Figure 1. Chicago Chiller Plant Annual Operating Load Profile (Four Chillers):
Hour-by-Hour Analysis vs. IPLV Estimates.
Chiller;Load;(%)
100 90 80 70 60 50 40 30 20 100
(IPLV=12%)
4%;of;operation
23%;of;operation
67%;of;operation
(IPLV=45%)
(IPLV=42%)
(IPLV=1%)
6%;of;operation
55°F;( 13°C);60°F;( 16°C);65°F;( 18°C);70°F;( 21°C);75°F;(24°C);80°F;( 27°C);85°F;( 30°C)
Entering;condenser;water;temperature
Source: Trane TRACE 700.
Figure 2. Dubai Chiller Plant Annual Operating Load Profile (Four Chillers):
Hour-by-Hour Analysis vs. IPLV Estimates.
Chiller;Load;(%)
100 90 80 70 60 50 40 30 20 100
(IPLV=12%)
1%;of;operation
20%;of;operation
78%;of;operation
(IPLV=45%)
(IPLV=42%)
(IPLV=1%)
1%;of;operation
55°F;( 13°C);60°F;( 16°C);65°F;( 18°C);70°F;( 21°C);75°F;(24°C);80°F;( 27°C);85°F;( 30°C)
Entering;condenser;water;temperature
Source: Trane TRACE 700.
Figure 3. Annual Operating Cost Comparison: Three Chiller Plant Design Alternatives.
Based on a 10,000-ton plant in Chicago.
$5,000,000
Yearly;utility;cost
$4,500,000
$4,000,000
$3,500,000
$3,000,000
$2,500,000
$2,000,000
$1,500,000
$2,477,975
$2,422,875
$2,502,760
$4,712,810
$4,597,654
$4,804,747
$4,402,283
$4,282,705
$4,600,825
Blended;rate;Time;of;day;rate
structure structure
Source: Trane TRACE 700.
n Alternative 1: Focus put on part-load value with variable-frequency drive
n Alternative 2: Focus put on full-load efficiency without variable-frequency drive
n Alternative 3: Focus put on full-load efficiency without variable-frequency drive and optimized cooling tower temperature
Minimum;demand;rate
structure
This analysis was based on
the following fixed and variable
assumptions for each plant:
• typical 10,000-ton district cooling
application with corresponding
diversity
• four 2,500-ton dual-compressor
chillers, each with a dedicated
water tower
• equally sized centrifugal chillers
piped in parallel
• cooling tower with 7 degrees F
approach and variable-speed drive
(VSD) with 10 F range
• variable air volume with reheat,
55 F supply-air temperature
• application of outside-air dry-bulb
economization per ASHRAE
Standard 90.1
• typical commercial business hours
and holiday schedule
This leaves weather and resultant
loads as the contributing variables.
The hour-by-hour, weather-dependent
analysis provides corresponding
chiller operating points with wet-bulb
temperatures as plotted in figures
1 and 2. Each operating hour for an
entire weather cycle, which is one
complete year of data, is plotted and
represented as individual points. With
four chillers available as required for
24 hours a day over 365 days, there
is a potential for 35,040 operating
points ( 8,760 hours in a year per
chiller). The number of points varies
with the cooling requirements and
climate conditions. When the resulting
dot plots are totaled, the percentage
of operation in each IPLV/NPLV ‘bin’
or ‘bucket’ is determined.
The plots show with graphical
clarity that actual multiple-chiller
plant operation does not line up with
IPLV rating values. Figures 1 and 2
display the information correlating
the capacity needed and the amount
of work by the compressor, or lift,
required. Lift is the pressure that
the centrifugal compressor must
overcome, so lower condensing
temperatures driven by lower wet-bulb temperatures result in lower
lift. Load and lift work in harmony
to generate the total work completed
by the chiller.
