ASHRAE Journal - August 2022 - 54

ASHRAE TECHNOLOGY AWARD CASE STUDIES
FIGURE 5 SHAP South secondary annual power consumption by monthly averages.
1,600,000
1,400,000
1,200,000
1,000,000
800,000
600,000
400,000
200,000
can automatically transition between mechanical and
free cooling without fear of losing the load. The chillers
provide the turn-down ability to start at less than 10% of
design load conditions. Even if the chiller were to struggle
to come online after transitioning out of free cooling
mode, the chiller immediately attempts to restart itself
and has the ability to pull down just as fast.
This project leveraged a new design process wherein
we focused on optimizing the system in the design
phase, rather than retroactively optimizing a system that
was over-engineered to meet design day load conditions.
This resulted in equipment being properly sized for the
various load conditions, while still providing additional
capacity for future loads. More importantly, this means
the equipment was selected based on effi ciency ratings
for the shoulder months and low-load conditions, and
also to operate the complete system most effi ciently during
design day.
The overall objective tasked by the customer was also
to achieve the hydronic design of the chilled water system
to realize a total plant system effi ciency of less than
0.50 kW/ton (0.14 kW/kW).
While the variable primary pumping concept was
fi rst implemented back in 1995, many modern designs
still use the primary/secondary pumping concept. The
inherent fl aw of the primary/secondary pumping concept
is low chilled water ∆T, which results in additional
chiller and pump energy being consumed. The variable
primary chilled water pumping confi guration at SHAP
South minimizes pump and chiller energy by continuously
providing exactly the amount of cooling as being
demanded by the load. Figure 1 shows the SHAP South
secondary annual power consumption. The values associated
with " Optimization " mode are recorded values
from the SHAP South secondary variable primary chilled
water system . The values associated with the " Baseline "
mode are based for a traditional primary/secondary
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chilled water system, and the values were calculated
based on best engineering practices and over
20 years of real-world experience.
Additionally, the condenser water loop was
designed to be variable fl ow. The same principle
applies on the condenser side; over-fl owing the
loop results in low condenser water ∆T. Varying
the fl ow through the condenser water loop as the
load conditions vary saves pump energy while
maintaining the chiller's safe operation.
As a result of optimizing the design phase and
hydronic confi guration, we were able to implement the
following sequencing and control algorithms:
* Selective sequencing of the chillers to operate the
most effi cient combination of chillers while satisfying
the dynamic load.
* Adaptive sequencing of the primary chilled water
and condenser water pumps to reduce the overall pump
group energy across the entire operating range of chilled
water and condenser water loop fl ows.
* Adaptive cooling tower fan sequencing to maximize
cooling surface area while meeting fl ow requirements of
the towers to reduce overall cooling tower fan energy.
* Resetting the condenser fl ow setpoint to leverage
the pump affi nity laws for pump energy savings while
simultaneously providing relief to the chiller compressor
for additional energy savings.
Environmental Impact
Chillers use HFC R-134a refrigerant, a zero ODP, low
GWP, nontoxic, nonfl ammable refrigerant with no
phaseout date at the time of design. Furthermore, they
use a shell and tube evaporator with a hybrid falling fi lm
design. In general, the falling fi lm design uses up to 30%
less refrigerant than the traditional fl ood evaporator.
By selecting more effi cient equipment for the Energy
Center, we are able to reduce our overall electrical
consumption.
Chillers' A-weighted sound power level was 79 dBA at
full load, very low for such a large piece of equipment.
Most centrifugal chillers operate around 90 dBA at full
load (a telephone dial tone is 80 dBA). Despite the chillers
being located in their own mechanical room, the
lower sound level is more palatable to operators working
in that space. The chillers operate at part load a majority
of the time so the sound level is signifi cantly reduced
from the stated level.
2022
Monthly Average Power (kWh)
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