey the fluids. This is why it is crucial that all of the refrigerant has flashed to vapor before it gets into the compressor. Liquid inside a compressor head can do some damage.
The metering device is kind of where the magic happens. This is what controls the flow of refrigerant basically. If your room or space is too hot and you adjust the room temperature – if the air conditioning unit needs to, the metering device will open up a bit more and allow more refrigerant to join in the fun. There are a variety of metering devices in use in refrigeation cycles. Some use orifice plates which is just a huge, flat plate with a bunch of holes in it, others use a valve that opens and closes solely based upon space temperature.
The condenser may be air or watercooled. Sometimes for a home unit this is the aluminum part on the outside. Usually a fan is installed as well that blows air over your condenser coil, helping give up some of the heat. For a building, this is where the condenser water system and cooling towers tie in.
The cooling towers are what you see in movies where steam is coming off of a roof from a fan. Or those nuclear towers you may see driving down the highway – those big towers you see putting off steam are nuclear power plant cooling towers
Alright. We covered the basic 4 step refrigeration cycle in this post. Ultra important for building engineers, maintenance and facility personnel, to have this down and understood. Knowing this cycle for homeowners or DIYer’s is cool because you kind of have a way better grasp of what is going on with your own units. More on refrigeration later I’m sure.