Energy Cogeneration Handbook
`
`Criteria for Central Plant Design
`
`Design criteria for central plants that facilitate energy conversion, utilization, and
`conservation; an evaluation of project alternatives and an examination of systems and
`their functions to achieve optimum overall design in the generation of heating, cooling,
`and electricity.
`
`.by
`
`GEORGE POLIMEROS
`
`INDUSTRIAL PRESS INC.
`200 Madison Avenue, New York, N.Y. 1015 7
`
`GZJKDKV!3112
`
`

`
`Library of Congress Cataloging in Publication Data
`
`Polimeros, George.
`Energy cogeneration handbook.
`
`Includes bibliographies and index.
`1. Cogeneration of electric power and heat.
`I. Title.
`1981
`TK1041.P64
`ISBN 0—8311—1130-5
`
`621 .1'9
`
`}
`so-zmss
`'
`
`FIRST PRINTING
`
`ENERGY COGENERATION HANDBOOK
`
`Copyright © 1981 by Industrial Press Inc., New York, N.Y. Printed in the United States of America. All
`rights reserved. This book, or parts thereof, may not be reproduced in any form without permission of the
`publishers.
`
`GZJKDKV!3112
`
`

`
`CHAPTER 3
`
`Chiller Arrangements
`
`Air conditioning accounts for a substantial portion of
`the annual energy consumption in most buildings. Re-
`frigeration constitutes a large sector of industrial energy
`applications.
`Indubitably, the efficiency of these opera-
`tions is of immense importance.
`their
`Since chillers are energy-consuming machines,
`many sizes, numbers and arrangements in various ap-
`plications are not only evidence of design flexibility in
`handling air conditioning or refrigeration loads, but are
`proof that design engineers consider seriously the question
`of energy use.
`p
`This chapter addresses the factors that influence chill-
`er selection and arrangement. There are many possible
`arrangements;
`those examined here are the four basic
`configurations exemplified in Fig. 3-1, as well as others
`shown in Figs. 3-8 through 3-16. These are somewhat
`complex, but appear often enough in actual practice
`to merit comment.
`
`3.! Basic Arrangement
`
`Two or more chillers may be employed, either in
`series or in parallel, in the four basic arrangements :1,
`b, c, and d of Fig. 3-1. A fifth, at series-parallel arrange-
`ment,
`is illustrated in Fig. 3-9, Section 3.6. Each ar-
`rangement presents the designer with specific choices,
`requiring a grasp of design objectives. Understanding
`the significance of the four basic arrangements will
`facilitate examination and analysis of the more involved
`cycles. These four arrangements deal with water side
`control and are predicated on full water flow through
`both evaporator and condenser and a fixed leaving chilled
`water temperature. <
`Central cooling plants invariably have more than one
`chiller. The justification for more than one chiller and the
`determination of the final number of chillers depend on
`a variety of factors.
`
`At the present time, machine size is limited by ship-
`ping dimensions and a lack of technical and operating
`experience for machines in excess of 7000-8000 tons.
`However,
`13,000-ton (45 760 kW) machines
`can be
`ordered.
`Large machines are shipped as components
`to be field-assembled. Undoubtedly, for plants in excess
`of 50,000-tons (176 000 kW) machines of greater than
`13,000-ton capacity can ‘be used; the huge refrigeration
`
`UN” A
`CONDENSER
`
`.
`
`'
`
`'
`
`PARALLEL
`(A)
`CONDENSERS
`PARALLEL
`CHILLERS
`
`UN|T A
`CONDENSER
`__?._..m.
`
`UNIT B
`| CONDENSER
`
`(:2)
`PARALLEL
`ccuoenseas
`SERIES CHILLERS
`
`55‘
`
`1
`‘
`
`.
`‘
`
`i
`
`55'
`
`unit A
`CHILLER
`
`__.:. 3
`50°
`‘l
`
`UNIT B
`CHILLER
`
`45‘
`-———
`
`uun A
`CONDENSER
`
`90°
`
`unrr B
`‘;
`.
`‘ CONDENSER
`
`95'
`_._..
`
`um A
`CHILLER
`
`50‘
`
`l
`
`um a
`CHILLER
`
`UNIT A
`CONDENSER
`
`°
`
`UNIT B
`CONDEN5-ER
`
`45"
`__>
`
`“
`
`UN|T B
`50'
`UNIT A
`CH|LL.ER "'i—" » CHILLER
`
`i __
`
`(C)
`SERIES CONDENSERS
`SERlES CH\LLERS
`(CONDENSER WATER
`PARALLEL TO
`CHlLLER wuss)
`
`(D)
`SERIES CHILLERS
`SERIES CONDENSERS
`(CONDENSER WATER
`COUNTERFLOW TO
`CHILLER WATER)
`
`Figure 3-1. Four basic chiller and condenser arrangements em-
`ploying conventional
`temperatures [temperatures given are F:
`95F (35C); 90F (32.2C); 85F (29.4C); 55F (12.8C); 50F (10C);
`45F (7.2 C)] .
`
`GZJKDKV!3112
`
`

`
`Ch. 3
`
`plants of the future will require machines of increased
`tonnage. Commercial demand is bound to determine
`ultimate size development. Open centrifugal compressors,
`rather than hermetic, are used in large central plants,
`when the need is for centrifugals, since hermetic centri-
`fugal machines are currently limited to approximately
`1200 tons (4224 kW).
`Spare or standby capacity is another important factor.
`A fully redundant unit
`is ordinarily too expensive to
`tolerate in commercial applications.
`In industrial work,
`it becomes a matter of process needs and weighing of
`acceptable alternatives. With no standby capability,
`a machine breakdown is liable to deprive the plant of
`its services for a few hours or many months. Decreasing
`the impact of the loss of one machine, not taking into
`consideration the size and total number of chillers,
`will
`insure minimal
`inconvenience except at
`times of
`top load conditions. Machine breakdowns are associ-
`ated with the concept of reliability, which for centri-
`fugal chillers is more than 99%.
`In general, repair of large machines may be too costly
`and the downtime prohibitive. Although even a small
`hermetic machine may be out of service for a long time,
`dismantling of large
`chillers
`is Very time-consuming
`and spare parts ‘for them may be harder to obtain and
`the need for factory assistance more probable.
`Efficiency and surging are two aspects that must be
`examined carefully. Large machines perform best at full
`load and become increasingly inefficient at diminishing
`part
`loads. At low-load ranges, centrifugal compressors
`are subject
`to surgingpand other operating difficulties.
`Thus,
`it
`is practical to select centrifugal machines for
`peak efficiency at prevailing ‘part loads, perhaps in the
`neighborhood of 75% load. Efficiency selection at part
`load makes surging a little less likely or, at least, shortens
`the periods of occurrence, since most machines operate
`at partial loads most of the time.
`If a substantial number of chillers are involved, the
`question of series versus parallel arrangement emerges
`from the beginning. A series arrangement is limited to
`three or four chillers, bearing in mind that a minimum
`six degrees of chilled water temperature difference is
`needed for each chiller. On the other hand, an unlimited
`number of chillers can be piped in parallel.
`The designer must strike aneconornic and operational
`balance in arriving at
`the total number of machines,
`considering power consumption, piping and valving econ-
`01113’, maintainability, total installed cost, operating costs,
`513306 limitations, the extent of automatic operation of
`the refrigerant, and water cycles and availability and
`capability of operating personnel.
`
`3.2 Arrangements (Fig. 3-1), Chillers in Parallel,
`Condensers in Parallel
`‘
`
`Both evaporators and condensers are piped in parallel,
`an arrangement with widespread‘ acceptance and one that
`designers
`are most
`familiar with, offering extensive
`flexibility and the ability to accommodate phased ad-
`ditions.
`
`A cardinal advantage of parallel flow is that it lends
`itself extremely well to variable flow, especially in pri-
`mary-secondary loops, and is the main reason for its
`popularity. As load decreases, condenser and chilled
`water pumps can be disengaged, saving horsepower and
`contributing to a more economical operation.
`Based on chiller heat transfer area of 6—8 ft” per ton
`(0.15—O.2l m2 /kW) and maximum water velocity of 12 ft
`per sec (3.64 m/s), units connected in parallel require
`two- or
`three-pass evaporators
`and,
`therefore, offer
`excellent heat
`transfer performance for a large chilled
`water range.
`In the last
`two decades, economics have
`tended to increase water velocities both in the chiller
`and condenser sections from 8 to approximately 10 ft
`per sec (2.4—3 m/s), at times, even higher.
`A great deal is known about this arrangement and many
`designers
`feel comfortable in using it. Nevertheless,
`there are some disadvantages of a rather serious nature,
`especially in the case of full system flow. To maintain
`full flow at partial loads, return chilled water must be
`circulated through all units, both active and inactive,
`with the non-refrigerated water being mixed with water
`that has passed through the evaporators and chilled to
`design temperature.
`The mixture, of course,
`is at a
`higher temperature than design.
`If design temperature
`water is desired,
`the active chiller must overchill the
`water passingthrough it. The problem of overchilling
`is examined a little further on in this section. In general,
`overchilling increases the risk _of an evaporator freeze-up
`because of lower suction temperatures and tends to in-
`crease horsepower
`requirements, and therefore oper-
`ating costs. Unpleasant alternatives to overchilling in-
`clude a variable chilled water supply temperature above
`design levels, if one machine is operated, or operation
`of both machines at very low loads, which requires more
`horsepower per ton and is entirely uneconomical.
`As for condensers in parallel,
`they present few diffiw
`culties. Water temperature problems do not exist, and
`lift is identical for all units. Whether condenser water
`is allowed to circulate through an inactive unit or not,
`does not affect the performance of the cooling tower.
`Reduced condenser water flow is desirable at times be-
`
`cause it
`
`reduces pumping requirements (see Section
`
`GZJKDKV!3112
`
`

`
`60
`
`4.10 “Optimization of Condenser and Chilled Water
`Flow”). Various techniques can be used to minimize
`low flow problems. The subjects of reduced water flow
`and variable primary-secondary flow are discussed in
`Chapter 7.
`Undoubtedly, a great deal more piping, valving, pumps,
`‘and instrumentation are needed for the parallel arrange-
`ment than for the series. However, pipe sizes may be
`reduced due to smaller flows through each branch. Piping
`in parallel is quite involved spatially and there is a greater
`abundance so that, in an overall sense, costs are high. On
`the other hand,
`series piping throughout
`the system
`handles full flow at all times and is full size, including the
`bypasses around the chillers. Costs may exceed those of
`the parallel arrangement. A piping layout, a flow diagram,
`and an accurate estimate will establish relative costs.
`When examining the question of overchilling more close-
`ly one comes to the conclusion that the problem is caused
`perhaps by an unrealistic requirement for constant flow in
`the primary circuit at all times, regardless of load size.
`Specification of constant flow is a holdover from past
`practices, when constant
`flow for small systems was
`acceptable economically and offered the hydraulic stabil-
`ity that most engineers prefer. Full flow in large systems
`‘ is wasteful of pumping horsepower and, in terms of wear
`and tear, of the machines and system in general.
`Overchilling may be eliminated by adhering to one of
`four main curative methods. First, all units may be op-
`erated together down to a small portion of their capacity
`in order to handle part loarfs.
`In doing so, the chillers
`
`T
`
`
`
`PERCENTPOWER
`
`10
`
`20
`
`3O
`
`6O 70
`4O 50
`PERCENT LOAD
`
`BO
`
`90
`
`1OO
`
`Figure 3-2. Typical reduced part load performance for centrifugal
`compressor system.
`
`Ch. 3
`
`sacrifice operating economy.
`illustrates the point:
`
`The following example
`
`Assume a 1000-ton (3520 kW) chiller requiring abgut
`1000 Bhp (746 kW) at design conditions. If two identical
`1000-ton (3520 kW) units are installed in parallel and at
`30% system load, each chiller must operate at 300 mm
`(1056 kW) and 330 Bhp (246 kW) (Point A, Fig. 3-2) 3
`total of 660 Bhp (492 kW). If the 30% system load were
`to be satisfied by operating only one machine, then it
`would operate at 60% capacity and 57% power (Point B)
`or 570 Bhp (425 kW) a clear saving of 90 Bhp (67 kW)_
`This approach introduces
`some complications.
`It
`requires automatic valves for each chiller if one pump is
`used, or a pump for each chiller with a check valve in its
`discharge. Furthermore, it is assumed that the pumps are
`non-overloading; if not, a pump bypass or a back pressure
`valve may be necessary to control the pump operating
`point.
`‘
`A second alternative to: overchilling is reduction of
`overall system flow by maintaining full flow only through
`the active units, if building flow requirements permit it
`and if pump transients are mfanageable.
`A third solution to accept chilled Water temperatures
`above design conditions, if it does not present control
`problems and can satisfy load conditions throughout
`the project. Ordinarily, the supply chilled water tem-
`perature is constant in large projects because buildings
`peak at different times and variable temperature control
`would hamper design flexibility. On the other hand,
`advocates of this solution maintain that higher design
`temperatures at part load can be accommodated if cool-
`ing coils are oversized when full load is anticipated in
`one building at the same time. Oversizing coils can be
`considered a viable design alternative, if it is applied to
`a limited number of coils.
`
`A fourth method is a series arrangement of evaporators,
`provided economic as well as hydraulic considerations
`and project design philosophy are favorable.
`Over-
`chilling is not possible since, at part load, chillers may
`be deactivated, while through flow is maintained.
`Up to this point, overchilling and methods to avoid
`it have been ; defined. The question remains as
`to
`Whether some -degree of overchilling may be usefully
`employed and; what may be its practical
`limits in
`application. Tables 3-1 through 3-4 examine two and
`three chillers piped in parallel, using 10F (5 .55 C) and
`18 F (IOC)
`temperature differentials between supply
`and return water.
`
`Table 3-1 indicates that two chillers in parallel with
`a 10F (5.55 C) temperature difference constitute a usable
`combination. However,
`the low 37F temperature is
`
`GZJKDKV!3112
`
`

`
`Ch. 3
`
`Table 3-1
`
`2 Ideal Performance of Two Chillers in Parallel
`[Design Chilled Water Supply 42F (5.6 C), Design Chilled Water Return 52F (1 1.1C)~l0F Temperature Differential]
`
`% Load % Load
`of
`of
`System
`Unit
`
`Supply
`Temp.,
`F
`
`Return % Load % Load
`Temp.,
`of
`of
`F
`System
`Unit
`
`Supply
`Temp.,
`F
`
`Return
`Temp.,
`F
`
`Both Chillers Operating
`
`50
`37.5
`25
`18.75
`12.5
`6.25
`
`100
`75
`50
`37.5
`25
`12.5
`
`42
`42
`42
`42
`42
`42
`
`52
`49.5
`47
`45.75
`44.5
`43.25
`
`50
`37.5
`25
`18.75
`12.5
`6.25
`
`100
`75
`50
`37.5
`25
`12.5
`
`First Chiller Operating, Second Chiller Bypassing
`(First chiller providing 42F water; mixture temperature increasing above 42 F)
`100
`50
`75
`37.5
`50
`25
`12.5
`25
`
`First Chiller Operating, Second Chiller Bypassing
`(First chiller overchilling, mixture temperature at 42 F)
`
`47
`45.75
`44.5
`43.25
`
`and may need resetting to 38——39F to
`problematic
`prevent evaporator freeze-up. Two and three chillers in
`parallel using an 18 F (lOC)
`temperature differential
`are unfeasible, since they require overchilling to 3l~
`28 F.
`'
`,
`
`In general, a 10 F (5.5SC) chilled water range may
`prove practical under
`limited conditions for
`smaller
`central installations of a few thousand tons with short
`
`Primary piping networks, and not more- than two chill-
`ers able to tolerate overchilling and not requiring close
`temperature control.
`If the number of chillers or the
`temperature differential
`increases
`to an appreciable
`extent, overchilling is not a useful design tool. Again,
`we conclude that an arrangement based on constant
`chilled water flow throughout
`the system at all times
`is not ordinarily attractive because constant
`flow at
`partial loads contributes to higher operating costs.
`
`3-3 Arrangement b (Fig. 3-ll, Chillers in Series,
`Condensers in Parallel
`
`Arrangement b is next in popularity to arrangement
`4- The condensers are piped in parallel, since no con-
`
`denser water temperature problem exists. Chillers in
`series deliver design temperature chilled water at all
`loads, entirely eliminating the overchilling problem of
`the parallel arrangement.
`The designer must decide during the early stages of
`a project whether series or parallel chiller flow is best-
`suited to his project, taking the special conditions of a
`particular job into consideration.
`If constant flow is
`envisaged for the primary chilled water circuit, the series
`arrangement is preferable. Under special circumstances,
`some variation in flow can be tolerated even in a series
`
`arrangement, although this violates the old taboo against
`varying flow through the chiller for fear of a freeze-up.
`Constant flow may be the result of staggered demand,
`when diversity remains fairly constant. Large temperature
`differentials can dovetail well into a constant flow sit-
`uation.
`'
`
`Although constant refrigeration loads do not neces-
`sarily accompany constant
`flo.w,
`they very often do.
`The series arrangement is strongly indicated when con-
`stant refrigeration loads are present.
`The point can be made that, at full-load maximum
`horsepower, savings can be effected by using series flow
`
`GZJKDKV!3112
`
`

`
`[Design Chilled Water Supply 42F (5.6 C), Design Chilled Water Return 52F (11.1 C)—10F Temperature Differential]
`
`Table 3-2
`Ideal Performance of Three Chillers in Parallel
`
`%
`
`Supply
`Temp.,
`F
`
`Return
`Temp , ,
`
`% Load
`of
`Unit
`
`Supply
`Temp.,
`F
`
`Return
`Temp .,
`F
`
`52
`49.5
`47
`45.75
`44.5
`43.25
`
`% Load
`of
`System
`All Chillers Operating
`100
`33.33
`75
`25
`50
`16.6
`12.5
`37.5
`25
`8.33
`4.16
`12.5
`
`% Load
`of
`Unit
`
`Supply
`Temp .,
`F
`
`Return
`Temp.,
`F
`
`% Load % Load
`of
`of
`System
`Unit
`
`Supply
`Temp. ,
`F
`
`Rctum
`Temp _,
`F
`
`33.33
`25
`16.66
`12.5
`8.33
`4.16
`
`100
`75
`50
`37.5
`.25
`12.5
`
`45.3}
`44.5
`43.25
`
`First Two Chillers Operating, Third Chiller Bypassing
`___.,..
`(First two chillers providing 42F water; mixture temperature increasing above 42 F)
`52
`100
`42
`100
`42
`52
`33.33
`33.33
`42
`25
`75
`42
`25
`75.
`49.5
`49.5
`42
`42
`47.62
`56.25
`18.75
`47.62
`18.75
`56.25
`37.5
`42
`12.5
`37.5
`42
`45.75
`45.75
`12.5
`42
`42
`6.25
`43.88
`18.75
`6.25
`18.75
`43.88
`
`First Two Chillers Operating, Third Chiller Bypassing
`(First two chillers overchilling; mixture temperature at 42F)
`100
`38.66
`48.66
`38.66
`33.33
`47
`25
`75
`39.5
`39.5
`18.75
`56.25
`40.13
`40.13
`45.75
`40.85
`37.5
`40.85
`44.5
`12.5
`6.25
`41.38
`41.38
`43.25
`18.75
`
`100
`75
`56.25
`37.5
`18.75
`
`45.33
`44.5
`43.25
`
`First Chiller Operating, Second Two Chillers Bypassing
`(First chiller overchilling; mixture temperature at 42F)
`35.33
`45.33
`45.33
`44.5
`37.83
`44.5
`43.25
`39.5
`43.25
`
`00D
`
`33.33
`25
`12.5
`
`100
`75
`37.5
`
`that, at part-load conditions with all compressors
`but
`operative,
`the difference between series and’ parallel
`flow diminishes substantially.
`Selection of the proper
`refrigerant on the part of the manufacturer is important
`because at a particular evaporating condition,
`the load
`carried by one unit versus the other can be greater,
`depending on which refrigerant is being used. However,
`the engineer is not
`in a position to select the refrig-
`erant and often the manufacturer chooses the refrigerant
`which will make his machine most competitive.
`Besides series arrangement and selection of proper
`refrigerant,
`there are two additional factors that affect
`energy consumption and power requirements in hp/ton,
`z'.e., suction temperature and the number of passes; At
`the higher temperature differentials of 15-20 F, (8.3-l1.lC)
`the series arrangement effects operating costs savings be-
`cause less chilled water need be circulated. The leading
`chiller (unit A, Fig. 3-1) operates at a very high suction
`temperature and hp/ton is reduced to a minimum. On
`
`the other hand, lagging chiller (unit B) operates at low
`suction temperature and high horsepower requirements.
`Nevertheless,
`the average hp/ton for the two machines
`is below that of an equal tonnage arrangement in par-
`allel. A chilled water range of 10 F (5.55C) or less for
`a series arrangement penalizes the application because of
`increased flow rate, velocity through the chiller becoming
`the limiting factor. The greater the number of machines
`in series, the greater the flow limitation. Ordinarily, more
`than three machines in series is disadvantageous.
`‘A
`minimum 6F (3.33C) chilled water temperature drop
`across each chiller is the rule of thumb. Conceivably,
`four machines in series could be considered with a 24F
`
`(l3.55C) differential. However, not all machines will be
`the same size. Most probably, the first two will be of one
`size and the last two of a different size.
`
`A series arrangement most often results in a single-pass
`evaporator, due to chiller flow and piping considerations.
`Single-pass evaporators are less efficient in heat transfer
`
`GZJKDKV!3112
`
`

`
`Ch. 3
`
`Table 3-3
`Ideal Performance of Two Chillers in Parallel
`[Design Chilled Water Supply 40 F (4.4 C), Design Chilled Water Return 58 F (14.4 C)—18F Temperature Differential]
`
`Supply
`Temp.,
`
`Return
`Supply
`Return % Load % Load
`Temp., 1
`Temp.,
`Temp,,
`of
`of
`F
`F
`F
`System Unit
`Both Chillers Operating
`
`% Load % Load
`of
`of
`System Unit
`
`Supply
`Temp.,
`F
`
`First Chiller Operating, Second Chiller Bypassing
`(First chiller providing 40F water; mixture temperature increasing above 4
`
`First Chiller Operating, Second Chiller Bypassing
`(First chiller overchilling, mixture temperature at 40F)
`31
`49
`33.25
`46.75
`35.5
`44.5
`37.75
`42.25
`
`than multiple-pass. Thus, single-pass evaporators for a
`particular leaving chilled water temperaturerequire either
`lower evaporator temperature and increased horsepower
`or
`larger heat
`transfer surface and,
`therefore, greater
`initial investment.
`
`In general, the greater the chilled water range, the more
`economically attractive a two-pass chiller becomes by re-
`ducing the horsepower gap between parallel and series
`chillers.
`I
`The designer of a series arrangement faces the problem
`of selecting identical or unequal chillers for the leading
`and follower unit.
`Equally sized chillers are desirable
`from a maintenance and logistical point of view, but,
`either case involves particular considerations in terms of
`performance and convenience.
`Figures 3-3 and 3-4 indicate some of the parameters
`that affect chiller selection.
`Loading the leading and follower unit properly in a
`series arrangement is of primary importance.
`Initial load
`balancing is required for optimum performance of chillers
`in series. When refrigeraion load is reduced or ‘when one
`
`the other continues to
`of the machines breaks down,
`handle its share of the load. Ordinarily, the leading
`machine assumes more than 50% of the load because of
`
`\
`
`VT 1
`
`J
`
`Slip,léavingchilledwaterternperuiure~evaporator
`
`
`
`temperatureF(C)
`
`l2l3l4l5|5|7lBIQ2O
`IOl|
`5578 Q
`(059)
`(0.52)
`(0.|3)
`(0.26)
`Sur(ace,Squore Feet per‘ Ton (m2IkW)
`Courtesy York Division afBorg-Warner Corp.
`Figure 3-3. Typical chiller performance.
`(Curves based on 16 ft
`length, 0.56" l.D. pipes; 10 ft/sec velocity. Dashed curves indicate
`a velocity of less than 200 ft/min.)
`
`GZJKDKV!3112
`
`

`
`[Design Chilled Water Supply 40F (4.4 C), Design Chilled Water Return 58F (14.4 C)—l8F Temperature Differential]
`
`Table 3-4
`Ideal Performance of Three Chillers in Parallel
`
`%
`
`Supply
`Temp.,
`F
`
`Return
`Temp .,
`F
`
`% Load
`of
`System
`
`% Load
`of
`Unit
`
`Return
`Temp.,
`F
`
`Unit No. 3
`% Load % Load
`Supply
`of
`of
`Temp . ,
`F
`System
`Unit
`
`Return
`Temp , ,
`F
`
`Supply Return
`Temp.,
`Temp.,
`F
`F
`
`Unit No. 2
`% Load % Load
`Supply
`of
`of
`Temp.,
`F
`Unit
`System
`All Chillers Opera ting
`100
`33.33
`25
`75
`50
`16.6
`12.5
`37.5
`8.33
`25
`12.5
`4.16
`
`40
`40
`40
`40
`40
`40
`
`46
`44.5
`43.37
`42.25
`41.12
`
`58
`53.5
`50.12
`46.75
`43.37
`
`First Two Chillers Operating, Third Chiller Bypassing
`(First two chillers providing 40F water; mixture temperature increasing above 40F)
`33.33
`100
`100
`33.33
`25
`75
`25
`75
`56.25
`18.75
`18.75
`56.25
`12.5
`37.5
`12.5
`37.5
`18.75
`6.25
`6.25
`18.75
`
`. First Two Chillers Operating, Third Chiller Bypassing
`(First two chillers overehilling; mixture temperature at 40 F)
`100
`33.33
`25
`75
`18.75
`56.25
`12.5
`37.5
`6.25
`18.75
`
`First Chiller Operating, Second Two Chillers Bypassing
`(First chiller overchilling; mixture temperature at 40F)
`28
`46
`46
`31
`44.5
`44.5
`42.25
`35.5
`42.25
`
`46
`44.5
`52.25
`
`100
`75
`37.5
`
`5 ME I co
`
`DNE<F/ASS COOLER
`
`TWO“PASS COOLER
`
`
`
`
`
`APPROX.em/row(aw/aw)
`
`42
`
`40
`(4.5)
`
`46
`
`48
`44
`2
`-
`—+
`(53),
`(5.7)
`LEAVING WATER TEMP-F(C)
`
`50
`'
`
`56
`54
`52
`:-
`+— ‘
`(11.1)
`, “ (133)
`
`Courtesy Carrier Corp.
`From Handbook of Air conditioning’
`System Design, © Mc Graw-Hill Book Co. Used with permission of
`McG'raw-Hill Book Co.
`
`Figure 34. Centrifugal compressor power-—refrigerant 11.
`
`its higher suction temperature. This implies that the
`follower machine can be smaller, resulting in cost reduc-
`tions.
`From a practical point of view,
`two identical
`machines at
`increased cost are preferred because this
`presents
`fewer maintenance problems.
`In selecting
`identical machines, and in order to produce identical
`speeds, it is necessary to divide the load at 55% for the
`leading machine and 45% for the follower machine as a
`
`first approximation, or at percentages that are not ap-
`preciably different. Centrifugal compressors are selected
`at a definite speed to satisfy a fixed design head. Since
`the leading and follower compressors have different heads,
`it
`is possible that different speeds are required if the
`55%/45% ratio is exceeded appreciably.
`.It follows that balancing both chillers at 50% unbal~
`ances loads and requires a different head and speed for
`each machine, precluding the possibility of sequencing
`the two machines as leading or follower units alternately.
`Two or more machines may be placed in series provided
`
`GZJKDKV!3112
`
`

`
`N
`
`”
`
`..
`
`H n H
`OPPOSED
`W"
`
`..
`
`OPPOSED
`
`A LEADING
`
`‘USlNG 4-WAY VALVES
`
`Figure 3-5. Two chillers in series without a spare unit.
`
`Ch. 3
`
`that a minimum of 6F (3 .33C) chilled water temperature
`
`drop is maintained across each chiller.
`Automatic speed resetting by a controller is difficult,
`and more so for motor than for turbine drive. It is pre-
`ferable that any seasonal speed adjustment be manual
`and that vane control be automatic so that there is less
`chance of the machines “hunting.”
`Constant flow through the machines promotes pump-
`ing smoothness and control stability, two distinctly de-
`sirable qualities, which are discussed in Chapter 7 as
`constant flow in the primary circuit.
`Unfortunately, friction of the two chillers in series
`is additive, most often resulting in higher heads, although
`not necessarily so for single pass evaporators. Relatively
`speaking,
`there are fewer pipes than in the parallel ar-
`rangement, although of a larger diameter since they must
`handle full
`flow.
`. The designer can compare costs of
`piping arrangements, and he must weigh the cost of a
`back-up unit
`in either parallel or series arrangement.
`Series arrangement piping costs may be reduced to some
`extent by using four-way Valves as Figs. 3-5 , 3-6, 3-7 and
`3-9 indicate.
`_
`
`Figure 3-5 shows single-pass and two-pass chillers ar-
`ranged in opposed or side-by-side positions. Four-way
`valves eliminate two gate valves in each case. The star-
`delta arrangement of Fig. 3-6 allows either unit A, B or
`C to become the spare but does not permit unit B in the
`leading position. Bypassing around each unit is pro-
`vided.
`The star-delta arrangement is capable of three
`combinations using ten gate valves. Four 4-way valves
`(Fig. 3-7) permit six possible combinations at consider-
`ably less cost. However, all six combinations may not be
`desirable and present difficult control problems.
`The arrangement presented in Fig. 3-8 has both chilled
`water and condenser water
`in series in counterflow
`fashion and possesses much flexibility in refrigeration
`load handling and. some variation in chilled water flow
`by employing more or fewer pumps as the circumstances
`demand. One chiller is used for low loads, two chillers
`for normal loads and three chillers for high loads. The
`third chiller serves as a spare when it is not used for high
`loads. Two or three pumps can be used, depending on
`load magnitude and system heat
`transfer character-
`istics.
`
`3.4 Arrangementc (Fig. 3-I), Chillers in Series,
`Condensers in Series (condenser water parallel to chilled
`Water)
`
`’
`'
`'
`This arr
`PF]-'13
`angement is. of theoretical llltefest 011135
`lead.
`mg machine (Unit A) has the lowest condensing
`
`-
`‘
`.
`.
`.
`.
`Figure 3-6. Two chillers in series with a spare unit in star-delta
`arrangement.
`
`GZJKDKV!3112
`
`

`
`FOLLOWER UNIT B,
`LEADING UNIT A,
`SPARE UNIT C
`‘
`
`LEADING UNITA, F LLOWER UNIT C,
`SPARE
`T B
`
`LEADING UNIT B, FOLLOWER UNIT A,
`SPA
`R UNIT C
`
`LEADING UNIT 8, FOLLOWER UNIT C,
`SPARE UNIT A
`
`LEADING UNIT C, FOLLOWER UNIT A,
`SPARE UNIT B
`
`LEADING UNIT C, FOLLOWER UNIT B,
`SPARE UNIT A
`
`Figure 3-7. Two chillers in series with a spare unit using 4-way valves.
`
`temperature and highest evaporator temperature. The
`follower
`(Unit B) has the highest condensing and the
`lowest evaporator
`temperatures.
`The lift (difference
`between condensing and evaporating temperatures) of
`the two units is distinctly diverse and, therefore,
`the
`heads of the two compressors will be quite opposed,
`creating problems of speed and control, and generally
`serious application problems. This arrangement cannot
`
`be recommended because it imposes severe design limit-
`ations.
`
`3.5 Arrangement d (Fig. 3-I), Chillers in Series,
`Condensers in Series (condenser water in counterflow
`to chiIIed water)
`
`this arrange—
`By following the counterflow principle,
`merit creates roughly equal lift
`(condensing temp. —
`
`GZJKDKV!3112
`
`

`
`CONDENSER WATER
`
`CHILLER UNIT
`NO 3
`
`> ‘
`
`V
`A
`
`V
`A
`
`BYPASS
`
`Figure 3-8. System load with flexibility in load handling and variation in chilled water flow possible because of use of multiple pumps.
`
`evaporator temp.) for both units, matching the highest
`condensing and evaporator temperatures in the leading
`machine (Unit A) to the lowest condensing and evap-
`orator temperatures in the follower machine (Unit B).
`Compressor head requirements are equalized at an equal
`load split.
`Condenser series flow can be utilized at great advantage
`where there is a dearth of condenser water and its temper-
`ature is low, as usually is the case with well water. A
`large condenser water
`temperature rise is practically
`mandatory.
`A valved bypass around each chiller and condenser is
`required for repairs andcleaning. As in any series ar-
`rangement,
`larger valves and pipes are required. The
`follower condenser and’: evaporator must be sized to do a
`leading unit job if exchange of duties between units is
`contemplated.
`
`3.6 Arrangement e, Series-Parallel
`
`The classic series-parallel arrangement is represented by
`the diagrams of Fig. 3-9. A tonnage of 25,000 is being
`handled in this fashion for the Capitol Building in Wash-
`ington,D.C.
`This arrangement is used extensively because it com-
`bines the most desirable features of both series and
`parallel arrangements, flexibility and economy, plus its
`Own advantage of a multiplicity of components and the
`implied potential -of combining these components to
`match loads and obtain maximum efficiency.
`Some disadvantages of the parallel arrangement also
`appear here. Constant system flow at partial system
`refrigeration load signifies circulating water
`through
`inactive chillers, requiring either overchilling or accept-
`ance of water temperatures higher than design.
`The overchilling operation means that at 50% load the
`deactivation sequence is Al - B1 or A2 - B2. Overchil-
`
`ling may be eliminated by deactivating A, - A2 first with
`B, and B2 still on the line, handling their share of the
`load.
`
`An alternate solution, reducing flow through the chill-
`ers at partial load, may cause pump head and water dis-
`tribution problems. Controlling the operation of the
`units becomes more complex in the sense that ideal
`performance would require identical performance from
`corresponding units in each series branch. Whereas one
`single branch may not be particularly difficult to con-
`trol, several series branches acting in concert demand
`more precision on the part of control and instrumen-
`tation systems.
`Introducing or withdrawing load from
`
`->4
`
`3
`
`>4
`
`USING 4~WAY VALVES
`
`Figure 3-9. Four chillers in series-parallel arrangement without a
`spare.
`
`GZJKDKV!3112
`
`

`
`68
`
`the system requires engagement or disengagement of
`refrigeration units in a particular pattern to maintain
`system design parameters with the least disturbance
`or inconvenience possible. A thermodynamic and op-
`erational analysis should provide the answer to the de-
`activation procedure for different control schemes. These
`are some of the questions that the designer must consider.
`Like the parallel,
`the parallel-series arrangement can
`accommodate additional series branches to match any
`capacity requirement,
`if the main pipes are sized ad-
`equately for the extra capacity. Oversizing of the main
`pipes must be provided if future growth is anticipated.
`The
`series-parallel arrangement can fit almost any type
`of refrigeration machine, be it a leading or follower unit,
`and can accommodate all practical drives.
`The designer is in a position to use a number of design
`tools to obtain the best chiller selection, including load
`