Troubleshooting a Screw Chiller – Adding Up Clues and Building Confidence

When I started my last post on the cooling tower level control
issue on a current project, I had planned to use a string of posts
to discuss what we learned on the project. But, it turned out that
Michael
Ivanovich (CSE Editor in Chief)
was putting together the April
issue, which focused on campus systems, and my pending string of
posts looked like a perfect article for the issue to him. So watch
for the April issue of CSE if you want to find out more about the
cooling tower issues I mentioned or just want to learn more about
campuses and engineering on them. Meanwhile, we’ve run into several
other interesting problems on the project that I thought I would
share.

The project is approaching substantial completion, so our focus has
been to work with the contractors to get systems stable, reliable
and robust, starting with the basic utility systems like the
cooling tower and related condenser water system and the steam and
condensate system. Getting these systems up lets us turn our focus
to the chilled water system and heating water system respectively.
In turn, stable operation of these systems allows us to focus on
the air handling equipment that serves the occupied areas in the
building, either directly in the case of the constant volume
systems, or via the terminal equipment in the case of the variable
volume systems. The project is a library that houses some very,
very rare documents, and thus requires very stable and somewhat
exotic control of humidity and temperature in order to
minimize the potential for degradation over time. Here is a picture
of the project site take from the perspective of a kite mounted
camera.

Being an aviation buff, the photo really appealed to me
.  It was taken by a UC Berkeley Building Sciences
professor, who, it turns out, was one of Ryan Stroupe’s professors.
Ryan is the person I work with when I teach at the Pacific Energy Center; small world. If
you want to see some more great photos from a wonderful
perspective, you should visit
Chris’ web site

In the photo, the project site is the building in the
center with the construction trailers off to the side. The picture
appears to have been taken fairly early in the project, before the
cooling towers had been set on the roof.

Returning to our technical discussion, we have been having some
difficulty getting the chilled water system to run steady state at
the design 39°F supply water temperature. The team has made
great progress in identifying and correcting the issues identified
to date, but we are not quite there yet. This is not unusual at all
and in my experience is simply part of the normal start-up process.
You fix the big issues which then allows the smaller issues to make
themselves known. Recently, we have been having occasional chiller
trips on low saturated suction temperature. Since I was on site in
and in the chiller room yesterday and one of the machines was
running at part load, I thought I would watch it for a while and
see what I could learn. To date, most of my experience with
chillers has been with centrifugal compressors, reciprocating
compressors, and absorption machines, so I was curious to spend a
little time watching how the screw compressors on these chillers
worked.

My first challenge was figuring out what I was looking at. This
is often the case out in the field; you find yourself confronted
with a machine that you are not familiar with but at the same time,
you need to understand what it is doing. For me, the trick is to
stay calm and remember that the machine is likely based on the same
principles as any other machine of its type and draw on that
experience. In this case, I was dealing with water cooled chiller
with a vapor compression type refrigeration cycle. The biggest
difference in my case was that the compressor on this machine was a
rotary screw compressor instead of the centrifugal compressor or
reciprocating compressor that I was more familiar with. (If you are
not familiar with screw compressors,
ESource has a good brochure
on their web site with some
diagrams of how they work. Even more information can be found on
one of the ASHRAE Equipment
handbooks
. For instance, Chapter 34 of the 2000 Equipment and
Systems Handbook has several pages of detailed information).

To understand what I was looking at, I used my general knowledge
of the refrigeration cycle and a bit of engineering logic. I’ve
inserted a picture of the actual machine I was looking at along
with a diagram of its refrigerant circuit that I subsequently
retrieved from the O&M manual to allow you to follow the
thought process I used to figure things out.

Standing there looking at the chiller, what I knew for sure was
that the compressor (circle A) was sitting on top. I was pretty
sure of that because it had the power connections to it and it, if
nothing else, it was the place where the operating sound was coming
from. I was able to deduce which tube bundle was the condenser
(circle B) and the evaporator (circle C) simply by looking at the
piping connections (which I assumed were correct; usually a good
assumption, but I have seen coils piped back wards and to the wrong
source, never a chiller though).

Having established a frame of reference, I then deduced that the
suction connection to the compressor would have to be a fairly
large line from the evaporator, which is the blue line in the
pictures (in reality, it is mostly behind the big cylindrical
muffler in the hot gas discharge line highlighted in red).

In a typical vapor compression cycle, the refrigerant moves from
the compressor to the condenser, so I looked for a large line
between the two (the red line in the pictures). Temperature would
have also been a clue.

Finally, the liquid refrigerant has to make its way back to the
evaporator via some sort of expansion device. I deduced that the
line highlighted in green was the liquid line because it had a
sight glass (red arrow), a filter/dryer, and went between the
condenser and evaporator. At first, I was not sure what the large
brass fitting above the sight glass was, but then I realized it was
the
expansion valve, probably an electronic one
given that the
machine is a current technology chiller and the fact that it had
some wiring connected to it.

Having gained an understanding of the circuit, I started to
watch the sight glass and noticed two things. One was that the

moisture indicator
 in it was green, which is a good sign
since water and refrigerant are not a good combination in the same
pipe. We had been watching the indicator on this particular circuit
a bit more than the others because one of the oil lines had been
damaged during construction. If the damage had been severe enough
to cause us to loose the holding charge in the machine, then
atmospheric moisture could have found its way into the system and
caused some corrosion. All evidence suggests that this has not
happened and that the contractor took care of the problem
immediately when they became aware of it. But a little
precautionary monitoring is probably in order just to be sure.

The other thing I noticed was that the sight glass seemed to
have a lot of bubbles – running white with foam at some times
-especially if the machine was loaded up. Below is a very low res
picture of what it looked like. It’s from a little video I took
with my camera to share with the contractor and chiller technician
to see what they thought. The video is more informative because you
can see the motion of the bubbles, which just look like a white
cloud here.

The bubbles caught my attention for a number of reasons. For one
thing, on most refrigeration systems, the sight glass should be
pretty clear most of the time, like looking through crystal clear
water. Bubbles, if they appear at all, will usually only persist
for a moment or two after a transient condition like a start-up.
The fact that I was seeing bubbles continuously and that they got
worse as the machine loaded made me wonder if the machine was low
on refrigerant. In the past, low refrigerant charge had correlated
with low suction pressure shut downs and bubbles in the sight
glass. But, given my lack of experience with screw chillers, I
wasn’t sure, so I looked for some more evidence.

One of the neat things about current technology electronics is
the amount of information they can put out our fingertips about the
machinery they serve.  The chiller I was working on was no
exception. It included a hand held interface that had all sorts of
useful operating data. The picture below is of one of menu
selections that covers operating temperatures.

Note that the SH.A temperature (which is the A circuit superheat
pointed to by the red arrow) shows up as a differential of over
32°F. A little bit of superheat in a refrigeration circuit is a
good thing because it protects the compressor from slugs of liquid.
But a lot of it means that the system is not making very good use
of the heat transfer surface it has available to it. (If you are
not familiar with superheat, I’ll put up a post in a day or so that
will give a familiar example in an effort to explain it, so tune in
then.)

At first, I thought what I was looking at was a temperature, not
a differential temperature, but then I noticed the
little delta symbol. Plus, if it really was superheat, it would be
physically impossible for it to be lower than the saturated suction
temperature, which shows up as 36.7°F in the picture (SST.A and
the blue arrow). So, to me, the high superheat collaborated my low
charge theory; if there was not enough charge in the system for the
load it was designed for – which would be reflected by the physical
size of the evaporator and the heat transfer surface it contained –
then the refrigerant that was available would be evaporated with
out using all of the heat transfer surface and the unused heat
transfer surface would superheat the resulting vapor more than
necessary to protect the compressor.

When I got back to the construction trailer, I took a look in
the O&M manual. The word search feature associated with .pdf
files allowed me to quickly peruse the electronic document on my
laptop (two more cool benefits of the electronic age) and discover
some more information that made me think my observation were worthy
of at least bringing up for consideration by the manufacturer. On
page 55, I discovered that while bubbles in the sight glass are not
an absolute indication of low charge, if the machine is fully
loaded and you see them, then it’s a distinct
possibility.

I had not run the procedure discussed in the manual to verify
the machine was fully loaded, but I was pretty sure it was because
the 2nd compressor kept cycling on and off as I watched things
happen. I interpreted that to mean that the load was at a point
where the smaller lead compressor was on the edge of not being able
to handle it and the controls were brining on more capacity on
occasion as a result.

On page 69, I found a troubleshooting table, which indicated
that if the compressor was cycling off on low saturated suction
temperature (which was the problem we were having), then one
possibility could be that the machine was low on charge.

On the plane home last night, with my confidence rising, I
decided to compose an e-mail to the contractor noting my
observations and suggesting that it may be worth having the vendor
take a look. I’ve already heard back from him and they’re on it
(hows that for responsive!).  So, we will soon
see if my suspicions were right or wrong;  stay tuned and I’ll
let you know. In the mean time, I hope my sharing the details of my
analysis helps you see how you can build off the knowledge you
have, trust fundamental physics, leverage modern technology, and
gain some insight into something you have never had first hand
experience with before.

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3 Responses to Troubleshooting a Screw Chiller – Adding Up Clues and Building Confidence

  1. hvac #5 says:

    havung same issue with scrolls unless the scrool is worn out since scrolls do drop i preformance ?

  2. Sreekanth says:

    What are the reasons for screw chiller air cooled one tripping at high amps?
    please send the answer to sreekanthrec@gmail.com

  3. khan says:

    photo, required the project site

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