ASHRAE Journal - January 2015 - 50

COLUMN ENGINEER'S NOTEBOOK

The third strategic factor, diversity assumptions, is
more problematic. It includes both the spatial and
temporal aspects of diversity. Diversity factors, in general reduce the magnitude of overall loads, because
they establish the extent to which peak load component values are not applicable over the entire extent
of the building operation. As an example, in a theater,
both the hall and the lobby can have a certain maximum occupant density, but, they almost certainly
will not have maximum occupancy simultaneously.
Similarly, certain areas of an office building may have
equipment power densities as high as 3 or 4 W/ft2 (0.28
to 0.37 W/m2), but almost certainly, the entire building
will not.
Determination of these diversity factors is an exercise that should involve the architect, engineer and
owner, to avoid future disagreement. Diversity factors
are independent of schedules and as such must be
reviewed with the schedules to ensure that the appropriate level of fluctuation is accounted for only once
(especially when the schedule is a percent of load type
of schedule). While agreed-upon schedules capture
known temporal variation of load components, diversity factors capture the uncertain variance of these
components. Diversity assumptions, like safety factors,
should be applied to the actual parameters that are
diversely allocated rather than any value resultant to a
subsequent calculation.
Diversity factors may also be applied in sequence
as the fraction of the building area to which they are
applied becomes greater, because the likelihood that
all served areas will be operating at peak intensity
becomes less as the area grows larger. From a systems
standpoint, this approach may mean that no diversity factor for plug loads is applied for single terminal units, while a moderate diversity factor (90%) is
applied to sizing trunk ducts, a 70% plug load diversity
factor is applied for serving central air-handling units
and a 50% factor is used for sizing the chiller plant.
The fourth strategic factor, redundancy factor,
reflects the need to upsize components or distribution
systems to accommodate continued operation during
a planned or unplanned component outage. A typical application of a redundancy factor is a design that
meets the heating load requirement with two boilers
each sized at 75% of the calculated heating load. Even
if one of the boilers fails, the building will remain
50

ASHRAE JOURNAL

ashrae.org

JAN UARY 2015

comfortable throughout most weather conditions
and will be, at least, minimally habitable in the most
extreme conditions. Redundancy factors almost always
involve meeting capacity requirements with more than
one piece of equipment. If the capacity requirement
is met by a large number of units, as is often the case
with a modular boiler plant, a prudent redundancy
requirement may be met without upsizing the plant to
any extent or affecting operating efficiency. Meeting
the load with more, smaller units, furthermore, may
increase part-load operating efficiency. Once again,
this factor is determined in concert with the entire
project team, including the owner.
The pervasive claim is that oversizing results in significant loss in energy efficiency, which is demonstrably not always the case. Oversized ducts and pipes have
significantly reduced pressure drop at part loads and
potentially entail less transport energy. The key is to
ensure that systems can operate effectively and efficiently at partial capacity at minimum expected loads.
Integral to this strategy is estimating the minimum
load at which components can be expected to operate.
Components and distribution systems should be investigated for their minimum part-load limits and design
revisions, if necessary, should be made to facilitate
meeting the minimum part loads. Detailed investigation of part-load operations strategies are beyond the
scope of this article, but some general strategies are
presented.
* The first part-load strategy is usually available in
most systems designed currently through the ubiquitous use of variable frequency drives. The minimum
part load of components that use VFDs is much lower
than that with other part load operating systems. Minimum speed for VFDs serving "VFD-ready" motors can
be as low as 10%.
* Another part-load strategy is to provide capacity
with multiple units instead of a single unit. This strategy is consistent with the redundancy strategy described
above. The capacity can be provided with units of various sizes, so that the minimum system part load will be
the minimum part load of the smallest component.
* A third strategy is to make sure that sensors and
control elements are of sufficient quality and accuracy
to operate at low part loads. Even moderate oversizing of pressure-independent VAV terminals can result
in overcooled space, or excessive use of reheat, if flow



ASHRAE Journal - January 2015

Table of Contents for the Digital Edition of ASHRAE Journal - January 2015

Contents
ASHRAE Journal - January 2015 - BB1
ASHRAE Journal - January 2015 - BB2
ASHRAE Journal - January 2015 - Cover1
ASHRAE Journal - January 2015 - Cover2
ASHRAE Journal - January 2015 - 1
ASHRAE Journal - January 2015 - 2
ASHRAE Journal - January 2015 - Contents
ASHRAE Journal - January 2015 - 4
ASHRAE Journal - January 2015 - 5
ASHRAE Journal - January 2015 - 6
ASHRAE Journal - January 2015 - 7
ASHRAE Journal - January 2015 - 8
ASHRAE Journal - January 2015 - 9
ASHRAE Journal - January 2015 - 10
ASHRAE Journal - January 2015 - 11
ASHRAE Journal - January 2015 - 12
ASHRAE Journal - January 2015 - 13
ASHRAE Journal - January 2015 - 14
ASHRAE Journal - January 2015 - 15
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ASHRAE Journal - January 2015 - 19
ASHRAE Journal - January 2015 - 20
ASHRAE Journal - January 2015 - 21
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ASHRAE Journal - January 2015 - 38
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ASHRAE Journal - January 2015 - Cover3
ASHRAE Journal - January 2015 - Cover4
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