ASHRAE Journal - November 2020 - 54
ENGINEERS NOTEBOOK
COLUMN
David Sellers
What to Consider
When Designing for N +1
BY DAVID SELLERS, P.E., MEMBER ASHRAE
The September 2020 ASHRAE Journal column by Nathan Ho, "Performance-Based
Approach to Laboratory Exhaust Systems," mentioned N + 1 redundancy for the
exhaust fans. In reviewing it, I recalled some lessons I learned regarding what that
truly means during my tenure as a system owner at a silicon wafer fabrication facility
(aka, a "fab").
In my experience the definition of N + 1 is "it
depends." Generally, the relationship is an expression
of redundancy. For Nathan's system, the N + 1 requirement was expressed in the context of fan redundancy.
It was met by designing a system that required two
identically sized fans to handle the design condition
and providing a third fan with the same capacity as
the other two. If one of the operating fans failed, the
reserve fan could be started to restore full capacity
while repairs were made.
The lab was not provided with a redundant distribution system. If one of the ducts imploded, the system
would be out of service until repairs could be made,
even though there was an extra fan. There are other N
relationships that address this. For example, 2N implies
that there is a totally independent, fully redundant
distribution system. And the concept is not just about
having redundancy; it's about seamless transition to the
redundant systems upon failure.
That means it is possible to "mathematically" address
N + 1 in the design solution only to discover your system
is not as redundant as the relationship implies when it
54
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N OVEM BER 2020
comes online due to system dynamics, and perhaps due
to the details of the definition of a "failure."
Prior to joining the team at the fab, I had limited exposure to industrial process sites. N + 1 had been about
having an extra chiller in the central plant that was as
large as the largest chiller. In a health-care environment, it might have been about complying with a code
requirement to have an extra boiler and related auxiliaries to allow the facility to continue to be heated on the
design day despite the failure of a prime mover. In both
scenarios, an operator had time to facilitate the transition to the redundant equipment before things went
"amok."
Within hours of arriving at the wafer fab, I learned
there may be little or no time for such an operator intervention in an industrial process environment. Designs
needed to address failure modes in the context of process requirements, safety, equipment performance
characteristics and system dynamics. This needed to be
complemented by an automation strategy that could
David Sellers, P.E., is senior engineer at Facility Dynamics Engineering's office in
Portland, Ore.
https://www.ashrae.org/
ASHRAE Journal - November 2020
Table of Contents for the Digital Edition of ASHRAE Journal - November 2020
Contents
ASHRAE Journal - November 2020 - Intro
ASHRAE Journal - November 2020 - Cover1
ASHRAE Journal - November 2020 - Cover2
ASHRAE Journal - November 2020 - 1
ASHRAE Journal - November 2020 - Contents
ASHRAE Journal - November 2020 - 3
ASHRAE Journal - November 2020 - 4
ASHRAE Journal - November 2020 - 5
ASHRAE Journal - November 2020 - 6
ASHRAE Journal - November 2020 - 7
ASHRAE Journal - November 2020 - 8
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ASHRAE Journal - November 2020 - 10
ASHRAE Journal - November 2020 - 11
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ASHRAE Journal - November 2020 - Cover3
ASHRAE Journal - November 2020 - Cover4
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