Hey there guys, Paul here from TheEngineeringMindset.com. In this video we're
going to be learning all about the electronic expansion valve. We'll look at how they
work, where to locate them, and why you should use them. Today's video is brought
to you by our partners over at Danfoss who have
kindly sponsored this video. Danfoss is committed to
providing the very best solutions for a wide range of applications, including chillers, cold
rooms and heat pumps. These solutions include their series of high quality, electronic
expansion valves, which are approved for use
with all common refrigerants, as well as a wide range of
capacities and pressure ranges. The compact and durable design
make them easy to install while enabling significant
energy and cost savings. You can check out their entire range of electronic expansion
valves by following the link in the video description below. Electronic expansion valves are used in refrigeration systems to
precisely control the flow of refrigerant into the evaporator. You can find these on
everything, including VRF units, inverter mini splits, heat pumps, chillers, AHU coils, et cetera. Now, we've previously covered how thermal expansion valves work.
These are very common in
refrigeration systems, but they are nowhere near
as efficient or precise as an electronic expansion valve. If you haven't watched the
video on how these work, then I highly recommend you do so. Links are in the video description below. Electronic expansion
valves are an evolution on the thermal expansion valve. They are much more sophisticated and allow the refrigeration system to operate much more
accurately and efficiently. You usually see the electronic
expansion valves listed with the acronym of either EEV or an EXV. They both mean the same thing.
It just varies by manufacturer. There are many benefits to using an electronic expansion valve and I've listed some of the
main points on screen now. The focus though, is usually that by using an electronic expansion valve you're going to reduce
the energy consumption of the refrigeration system, as well as gaining much
better performance. What do electronic
expansion valves look like? There are many different designs for an electronic expansion valve and I've put a few examples on screen now so you can see how
their appearance varies. The difference in design
depends on the type of system, the refrigerant used, and the
pressure it's working with. As you can imagine, a small split AC unit isn't going to need the same valve as a high pressure industrial application. So the design is going to vary, although the basic working principle
is essentially the same.
Main parts of the
electronic expansion valve. As we just saw, there are
many different designs for electronic expansion
valves, but we're going to focus on a simplified design to help
you with your base knowledge. This design uses a permanent
magnet stepper motor. At the head of the valve, we
have the stepper motor body, which contains the copper coils. These are used to generate
an electromagnetic field, which will be used to
control the valve's position. Sitting concentrically
within the main body of the stepper motor
is a permanent magnet. This permanent magnet is affected by the electromagnetic field of the coils and the change in polarity
of the electromagnetic field will cause the permanent magnet to rotate. Attached to the permanent
magnet there's a shaft. Now, some designs will use a gear assembly between the motor and the shaft, but as we're only looking
at a simplified example, we're going to stick to
a directly coupled shaft.
On the shaft is a thread. This thread sits within a holder which is attached to the valve body. At the end of the shaft is a valve needle. Then we have the valve seat which the needle moves into and out of to close and open the valve and control the flow of refrigerant. We'll also have some
temperature and pressure sensors to take measurements of the refrigerant in order to calculate the superheat. These will be connected to the controller. The controller is then
connected to the stepper motor and will tell it how much to open or close to maintain the correct superheat. So how does an electronic
expansion valve work? If we have a look at a
typical refrigeration system you can see the main parts
being the evaporator, the compressor, the condenser,
and the expansion valve.
The evaporator collects
all the unwanted heat from the building and transfers
this into the refrigerant, which causes the refrigerant
to boil and evaporate. The compressor then sucks in
this evaporated refrigerant, which is a low pressure, low temperature and slightly superheated vapor. It then compresses this
into a smaller volume, which causes it to be a high pressure, high temperature, super-heated vapor. The refrigerant then
moves to the condenser where the unwanted heat is
pulled out of the refrigerant and rejected into the atmosphere.
This causes the refrigerant
to condense into a liquid so when it leaves the condenser it will be a high pressure, medium
temperature, saturated liquid. It then passes down into
the expansion valve. The expansion valve causes
a pressure difference between the condenser and the evaporator. It holds back the high
pressure liquid refrigerant and decides how much to let
through into the evaporator.
As the high pressure
liquid refrigerant bursts through the small gap
between the valve seat and the needle, it drops in pressure. This will result in some of
the refrigerant vaporizing and the rest will continue
through as a liquid. If you want to visualize this, it's similar to a water
bottle spray nozzle. As you pull the trigger, the high pressure liquid is
forced through a small orifice into a much lower pressure atmosphere, which causes the water to become
part liquid and part vapor. This mixture of liquid
vapor refrigerant is sprayed into the evaporator,
where it will absorb heat from the air or water
which surrounds the outside of the pipe which forms
the evaporator coil. In this case we have a fan which is blowing air
across the evaporator coil. As the refrigerant passes
through the evaporator coil it is exposed to thermal energy which will cause it to undergo
a complete phase change from a liquid to a saturated vapor still at a low pressure and temperature.
During this change, there'll be little to no temperature change
because of the latent heat and instead it will increase
in entropy and enthalpy. As thermal energy is still
being applied to the vapor towards the exit of the evaporator, there is no liquid leftover
to change into a gas form, so the refrigerant will
instead start to increase in temperature and become superheated. After the evaporator, we'll
find a pressure transducer and a temperature sensor which
constantly take measurements and sends this data to the controller. Not all electronic expansion
valves will use this method. It really depends on the
manufacturer and the system. Some simple ones will
use a single thermistor at the evaporator outlet. Some will use two thermistors, one at the inlet and
another one at the outlet and then use a temperature
differential as the superheat. But we're going to focus on the pressure and temperature method in this example, as it's a very reliable method and one you'll likely come
across in the real world. The pressure is measured and
converted by the controller using stored data for the
refrigerant being used in the system to find the
saturation temperature.
This is then compared to the
actual temperature measurement. The difference between the two
is the operating superheat. The controller then decides whether the expansion valve should open to let more refrigerant
into the evaporator or close to reduce the amount entering. Once decided, the controller
then sends electrical signals to the expansion valve stepper
motor to energize the coils and create an electromagnetic
field and vary its polarity. This forces the permanent
magnet to rotate clockwise or counter-clockwise depending
on the polarity generated. Each time a signal is sent, the permanent magnet will rotate
a fraction of a revolution to provide very accurate
adjustments to the superheat. The shaft, which is connected
to the permanent magnet, will also rotate and as it
does so the threaded section inside the holder will cause
the assembly to be pulled down.
As the assembly is pulled down, it will cause the valve
needle to reach the seat, reducing the gap between and restricting the flow of refrigerant. When the signal is sent to open the valve, the assembly will rotate
in the opposite direction and the threaded section
pulls the assembly up. This opens the valve and moves
the needle away from the seat allowing more refrigerant to flow. The controller is
constantly taking pressure and temperature measurements
and sending signals to adjust the valve position
to suit the current load. When the cooling load
increases, the refrigerant within the evaporator is
going to boil off much quicker and the suction line pressure
and temperature will increase. The expansion valve senses this and opens to allow more refrigerant in. When the cooling load decreases, the refrigerant will boil off more slowly and the suction line pressure
and temperature decreases. The expansion valve
will then begin to close to allow less refrigerant
into the evaporator and maintain the correct superheat.
Just before we go, I want
to give one last shout-out to our partners over at
Danfoss, remind you to check out their electronic expansion
valves by following the link in the video description below. Okay guys, that's it for this video. Thank you very much for watching. I hope you've enjoyed
this and it's helped you. If so, please don't forget
to like, subscribe and share. If you have any questions, leave them in the comments section below. Also, don't forget to follow us on Facebook, Twitter,
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TheEngineeringMindset.com. Once again, thanks for watching..
