Article RAC

 

THE 4-WAY CYCLE INVERSION VALVE

A vast number of heat pumps have been installed everywhere since the first oil crisis of 1973. Most of these heat pumps are fitted with a 4- way cycle inversion valve used either to change the pumps to summer mode (cooling) or to defrost the external unit in winter mode (heating).

The object of this chapter is to study the operation of the 4-way cycle inversion valve (4WV) fitted to the majority of standard air-air heat pumps, as well as those defrost systems that use cycle inversion (see Refrepair Manual, page 433). This should allow effective remedial action to be taken in the face of the commonest faults.

A) Operation of the 4WV:

Let's examine the diagram of one of these valves. It is made up of a 4-way main valve body and a small 3-way solenoid valve located on the body of the main valve.

For the moment, let's just consider the main 4-way valve.

Firstly, we should observe that of the four connections of the main valve, three are placed side by side (the suction side of the compressor is always connected to the middle one) and that the fourth connection is on its own on the other side of the valve (the compressor discharge is always connected there).

The discharge (point 1) and suction (point 2) of the compressor are ALWAYS connected as shown in the diagram.

Note that the discharge connection may be offset on some models of 4WV.

Inside the main valve body the connections between the different ports are achieved using a moving slide (point 3) running between two pistons (point 4). Both pistons possess a small orifice (point 5), and both are equipped with a needle (point 6). Finally, three capillaries are connected to the main valve body, as shown in the diagram, and to the pilot solenoid valve.

Finally, three capillaries are connected to the main valve body, as shown in the diagram, and to the pilot solenoid valve.

All the components that we've just mentioned have a role in the operation of the 4WV. Therefore, if one of these components fails, it can be the cause of an operating problem that is very difficult to detect unless the operating principles of this valve are well understood.

Let's now look at how the main valve operates…

 

If there is a voltage supplied to the small pilot solenoid before the valve is installed, a distinct "clicking" can be heard. However, the slide does not move. To make the slide move inside the main valve, it is essential that the HP and LP pressures are being exerted inside the main valve.

 

Let's look at why this is…

The compressor discharge (HP) and suction (LP) are always connected as shown opposite. For the moment, we'll simulate the operation of the 3-way pilot solenoid by using two manual valves. One of these is closed (point 5), and the other open (point 6).

At the valve's centre, the HP exerts a force on both pistons, which tries to push the slide to the left (point 1), and at the same time to the right (point 2). Since these forces are identical, they cancel each other out.

Remember that there is a small orifice in each of the pistons. HP can therefore be exerted behind the left-hand piston (point 3) and it then pushes the slide to the right.

At the same time the HP also is exerted behind the right hand piston (point 4). But since valve 6 is open and the diameter of the connecting capillary much larger than the diameter of the piston orifice, the molecules of HP vapour passing through the orifice are quickly drawn into the LP. The pressure behind the right hand piston (point 4) is in equilibrium with the LP.

So with the greater force (that due to the HP) pushing to the right, the slide moves to the right and connects the compressor discharge with the left-hand pipework (point 7), and the suction side with the right hand pipework (point 8).

If the HP is now being exerted to the right of the slide (valve 6 closed) and the LP is exerted on the left (valve 5 open), the superior force is pushing to the left and the slide moves to the left.

When this happens, the slide connects the compressor discharge with the right hand pipework (point 8) and the suction side with the pipework to the left (point 7), that is, the exact reverse of situation in the previous diagram.

Remember that the pressure difference between HP and LP causes the slide to move.

Of course, it would not be practical to use two manual valves to reverse the operating cycle. That's why we're going to study in a little more depth how the 3-way pilot solenoid valve responsible for automating the cycle inversion operates.