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

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

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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.

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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

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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

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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,

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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.

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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

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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

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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.

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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

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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.

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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

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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

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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

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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

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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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