ASHRAE Journal - January 2014 - 52

COLUMN ENGINEER'S NOTEBOOK
Kent W. Peterson
Improving Performance
Of Large Chilled Water Plants
BY KENT W. PETERSON, P.E., PRESIDENTIAL MEMBER/FELLOW ASHRAE
Although large campus central chilled water plants can be designed to be
energy efficient, the most impact on the overall system performance often
is how the connected building systems are designed to interface with the
control plant.
Improving Chilled Water ∆T
Many large central chilled water systems depend on
high chilled water temperature differential, ∆T, to minimize
pumping energy and optimize chilled water thermal
storage capacity. Buildings directly connected to central
chilled water distribution systems should be designed to
minimize pumping energy and maximize return chilled
water return temperature to the central plant. High ∆T is
achieved with proper coil and control valve selection, piping
and pumping design and supply water control.
Maximizing the ∆T between the chilled water supply
and return will maximize the cooling load that can be met
with a given chilled water flow rate. Chilled water ∆T is
primarily determined by cooling coil effectiveness at the
loads and is not something that can be achieved with controls
or control sequences at the central chiller plant.
Cooling Coils
Maximizing cooling coil performance is crucial for the
entire chilled water system operation. Chilled water ∆T
will be determined by how well the terminal devices
perform. Cooling coils should be selected to satisfy the
load, considering the expected supply water temperature
delivered to the coil.1 Temperature gain in the
distribution system as well as heat exchangers should
be considered. Chilled water temperature at the cooling
coil inlet can sometimes be several degrees higher
than the supply temperature leaving the central plant.
The return water temperature and leaving air condition
at each coil depends on coil configuration, airflow
across the coil, entering air enthalpy and entering water
temperature.
52
A SHR A E J O U RNA L
ashrae.org JAN UARY 2014
When designing new buildings to connect to an exist-
ing central plant, it is many times best to use an 8 row/10
fins per inch coil.2
When evaluating the potential for connecting an
existing building to a high ∆T central chilled water
system, careful evaluation of the existing coils is prudent
when considering a potential lower chilled water
supply temperature and the coils ability to meet the
system ∆T requirements. Generic AHRI-certified rating
and selection programs (available from several
coil manufacturers) can be used to model existing coil
conditions/construction. The coil construction can
be matched and the impacts of the different chilled
water supply temperature can then be modeled for
the existing coils. Many times the existing coils may
not need to be changed out when the higher ∆T central
plant has a lower supply water temperature than
the original coils within the existing building being
connected to the plant.
Coil performance is generally based on mean tem-
perature differential of the supply and return water
temperatures at the coil. For a given coil selection and
load, the warmer the supply water, the more water the
coil needs to meet the load, resulting in a lower return
water temperature. Figure 1 shows the effect on ∆T for a
cooling coil at different entering water conditions with
a constant load. ∆T will also degrade as the entering air
temperature approaches the design return water temperature
during part load conditions.
Kent W. Peterson, P.E., is chief engineer/COO at P2S Engineering in Long Beach, Calif.
He is former chair of Standard 189.1.

ASHRAE Journal - January 2014

Table of Contents for the Digital Edition of ASHRAE Journal - January 2014

ASHRAE Journal - January 2014
Contents
Commentary
Industry News
Letters
Meetings and Shows
Effect of Heat Rejection Load and Wet Bulb on Cooling Tower Performance
Shaping the Next... Building and Energy
Texas Hospital Central Plant Redesign
Detecting Faults in Hong Kong High-Rise
Engineer's Notebook
HVAC Applications
Refrigeration Applications
Energy Modeling
The Performance Gap
Data Centers
People
Classified Advertising
Advertisers Index
ASHRAE Journal - January 2014 - B1
ASHRAE Journal - January 2014 - B2
ASHRAE Journal - January 2014 - ASHRAE Journal - January 2014
ASHRAE Journal - January 2014 - Cover2
ASHRAE Journal - January 2014 - 1
ASHRAE Journal - January 2014 - 2
ASHRAE Journal - January 2014 - Contents
ASHRAE Journal - January 2014 - Commentary
ASHRAE Journal - January 2014 - 5
ASHRAE Journal - January 2014 - Industry News
ASHRAE Journal - January 2014 - 7
ASHRAE Journal - January 2014 - 8
ASHRAE Journal - January 2014 - 9
ASHRAE Journal - January 2014 - 10
ASHRAE Journal - January 2014 - 11
ASHRAE Journal - January 2014 - 12
ASHRAE Journal - January 2014 - Letters
ASHRAE Journal - January 2014 - Meetings and Shows
ASHRAE Journal - January 2014 - 15
ASHRAE Journal - January 2014 - Effect of Heat Rejection Load and Wet Bulb on Cooling Tower Performance
ASHRAE Journal - January 2014 - 17
ASHRAE Journal - January 2014 - 18
ASHRAE Journal - January 2014 - 19
ASHRAE Journal - January 2014 - 20
ASHRAE Journal - January 2014 - 21
ASHRAE Journal - January 2014 - 22
ASHRAE Journal - January 2014 - 23
ASHRAE Journal - January 2014 - Shaping the Next... Building and Energy
ASHRAE Journal - January 2014 - 25
ASHRAE Journal - January 2014 - 26
ASHRAE Journal - January 2014 - 27
ASHRAE Journal - January 2014 - 28
ASHRAE Journal - January 2014 - 29
ASHRAE Journal - January 2014 - 30
ASHRAE Journal - January 2014 - 31
ASHRAE Journal - January 2014 - 32
ASHRAE Journal - January 2014 - 33
ASHRAE Journal - January 2014 - 34
ASHRAE Journal - January 2014 - 35
ASHRAE Journal - January 2014 - Texas Hospital Central Plant Redesign
ASHRAE Journal - January 2014 - 37
ASHRAE Journal - January 2014 - 38
ASHRAE Journal - January 2014 - 39
ASHRAE Journal - January 2014 - 40
ASHRAE Journal - January 2014 - 41
ASHRAE Journal - January 2014 - 42
ASHRAE Journal - January 2014 - 43
ASHRAE Journal - January 2014 - 44
ASHRAE Journal - January 2014 - 45
ASHRAE Journal - January 2014 - Detecting Faults in Hong Kong High-Rise
ASHRAE Journal - January 2014 - 47
ASHRAE Journal - January 2014 - 48
ASHRAE Journal - January 2014 - 49
ASHRAE Journal - January 2014 - 50
ASHRAE Journal - January 2014 - 51
ASHRAE Journal - January 2014 - Engineer's Notebook
ASHRAE Journal - January 2014 - 53
ASHRAE Journal - January 2014 - 54
ASHRAE Journal - January 2014 - 55
ASHRAE Journal - January 2014 - 56
ASHRAE Journal - January 2014 - 57
ASHRAE Journal - January 2014 - HVAC Applications
ASHRAE Journal - January 2014 - 59
ASHRAE Journal - January 2014 - 60
ASHRAE Journal - January 2014 - 61
ASHRAE Journal - January 2014 - 62
ASHRAE Journal - January 2014 - 63
ASHRAE Journal - January 2014 - Refrigeration Applications
ASHRAE Journal - January 2014 - Energy Modeling
ASHRAE Journal - January 2014 - 66
ASHRAE Journal - January 2014 - 67
ASHRAE Journal - January 2014 - The Performance Gap
ASHRAE Journal - January 2014 - 69
ASHRAE Journal - January 2014 - 70
ASHRAE Journal - January 2014 - 71
ASHRAE Journal - January 2014 - 72
ASHRAE Journal - January 2014 - 73
ASHRAE Journal - January 2014 - Data Centers
ASHRAE Journal - January 2014 - 75
ASHRAE Journal - January 2014 - 76
ASHRAE Journal - January 2014 - People
ASHRAE Journal - January 2014 - Classified Advertising
ASHRAE Journal - January 2014 - Advertisers Index
ASHRAE Journal - January 2014 - 80
ASHRAE Journal - January 2014 - Cover3
ASHRAE Journal - January 2014 - Cover4
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