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2/744 HPAC ENGINEERING OCTOBER 2011

DESIGN CRITERIA FOR PRIMARY/SECONDARY AND PRIMARY-LOOP-ONLY SYSTEMS

tion that in real-life conditions, the

relative chilled-water-flow variation

in the plant is changing in direct pro-

portion to the relative plant cooling

load, or W ≈ Q 1:

WFTDRREQ = (CPDCL ÷ CPCLMFC)

× SCSCF (3)

where:

CPDCL = chiller-plant design cool-

ing load in tons

CPCLMFC = chiller-plant cooling

load in tons at the end of mechanical

cooling season or at the load associ-

ated with implementation of various

control strategies (i.e., reset chilled-

water temperature control, etc.)

SCSCF = system control strategy

correction factor, which is equal to

or less than 1. SCSCF equals 1 for

systems that do not utilize reset

chilled-water temperature control at

the chiller plant or variable airflow

rate at terminal units, etc. Otherwise,

As a result, the available chilled-

water-flow turndown ratio (which

represents system water flow con-

trollable range shown in Figure 2) for

the PLOVF system (WFTDRAVLPLOVF)

should be reduced as compared with

WFTDRAVLP/S for the P/S system by

the overall system operational safety

factor (OSOSF) of 0.82 to 0.9. Thus:

WFTDRAVLP/S = WFTDRDEAVL

WFTDRAVLPLOVF = (0.82 to 0.9) ×

WFTDRDEAVL

Required Evaporator Chilled-

Water-Flow Turndown Ratio

Required evaporator chilled-wa-

ter-flow turndown ratio (WFTDRREQ)

is another important parameter while

selecting P/S or PLOVF systems for a

chiller-plant application. WFTDRREQ

for a chilled-water plant can be eval-uated for P/S and PLOVF systems

from the following equation, which

is based on the conservative assump-

determined from Equation 1.

Schematical representation of

comparative available chilled-water

turndown ratios for P/S and PLOVF

systems is shown in Figure 2. Be-

cause of the independent-loops ar-

rangement of the two-loop system

and its unique architecture, the P/S

system is able to utilize all of the

available evaporator chilled-water-

flow turndown ratio. On the otherhand, a PLOVF system, because of

the specifics of its architecture and

rigid dependency of the flows via

the generation and distribution pip-

ing loop system, is able to utilize

less than full available evaporator

chilled-water turndown ratio. Figure

2 outlines suggested parameters, in-

cluding operational safety factors

for F1HL and F1LL, to avoid PLOVF-

system shutdown on high and low

chiller evaporator flows or prema-ture addition or removal of a chiller

and associated ancillary equipment

from the line.

1.1 to 1.05 F1LL

Notes:F1HL—Evaporator allowable high-limit water-flow rate, gpmF1LL—Evaporator allowable low-limit water-flow rate, gpm∆T

DEMAX, ∆T

DEMIN—Maximum or minimum evaporator

design chilled-water temperature, respectively, °F∆T

DEDP—Distribution piping system design temperature

differential, °FWFTDR

AVL P/S, WFTDR

AVL PLOVF—Available chilled-water-flow

turndown ratio for P/S and PLOVF systems, respectivelyOSOSF—Overall system operational safety factor forP/S and PLOVF systems

F1HL F1HL

F1LL F1LL∆TDEMAX

∆TDEMIN

System operationalsafety factor for F1HL

WFTDRAVL P/S

=

1.0 × (F1HL/F1LL) ∆ T D E D P

0.9 to 0.95 F1HL

WFTDRAVL PLOVF

=(0.9 to 0.95) ×

F1HL/(1.1 to 1.05) ×F1LL = (0.82 to 0.9) ×

(F1HL/F1LL)

OSOSF = 0.82 to 0.9

P/S systemwater flow

controllable range

PLOVF systemwater flow

controllable range

System operationalsafety factor for F1LL

OSOSF = 1.0

FIGURE 2. Chiller evaporator comparativeavailable chilled-water turndown ratio

schematical representation for P/S and

PLOVF systems.

Notes and symbols: 1 and 2 — Series chiller-evaporator arrangement bypass pipes3 — Chiller-plant decoupling pipe

4 — P/S- or PLOVF-system primary-loop pump

5 — P/S-system secondary-loop pump

6 — Chiller-evaporator arrangement A and B have equal cooling loads

7 — For the specifics of control system arrangements, see1

W — Chiller-plant cumulative relative water-flow rate via chillers’ evaporators

WCPGS

, WCPDS

— Chiller-plant water flow rate via generation and distribution systems, respectively

TINT

— Intermediate chilled-water temperature between chillers 1 and 2

— Additional electrical valves for series chiller-evaporator control arrangements

VFD — Variable-speed pumps control arrangement1

Relative parameters are shown overlined

VFD

VFD

VFD

VFD

Load

Load

Chiller #1

Chiller #1 Chiller #2

Chiller #2

B. Series chillers evaporation arrangement

A. Parallel chillers evaporation arrangementW

CPGSW

CPDS

WCPGS

WCPDS

Distribution SystemGeneration system

Distribution SystemGeneration system

W/2

W/2

W W

W

W

4

2

1

4 5

5

3

3

TINT

FIGURE 1. Examples of a parallel chillers evaporators arrangement and a series chiller

evaporator arrangement.


3/7

SCSCF should be assumed to be less

than 1.

Reset Temperature Control and

Required Chiller-Plant Chilled-

Water Turndown Ratio

Figure 3 is illustrative of the impact

of the reset chilled-water-tempera-

ture control strategy. This is analyzed

for the cooling coil (which is assumed

to be maintained at ideal clean heat-

exchanger-surface conditions) with a

design load of 28.5 tons.1 The air-han-

dling units are operating with con-

stant airflow; we assumed a minimum

mechanical cooling load of 2.9 tons.

The upper graph shows four op-

tional control strategies related to

chilled-water supply temperature.

Option 1 is associated with constant

relative chilled-water temperature T1

= 1 (T1 = 40°F) over the system’s en-

tire operational hours. (Note: relative

parameters are shown overlined.)

Option 2 resets relative chilled-water

temperature from T1 = 40ºF (T1 = 1)

at design conditions with relative

cooling load Q = 1 to T1 = 47.5ºF (T1 =

1.19) at Q = 0.1.

Options 3 and 4 are a combination

of options 1 and 2. Option 3 main-

tains design chilled-water tempera-

ture of 40°F (T1 = 1) until Q is reduced

to 0.52. After that, it gradually resets

relative chilled-water temperature to

47.5°F (T1 = 1.19) at Q = 0.1. Option

4 maintains relative design chilled-

water temperature of 40°F (T1 = 1)

until Q is reduced to 0.29 and after

that gradually resets relative chilled-water temperature to 47.5°F (T1 =

1.19) at Q = 0.1.

The graph in the middle of Figure

3 depicts relative temperature-dif-

ferential variations associated with

control strategies shown in the upper

graph. The relative temperature dif-

ferential (∆T) increases for Option 1

from ∆T = 1 at Q = 1 to ∆T = 1.25 (or

by a factor of 1.25) when the relative

cooling load, Q, decreases from the

design value of 1 to 0.1; ∆T decreasesfor options 2, 3, and 4, respectively,

from 1 at design load Q = 1 to 0.2 at Q

= 0.1 or by the factor of 5.

The bottom graph in Figure 3

shows relative chilled-water-flow-

rate (W) variation as a function of the

relative cooling load Q. Under the

Option 1 chilled-water-temperature

control strategy, W reduces from 1

to 0.08 when Q is lowered from 1 at

design conditions to 0.1, respectively.

At the same time, options 2, 3, and 4experience reduction of W from 1 at

design conditions to 0.5 at the end of

the mechanical cooling season.

Thus, Figure 3 indicates that the

reset chilled-water temperature con-

trol has a pronounced impact on

WFTDRREQ in the chiller plant. The

unchanged magnitude of chilled-

water temperature T1 = 40°F for the

considered conditions (Option 1)

leads to the reduction of the relative

chilled-water flow from 1 to 0.08 (re-sulting in WFTDRREQ = 12.5), while

Q fluctuates from 1 to 0.1. Option 2

would result in W variation from 1 to

OCTOBER 2011 HPAC ENGINEERING 45


Relative cooling load, Q

1.20

1.15

1.10

1.05

1.00

0.95

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

1.2

1.0

0.8

0.6

0.4

0.2

0.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Option 1

Option 2

Option 3

Option 4

Option 1

Option 2

Option 3

Option 4


0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1


R e l a t i v e c h i l l e d - w a t e r

t e m p e r a t u r e d i f f e r e

n t i a l , ∆ T

R e l a t i v e c h i l l e d - w a t e r

fl o w r a t e ,

W

R e l a t i v e s u p p l y

c h i l l e d - w a t e r t e m p

e r a t u r e ,

T 1

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Notes: Supply chilled water temperature at design relative cooling load Q = 1 is assumed to be 40°FOption 1: Constant chilled-water temperature maintained at its design level VFTDR

REQ = 12.5

Option 2: Linear reset chilled-water temperature control from design conditionsto 0.1 relative cooling load (WFTDR

REQ = 2)

Option 3: Reset chilled-water temperature control at 0.52 relative cooling load and below (WFTDRREQ

= 2.1)

Option 4: Reset chilled-water temperature control at 0.29 relative cooling load and below (WFTDRREQ = 4.2)

Option 1

Option 2

Option 3

Option 4

FIGURE 3. Relative chilled-water-plant parameters at various cooling loads and supply

chilled-water temperature control strategies.


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0.5 (WFTDRREQ = 2).

Under Option 3, the magnitude of

W is reduced from 1 to 0.47 (WFT-

DRREQ = 2.1) at Q = 0.52 and then W

increases to 0.5 (WFTDRREQ = 2) at Q

= 0.1. Finally, under Option 4, W is re-

duced from W = 1 to W = 0.24 (WFT-

DRREQ = 4.2) and then W increases to

W = 0.5 (WFTDRREQ = 2). The higher

magnitude of WFTDRREQ should be

selected between the two turndown

ratio values that relate to options 3

and 4 to establish the resulting value

of WFTDRREQ for the system.

Chiller-Water-Plant Required

Number of Chillers

The number of chillers sharing the

plant load at a given chiller’s load

safety factor should be selected for

both P/S and PLOVF systems with

the purpose of making WFTDRAVL

equal or higher than WFTDRREQ to

eliminate water flow via the decou-

pling pipe and to optimize chiller-

plant electrical energy use.

The required number of chillers

for the plant optimal operation can

be calculated from the following

equation:

NREQ = (WFTDRREQ ÷ WFTDRAVL) ×

CPLSF (4)

where:

WFTDRAVL = available chilled-wa-

ter turndown ratio for a chiller plant

with P/S and PLOVF systems

WFTDRREQ = required chilled-wa-

ter turndown ratio for a chiller plant

with P/S and PLOVF systems

CPLSF = chiller-plant load safety

factor

If the installed number of chill-

ers in the plant (NI) is less than NREQ,

then WFTDRRE Q is greater than

WFTDRAVLP/S & PLOVF and the applica-

tion of the P/S system will be ben-

eficial from the energy-conservation

point of view as compared with a

PLOVF system. If NI is greater than or

equal to NREQ, then WFTDRREQ is less

than or equal to WFTDRAVLP/S & PLOVF,

and both P/S and PLOVF systems are

equally energy-efficient.

Chilled-Water Plants With Parallel

and Series Chiller Evaporators

Schematical representations of the

chiller plant with a parallel and series

connection of the chillers’ evapora-

tors and P/S and PLOVF systems are

shown in Figure 1.

Chilled-water systems with par-

allel chiller-evaporator connections

(Figure 1A) are common. The chiller’s

plant load and the relative water-flow

rate is equally (W ÷ 2) shared by a

number of chillers on line until it is

reduced to a single chiller. The paral-

lel chiller arrangement is relatively

46 HPAC ENGINEERING OCTOBER 2011

Circle 177



5/7OCTOBER 2011 HPAC ENGINEERING 47


the entire mechanical cooling season.

We considered four control strat-

egy options outlined earlier in Figure

3. The conditions shown in Table 1 for

a P/S system under Option 1 (without

chilled-water reset temperature con-

trol) will require the installation of

nine 67-ton chillers. The same option

for a PLOVF system will require the

unit cooling-coil parameters will not

change. Each table also includes two

values of OSOSF for PLOVF system

that are either 0.9 (Option 1A) or 0.82

(Option 1B). Required number of chill-

ers is calculated utilizing Equation 4 to

satisfy the conditions under which a

chiller plant will not have water flow-

ing through the decoupling pipe for

simple in operation and maintenance.

Chilled-water systems with series

chiller-evaporator connections are

mostly utilized in custom-made appli-

cations. The same series connection

is equally applicable for a P/S and

PLOVF system and variable-speed

pump control. The series-chillers ar-

rangement (assuming that identical

chillers to be employed in both series

and parallel arrangements) requires

a substantial increase of the pressure

drop via the evaporator (because of

a twofold W increase in relative wa-

ter flow via a chiller at design con-

ditions). In this instance, the design

pressure drop will be approximately

eight times higher in a series ar-

rangement (Figure 1B) than in a par-

allel arrangement (Figure 1A).

The series arrangement is less

dependable in operation and will

require employment of additional

electrically operated control valves

to remove/add a chiller from/to the

line to adjust the load or to isolate

a failed chiller (Figure 1B). The se-

ries arrangement will make the reset

chilled-water temperature control

more challenging to implement, even

for a P/S system because of the in-

troduction of the additional variable

parameter (TINT in Figure 1B) repre-

senting the temperature of chilled

water leaving Chiller 1 and entering

Chiller 2.

When the identical and equal

number of chillers with similar

load-sharing strategy is utilized, the

chilled-water turndown ratio will bethe same for both parallel and series

chillers’ evaporator arrangements.

Specifying Chillers

Tables 1 and 2 demonstrate com-

parative chiller-plant analysis of the

required number of chillers for P/S

and PLOVF systems. The data in these

tables are related to two sets of values

for distribution-piping-system design

temperature differential: ∆TDEDP =

15°F (Table 1) and ∆TDEDP = 10°F (Table2). For the purpose of analysis of the

∆TDEDP impact on required number

of chillers, we assumed that terminal-

TABLE 1. Comparative chiller-plant turndown ratios and required number of chillers for

P/S and PLOVF systems (∆ T DEDP = 15°F).

Parameter

P/SOption 1

PLOVFOption 1A

PLOVFOption 1B

P/SOption 2

P/SOption 3

P/SOption 4

Overall system operationalsafety factor, OSOSF

1 0.90 0.82 1 1 1

WFTDRREQ 12.5 12.5 12.5 2 2.1 4.2WFTDRAVL. DES 1.5 1.3 1.2 1.5 1.5 1.5

Required number ofchillers, NREQ

8.4 9.3 10.2 1.3 1.4 2.8

Installed number ofchillers, NI

9 10 11 2 2 3

Required individual chillersdesign load, tons

67 60 55 300 300 200

Notes:1. Chiller plant design cooling load, tons: 6002. Installed chiller plant cooling load capacity, tons: 6003. Assumed chiller-plant load safety factor: 14. Assumed maximum chiller evaporator design chilled-water temperature differential

(∆TDEMAX) = 22.4 °F

5. Assumed distribution piping system design temperature differential (∆TDEDP) = 15 °F6. Control options 1, 2, 3 and 4 are in reference to Figure 37. AHUs serviced by the chiller plant are assumed to be operating with constant-air-flow control

arrangement8. Reset chilled water temperature control is assumed to be applicable only for P/S system1

TABLE 2. Comparative chiller-plant turndown ratios and required number of chillers for

P/S and PLOVF systems (∆ T DEDP = 10°F).

Parameter

P/SOption 1

PLOVFOption 1A

PLOVFOption 1B

P/SOption 2

P/SOption 3

P/SOption 4

Overall system operationalsafety factor, OSOSF

1 0.90 0.82 1 1 1

WFTDRREQ 12.5 12.5 12.5 2 2.1 4.2

WFTDRAVL. DES 2.2 2.0 1.8 2.2 2.2 2.2

Required number of

chillers, NREQ5.6 6.2 6.8 0.9 0.9 1.9

Installed number ofchillers, NI

6 7 7 1 1 2

Required individual chillersdesign load, tons

100 86 86 600 600 300

Notes:1. Chiller-plant design cooling load, tons: 6002. Installed chiller-plant cooling-load capacity, tons: 6003. Assumed chiller-plant load safety factor: 14. Assumed maximum chiller evaporator design chilled water temperature differential

(∆TDEMAX) = 22.4 °F5. Assumed distribution piping system design temperature differential (∆TDEDP) = 10 °F6. Control options 1, 2, 3 and 4 are in reference to Figure 37. AHUs serviced by the chiller plant are assumed to be operating with constant-air-flow control

arrangement

8. Reset chilled water temperature control is assumed to be applicable only for P/S system1


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48 HPAC ENGINEERING OCTOBER 2011

installation of either 10 60-ton chill-

ers (Option 1A) or 11 55-ton chillers

(Option 1B). Thus, the PLOVF system

will utilize a higher number of chill-

ers to satisfy given conditions.

Table 1 further indicates that the

number of installed chillers (NI) for a

P/S system under Option 2 (assuming

continual reset water-temperature

control through the entire cooling

season) could be reduced to two 300-

ton chillers. The number of installed

chillers (NI) under options 3 and 4

(assuming reset water-temperature

control during off-design conditions)

for a P/S system could be reduced to

two 300-ton chillers or three 200-ton

chillers, respectively.

Table 2 shows the number of chill-

ers required for the system with

∆TDEDP = 10ºF could be reduced by

the factor of 1.5 compared with the

system with ∆TDEDP = 15ºF because

of the higher WFTDRAVL value cal-

culated from Equation 1. Still, the

number required for a P/S system

under options 2, 3, and 4 remains

substantially lower than for a PLOVF

system.

In a real-life situation, it is unlikely

(perhaps, with the exception of mod-

ular chillers) that the number of chill-

ers would be increased to nine or 10

(Table 1) or to six or seven (Table 2)

for a chiller plant with P/S and PLOVF

systems because of the additional

cost associated with the multiple

chillers and their ancillary equipment

(cooling towers, condenser pumps,

controls, etc.). Because of that, two

300-ton chillers are most likely to be

installed for the considered plant.

This is equivalent of the condi-

tions when NI is less than NREQ and

WFTDRREQ is greater than WFT-

DRAVL for both P/S and PLOVF sys-

tems. Under these scenarios, a P/S

system will provide lower annual

electrical energy usage for the chiller

plant compared with a PLOVF sys-

tem. As an additional benefit, a P/S

system will reduce the number of

times a system swtiches

from two-

chiller to single-chiller operation and vice versa by about 10 percent to 18

percent, compared to a PLOVF sys-

tem with an OSOSF of 0.9 or 0.82,

respectively.

Summary

This investigation specifies design

criteria and required conditions at

which P/S and PLOVF systems will

have no energy wasteful chilled-wa-

ter flow via the decoupling pipe of

the chiller plants with parallel andseries chiller-evaporator arrange-

ments and essentially operate as sin-

gle-loop systems. As long as both P/SCircle 178



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and PLOVF systems operate in this

mode (assuming both systems have

the same major parameters, identi-

cal chiller and evaporator arrange-

ments, equal design chilled-water-

pump horsepower, overall flow, head

pressure, variable-frequency-drive

controls, etc.), their annual electrical

energy use will be optimal and equal.

The other findings of the article are

as follows:

• Satisfying the above conditions

requires a larger number of chillers

with a lower design cooling capacity

as compared with current engineer-

ing practice for both P/S and PLOVF

systems and may call for the employ-

ment of modular chillers.

• The procedure of selecting and

specifying available and required

chilled-water turndown ratio for the

evaporators of a chiller plant with

P/S or PLOVF systems is presented

to optimize electrical energy use.

• The number of chillers for P/S

or PLOVF systems is selected based

on the optimal distribution-piping-

system temperature differential

and chiller-plant load safety factor

to match the required and available

magnitudes of the chilled-water turn-

down ratio.

• A P/S system requires a lower

number of chillers in a chiller plant

compared with a PLOVF system

because of the higher controllable

range of water-flow rate for P/S sys-

tem and the ability to realize a reset

chilled-water-temperature control

strategy.• A PLOVF system compared with

a P/S system—assuming both sys-

tems have an equal number of chill-

ers and chiller capacities—will need

more frequent changes in the num-

ber of operating chillers and their as-

sociated ancillary equipment over the

cooling season because of the lower

available magnitude of chilled-water

turndown ratio.

• The similar procedures and de-

sign criteria are equally applicable forthe selection of available and required

magnitudes of a hot water boiler-plant

turndown ratio, as well as a number

of boilers in the central boiler plant

with P/S or PLOVF systems.

References

1) Burd, A., & Burd, G. (2010, De-

cember). Primary/secondary-loop vs.

primary-loop-only systems. HPAC

Engineering , pp. 36-45. Available at

http://bit.ly/Burd_1210

Did you find this art icle useful? Send

comments and suggestions to Senior

Editor Ron Rajecki at ron.rajecki@

penton.com.

Circle 179


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1011 HPACEngineering Burd