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A four-way reversing valve has four pipe connections, three of them on one side and the fourth on the opposite side, whereby the three copper connectors have a larger diameter than the one on the opposite side. The middle one of the three large-diameter connectors is permanently on the suction side, and the single small-diameter connector is permanently on the high pressure side. Since the remaining two may be on either the suction or the high pressure side – depending on how the unit is currently switched – they are made the same size as the permanent suction connection in order to take account of pressure drops. A four-way valve also possesses a solenoid pilot valve with a coil that can be powered to change the direction of flow of the refrigerant. There are small pilot connections between the small-diameter valve connection and the solenoid pilot valve and from there to the central large-diameter connector.
Pressure drops and dimensioning
Pressure drops are always important when it comes to dimensioning valve systems. An excessive pressure drop will usually have a deleterious effect on the heat pump’s energy efficiency, while too small a drop can, for example, disrupt the otherwise stable behaviour of a servo-operated solenoid valve. Neither of these considerations is so very critical for four-way valves. Minimal pressure drops on the pressure or suction side are not a problem for four-way reversing valves, because the slider mechanism that actually switches over the valve is driven by the pressure differential between the high-pressure and low-pressure sides of the refrigeration plant. This is the obvious approach here because this type of valve is located at the interface between the high and low pressures, which is not normally the case for the valves in a conventional dry-expansion heat pump that the external pressure. The actual pressure drop between, say, the refrigerant inlet and outlet on the suction side is therefore not crucial to reliable valve function. The question of “excessive pressure drop” is also barely a consideration for Danfoss-Saginomiya four-way valves provided the design is primarily geared to the dimensions of the suction line. If the four-way valve is chosen to suit the size of this pipe then you will generally get a valve with quite moderate pressure drops, but it will still be advisable to consult appropriate performance tables to check that the selected valve will be large enough. The primary criterion should remain the diameter of the pipe on the suction side, otherwise the heat pump manufacturer will have to install reducers on three connections, which would not only be extremely cost-intensive but also rather unsightly.
This type of valve is integrated into both the hot-gas and the suction line of a heat pump, whereby the two permanent connections – i.e. the pipeline that always carries the hot gas whatever the valve setting and the one that is always the suction line – are particularly easy to distinguish for the installation. The hot-gas line from the compressor goes to the narrow connector on the four-way valve while the suction line to the compressor attaches to the middle one of the wider connectors. These two pipelines between the valve and the compressor never change their function. In this context it is important to note that that the external pressure equalization of an expansion valve must always be connected to the permanent suction line, that is, the line from the middle one of the large-diameter connections on the four-way valves. If this is not done then the external pressure equalization will be subjected to far too high a pressure, which will not only prevent the expansion valve from functioning (by forcibly holding it shut) but may also permanently damage it. There remain the two outer large-diameter connections, which may sometimes be on the high and sometimes on the low-pressure side.z
If the valve is installed using standard soldering methods (copper-copper joint, brazing alloy such as SilFos 15) you should be sure to observe the following: The genuine copper connecting pieces that you will generally find on this type of valve are very suitable for soldering to copper pipes, but the excellent thermal conductivity of copper and the precise fit of the slider mechanism make it exceptionally important to minimise the heat applied to the valve during the soldering process. You should therefore always keep a cold wet cloth wrapped around the valve while it is being soldered into place. Once this hurdle has been overcome you will find the four-way valve to be a reliably functioning component for the entire life-cycle of your heat pump.
Four-way valves are components that offer an extremely efficient way of defrosting air-to-air and air-to-water heat pumps. Complete reversal of the cycle makes it possible to also use the system for cooling. The design is not at all complicated and follows from the dimensions of the suction line, with no need to worry about minimum pressure drops.