ASHRAE Journal - November 2020 - 58

COLUMN ENGINEER'S NOTEBOOK

Static

FIGURE 2 Recreation of the sketch I drew to help solve the fab crashes.

Design

Each Fan
Operating
Point

Where You End Up if
One of Two Fans Fail

One Fan, Full Speed
Two Fans, Reduced Speed
Flow
Where You End Up if One of One Fan Fails

* Discard or recycle product that was in the contaminated areas; and
* Monitor and document cleanroom quality to verify
that the space was back to production standards.
Bottom line: a crash cost about $10,000 when you took
lost work, lost product, laundry and cleanup supplies
into account.

Spinning Reserve
Early in my career, a newly installed chilled water-cooled MRI machine
would trip out when the lead distribution pump in the central plant I had
designed and programmed was changed. That happened because I had
not considered the dynamics of the system when I punched the lead-lag
sequence into the control system.
The sequence looked at the run time on the lead pump, and if it had reached
an even multiple of 1,000, shut that pump down and started the other pump. As a
result, there was a brief loss of flow in the distribution system. Prior to the installation of the MRI equipment, the thermal inertia of the system easily masked the
short loss of flow. But not so for the hypersensitive MRI flow switch.
The solution was to think about it like an experienced operator charged with
making the change. In doing that, I realized I should start the lag pump and ramp
it up while slowing down the lead pump. And, I should not shut down the lead
pump until the lag pump had the situation under control (a lesson passed on to
me by an experienced operator).
That process created what I now know is called "spinning reserve" and protected the system from a total loss of flow during the transition. Learning to think
like an experienced operator can be a very valuable asset when writing control
sequences.
PHOTO 2 The EPI scrubbers. The tall cylinder just right of center is the process
exhaust scrubber. One of the two process exhaust fans is visible to its right.

N + 1 and Spinning Reserve

About one month into my tenure, after a week with one
or more crashes per day, I found myself in a meeting with
about forty people in attendance, including the person
from corporate headquarters responsible for the project.
There was a lot of discussion about increasing the time
delays on the safeties that triggered a fab evacuation and
reducing the transition time required to bring on the
backup fan. All of them had implications in terms of cost
and schedule, and none of them ensured success.
As I listened to the discussion, it reminded me of a
problem (of my own making) I had experienced with
a hospital chilled water system. I solved it by creating spinning reserve (see "Spinning Reserve" sidebar).
Recalling that experience, I started to wonder what
would happen if we created spinning reserve in the
process exhaust system. Thinking it through, I made a
sketch similar to what is shown in Figure 2 and showed it
to my supervisor.
Right about then, the person from corporate headquarters, very diplomatically, asked if there might be an alternative control strategy that would address the problem.
After encouragement from my supervisor, I screwed up

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my courage and showed the people the sketch. After a bit
of discussion, it was suggested we go out and try it.
It was late in the day, and we were only running one
shift, so the risk level was low. And if it worked, once the
control algorithms were modified we would have a solution to the problem.
So off most of us went to the scrubber pad (Photo 2) and
up the ladders to the platform where the exhaust fans
and their VFDs were located. I was extremely nervous.

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ASHRAE Journal - November 2020

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