Components for heat pumps – part 5: four-way reversing valves

Sunday, 1 July 2012

Four-way reversing valves are used to completely reverse the cycle of one-to-one heat pump systems. Such valves may be used to facilitate using the system for both heating and cooling, or to provide an effective and energetically optimized defrosting method.

Fourways Danfoss

Construction

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.

For the following description we shall suppose that the small (high pressure) connection is pointing upwards, the three other connections are pointing downwards, and the small solenoid pilot valve and its coil can be seen at the front of the unit. A standard four-way valve has only two switch positions – there are no intermediate positions. In switch position 1 there is no voltage on the coil of the solenoid pilot valve. In this position, highly pressurised hot gas from the pilot line of the narrower connector enters the chamber containing the sliding mechanism from the right. At the same time the pressure at the left-hand side of the slider chamber can be relieved via the permanent suction connection, discharging the gas to the low pressure side. This displaces the slider towards the left and opens the main paths at the top, leading down to the right and from left to the centre. In switch position 2 the hot gas makes its way from the top towards the left, while suction gas is simultaneously allowed to flow downwards from the right into the centre. This is achieved by applying mains voltage to activate the coil of the solenoid pilot valve so that high pressure gas is introduced into the slider chamber from the left. This allows the pressure on the right to be relieved via the central main connection at the bottom, causing the slider to move to the right.

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.

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 minimize 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.

Application

Four-way valves are used to reverse the cycle of one-to-one heat pumps. This makes the evaporator into a condenser and the condenser into an evaporator. This possibility is also very welcome for air-to-air heat pumps that are used in summer for cooling and in transitional periods for heating. While the four-way valve will hardly be needed in summer, while the system is in cooling mode, in winter it may be necessary to reverse the cycle in order to defrost the system. Four-way valves offer an extremely efficient way of defrosting both air-to-air and air-to-water heat pumps. Reversing the cycle of a one-to-one system enables the evaporator, which has become a condenser, to be defrosted from inside. This means that there is no need for heat to be applied to the evaporator package by providing an electric heater, instead the hot gas is directed through the pipes that have become iced up. This defrosts the system extremely well and can scarcely be beaten for speed, energy consumption and targeted heating.

Conclusion

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.