sponsored by danfoss when we look at a typical refrigeration system we have the compressor the condenser the expansion valve and the evaporator the condenser and the evaporator are both heat exchangers the condenser sits outside the property and the evaporator sits inside the room being cooled the compressor circulates a refrigerant in the pipework which cycles between all of these components the expansion valve regulates the flow of refrigerant the refrigerant is a specially designed fluid which can change between being a liquid and a gas very easily and that's because it has a very low boiling point for
example refrigerant r410a will boil at negative 48 degrees celsius or negative 55 degrees fahrenheit although this will change with pressure but if the refrigerant is inside a pipe within a room and the room is at 30 degrees celsius or 86 degrees fahrenheit then this will be more than enough to cause it to boil from a liquid into a vapor by the way if you want some refrigerant charts you can download these for free from our website i'll link to these in the video description down below so the refrigerant is entering the evaporator as a low
pressure low temperature liquid vapor mixture it flows on the inside of the pipe and at the same time a fan is moving the ambient air of the room over the outside of the pipe this causes the refrigerant to boil it will absorb the heat from the air through the pipe wall and then it evaporates as the refrigerant evaporates it carries away the heat it's similar to when we boil water the water heats up it evaporates and turns into steam and carries this heat away so the air on the outside is entering hot and it's leaving
cold the refrigerant continues to travel through the evaporator picking up more heat which boils it further until it becomes slightly super heated this basically means it's now completely gas and doesn't contain any droplets of liquid refrigerant liquid damages the compressor so we want to ensure it can't reach the compressor the compressor sucks in the refrigerant which is leaving the evaporator as a low pressure low temperature slightly superheated vapor it then squeezes this together into a much smaller space and so the pressure increases all the heat which was picked up in the evaporator is now contained
in a much smaller volume so the temperature will increase this is very important and we will see wire in just a moment but if you've ever pumped up a bicycle tire you will notice the pump becomes hotter as it's compressing the air the refrigerant is then pushed into the condenser where it enters as a high pressure high temperature superheated vapor the condenser is located outside and the outside air might be very warm on a summer's day as heat always flows from hot to cold we need to ensure that the refrigerant is hotter than the outside
air otherwise the unwanted heat will not be able to leave the refrigerant and leave our system a fan will help to blow ambient air over the condenser to remove this heat so the outside air is entering warm picking up heat and leaving hot as the heat is removed by the air this causes the refrigerant to condense back into a liquid it's a little bit like when steam hits the window on a cold day the window is cold so it condenses the steam from a vapor into a liquid and then this liquid starts to run down
the window the refrigerant therefore exits as a high pressure medium temperature saturated liquid and this heads to the expansion valve the expansion valve needs to regulate the amount of refrigerant entering the evaporator to control this superheat if it allows too much refrigerant to flow it can flood the evaporator so the refrigerant doesn't evaporate a liquid reaches the compressor and possibly destroys it but if it lets in too little refrigerant the system won't provide enough cooling and so it operates inefficiently the expansion valve regulates the refrigerant by only allowing it to pass through a small hole
this creates a large pressure difference across the valve one side is the high pressure liquid the other side is almost empty the small hole causes it to almost spray into the evaporator as a part liquid part vapor mixture the refrigerant will expand to try and fill the empty space on the other side and this will cause it to drop in pressure as well as temperature it's a little bit like spraying a deodorant can or a spray paint cam it flows through a small hole causing it to expand as a part vapor and part liquid mixture
and then we can feel the can become cooler now the original method to control this was by using a fixed orifice device and you might still find these on the back of your refrigerators the hole within this device was a fixed size so the entire system would simply turn on and off to meet the cooling demand it does work but we can't maintain a stable temperature control because the system is simply turning on and off the next evolution was the thermostatic expansion valve this is still widely used today and it senses the superheat at the
outlet through a bulb which is filled with another refrigerant this other refrigerant will expand to close the valve when the superheat is too high and it condenses to open the valve when the superheat is too low this type of valve needs to be manually calculated and adjusted by a technician it does work very well but it's prone to mistakes it also takes a long time to calculate and set up and it is also only adjusted once per service visit which is not ideal it will also not allow peak performance however it does work very well
and these will continue to be used for many years and so the latest evolution is to use electronic expansion valves these use a controller to measure the exact temperature and pressure and adjust the valve position automatically in real time to ensure optimal performance and maximum system efficiency okay that's it for this video but to continue learning about hvac and refrigeration engineering click on one of the videos on screen now and i'll catch you there for the next lesson don't forget to follow us on facebook twitter linkedin instagram as well as the engineeringmindset.com
