Please note that the 4 links from the main valve, 3 we find them on one side with the center always

This post is in response to the demands of two readers John and Flavio who asked for some clarification on the operation of the 4-way valve. These valves are used mostly in the conditioning heat pumps to allow you to reverse the cycle going from summer mode (cooling) system to heat (heating). Today more and more often are used in refrigeration such as defrost system seen that this system is more efficient and do not have the need to use large electric resistances.
From the drawing it is deduced that it is composed by: A 4-way main valve and a small 3-way pilot valve which is usually gea grasso mounted on the main valve body. We start talking about the main valve 4-way:
Please note that the 4 links from the main valve, 3 we find them on one side with the center always the suction of the compressor, and the fourth on 'the other side which is always the unloading of the compressor.
The internal passages are switchable through a mobile group (position 3) of this mobile group also belong 2 pistons (position 4) Each of the two pistons is provided with an orifice (position 5), and also there is a needle valve on each of 2 pistons (position 6), in the last 3 capillaries (position 7) are welded to the main valve body and connected to the pilot valve. Now to understand this better by taking a valve connected to the circuit, and simply feeding the coil would hear only the ticking of the pilot 3-way, but in the main valve does not happen anything at this because it needs to work for strength that are present both inside l 'is the high pressure and low pressure.
The HP sends, and the suction of the compressor are always connected as above represented now by way of example to represent better the operation will replace the 3-way pilot valve with manual taps 2 (reference 5-6) Now to the actual operation and right: At the center of the valve HP exerts a force on the second piston that drives the group to the left (Reference 1) and at the same time to the right (Reference 2) and what makes this group ?? Since these two forces cancel each identical. Recall, however, 'that the two pistons have an orifice, so the HP can pass behind the piston to the left (reference 3), and then there will be a push of the group to the right. Simultaneously, the HP also passes behind the piston gea grasso to the right (reference 4), but since the valve 6 is open and the diameter of the capillary tube is much larger than the diameter of the orifice on the piston, this means that the mass of gas that HP through this orifice is immediately sucked by BP. The pressure behind the piston to the right (reference 4) stabilizes with BP. At the end the greatest force due to HP pushes the group to the right, placing in communication with the compressor discharge pipe of the left (reference 7) and the suction pipe with the right (reference 8).
Now applying gea grasso the HP on the right (the valve 6 is closed) and BP left (valve 5 open) the dominant force pushes the group to the left which connects the outlet pipe of the right (reference 8) and the suction with the piping of the left (reference 7)
From the description it is clear that the operation of the 4-way valve is possible due to the pressure difference between the HP and BP. For this work using the pilot valve 3 way. The main street is the way in which the municipality is always connected to BP.
When the coil is not supplied via the right is closed and the path of the left is in communication with the BP, vice versa when the coil is energized via the right is put in communication with the BP and the route of the left is closed.
Not being fed the coil of the solenoid pilot his way to the left as well as the left part of the group, is put in communication with the BP (we always keep in mind that the diameter gea grasso of the orifice on the piston is much smaller than the diameter of the capillaries connecting ) Since the HP is on the right of the group the difference in pressure tends to push abruptly to the right of the main valve. Arriving at the bottom left the pin on the piston (Reference A) closes the orifice of connection of the capillary so that interrupts the passage of gas, at that point become useless. At this point the pressure goes back to equilibrate with the HP to the left of the group but since the HP is located equally to the right, the group will no longer change position.
Now feeding the coil gets in common