ASHRAE Journal - December 2019 - 58

COLUMN ENGINEER'S NOTEBOOK

example project.
* Compressor type.
Other factors such as space required, sound power,
and weight can also be factors on some projects, but
not this particular project. Minimum lift is a factor on
projects with integrated waterside economizers, but this
project included airside economizers so minimum lift
was not a factor.
In this case, the design team selected Option 1 for the
following reasons (Table 1, Page 57):
* It cost the least to purchase and install. Option 3 has
the lowest price for the chiller, but it has an open drive
so there is a secondary cost to install a cooling system in
the chiller room.
* It is likely to be the most energy efficient option
because it has the lowest NPLV, unloads well, and has a
low evaporator turndown, somewhat offset by higher
condenser water pump energy.
* It has screw compressors, which generally are less
expensive to maintain than centrifugal compressors,
especially those without magnetic bearings.

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TABLE 2 Maximum AHU sound power levels.
MAXIMUM SOUND POWER LEVEL (DB)

AH-1, 2

Frequency
Supply Air
Opening

63
90

125
88

250
96

Return Air
Opening

500 1,000 2,000 4,000
84
80
79
75
82

Example 2: Custom Air-Handling Unit Selection
Custom air-handing units (AHUs) significantly benefit
from performance-based bidding because they have so
many design options. Bid specs must include some limits,
e.g., maximum coil and filter face velocities, the minimum number of fans in fan arrays, the minimum number of variable frequency drives (VFD), maximum width,
length and height, etc. Another key element to specify is
maximum sound power by octave band at the unit return
air inlet and supply air outlet, data determined by the
project acoustical consultant based on the actual layout
of the AHUs and proximity to occupied spaces (Table 2).
AHU manufacturers can meet noise targets in many ways,
including increasing the number of fans in the array,
increasing the size of internal elements such as coils,
filters, dampers, air openings, etc., to minimize internal
pressure drop, or adding sound attenuators.
Meeting acoustic goals played a role in the AHU bid
summarized in Table 3.† The manufacturer of AHU
Option 3 probably tweaked the number of fans and
internal equipment sizes and pressure drops until the
AHU was able to meet the specified sound power limits
without requiring a supply air sound attenuator. The
result was a reasonably priced AHU with better energy
performance than the competition. Option 1 had the
lowest price but significantly worse energy performance
due primarily to higher internal component pressure
drops, including that of a supply air sound attenuator.
The project team opted to select Option 3 accordingly.

Conclusions
Selecting equipment using performance-based bids
has several advantages, including:
* Without the pressure of the conventional low-bid
process, equipment can be selected based not only on
low price, but also on features and performance that
the design team and owner believe justifies the price
premium. Energy efficiency and manufacturer's reputa† The

actual bid form includes much more information than shown
here. Rows not relevant to this article were eliminated

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ASHRAE Journal - December 2019

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