Those Chillers are Rubbish!

[Grizzly]

Ever had it said?
OK.
Now I am going to tell you a story!

I hope you are sitting comfortably?

Recently I arrive on-site to carry out what I believe is a scheduled Maintenance Visit.
On 2 Modern Chillers mounted on a large Office Block roof. These are connected to the Trane BMS via the normal remote run signals.

On arrival I notice several vehicles from the Facilities Company that "we" are contracted to.
( Normally the site is only visited by 1 of their engineers once a week.
And we are contracted to 4 visits a year.)

So I contact the Office Block representative who is responsible for any Contractor whilst they are on-site.
I am met at the door by this young lady.
Who says "Oh Good, have you come to fix the Chillers?
They have become a real pain as they are always breaking down lately.

This comes as somewhat of a surprise to me!
Because although the chillers are made by a rival company.
They are non the less very good and reliable.

I also knew from my previous visits that they are well over sized and therefore more than capable of handling any loads required of them.
In fact normally only 1 of the 2 chillers ever ran at any given time!

So I answered.
That although i was only supposed to be carrying out a "Maintenance Log".
I would be more than happy to investigate their problem further.

When i get to the rooftop Plant room.
Which houses the Boilers, Air Handling Units, Chilled Water Pumps etc.
With the 2 chillers Just outside through a pair of double doors.
I meet a Controls Engineer from the Facilities Company with his laptop connected via the Trane Outstation.

We will call him Dave ( because I cannot remember his real name.) Dave turns out to be one clued up guy and helpful with it.

So we get to talking:-

The initial diagnosis is that periodically an engineer will be called to site to investigate loss of cooling to the 3 floors of offices.

He normally would find the Chiller Flow Fault alarm illuminated on the Main Control panel within this plant room.
This control panel controls every thing other than the chillers. Which it just sends a run signal to or indicates an alarm condition.

Resulting in a manual restart of the chillers followed by a reset within the main control panel.

Apparently one-time whilst resetting the system the engineer observed the whole of the main control panel lit up like a set of Christmas Tree Lights!

With all the indication lamps switching on and off before the panel locking out on Chilled Water Flow Fault.

My first action was to interrogate the chiller alarm history. I drew a blank sure enough flow switch failure was observed. but when checked the flow switch operated as it should.
I go back to Dave, who in the meantime has discovered that Trend is programmed so that once a week.
The running Chilled Water Pump is stopped and the 2nd one starts up. To balance the running hours.... Acceptable standard practise!

Only when he triggers the change over nothing happens. So I investigate further, only to find that the 2nd pump has tripped its MCB/Overload. Not sure how their engineers missed that one? The pump metered out perfectly?

Having reset everything Dave and I agree to go for another test run of the Pump changeover.

Pump changes over!
Suddenly the panel lights start flashing with cont actors chattering, it's the Christmas tree Lights all over again. pretty scary it is too!

Whilst all this is happening I noticed that over in the far corner of the plant room.
The small chilled water pressurisation unit was switching it's pump on and off. in unison with the Main Panel Lights and Contacts.

Eureka!
This unit had 2 circuits each with a pressure gauge and it's own pressure switch to operate the "top-up" pump.
Further investigation revealed that when the r/h circuit (which has a small leak) dropped to 1bar the pump switched on.
When the pump rose above 1bar it switched off!
No differential (factory set and unable to adjust without a major strip down).

So this tiny little pressurisation unit was causing the chilled water circuit pressure to fluctuate sufficiently.
That when the system pressure was low enough the flow switch paddle would flick back-wards and for-wards.
In time with it's pump which in turn was switching the main control panel and pumps on and off!
Hence the Christmas Tree Lights.

And Everyone said " THOSE CHILLERS ARE RUBBISH!"

"Thanks to Dave"
Him and I walked out of there feeling pretty happy with ourselves that day

Grizzly