tonnage plants. While it is often assumed
that cooling towers should be driven
to the lowest possible point to take
advantage of the VSD, one must ask,
At what cost? Blended rates and bin-type rating values like IPLV display this
option to yield paybacks.
Unfortunately, when these systems
are installed, real-world operation yields
significantly different bottom lines –
underscoring the need to avoid shortcuts
and perform detailed analysis. The
interaction and performance of pumps,
towers, chillers, weather, load patterns
and utility rates must be considered
together to ensure designs deliver the
intended results. This is especially true
as the results may seem counterintuitive.
Our industry is often misled by
claims of analysis simplicity, and
the IPLV/NPLV and blended rate
analyses represent two significant
culprits. Though they are attractive
analysis methods, their results can be
devastating to a building owner and the
environment. Inaccurate information is
paid for by building owners in higher
utility bills and by the environment
through millions of pounds of
unnecessary carbon dioxide emissions
over the life of the equipment. A careful
performance analysis is always needed.
As has been shown, regardless of VSD
applications, ensuring positive bottom
lines for large-tonnage chiller plants
requires the use of full-load efficiency
ratings, not IPLV/NPLV.
Using the IPLV Rating
Integrated part-load value (IPLV) evaluates a
single chiller application only. It uses a weighted
average of four distinct chiller operating points, with
assumptions of load and lift reductions – which is
the amount of refrigeration tons at a specific entering
condenser water temperature.
The Air-Conditioning, Heating and Refrigeration
Institute (AHRI) weighting points for IPLV are
as follows:
• 100 percent load, 85 F ( 29. 4 C) condenser
water temp, representing 1 percent
operating hours
• 75 percent load, 75 F ( 23. 9 C) condenser water
temp, representing 42 percent operating hours
Authors’ Note: This article is based
on a study by Geister and Thompson
titled “A Closer Look at Chiller Ratings,”
published in the December 2009
ASHRAE Journal. Readers interested in a
more detailed examination of IPLV may
also find that article informative.
Mike Thompson is the global eader of refrigerant strategy for the Trane, Thermo King and Hussman brands of Ingersoll-Rand plc. He has been with Trane for 19
years. A former IDEA board member, Thompson is active in a wide range of
HVAC industry organizations including ASHRAE,
and he represents Ingersoll Rand in the global
Montreal Protocol meetings. He can be contacted at mthompson@trane.com.
Justin Wieman is Trane’s centrifu- gal chiller strategic projects manag- er. With the company for 10 years, he has held a number of positions within Trane’s centrifugal chiller
group and has developed lifecycle
software programs such as TRACE 700. Wieman
is an active member of ASHRAE. His email
address is jwieman@trane.com.
W. Ryan Geister is the global portfolio leader for Trane’s centrif- ugal chillers. During the past 14 years with Trane, he has helped evelop and support the company’s design and analysis tools,
served as an instructor on HVAC systems in the
Trane Graduate Training Program and held multiple chiller management positions. Geister is an
active member of ASHRAE and sits on the AHRI
Compliance Committee. He can be reached at
rgeister@trane.com.
• 50 percent load, 65 F ( 18. 3 C) condenser water
temp, representing 45 percent operating hours
• 25 percent load, 65 F ( 18. 3 C) condenser water
temp, representing 12 percent operating hours
The calculation assumes that only 1 percent of
the chiller operation occurs at high loads and high
condenser water temperatures, and 57 percent of
the chiller operating hours are at 50 percent load or
less with operation at 65 F ( 18. 3 C) condenser water
temperature. Large-tonnage chiller plants rarely ever
run at these conditions.
The right use of IPLV is to compare a chiller’s ability to
unload at reduced loads and lifts, enabling a side-by-side comparison. The wrong use of IPLV is to assume
any relationship with resultant energy savings.
