Ashrae D 9305 20021125
Ashrae D 9305 20021125
Ashrae D 9305 20021125
JOURNAL
The following article was published in ASHRAE Journal, November 1999. Copyright 1999 American Society of Heating, Refrigerating and AirConditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper
form without permission of ASHRAE.
Achieving High
Chilled-Water Delta Ts
Donald P. Fiorino, P.E.
Member ASHRAE
Consider a building served by a variable flow hydronic cooling system (with partial storage) that was designed for a 15F
(8.3C) chilled-water Delta T. The buildings design cooling load
is 1,500 tons (1119 kJ/s); the design pressure drop in its secondary chilled-water circuit is 150 ft (249 kPa); and its served
by two 75 hp (56 kJ/s) secondary chilled-water distribution
pumps (each capable of 1,500 gpm [94 L/s] chilled-water flow
[maximum]). The design capacity of its chilled-water storage
tank is 500 tons (384 kJ/s) sustained over a 10-hour discharge
periodequating to 5,000 ton-hours (63.4 MJ). Two 500 ton
(384 kJ/s) water chillers (with three-pass evaporators selected
at 7.5 fps [2.2 m/s] tube velocity) cool the building in conjunction with the chilled-water storage tank during the discharge
cycle, then recharge the chilled-water storage tank during the
14-hour charge cycle. Consider further that, due to several common shortcomings, the actual chilled-water Delta T is only 10F
(5.6C) at peak cooling conditions. What will be the effect on
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C H I L L E D -WAT E R S U P P LY
Electrical Distribution
Hydronic Cooling
Sym p tom s
Prob lem s
L ow Power Fa ctor
L o w De l t a 6
Root
Ca use
L a rg e, Und erloa d ed
Ind uction Motors
Solution
Delta 6 Correction a t
Cooling L oa d s
Benefits
standard four-row, A-frame cooling coils with 10F (5.6C) design chilled-water Delta Ts and that contain standard three-way
bypass control valves are frequently furnished and installed in
variable flow hydronic cooling systems. The consistent and
predictable results are low/below-design chilled-water Delta Ts
in the affected systems, plus associated performance problems.
Thorough specifications that require the manufacturer to furnish optional six-row cooling coils and optional two-way control valves will preclude this from happening.
25
Also, its important to realize that 200% of cooling coil design chilled-water flow will only yield 115% of design cooling
coil output (Figure 2). Furthermore, this condition will result in
40% lower-than-design cooling coil Delta Ts. It will also starve
other terminal devices of necessary chilled-water flow. So twoposition control valves, as well as modulating control valves
that remain 100% open under all load conditions (due to dirty
cooling coils/filters, sub-set thermostats, etc.), are especially
harmful in variable flow hydronic systems.
Objectives
Design or above-design chilled-water Delta Ts are assured from 1% to 100% load conditions in properly-designed,
installed, controlled, and maintained variable flow hydronic
cooling systems.
A design Delta T of 15F (8.3C) to 18F (10.0C) is readily
achievable in comfort cooling applications with 42F (5.6C)
entering chilled-water temperature, 80F (26.7C) mixed entering air temperature, and 53F (11.7C) leaving air temperature.
These objectives can be achieved by proper application of
cooling coils, control valves, control systems, distribution
pumps, and piping systems.
Approach
To apply resources most effectively, its important to realize
that chilled-water Delta Ts are determined by the various terminal devices, a.k.a., cooling loadsand not by the central waterchilling plant. So, to effectively raise chilled-water Delta Ts, the
design, installation, operation, and maintenance of the cooling
coils, control valves, and control systems located in equipment
rooms throughout facilities must be addressed. This is the fundamental solution that provides high leverage and yields enduring improvements.
After this is understood, it is easy to realize that installing
more water chillers, larger chilled-water pumps, and/or larger
chilled-water pipelines at the central water-chilling plant will
not overcome the poor performance problems associated with
low/below-design chilled-water Delta Ts in variable flow hydronic cooling systems. That symptomatic approach is capitalintensive, energy-intensive, and misses the fundamental
problem. This is mentioned because the symptoms of low/below-design chilled-water Delta Ts, i.e., poor cooling performance,
are frequently misinterpreted as evidence of inadequate capacity at the central water-chilling plant (refer to the example given
at the beginning and the case study cited in the reference section of this article).
Best Practices
Following are 25 best practices to achieve high chilled-water Delta Ts. They range from component selection criteria to
distribution system configuration guidelines and are applicable
to new installations as well as retrofit projects. All have been
successfully implemented at a 6.8 106 ft2 (631.7 103 m2) semiconductor manufacturing complex in Dallas. Their leveraged, synergistic effect improved that large facilitys peak composite Delta
T by 50%from 12F (6.7C) in 1993 to 18F (10.0C) in 1998.
1. Schedule cooling/dehumidifying coils for high Delta Ts.
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pump(s) should be smaller than control valves in branch circuits furthest from the chilled-water pumps. This will enable the
former to waste the excess differential pressure that will be
present in the branch circuits nearest to the chilled water pumps.
Once this is accomplished, external balancing devices are
unnecessary. In fact, external balancing devices reduce control
valve authority and are counterproductive. Furthermore, they
add unnecessary branch circuit pressure drops. The only place
external balancing devices are really needed in variable flow
hydronic cooling systems are at piping manifolds serving large,
stacked coil air-handling units.
6. Specify digital control. A properly functioning digital control system is more accurate than a properly functioning pneumatic control system and will position chilled-water control
valves with less offset and drift.
7. Use chilled-water several times before allowing it to return. Chilled-water is not required to return to the water chilling
plant after a single pass through one terminal device (Figure 3).
For example: 1) use the 56F (13.3C) chilled-water returning from
cooling/dehumidifying coils as the supply for sensible cooling
coils; 2) then use the 64F (17.9C) chilled-water returning from
the sensible cooling coils as supply for water-cooled equipment,
e.g., vacuum pumps, air compressors, etc. in order to maximize
the chilled-water Delta T. However, be sure that the 74F (23.3C)
chilled-water returning from water-cooled equipment returns directly to the water chilling plant without further recycling.
8. Use chilled-glycol in low temperature/humidity applications. In practice, 5F (2.8C) is about the closest approach to
the leaving air temperature that a single cooling/dehumidifying coil can provide while still yielding a high chilled-water
Delta T according to the cooling coil selection criteria recommended earlier. For a variable flow hydronic cooling system
with a 42F (5.6C) chilled-water supply temperature, this will
favor the use of chilled-glycol in applications requiring 47F
(8.3C) or lower leaving air temperatures, e.g., makeup air units
for clean rooms with 44F (6.7C) dew point requirements.
This practice prevents one low temperature/humidity application from requiring that the chilled-water temperature be subset for the entire facility.
9. Omit unnecessary water-to-water heat exchangers. As a
minimum, there will be a 2F (1.1C) temperature difference between the hot inlet and cold outlet at a cooling heat exchanger.
If the hot and cold pressures are in the same ANSI class and if
the hot and cold fluids are identical, it is better to omit the
cooling heat exchanger and blend the hot and cold fluids to
produce the desired hot outlet temperature (Figure 4) and elevate the chilled-water Delta T. Configure the piping so that the
excess water from the hot outlet, i.e., the warmest water, returns
to the water chilling plant.
10. Treat sensible cooling and cooling/dehumidifying separately. In order to sub-cool the entering air to saturation and
remove moisture, a cooling/dehumidifying coil will generally
require a lower entering water temperature than a sensible cooling coil. Therefore, in applications such as clean rooms and
computer rooms, its desirable to install sensible cooling coils
to control the temperature of the large volume of re-circulated
air and install separate side-stream cooling/dehumidifying coils
November 1999
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outdoor air applications requiring a large airside Delta T and reheating. However, increase coil face areas in order to reduce air
velocities, avoid increased airside pressure drops, and avoid
increased fan energy consumption.
13. Replace three-way bypass control valves. As discussed
earlier, these are detrimental in variable flow hydronic cooling
systems and must be replaced with two-way control valves
wherever they are encountered. This is because three-way bypass control valves allow chilled-water to bypass the cooling
coil and return to the water chilling plant at supply temperature.
Three-way bypass control valves also permit increased total
flow at partial cooling loads. Because three-way bypass control valves are normally furnished with less powerful actuators
than two-way control valves, simply plugging the bypass port
generally will not be adequate.
14. Close control valves when air-handling unit fans are off.
When the fan in an air-handling unit is off, the respective chilledwater control valve must close. Otherwise, chilled-water will
return to the water chilling plant at supply temperature. This
best practice is so obvious that its often overlooked. However,
it should not preclude opening the chilled-water control valve
as a last-ditch freeze protection measure in the event of a preheat system malfunction.
15. Calibrate temperature and humidity sensors. If these
read high, chilled-water control valves will open further than
necessary and Delta Ts will be reduced. Poor sensor location,
e.g., an air temperature sensor exposed to direct sunlight, can
have the same effect.
16. Protect temperature and humidity setpoints. The best
practice is to enter the setpoints into an appropriate software
program that has password protection. Otherwise, occupants or
mechanics attempting quick fixes to cooling problems are likely
to lower the setpoints. The root causes of the cooling problems,
e.g., inadequate air distribution, will be difficult to solve if ran28
ASHRAE Journal
dom setpoint changes are permitted, and zone temperature/humidity setpoints will most likely remain sub-set. The case study
cited earlier describes how extensive this problem can become.
17. Minimize fouling and airflow restrictions. Inadequate
airside flow and internal or external fouling of cooling coils
inhibit heat transfer and require more chilled-water flow than
necessary, thereby lowering Delta Ts. Appropriate best practices include use of a fouling factor in cooling coil selections;
periodic cleaning of the exterior surfaces of cooling coil tubes/
fins; continuous water treatment to preclude corrosion, biofouling, and scaling on the interior surfaces of cooling coil
tubes; and replacement of air filters as they become clogged.
18. Reduce distribution pump speeds at partial cooling
loads. If distribution pump speeds remain constant year-round,
chilled-water pressure differentials will rise when chilled-water flow rates decrease at partial cooling loads. This may force
chilled-water control valves open if they have inadequate actuators, thereby lowering Delta Ts. The best practice is to
equip distribution pumps with variable speed drives and modulate their speeds to maintain chilled-water pressure differentials in the most remote branch piping circuits at pre-set levels. The control valves will provide better control with constant chilled-water pressure differentials and annual distribution pump energy will be reduced considerably.
19. Use multizone and primary/secondary distribution pumping. Consider a central water chilling plant that serves two buildings. Building 1 requires 30 psi (50 kPa) differential pressure at
its service entrance under design cooling conditions and BuildNovember 1999
C H I L L E D -WAT E R S U P P LY
ing 2 requires 50 psi (83 kPa).
tems. Reverse return and loopIt is best for the central water
style chilled-water distribution
Delta T Techniques
chilling plant to maintain a 5
systems provide more uniform
psi (8 kPa) primary differential
branch circuit pressure differ In existing hydronic cooling systems, high, e.g., 15F
pressure at both building serentials than linear, direct-re(8.3C) or greater, Delta Ts reduce chilled-water flow
vice entrances and equip each
turn chilled-water distribution
rates and pressure drops, as well as pumping energy
building with an appropriately
systems. This promotes stable
and operating costs.
sized variable speed second In new hydronic cooling systems, high, e.g., 15F (8.3C)
control valve performance and
ary chilled-water distribution
or greater, Delta Ts permit smaller pumps and piping to
high chilled-water Delta Ts.
be installed.
pump station.
23. Eliminate constant
This compares favorably to
speed booster pumps. Con Adopting standard rating conditions for water-chilling
an all-primary distribution sysstant speed chilled-water
packages, i.e, 10F (5.6C) chilled- water Delta Ts, as
tem with the water chilling
booster pumps are detrimenthe design Delta T for variable-flow hydronic cooling
plant maintaining a 50 psi (83
tal in variable flow hydronic
systems will result in low Delta Ts.
kPa) pressure differential at the
cooling systems and should be
One error or omission in project drawings/specificaservice entrance to both buildequipped with variable speed
tions may result in low/below-design chilled-water Delta
ings. This approach is particudrives wherever they are enTs.
larly effective if Building 1 has
countered. Otherwise, they
Low/below-design chilled-water Delta Ts reduce the
a larger cooling load than
rob chilled water from adjaability of heat exchangers and pipes to deliver hydronic
Building 2, because as much
cent cooling coils and heat excoolingjust like low power factors reduce the ability
as 20 psi (33 kPa) of excess
changers at high flow condiof transformers and conductors to deliver electrical
pressure differential must be
tions and generate high
power.
continuously wasted by the
chilled-water pressure differen The chilled-water Delta T should be equal-to-design at
chilled-water control valves in
tials at low flow conditions.
full-load and greater-than-design at part-load in a variBuilding 1.
Neither case is conducive to
able-flow hydronic cooling system.
20. Reduce pressure difhigh chilled-water Delta Ts.
Chilled-water Delta Ts are determined by a buildings
ferentials at partial cooling
24. Replace marginal coolvarious terminal devices, a.k.a., cooling loads, not by
loads. A cooling coil can proing coils and heat exchangits central water-chilling plant.
vide 80% of design cooling
ers. In every variable flow hy High chilled-water Delta Ts result from proper design,
output with 50% of design
dronic cooling system, a few
installation, operation, and maintenance of cooling coils,
chilled-water flow. Also, presmarginal cooling coils or heat
control valves, control systems, distribution pumps,
sure drops at 50% of design
exchangers exist. These reand distribution piping.
chilled-water flow are only
quire lower chilled-water sup More water chillers, larger chilled-water pumps, and/or
25% of those at 100% of deply temperatures and/or
larger chilled-water piping will not overcome the persign chilled-water flow. This
higher chilled-water pressure
formance problems resulting from low/below-design
means that chilled-water difdifferentials than all of the othchilled-water Delta Ts.
ferential pressure setpoints
ers due to under-sizing, overcan be reduced at partial coolloading, control valve too
ing loads without adverse impact on cooling coil performance. small, etc. As these bad actors are replaced, cooling perforIf the distribution pumps are equipped with variable speed mance will improve and Delta Ts will increase.
drives, this best practice saves considerable distribution pump25. Monitor Delta Ts. If you cant measure it, you cant
ing energy on an annual basis. It also promotes high Delta Ts manage it. So provide chilled-water temperature indicators/senand stable control valve performance. However, be sure that sors at the inlets and outlets of all cooling coils and heat excooling coils and heat exchangers serving constant, year-round changers, as well as at branch points in distribution systems.
cooling loads, e.g., clean rooms and computer rooms, are se- With the information that these devices provide, low/belowlected for the lowest expected chilled-water pressure differential. design chilled-water Delta Ts will become readily evident and
21. Elevate supply temperatures at reduced cooling loads. can be traced quickly to their source(s).
When the outdoor air dew point is below the desired indoor air
dew point, only sensible cooling is required and the chilled-water Cost of Ownership
supply temperature can be raised without adverse impact on
So, what are the first cost implications of measures to assure
cooling coil performance. This practice will promote high Delta high chilled-water Delta Ts? In general, eight-row cooling coils
Ts and help preclude laminar chilled-water flow conditions inside will cost more than six-row cooling coils, particularly when enlightly-loaded cooling coils. However, be sure that cooling coils larged face areas are provided to avoid increased airside presand heat exchangers serving constant, year-round cooling loads, sure drops. Also, industrial-quality control valves with robust
e.g., clean rooms and computer rooms, are selected for the high- actuators will cost more than commercial- quality control valves
est expected chilled-water supply temperature.
with less robust actuators, but omitting the external balancing
22. Design reverse-return and loop-style distribution sys- devices required for the latter will mitigate the first cost differNovember 1999
ASHRAE Journal
29
ence. On the other hand, a hydronic cooling system with a design Delta T of 15F
(8.3C) will require smaller distribution
pumps and piping (with a lower first cost)
than a hydronic cooling system with a
10F (5.6F) design Delta T. The system
with the higher Delta T will also require a
smaller chilled-water storage tank (with a
lower first cost). These considerations will
tend to tilt the scale towards lower first
cost for high Delta T designs in large hydronic cooling systems having fewer/
larger air-handling units, more distribution pumps, greater lengths of distribution piping, and chilled-water storage
tanks. Good candidates include district
cooling systems that serve large campuses with multiple buildings.
Operating cost considerations clearly
favor hydronic cooling systems having
higher chilled-water Delta Ts due to lower
chilled-water flow rates and pressure
drops year-round. Also, as mentioned
earlier, providing cooling coils with enlarged face areas will preclude increased
airside pressure drops (and increased fan
energy consumption). Large hydronic
cooling systems have the potential to realize the greatest operating cost savings
from high Delta T designs.
In all cases, specific application conditions will determine first cost and operating cost differences resulting from alternate hydronic cooling system design
and operation practices.
References
1. Kirsner, W. 1995. Troubleshooting chilledwater distribution problems at NASA Johnson
Space Center. Heating/Piping/Air Conditioning 67(2):5159.
Bibliography
Hansen, E.G. 1985. Hydronic System Design and Operation. New York: McGraw-Hill.
Hansen, E.G. 1998. Letter to the editor,
Re: Low Delta T. ASHRAE Journal
40(11):19.
Kirsner, W. 1998. Low delta t central plant
syndrome. ASHRAE Journal 40(1):3742.
Avery, G. 1998. Controlling chillers in
variable flow systems. ASHRAE Journal
40(2): 4245.
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November 1999