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4.0 GENERAL DISCUSSION: STRATEGIES FOR VARIABLE FLOW CHILLED WATER GENERATION AND DISTRIBUTION
This report, for the most part, discusses general concepts,
techniques and strategies associated with variable flow
chilled water generation and distribution systems.
Comparisons are made with constant flow hydronic systems.
In general, emphasis is on large system applications, such as
might be relevant to a College Campus, City Civic Center, or
group of Industrial Buildings.
4.1.2Type A.1 Central Plant and Distribution – (Refer to SK-4.1.2)
This is a conventional central plant concept in which all
chilled water generation and pumping for the water
distribution systems is centralized and water distribution
piping is dead-ended. An exception to completely centralized
pumping would be large buildings where booster pumps
could be utilized to minimize pumping power in the central
plant.
Advantages include:
a. Installed equipment can be downsized to take
advantage of the cooling load diversity between all
buildings served by the central plant.
b. Maintenance can be centralized.
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c. Demolition of and/or changeover from water systems
within buildings can be accomplished with a minimum of
disruption.
Disadvantages included:
a. Relatively high initial cost compared to interconnected
buildings or stand-alone buildings.
b. A significant central plant structure must be provided on
the campus. If not entirely built during initial phases of
construction, space must be reserved for its ultimate
configuration.
c. Pumping costs are relatively high.
d. A break in one of the dead end distribution mains can
prevent some or all of the buildings on the dead ended
main from receiving water.
e. Significant upgrade of distribution piping is required. A
good deal of piping will be direct buried, resulting in
disruption.
4.1.3Type A.2 Central Plant and Distribution – (Refer to SK-4.1.3)
This is the same as the Type A.1 Central Plant with
distribution, except that water distribution is not dead-ended
but is through looped mains. The advantage is that a break
in a water distribution main can be isolated using sectional
shutoff valves, and buildings can continue to receive chilled
water. A disadvantage is an increase in initial cost.
4 - 2
4.1.4Type B.1 Central Plant and Distribution – (Refer to SK-4.1.4)
This is the same as the Type A.1 Central Plant with
distribution, except that each building system provides for its
own distribution pumping.
Advantages include:
a. Water pumping costs can be significantly decreased.
Building pumps located close to the central plant are not
required to have the capacity to handle the bulk of the
distribution system pressure loss.
b. Building pumps are shut off when a building is not in
use.
c. Building pumps are provided only at the time that a
building is connected into the distribution pumping
system.
Disadvantages included:
a. Some decentralization of maintenance.
b. Greater inconvenience can be encountered when
demolition or changeover from local water chilling to
central plant water chilling occurs within a building.
4.1.5Type B.2 Central Plant and Distribution – (Refer to SK-4.1.5)
This is the same as Type B.1 Central Plant with distribution
except that water distribution is not dead-ended but is
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through looped mains. The advantage is that a break in a
chilled water distribution main can be isolated using sectional
shutoff valves, and buildings can continue to receive chilled
water. A disadvantage is an increase in initial cost.
4.1.6Interconnected Systems – (Refer to SK-4.1.6)
Interconnected systems provide some of the advantages of
central plants and mitigate some of the disadvantages.
Advantages include:
a. Initial costs can be less than those for a central plant
system, particularly if existing building water chillers can
be incorporated into an interconnected system.
b. Diversity can be achieved, although not necessarily as
great as what could be obtained with a single central
plant.
c. Water pumping costs can be reduced, particularly when
compared to a Type A.1 or Type A.2 central plant.
d. Building water systems can be selectively turned off
during periods of light loads and still permit every
building to obtain water.
e. A broken interconnection main will not necessarily shut
down a building, since building chillers can be
disconnected from the interconnection mains and can
function to serve only the buildings in which they are
located.
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f. A relatively large and significant central plant structure
could be avoided. In addition to in-building chillers and
boilers, relatively small “load centers,” similar to small
central plants, could be utilized to eliminate some of the
in-building equipment and permit more convenience
demolitions and/or changeovers.
Disadvantages include:
a. A building pumping system must have the pumping
head capacity to handle the most severe requirement of
any building on its interconnected system. This can
reduce pumping savings.
b. Maintenance procedures must be centralized.
c. Substituting new chillers for existing equipment can
create changeover problems resulting in shutdown or
requiring temporary service.
4.2 Variable Flow vs. Constant Flow
The previous subsection of this report discusses general
alternatives for chilled water distribution arrangements.
Variable flow chilled water distribution systems provide many
advantages over constant flow systems for central plants and
are recommended. Variable flow is mandated for
interconnected systems if effective operation for them is to
be achieved.
4 - 5
4.2.1Constant Flow System Characteristics
SK-4.2.1 schematically indicates piping connections to typical
constant flow coils. Design flow for constant flow distribution
systems is based on the sum of the maximum instantaneous
flow requirement for each separate air handling unit coil and
other water flow using apparatus. Normally, this flow does
not decrease as the load on the distribution system falls. For
a large constant flow distribution system with a 75 percent
diversity and a 12 degrees F. average temperature rise
through coils, the design temperature differential across
chillers would only be 9 degrees F. For large buildings or for
large systems when the design temperature drop through
chillers matches the temperature rise through coils, then the
chillers can never be fully loaded (except perhaps for a fast
pulldown on startup) or the cooling coils are undersized.
A disadvantage with constant flow systems have chiller piped
in parallel is that water bypasses a chiller or circulates
through an inactive chiller when a chiller is turned off during
light loads. This results in a rise in the chilled water
distribution system water temperature unless the chilled
water temperature supplied from operating chillers is
deliberately depressed. Positioning chillers for series flow,
which is an arrangement frequently used for in-building
systems, overcomes this problem.
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A concept utilized with some campus chilled water
distribution systems is to provide the distribution system with
a greater temperature differential than the temperature
differentials used in buildings. This is accomplished using
secondary building pumps which blend distribution system
supply water with building return water to provide a building
supply water temperature which is higher than the
distribution supply water temperature. An example would be
a design distribution chilled water temperature rise from 40
degrees F to 55 degrees F in conjunction with a design
building temperature rise from 45 degrees F to 55 degrees F.
In this case, the distribution system flow is only 2/3 that of
building flows. This concept is applicable to both constant
flow and variable flow systems, although historically its
greatest application has been with constant flow systems. A
justification might be an existing distribution piping which is
inadequate in capacity to serve new buildings. This
justification usually does not exist since building coils
supplied with the lower distribution system temperature and
with decreased flow will normally perform better than when
supplied with chilled water and correspondingly greater flow.
4.2.2 Variable Flow System Characteristics
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SK-4.2.2 schematically indicates piping connections to typical
variable flow coils. Design flow for a variable flow distribution
system is based on its block load, and theoretically, the flow
will vary downwards roughly in proportion to any decrease in
system load. For a large variable flow distribution system
serving a group of buildings having 75 percent diversity, the
design flow would be 75 percent of that for a constant flow
system. Normally, the design temperature drop through
chillers would match the average design temperature rise
through coils.
A shortage of distribution capacity due to an unanticipated
addition of buildings which conceivably might develop over
an extended period of time could result in performance
problems under maximum system load conditions. However,
unlike a constant flow system, the variable flow system would
constantly rebalance flows to match load demands on coils,
as the load on the distribution system would decrease to an
average condition.
4.3 Coil Characteristics for Variable Flow
4.3.1No Secondary Coil Pumps
Heat transfer across a chilled water cooling coil is influenced
by the area and configuration of the coil, the materials of
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construction, the air side and water side heat transfer film
coefficients and the mean temperature difference between
the air and water. The mean temperature difference can
have significant influence.
SK-4.3.1A shows two examples of the relationship between
supply air and chilled water temperatures as these media
flow across and through a coil. Both relate to identical design
load conditions and to variable chilled water flow; one is
based on return air or space temperature control of the
automatic coil valve and the other is based on supply air
temperature control of the automatic coil valve. The
diagrams are based on no partial phase changes, in other
words, no condensation of water vapor. The presence of
condensed water vapor on the coil surface decreases the
resistance of the airside film, improves heat transfer and
reduces chilled water flow.
The area between the supply air and the chilled water
temperature curves is proportional to the mean temperature
difference. It can be seen from SK-4.3.1A that for return air
(or space) temperature control, the area actually increases as
the load decreases, but that the area decreases for supply air
temperature control. This indicates that there is a greater
reduction in chilled water flow at reduced loads for constant
volume single zone systems controlled from return air or
space temperature than for variable air volume, double duct
4 - 9
or multi-zone systems controlled from supply air. The above
discussion recognizes that the resistances of water and air
films increase as flows are diminished; however, the influence
may not be as great as the change in mean temperature
difference.
SK-4.3.1B shows generalized relationships of water flow to
reduced load for constant volume return (or space)
temperature control and for variable volume supply air
temperature control. The greater reductions in water flow at
reduced loads for return air (or space) temperature control is
clearly indicated. It should be noted that laminar chilled
water flow occurs very roughly at 20 percent load. When this
transition area is entered, the resistance of the chilled water
film radically increases and heat transfer is greatly and
adversely affected. The same can be true for significantly
reduced airflows, but a determination of when this condition
can occur is much more difficult.
In order to achieve effective variable flow with respect to air
circulation cooling systems having supply air temperature
control, the following recommendations are presented:
Never permit a controller to be adjusted below its design
set point. For fixed set point control, utilize a narrow
proportional band.
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If feasible, use a proportioning controller with an
integrating mode to eliminate offset at low loads.
State of California energy conservation regulations
which call for the readjustment of a cold deck
temperature from a selected zone having a greatest
need for cooling will be helpful towards eliminating
excessive offset. The same is true of readjusting a cold
deck temperature inversely from outdoor air
temperature or from quantity of airflow.
Positively close the automatic coil valve.
4.3.2Secondary Coil Pumps
There are occasions when a secondary coil pump is desirable
for use with a variable flow chilled water distribution system.
The secondary coil pump permits all portions of a cooling coil
to be relatively warm at light loads, and if desired, can permit
a cooling coil to be relatively warm and to enable a relatively
high dew point supply air temperature to be produced at a
design load condition. Secondary coil pump applications can
be desirable where higher than normal humidity space
conditions are required, such as for surgeries, some data
processing spaces, printing and paper storage areas, library
and museum storage areas, etc. Secondary coil pump
applications can be desirable where higher than normal
humidity space conditions are required, such as for surgeries,
4 - 11
some data processing spaces, printing and paper storage
areas, library and museum storage areas, etc. Secondary coil
pump applications are frequently desired in some area of the
United States where freeze-ups can occur with 100 percent
outdoor air units. SK-4.3.2A illustrates the application of a
secondary coil pump with a variable flow chilled water
distribution system.
SK-4.3.2B shows a generalized relationship of branch water
flow vs. percent load for return air control and for supply air
control systems where a secondary coil pump is used. Please
note that under reduced loads, the reduction in water flow is
not as great as the reduction in load for a supply air control
system. It is recommended that the use of secondary coil
pump applications for supply air control systems be limited
for variable flow chilled water applications.
4.4 Variable Flow Pumping Arrangement – No Secondary Pumps
4.4.1Variable flow chilled water circulation pumping systems are
commonly used with in-building chiller systems of moderate
size. Initial costs are lower than for systems, which utilize
secondary pumps, but pumping costs generally are higher.
SK-4.4.1A is a sketch of a single chiller and pump
arrangement which illustrates appurtenances desired for
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proper control of a variable flow pumping system without
secondary pumping. Objectives are to provide constant flow
through the chiller and variable flow through the distribution
system, in which case the pressure differential across the
distribution system is controlled at a specific location. In SK-
4.4.1A, Valve B modulates to provide constant chiller flow.
Valve B would be pneumatically actuated and could be
controlled from a controller, which senses pressure
differential across the chiller. Valve A also would be
pneumatically actuated and controlled from differential
pressure at a predetermined location somewhere in the
distribution system.
Many past projects have attempted to eliminate Valve B, or
it’s equivalent. This can produce problems. For one
situation, if Valve A is located where shown and is controlled
to provide a constant pressure differential at point A-A, then
the pressure differential at point B-B can increase
significantly at light loads due to decreased pressure drop in
the distribution system. For another situation, if Valve A is
located where shown and is controlled to provide a constant
pressure differential at point B-B, then it must open wide at
low loads to attempt to produce excessive flow and pressure
drop through the chiller.
4 - 13
This would be necessary to drop the differential pressure at
point A-A equivalent to the decrease in distribution system
pressure drop. Locating Valve A at the end of the distribution
system (point B-B) and controlled to provide a constant
differential pressure at that point would be satisfactory for a
single pump system with some increases in distribution pipe
sizes, but not for a multiple pump system.
An alternative to the use of Valve B would be to substitute a
constant flow control, such as manufactured by Griswold, and
this is indicated in SK-4.4.1B. This is actually preferred over
Valve B, since it provides the same results at lesser cost. For
the balance of this report, the flow control arrangement will
be indicated.
Some past installations have attempted the use of a self-
contained control valve for Valve A in lieu of a pneumatically
powered automatic modulating valve controlled from a
proportioning differential pressure controller. Self-contained
control valves frequently provide unsatisfactory results for
this application due to their relatively poor regulation and
repeatability.
4.4.2SK-4.4.2 shows a two-pump/chiller arrangement, which is also
typical for more than two chillers. Chillers, along with their
respective pumps, can be turned on and off as the load
4 - 14
changes significantly. For projects with selective small
cooling loads operating during off-normal hours, a relatively
small chiller can run by itself. (During these times, the
automatic coil valves of normal operating hour air handling
units would be tightly closed.)
SK-4.4.2 shows pumps located on the supply side of chillers.
This is the preferred arrangement for high rise buildings
and/or extensive distribution systems where high pump
discharge pressure could otherwise require a special high
pressure water side design working pressure for the chillers.
4.5 Variable Flow Pumping Arrangement – Primary and Secondary
Pumping
4.5.1Type A Central Plant - (Refer to SK-4.5.1)
This equipment and piping arrangement is frequently
considered for central plants and large buildings. Constant
flow is provided through the distribution system
Chillers along with their respective primary pumps can be
turned on and off as the system load changes. A small chiller
operating by itself during off-normal hours can provide
cooling for selective applications such as computer rooms,
security facilities, etc.
4 - 15
Secondary pumps can be controlled to provide a constant
pressure differential at a selected point in the distribution
system, or from the average of several selected points, or
from any one of several selected points which require a
minimum pressure differential. A strategy for establishing a
pressure differential could be to provide a setting, which
would accommodate coils in small and moderate sized
buildings but would require booster pumps in large buildings.
This could provide savings in the cost of operating secondary
pumps.
Strategies for operating the system should be such that
reverse flow is not permitted at point A-A. A strategy for
utilization of constant speed secondary pumps could be to
provide these pumps in number and flow capacity to match
the primary pumps. This strategy would match the operation
of a primary pump with its equivalent secondary pump.
4.5.2Type B Central Plant – (Refer to SK-4.5.2)
This is similar to the Type A central plant, except that
secondary pumps are located in buildings and not in the
central plant. A principal advantage is that pumping costs are
less for the Type B central plant, since each building
secondary pumping system is selected for its specific building
4 - 16
pressure differential plus only that portion of the distribution
system pressure drop which exists between the central plant
and the building.
Strategy for operating the Type B central plant should be to
have a sufficient number of primary pumps with chillers in
operation to prevent reverse flow at point A-A.
4.6 Mixed Chiller Operation
There are chiller systems which require simultaneous
operation of different types of chillers and at the same time
provide base loading of one chiller type for energy cost
savings.
An example would be a system featuring a standard electric
water chiller plus a chiller with a double-bundled condenser to
provide hot water for space heating. When the heating is
needed and the cooling load is greater than the heating load,
the standard chiller operates to accommodate the balance of
the cooling load. This operation is desirable since the
operating cost of the double-bundled chiller is higher than
that of the standard chiller per unit of cooling.
Another example is the cogeneration plant with a topping
cycle using one or more single stage absorption chillers to
4 - 17
utilize waste heat, but also requiring electric chillers to
supplement cooling provided by the absorption chillers during
warm weather. In this case, the absorption chiller operates
only at the capacity to utilize waste heat, and the electric
chiller operates to provide the balance of the cooling load.
Additional examples could relate to strategies for operating
thermal energy storage (TES) cooling systems, which utilize
heat exchangers piped into a system, and which also
incorporates standard chillers.
Examples in this section of the report relate to a mix of
absorption chilling with electric chilling. In these examples,
the absorption chilling would be base loaded during electric
utility on-peak periods and electric chilling would be base
loaded during electric utility low-peak periods.
The examples relate to chillers piped in parallel. Small and
moderately sized chiller plants can utilize chillers piped in
series, and this can result in simplified controls to provide
base loading of one chiller type.
4.6.1No Secondary Pumps – (Refer to SK-4.6.1)
Pressure differential in the chilled water distribution system is
the overriding control of the two automatic bypass valves,
4 - 18
and in the respect, the system shown on SK-4.6.1 is the same
as the system shown on SK-4.4.2.
When base loading of one of the chillers is desired, its
automatic bypass valve is modulated towards the closed
position. A limit to the closure is the return water temperature
entering the chiller, which is not allowed to exceed maximum
design conditions.
The chiller which is not base loaded has its automatic bypass
valve controlled from distribution system differential
pressure, and its portion of the total cooling load is that which
is not handled by the base loaded machine.
Both chillers produce the same chilled water supply
temperature (example: 42 degrees F). The chiller which is
not base loaded has a lower entering water temperature
compared to the base loaded chiller.
4.6.2Primary and Secondary Pumping – (Refer to SK-4.6.2)
When base loading of one or more of the chillers is desired,
its automatic bypass valve is modulated toward the closed
position.
4 - 19
The non-base loaded chiller (or chillers) has its bypass valve
controlled to prevent reverse flow at point A-A.
All chillers produce the same chilled water supply
temperature (example: 42 degrees F). The chiller (or chillers),
which is not base loaded has a lower entering water
temperature compared to the base loaded chiller.
4.7 Interconnections
4.7.1Multiple Buildings: No Secondary Pumping – (Refer to SK-
4.7.1)
With this arrangement, any chiller with its pump is capable of
circulating chilled water to any cooling coil throughout the
assembly of buildings. Every chilled water pump must be
selected with a pumping head capable of circulating water to
any coil under design conditions. During light loads (for
example during other than normal operating hours when
most air handlers and their cooling coils are shut off), one
chiller with its pump may provide chilled water service to the
air handling unit cooling coils still in operation.
Flow controls function to automatically dissipate excess
pumping differential pressure as flows very through the
building and distribution piping systems. Mixed chiller
4 - 20
operation, when applicable, is obtained by base loading or
assigning a priority to the appropriate chiller. Differential
pressure control of automatic bypass valves can be provided
using several strategies, including the following:
a. Single controller, strategically located, which
simultaneously controls all valves, except that it
functions as an override for the bypass valve on a base
loaded chiller.
b. Several controllers in various buildings. The controller
requiring the greater differential pressure
simultaneously controls all valves, except that it
functions as an override for the bypass valve on a base
loaded chiller.
c. Separate controller for each building having its own
chillers, each controller only controls the automatic by
pass valves in its building, except that it functions as an
override for the bypass valve on a base loaded chiller.
Even though each controller may have a somewhat
different set point, no chiller can function beyond its
design capacity due to the limitations in its flow imposed
by its flow control.
Part load operation of chillers will be affected by the number
of chillers permitted to run at one time. Operation or non-
operation of chillers can be automatically implemented based
4 - 21
on the part load capacities of chillers in operation after initial
startup. A preferred method is to select the chillers to
operate based on predicted outside temperature conditions
and occupancy; in other words, experience and chiller
unloading characteristics.
4.7.2Type A Central Plant; Interconnection with Separate Building
Chiller Plant – (Refer to SK-4.7.2)
There are circumstances when a central plant is utilized, and
it is desired to augment its capacity using a connection
between the central plant distribution loop and a building
with a significant water chilling installation. Such an
arrangement can also permit the central plant to be a
standby for the building chillers and also permit the building
chillers to be shut down during light loads.
SK-4.7.2 shows a central plant, a central plant piping
distribution system and a building with its own chillers
connected to the distribution system through booster chilled
water pumps. The booster pumps are necessary when the
pressure differential produced by the building chilled water
pumps is less than the pressure difference in the distribution
piping system.
4 - 22
The pressure differential in the distribution piping system
controls the capacity of the central plant secondary chilled
water pumps. The capacity of the secondary pumps is
automatically limited to prevent reverse flow at point A-A.
When the pressure differential in the piping distribution
system falls due to the capacity limitation of the central plant
secondary chilled water pumps, then the building booster
chilled water pumps operate and are automatically controlled
in capacity by the pressure differential in the piping
distribution system.
The amount of chilled water contributed to the piping
distribution system for the building is affected by the number
of central plant chillers permitted to operate.
When the building chillers are shut down, opening of the
automatic chiller shutdown valve permits the building to be
supplied with chilled water from the central plant through the
piping distribution system.
4.7.3Type B Central Plant; Interconnection with Separate Building
Chiller Plant – (Refer to SK-4.7.3)
There are circumstances when a central plant is utilized, and
it is desired to augment its capacity using a connection
4 - 23
between the central plant distribution loop and a building
with a significant water chilling installation.
SK-4.7.3 shows a central plant, a central plant piping
distribution system and a building with its own chillers
connected to the distribution system. Building chilled water
booster pumps are not needed to supplement chilled water
flow in the distribution system with building chilled water.
However, the building cannot be supplied with chilled water
from the distribution piping system unless secondary building
chilled water pumps are added.
The pressure differential in the central plant piping
distribution system is not controlled. When the chilled water
flow at point A-A in the central plant primary loop approaches
zero, the building automatic supplementary capacity valve
modulates open to prevent reverse flow at point A-A. The
supplementary capacity valve remains closed at all other
times.
The amount of chilled water contributed to the piping
distribution system form the building is affected by the
number of central plant chillers permitted to operate.
4.7.4Interconnection with Separate Building Chiller Plant which has
Limited Capacity.
4 - 24
There are circumstances when a building chiller plant lacks
capacity to handle its building load under maximum design
conditions, and it is necessary to supplements its capacity
from interconnecting chilled water distribution mains.
An example might be a building chiller plant, which has an
old, poorly functioning original absorption chiller and a
relatively new electric chiller, which replaced another original
absorption chiller. It is desired to keep the electric chiller in
operation, remove the remaining absorption chiller and make
up an deficiency in building chiller capacity by obtaining
supplementary cooling through connections to a campus
chilled water distribution system.
SK-4.7.4A is applicable to a building chiller plant and a
building chilled water distribution system, which is connected
to a campus chilled water distribution system. In this case,
the building chiller plant operates at a pressure differential
less than the pressure differential of the campus distribution
system. As the building cooling demand increases, the
automatic bypass valve modulates closed. On a further
increase in building cooling demand, the pressure differential
in the building chilled water distribution system continues to
fall, and this causes the automatic pressure regulating valve
to modulate open. This supplements chilled water flow to the
4 - 25
building which chilled water from the campus distributions
system. The automatic pressure-regulating valve is closed
whenever the building chiller is shut down.
SL-4.7.4B also is applicable to a building chiller plant and a
building chilled water distribution system, which is connected
to a campus chilled water distribution system; however, in
this case, the building chilled water plant operates at a
pressure differential greater than or equal to the pressure
differential of the campus distribution system. As the building
cooling demand increases, the automatic bypass valve
modulates to the closed position. On a further increase in
building cooling demand, the pressure differential in the
building chilled water distribution system continues to fall,
and this causes the booster chilled water pump to start and
to have its flow capacity proportionally increased. The
booster pump shutoff valve closes whenever the building
chiller is shut down.
An option permits the building chiller to supplement the
campus chilled water distribution system when and if the
building load demand is less than the chiller capacity. In this
event, the booster chilled water pump does not operate, and
the optional pressure regulating valve modulates open to the
extent that the pressure differential in the building
4 - 26
distribution system decreases and causes the automatic
bypass valve to close.
4.8 Thermal Energy Storage (TES) for Cooling.
Due to large storage volume requirements, chilled water
storage is not considered a practical option by this report for
most installations.
Most TES cooling system constitutes the major component of
an installation, which produces water chilling for distribution
in a building or a building complex. This will not be the
situation if stand alone building cooling systems are not to be
considered. If a TES cooling system is to be a consideration,
it will be an increment in the large central water chilling plant
or in a group of building water chilling systems
interconnected through chilled water distribution mains. The
capacity of the TES cooling increment will depend on the
particular college construction program with includes the
addition of the increment to the cooling system.
The concept of the TES cooling system being a relatively
small increment in a large water chilling and distribution
system offers some advantages. It can be fully and effectively
utilized under relatively low campus cooling load conditions.
Also, TES cooling systems that use ice can operate under a
4 - 27
full storage strategy in conjunction with conventional chiller
handling campus cooling loads during off-peak and perhaps
mid-peak electric utility time periods.
The following are comments on various TES cooling systems,
which may be applicable and which do not utilize chilled
water storage.
4.8.2Ice Builder - (Refer to SK-4.8.2A and SK-4.8.2B)
An ice builder system utilizes coils in a non-pressurized
(open) water tank to freeze the water for storage purposes.
Refrigerant such a HFC-134a in the coils with circulating
provided by direct expansion, flooding or pump recirculation
in conjunction with a refrigeration compressor can provide
the cooling for freezing. For large tank installations,
glycol/water solution in conjunction with a chiller can be used
to provide the cooling. The arrangement presented in this
discussion utilizes the glycol/water solution and chiller
system.
SK-4.8.2A illustrates the ice builder system functioning in a
full storage extraction (electrical on-peak) mode. Partial
storage (load leveling) operation also can be implemented by
operating the glycol chiller during extraction. The heat
exchanger functions as a chiller in the chilled water
4 - 28
distribution system, and the chilled water supply temperature
from the heat exchanger it controlled through operation of
the automatic temperature valve.
SK-4.8.2B illustrates the ice building system functioning in a
full storage (electrical off-peak and perhaps mid-peak)
charging mode.
4.8.3Ice Modules -(Refer to SK-4.8.3A and SK-4.8.3B)
Ice modules TES systems are marketed in several forms; all
use an ethylene glycol/water solution under pressure, which
is cooled by one or more chillers to freeze water for storage.
During extraction, ice cools the pressurized glycol solution,
which is circulated to the cooling load(s). For a campus chilled
water distribution system, a heat exchanger must be used to
separate the glycol solution system from the chilled water
system.
One such ice modules system is identified as a CALMAC ice
bank and is marketed by Trane. This system utilizes multiple
open polyethylene tanks containing water through which
spiral wound polyethylene tubing heat exchangers permit the
glycol solution to be circulated.
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SK-4.8.3A is a generalized illustration, which represents all
three examples of ice modules. The sketch indicates
operation under full storage extraction. In effect, the heat
exchanger functions as a water chiller, and its chilled water
supply temperature is controlled by operation of the
automatic temperature valve. Partial storage operation can
be achieved by operating the glycol solution chiller with a
supply temperature slightly higher than the design entering
glycol solution temperature to the heat exchanger. If partial
storage operation is desired, a glycol chiller utilizing a screw
refrigeration compressor is suggested.
SK-4.8.3B illustrates the ice modules system functioning in a
full storage charging mode.
4.9 Existing Constant Flow Buildings Used with Variable Flow
Distribution Systems.
Some existing buildings having air handling unit cooling coils
designed for constant flow operation can also operate
satisfactorily when connected to a variable flow campus
chilled water distribution system. This is possible without
changing automatic coil valves or providing extensive
modifications to chilled water piping distribution within the
building.
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Buildings having constant flow systems, which could be
candidates for connecting with a variable flow campus chilled
water distribution system could have conventional single
zone air handling units or could have double-duct or multi-
zone air handling units serving classroom and administration
areas. The double-duct or multi-zone systems should serve
both exterior and interior zones and be designed to provide
generous air quantities to interior spaces.
4.9.1Connection to Type A Distribution System - (Refer to SK-
4.9.1)
A building pump is provided, in effect, to de-couple the
building piping from the pressure differentials of a Type A
variable flow chilled water piping system. Temperature
Controller TC 1 controls automatic modulating Valve TV 1 to
produce a constant building return water temperature as
measured by Temperature Sensor TS 1. The building chilled
water supply temperature as measured by Temperature
Sensor TS 2 varies inversely with the building load, and the
chilled water branch flow to the building form the campus
distribution system also varies inversely with the building
load. Temperature Controller TC 1 should be a proportioning
type with an integrating mode to minimize offset.
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4.9.2Connection to Type B Distribution System - (Refer to SK-
4.9.2)
A Type B variable flow chilled water piping system requires a
building pump or pumps to provide chilled water distribution
within the building. Temperature Controller TC 1 controls
automatic modulating valves TV 1A and TV 1B to produce a
constant building return water temperature as measured by
Temperature Sensor TS 1. (Refer to AUTOMATIC VALVE
SCHEDULE on SK-4.9.2 for automatic valve sequencing.) The
building chilled water supply temperature as measured by
Temperature Sensor TS 2 varies inversely with the building
load, and the chilled water branch flow to the building from
the campus distribution system also varies inversely with the
building load. Temperature controller TC 1 should be a
proportioning type with an integrating mode to minimize
offset.
4.10 Compression and Expansion Tank Application
A compression tank or expansion tank application has two
primary functions. One is to permit the occurrence of thermal
expansion and contraction in a closed hydronic system
without require bleedoff or makeup of the hydronic system
fluid, and the second is to fix one point in the hydronic
system where the working pressure can always be defined.
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This point is where the compression or expansion tank
connects to the hydronic system.
This point of connection in conjunction with the pressure
maintained in the compression or expansion tank should
permit an above atmospheric pressure in all portions of the
closed hydronic system whether it is operating or shut down.
For a central plant application, the preferred location of a
compression/expansion tank is within the central plant and
not in one of the buildings connecting to the central plant
distribution system. This permits a building to be valved-off
without affecting the influence of the tank on the balance of
the distribution system. The desired location for a central
plant connection is the chilled water return line. The desired
location for interconnected buildings without a central plant is
in the building that has the least likelihood of being valved-
off.
There should be only one tank connection to a hydronic
system. An exception is when one portion of a hydronic
system is to be valved-off at times and operated separately,
in which case it should have its own tank that, in effect, is
deactivated when all portions of the hydronic system are
operating.
4.10.1 Tank Alternatives - (Refer to SK-4.10.1)
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Tank size is a function of the net volumetric
expansion/contraction of fluid in the hydronic system due to
thermal changes and to its maintained pressures.
A 20 psig working pressure must be maintained at the top of
the tallest building of an interconnected hydronic system.
If it were possible to utilize an open (atmospheric) expansion
tank for the hydronic system, its volume would be
approximately 1.6 time the net thermal expansion of the
hydronic system.
If a compression tank were to be located in the central plant
and pre-charged with nitrogen or compressed air to 90 psig,
its volume would be approximately 6.2 times the net thermal
expansion of the hydronic system. This is based on a 20 psig
maximum permissible pressure variation in the tank for
hydronic system volume changes due to the thermal
expansion and contraction.
If a compression tank were to be located in the central plant
and not pre-charged (i.e. the tank filled to 90 psig when the
tank initially is an atmospheric pressure), its volume would be
over 40 times the net thermal expansion of the hydronic
system. This is based on a 20 psig maximum permissible
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pressure variation in the tank for hydronic system volume
due to thermal expansion and contraction.
4.10.2 Recommended Tank Arrangement - (Refer to SK-4.10.2)
If a central plant for the campus is to be constructed, the
recommended tank arrangement is as indicated on the
sketch. An open expansion tank is utilized and a small boiler
feed pump operates infrequently to maintain hydronic system
pressure. Advantages included (1) lowest initial cost and
smallest tank size (2) ability to maintain a closely regulated
pressure at the point where the expansion tank connects to
the hydronic system and (3) easy and convenient means to
readjust the operating pressure of the hydronic system, if this
should be desired.
Pre-charged bladder type compression tanks, are not
recommended for a large central plant installation due to
their anticipated relatively high cost. However, they should be
considered for interconnected systems where several
separate hydronic systems would have a relatively low water
volume.
4.11 Readjusting the Chilled Water Supply Temperature
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A strategy for some central plant operations is to raise the
chilled water temperature during low cooling loads. In most
situations, the motive is to decrease energy consumed by
chillers. In some cases, “free cooling” using a cooling tower
during low outdoor wet bulb temperature conditions can
produce water cool enough to permit air conditioning during
light load conditions. Administration areas and classrooms are
spaces, which typically can tolerate increased chilled water
supply temperatures during overall low load conditions.
Laboratory spaces frequently cannot tolerate a significant
chilled water supply temperature warm up.
Chillers with constant flow chilled water distribution systems
will decrease their energy consumption when the chilled
water supply temperature is raised. However, even here, the
total HVAC energy consumption may not decrease. If variable
air volume (VAV) air distribution systems are utilized, the
higher chilled water supply temperature may require
increases to supply air quantities, resulting in a need for
increased fan power.
For variable flow chilled water distribution systems, which
function effectively, readjusting the chilled water supply
temperature upwards can result in an energy penalty,
particularly for large systems.
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4.11.1 Type A Central Plant and Distribution - (Refer to SK-4.11.1)
This discussion is based on the use of variable speed
secondary pumps. Theoretically, the flow reduces in
proportion to the load, and the pump horsepower would also
decrease, but not necessarily in the same proportion due to
an increase in drive losses and perhaps a decrease in pump
efficiency. However, the pressure differential which the
variable speed secondary pump(s) must produce at a
reduced chilled water flow decreases, and this reduction is
influenced by the location in the chilled water distribution
systems of the pressure sensors which control the speed of
the pumps. The combination of reduced chilled water flow
and a reduced pressure differential requirement can result in
a decreased systems power requirement compared to that
which would result if the chilled water supply temperature
were to be raised. This is particularly the situation for
extensive chilled distribution systems.
4.11.2 Type B Central Plant and Distribution – (Refer to SK-
4.11.2)
This discussion is based on the use of variable speed building
pumps and no secondary chilled water pumps in the central
plant. Theoretically, the flow reduces the proportion to the
load. Furthermore, the pressure drops in the distribution
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mains decrease exponentially as the flow decreases. As a
result, power requirements for building pumps located further
from the central plant progressively become less. The
reductions in building pump power requirements due to
reduced flows can be greater than the decrease in chiller
power which would occur without these flow reductions if the
chilled water supply temperature were to be raised.
4.11.3 Laboratory and Special HVAC Systems
As mentioned earlier, laboratory spaces frequently cannot
tolerate a significant chilled water temperature warm-up.
When such a situation occurs, solutions are to utilize a
booster chiller or to divorce the laboratory from the central
plant and utilize a dedicated chiller or refrigeration system.
4.12 Automatic Valves
In general, automatic valves should have pneumatic
actuators, which are the spring-opposed diaphragm type.
Every automatic valve should have an actuator, which has
the diaphragm area and construction necessary to smoothly
move the valve at a proper stroking speed through its full
travel against the maximum hydronic system pressure
differentials, which can occur across the valve.
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In general, butterfly valves for sizes 2-1/2-inches and larger
with proper actuators can be used for throttling and shutoff.
Properly sized and constructed butterfly valves can have
turndown ratios of 100/1 or better. Globe valves can be used
for sizes 2-inches and smaller. Where pressure drops or
turndown ratios are critical for valves in sizes 1-inch through
2-inch, V-port ball valves should be considered. Effective
automatic valve operation should not be expected when an
existing 3-way valve is modified to provide 2-way service by
plugging one of its ports.
Consideration should be given towards referencing automatic
valves to the products of a manufacturer whose primary
business is the production and distribution of automatic
valves and actuators. Referencing to a control systems
manufacturer or supplier will not necessarily result in
optimum valve selections or products which result in the
greatest value for the money spent.
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