Strange problem

[Peter_1]

We encountered today a strange problem in a unit and I must admit, my son of 21 found could pinpoint as first the problem.

Problem why client called me: on a NH3 pump system, the meat-cutting room evaporator came completely in ice. They also thought that the room sometimes became to cold.
They already had closed the liquid line to that room so that the evaporator could defrost.
The client asked me to check also the thermostat if it was perhaps blocking.
There was another problem I another room with a fan that was tripping on over-current.

We went to that room and it was almost complete de-iced. So we had to believe him it was iced before.
My son started with the tripping fan, I with searching the fault – if there was one – with the icing evaporator.

So I introduced a defrost cycle in the switch-cupboard on the other side of the factory whereby the 2 SV’s (on the inlet and outlet of the evaporator) closed and the evaporator closes simultaneously.
The 2 SV’s where completely under ice (as almost all the regulating devices over there) Under all those is even a drip pan installed.
In the meantime while defrosting, I went to my son to look if everything was going fine. The tech of the slaughtery had installed a too small over-current protector so there was in fact no problem.

In the meantime the defrost cycle was finished.

I opened the cover of the Danfoss thermostat (the big industrial ones), installed 5 m high and water flew out of it. Oeps, this wasn’t normal
The contacts were completely wet. The cable of the thermostat went through the ceiling and came together with the other wires of the 2 SV’s, the fan wires and a wire for the temperature registration in a connection box on the false ceiling.

I took my screwdriver and forced the contacts – which were really wet – open and close.
I didn’t hear that specific ‘clic’ from the SV’s who had to open and close. They were right behind me, so I had to hear them.

I took my magnetic tester - with the build-in light that lights up if a magnetic field is detected - to see if the coil was energised. One was energised, the other not.
I released 2 wires from the thermostat and still the coil remained energised. . Even the auxiliary relay kept energised. When we switched off the rocker switch on the front panel, the auxiliary relay kept energised, the only way we could switch it off was tripping the braker of 16AMPS.
This was strange, isn’t it?

We concluded that the contacts in the thermostat kept hanging due to the water (which was on the contacts)
Shut off the power of that room and installed a new Ranco thermostat.

Energised again and problem wasn’t gone at all.
Released again 2 wires and the coil remained still energised.

We went back to the switch-cupboard to look deeper to the drawings. You have to know that this plant was installed in 1983 and that many guys already changed very much in this cupboard without changing the drawings. We thought that there has to be somewhere a relay which took over the power.
So we went on the false ceiling which took almost 1 hour to find the exact location of the junction box. We even had to remove a plate on the roof to have access to this ceiling.
We thought that there must be somewhere a relay in it or that some wires were burned together. Opened the box which looked as new.

In the meantime, my son was searching and measuring in the switch-cupboard as he called me suddenly on his phone and said proudly “ Dad, I know where the fault is but you have to come to help me a little bit higher to measure something”

Do you know what went wrong? The answer is in the story above and in the drawing.
Remember that this NH3 plant was build in 1983 and that there were never problems before like this.


One picture is the drawing and the second one the 2 SV's as we found them completely under ice.
The one in front in the picture is the SV for th eliquid and the one behind is the PM valve with pressure regulating valve on it and the SV to shut the valve off.

The diode (rectifier in English I think) is connected to a contact of an auxiliary relay. This relay is energised when a push button switch is activated for the main lamptest.

The rocker switch has 3 positions Off, fans automatic and continuous.