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. 4.1.2 Type 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. 4 - 1
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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.
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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.
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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
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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,
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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.
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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