xiaohong liao et. al. 2007

International Journal of Refrigeration 30 (2007) 904e911www.elsevier.com/locate/ijrefrig

Absorption chiller crystallization control strategiesfor integrated cooling heating and power systems

Xiaohong Liaoa,*, Reinhard Radermacherb

aUnited Technologies Research Center, 411 Silver Lane, MS 129-17, East Hartford, CT 06108, United StatesbUniversity of Maryland, College Park, MD 20742, United States

Received 5 July 2006; received in revised form 26 September 2006; accepted 30 October 2006

Available online 19 January 2007

Abstract

The concept of an air-cooled absorption chiller system is attractive because the cooling tower and the associated installationand maintenance issues can be avoided. However, crystallization of the LiBreH2O solution then becomes the main challenge inthe operation of the chiller, since the air-cooled absorber tends to operate at a higher temperature and concentration level than thewater-cooled absorber due to the relative heat transfer characteristics of the coolant. This leads to crystallization of the workingfluid. The paper focuses on the crystallization issues and control strategies in LiBreH2O air-cooled absorption chillers. As aresult a novel application opportunity is proposed for the integration of absorption chillers into cooling, heating and power(CHP) systems. This new methodology allows for air cooler operation while avoiding crystallization.� 2006 Elsevier Ltd and IIR. All rights reserved.

Keywords: Air conditioning; Trigeneration; Absorption system; Water; Lithium bromide; Process; Regulation; Reduction; Crystallisation

Prevention de la cristallisation dans les refroidisseurs aabsorption utilises dans les systemes integres de chauffage,

de refroidissement et de production d’energie

Mots cles : Conditionnement d’air ; Trigeneration ; Systeme a absorption ; Eau ; Bromure de lithium ; Procede ; Controle ; Reduction ;

Cristallisation

1. Introduction

Absorption chilling is a key technology in the CHP port-folio since it offers significant opportunities to transformwaste heat into cooling [1]. The motivation for an air-cooled

* Corresponding author. Tel.: þ1 860 610 7408; fax: þ1 860 998

8316.

E-mail address: [email protected] (X. Liao).

0140-7007/$35.00 � 2006 Elsevier Ltd and IIR. All rights reserved.

doi:10.1016/j.ijrefrig.2006.10.009

option is to use air, a free coolant, to remove the heat of con-densation and absorption processes. As a result, the coolingtower, cooling water, and the associated maintenance, thewinterizing procedure, and Legionella concerns are elimi-nated. Places in the world where water is a precious com-modity can particularly benefit from this design.

However, there is little literature on air-cooled absorptionchillers, mainly due to the unavailability of a commercializedproduct, and most publications are restricted to purely

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905X. Liao, R. Radermacher / International Journal of Refrigeration 30 (2007) 904e911

Nomenclature

CHP Cooling, heating and powerCOP Coefficient of performanceDX Direct expansionEES Engineering Equation SolverHVAC Heating, ventilation and air-conditioning

LiBr Lithium bromideRT Refrigeration tonRTU Roof top unitVAV Variable air volume

theoretical simulation. Salim [2] simulated a 7 kW coolingcapacity automotive LiBr absorption air-conditioner. Alvaand Gonz�alez [3] simulated an air-cooled solar-assisted ab-sorption system with the cooling loads in the range of 10.5,14 and 17.5 kW, and compared its performance to the perfor-mance of a water-cooled system. Izquierdo et al. [4] calcu-lated the operating parameters of a single-effect anda double-effect LiBr air-cooled absorption system drivenby solar energy with the objective of crystallization preven-tion. Florides et al. [5] mentioned several causes of crystal-lization occurring in a water-cooled LiBr absorptionmachine. There is also research using chemical additivesto shift the crystallization line to higher temperatures [6].However, all of the suitable chemicals exhibit negative char-acteristics that effectively limit their practical application.

In this research, a new approach to avoid crystallization eimplementing temperature control strategies, as well as anapplication in the CHP area, are proposed by the authors.The analysis and concept presented are based on verifiedcomputer models.

2. Single-effect LiBr air-cooled absorption chillers

An EES (Engineering Equation Solver [7]) model ofa 63 kW (18 refrigeration ton (RT), 1 RT¼ 3.516 kW)

single-effect LiBreH2O air-cooled system is developedto investigate the performance, crystallization issues andprecautions of air-cooled absorption chillers. It has beenvalidated in accordance with experiments on a 63 kWwater-cooled absorption chiller under comparable condi-tions. The chiller is installed on the campus of Universityof Maryland and fired by microturbine exhaust. Fig. 1 is aflow diagram for the absorption chiller.

2.1. Crystallization causes and precautions

In absorption chillers, usually the crystallization line forlithium bromide and water is very close to the working con-centrations needed for practical LiBr/H2O absorptionchillers, such as Point A in Fig. 2. If the solution concentra-tion is too high or the solution temperature is reduced toolow, Point A migrates to Point B and crystallization mayoccur, interrupting machine operation.

The actual location within the chiller is decided by themechanical structure of pipes and fittings; but crystallizationis most likely to occur in the strong solution entering theabsorber; that is Point 6 in Fig. 1, the concentrated solutionat the lowest temperature. Crystallization must be avoidedbecause the formation of slush in the piping network formsa solid, blocking the flow very quickly. If this occurs, the

1

Condenser

2

43

5

6

7

8

9

10

1112

13

14

15

16

1718

Desorber (generator)

Solution HeatExchanger

AbsorberEvaporator

PumpThrottling Valve

Throttling Valve

Concentrated SolutionVulnerable tocrystallization

Refrigerant

Solution

Fig. 1. Diagram of single-effect air-cooled absorption chiller.

906 X. Liao, R. Radermacher / International Journal of Refrigeration 30 (2007) 904e911

Crystallizationcurve

A

B10 20 30 40 50 60 70 80 90 100 110 120 130

Solution Temperature (°C)

0

10

20

30

40

50

60

80

70

Refrig

erant Tem

peratu

re (°C)

Water

030%

40%

50%

60%

70%

1

2

345

10

20

30

40

Sa

tu

ra

tio

n P

re

ss

ure

(k

Pa

)

Fig. 2. The property chart of LiBr/H2O solution with crystallization curve.

concentrated solution temperature needs to be raised signif-icantly above its saturation point, so that the salt crystals willdissolve within a reasonable time, freeing the machine. Torecover absorber operation after crystallization is very laborintensive and time consuming.

The big difference between water-cooled and air-cooledLiBrewater absorption chillers is the temperature of theabsorber. With air-cooling, one cannot always achieve a tem-perature of the solution in the absorber sufficiently low tomaintain the desired evaporator pressure. The only way tocompensate for the high absorber temperature is to increasethe concentration of LiBr in the solution, but that brings itcloser to crystallization [8].

One of the following six causes or a combination of thesecauses may trigger crystallization of air-cooled absorptionchillers, and the associated precautions are also suggested:

1. High ambient temperature: the air-cooled absorberstend to run hotter than water-cooled units due to therelatively poor heat transfer characteristics of air, andthe fact that the dry bulb temperature is equal to orhigher than the wet bulb temperature. Fig. 3 gives theoverall trend of how each point changes when the am-bient temperature increases in the solution field. Thesystem operates with the same exhaust temperatureand chilled water temperature setting in both cases.When the cooling water or heat sink temperature in-creases from 25 �C (dashed lines) to 35 �C (solid lines)crystallization occurs. This situation can be preventedby raising the chilled water temperature setting orreducing the amount of heat supplied to the desorber.

2. Low ambient temperature and full load [5]: this combi-nation may also cause crystallization because LiBrsolution concentration would be relatively high whilethe solution temperature is low. Fortunately for an

air-cooled chiller it is easy to prevent overcooling byreducing cooling airflow rate.

3. Air leak into the machine [5] or non-absorbable gasesproduced during corrosion: both deteriorate the heattransfer effectiveness of the absorber and cause highersystem pressure, decreased capacity and COP, andhigher crystallization probability. As a precaution tothis issue, the system should be evacuated routinely.

4. Too much heat input to the desorber: the exhausttemperature or the exhaust flow rate is too high, whichresults in increased solution concentrations to the pointwhere crystallization may occur. As a precaution to thisissue, the exhaust temperature or flow rate into thedesorber should be maintained within a specific range.Fig. 4 shows the overall trend of how each point changeswhen the exhaust temperature increases in the solution

0 20 40 60 80 100 1200.5

1

2

5

10

15

System

P

ressu

re (kP

a)

Crystallization curve

Crystallization happens !

cycle @ 25°C

cycle @ 35°C

Temperature (°C)

Fig. 3. The solution field of absorption cycle at ambient 25 �C(dashed line) and at ambient 35 �C (solid line).


field. The system operates with given ambient tempera-ture, chilled water temperature setting and flow rates, butwhen the exhaust temperature increases from thedesigned value of 280 �C (dashed lines) to 320 �C (solidlines), crystallization occurs. The precaution is to main-tain the correct heat input to the desorber.

5. Failed dilution after shutdown [5]: during normal shut-down, the machine undergoes an automatic dilutioncycle that lowers the concentration of the solutionthroughout the machine. In such a case, the machinemay cool to ambient temperature without crystalliza-tion occurring anywhere. Crystallization is most likelyto occur when the machine is stopped due to poweroutage while operating at full load, when highly con-centrated solution is present in the solution heatexchanger.

6. Chilled water supply temperature is set too low whenthe weather and/or exhaust are too hot. So the pre-caution is to set and maintain the correct chilled watersupply temperature.

3. Control strategies

3.1. Chilled water temperature control

A new control strategy is proposed by the authors to pre-vent crystallization. Though hot weather may cause crystal-lization in an air-cooled chiller, increasing the chilled watertemperature settings can avoid crystallization, and the by-products include improved cooling capacity and COP.

Assume the chiller is installed in a CHP system, utilizingthe waste heat from the prime mover as heat input to the de-sorber. Further assume, that the exhaust gas temperature andflow rate are fixed. Then for a certain ambient temperaturethe minimum chilled water temperature is determined bythe following two requirements: (1) the refrigerant (water)should not freeze; (2) the system should not be crystallized.Fig. 5 shows the relation between the two temperatures,

0 10 20 30 40 50 60 70 80 90 100 1100.5

1.0

2.0

5.0

10.0

System

P

ressu

re (kP

a)

Crystallization curve

Crystallization happens !

cycle @ 280°C

cycle @ 320°C

Temperature (°C)

Fig. 4. The solution field of absorption cycle at exhaust 280 �C(dashed line) and at exhaust 320 �C (solid line).

assuming the exhaust to the chiller is set at 280 �C and ofconstant flow rate. The minimum chilled water temperatureis defined by the crystallization prevention margin (forexample 1% to the crystallization curve).

The chilled water temperature control strategy is shownin Fig. 6. The darker zone in the three-dimensional figure(Fig. 6a) represents the infeasible chilled water supplytemperature and ambient temperature combination. Theair-cooled chiller should not operate in this zone to avoidcrystallization; while the lighter zone is safe for the chilleroperation, and the chiller can have higher cooling capacityas well as COP when the weather is cooler and the chilledwater temperature is higher. The dashed lines and the starsymbol (+) show how to look up the corresponding temper-ature setting and cooling capacity under a certain ambienttemperature. For example, when the ambient temperatureis at 35 �C, the minimum chilled water supply temperatureshould be set at 8 �C, and then the chiller can achieve58 kW cooling capacity. Note: all numbers are based ona chiller with 63 kW cooling capacity. As the counterpartof Fig. 6a, Fig. 6b plots the cooling capacity contour todisplay the same conclusion quantitatively.

In a word, the proposed control strategy is to increase thechilled water temperature setting to avoid crystallizationwhen the ambient temperature is too high.

3.2. Exhaust temperature control

In order to improve heat utilization in a CHP system, theabsorption chiller should take advantage of the exhaust ofprime mover that is as hot as the component materials canwithstand; otherwise fresh air is required to cool down theexhaust.

When the chilled water temperature is fixed with a con-stant flow rate, the highest exhaust temperature is alsorestricted by the ambient temperature, see Fig. 7. The darkerzone represents the infeasible temperature combination. Theair-cooled chiller should not run in this zone to avoid

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 404

5

6

7

8

9

10

11

12

Normal system operation

System crystallizes

Ambient Temperature (°C)

Min

. C

hilled

W

ater S

up

ply T

em

p (°C

)

Fig. 5. Minimum chilled water supply temperature (at exhaust

280 �C and with constant flow rate).


Chilled W

ater Supply Temp. (°

C)

25 28 31 34 37 400

2

4

6

8

10

12

14

16

18

Crystallization Prevention Margin

67 kW

65 kW63 kW 61

kW 60 kW 58

kW 56 kW

Ambient Temperature (°C)

225

30

35

400

10

20

30

40

50

60

70

3 4 5 6 7 89 10 11 12

Am

bient Temp. (°C

)

Ch

iller C

oo

lin

g C

ap

acity (kW

)

Ch

illed

W

ater S

up

ply Tem

p. (°C

)

(a)

(b)

Fig. 6. Chiller cooling capacity map over the chilled water and ambient temperature combination (at exhaust 280 �C and with constant flow

rate). (a) Chiller cooling capacity 3D plot for a 63 kW commercial chiller. (b) Chiller cooling capacity contour.

crystallization; while the lighter zone is safe for the chilleroperation, and the chiller can obtain higher cooling capacityand COP when the exhaust is hotter and the weather iscolder. For example, the dashed lines and the star symbol(+) in Fig. 7 show how to look up the corresponding temper-ature setting and cooling capacity under a certain chilled wa-ter temperature. For example, when the exhaust temperatureis at 300 �C, the chiller cannot run in weather hotter than29 �C, otherwise the crystallization may occur. At that com-bination, the chiller can obtain 68.2 kW cooling capacity.

In summary, when the ambient temperature is hot enoughto cause crystallization, it can be successfully prevented by

reducing the exhaust inlet temperature, assuming that the ex-haust flow rate is constant. But the cooling capacity and COPof an absorption chiller will be compromised consequently.

4. Suggested applications

4.1. Conventional roof top unit baseline

Many commercial buildings utilize conventional roof topunits (RTUs) to supply cooling, which use a conventional va-por compression cycle to cool air through a direct expansion


3632 3430282624220

10

20

30

40

50

60

70

80

90

290280300 310 320

330

Ch

iller C

oo

lin

g C

ap

actiy (kW

)

Ambient Temp. (°C)Exhaust Temp. (°C

)

Fig. 7. Chiller cooling capacity (chilled water supply temperature at 7 �C) over the exhaust and ambient temperature combination.

(DX) coil. Then the supply air is distributed via variable airvolume (VAV) boxes or other terminal units that modulateair volume distribution throughout each conditioned roombased on wall-mounted thermostats, adjusted by the buildingoccupants. Electric reheats within these VAV boxes providelocalized heating when required. Fig. 8 describes the relationof RTU, VAV and the conditioned space (building).

Consider an analysis of a conventional vapor compressionRTU for a commercial building, where the total 7.08 m3 s�1

mixed air consists of 80% return air and 20% outdoor air. Themixed air is cooled and dehumidified by a DX coil, then re-heated to the supply air, because the DX coil must overcoolthe air to achieve the required dehumidification. The wholeprocess is illustrated in Fig. 9. The energy consumption toprocess 1 kg of supply air is 21.3 (cooling and dehumidifyingby the DX coil)þ 3.1 (reheat)¼ 24.4 kJ kg�1 of dry supplyair. The moisture suppression required is 2.63 g kg�1 of drysupply air.

DX Coil

Roof Top UnitRoof Top Unit

Fan

Conditioned SpaceConditioned Space

Supply AirSupply Air

Return AirReturn Air

Outdoor AirOutdoor Air

Exhaust AirExhaust Air

Economizer Dampers

VAV w/reheating

Mixed AirMixed Air

Ceiling

Roof

13°C13°C

16°C16°C

To other rooms

Fig. 8. Roof top unit and the conditioned space.


It is believed that the dehumidification method results inextra energy consumption due to the overcooling and reheat-ing. Ironically, the reheating is still needed even in summerwhen the RTU is working!

4.2. CHP-single-effect LiBr air-cooled absorptionchillereHVAC combination

CHP introduces an innovative way to supply cooling,heating and power for a building; however, the capital costof CHP equipment and the load fluctuation of a typical com-mercial building restrict the advantage of designing a unitsized at the peak load. Therefore a conventional HVACsystem is still needed.

10 15 20 25 30 35 400.005

0.010

0.015

0.020

0.025

T (°C)

Hu

mid

ity R

atio

(kg

/kg

)

20%

40%

60%

80%

100%

21.3 + 3.1

SA

MA

OA

RA

= 24.4 kJ/kg

2.63

g/k

g

OA: outdoor airMA: mixed airRA: return airSA: supply air

Fig. 9. The conventional vapor compression air-conditioning used

to remove moisture and control air temperature with reheat.

The goal for the proposed CHP integration is to obtainmore operating hours out of the CHP equipment at fullcapacity, because considerable financial benefits can beachieved through maximizing the operating hours of theunit so that the cost saving achieved through the recoveryof waste heat can help to repay its higher initial capitalcost. So the best way is to let CHP take care of the baseload and conventional HVAC systems pick up the residualloads [9].

Based on extensive energy analysis of CHP system at dif-ferent geographic locations, building load, and desiccantunit and economizer combination, a building application isproposed as shown in Fig. 10: outdoor air is mixedwith the return air , and becomes the mixed air . Afterbeing dehumidified by the desiccant wheel, it reaches a hot-ter and drier state point , which satisfies the humidity con-tent of building supply air. Both the chilled water coil of theair-cooled absorption chiller and the DX coil step take careof the sensible load only, and decrease the air temperature to

and , respectively. Its psychrometric chart is shown inFig. 11.

For a certain air-conditioning application, the process airafter the desiccant wheel, state point , is always hot andwith constant flow rate, and no more latent load needs tobe removed afterwards. This is ideal to deploy the afore-mentioned chilled water temperature control strategy, sincethe chilled water temperature can be set at higher than theconventional setting 7 �C. A CHP system with air-cooledabsorption chiller in Fig. 10 can completely satisfy the entirelatent load and part of the sensible load, and no reheat isneeded in the entire cooling season e normally 2039 h inCollege Park MD, 1583 h in Hartford CT, 4115 h in Phoenix

Legend: Outdoor air: Return air: Mixed air: Air after desiccant wheel: Air after CHP coil: Supply air: Building exhaust

Ab

so

rp

tio

n

Ch

iller

MExhaust Gas

Exh

au

st G

as

DX

C

oil

Power

Electricity

1 4

8

5 6 7

3

Natural Gas

Ch

illed

W

ater

CH

P C

oil

Ab

so

rp

tio

n

Ch

iller

De

sic

ca

nt W

he

el

Fig. 10. Proposed CHP application without an enthalpy wheel in RTU (roof top unit).


AZ and 5785 h in Miami FL per year according to the data-base in EnergyPlus Energy Simulation Software [10].

The conventional HVAC unit can be much smaller andconsume less energy than the baseline design, a roof topunit only system. Since it can use a higher evaporation tem-perature, as it only needs to satisfy the sensible load, theCOP can be higher, which consequently yields additionalbenefits of reduced capital cost, compact size, smaller resis-tance to air due to smaller coils, and smaller fan power re-quirement. In addition, since there will be no condensationon the coil the problem of mold growth is eliminated. Theannual primary energy consumption of the proposed hybridsystem is 60% of the baseline.

Inlet air-cooling for the microturbine would be helpful interms of maintaining capacity rating at outdoor air condi-tions above 15 �C, however, that this was not a focus ofthis work. Economic evaluation of the proposed hybrid sys-tem is not a part of this paper, but the related analysis can befound in Czachorski’s research [11].

5. Conclusion

The concept of an air-cooled system is attractive becausethe cooling tower and the associated installation and mainte-nance issues can be avoided. However, crystallization thenbecomes the main obstacle in the operation of the unit. Sixcauses may trigger crystallization: (1) high ambient temper-ature; (2) low ambient temperature with full load; (3) air leakinto the machine or non-absorbable gases produced duringcorrosion; (4) too much heat input to the desorber; (5) faileddilution after shutdown; and (6) chilled water supply

10 15 20 25 30 35 40 450.005

0.007

0.009

0.011

0.013

0.015

0.017

0.019

0.021

0.023

0.025

Hu

mid

ity R

atio

(kg

/kg

)

20%

40%

60%

80%

(1)

(5)(3)

(4)

(7) (6)

Fig. 11. Psychrometric chart for the proposed application.

temperature is set too low when the weather and/or exhaustare too hot. Novel temperature control strategies togetherwith a well-chosen application are proposed to effectivelyprevent the occurrence of crystallization, which are to in-crease the chilled water temperature settings or to reducethe exhaust temperature according to the maps developedin this research. Finally, a novel CHP building applicationis suggested as an example for taking advantage of this pro-posed method with considerable potential for improvedeconomics.

Acknowledgements

The authors would like to acknowledge the support of theCenter for Environmental Energy Engineering (CEEE) atthe University of Maryland.

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