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Applied Thermal Engineering 27 (2007) 2026–2032

Cooling performance of a hybrid refrigeration system designedfor telecommunication equipment rooms

Jongmin Choi a, Jongug Jeon b, Yongchan Kim b,*

a Department of Mechanical Engineering, Hanbat National University, Duckmyung-Dong, Yusung-Gu, Daejeon 305-719, Republic of Koreab Department of Mechanical Engineering, Korea University, Anam-Dong, Sungbuk-Gu, Seoul 136-701, Republic of Korea

Received 17 June 2006; accepted 8 December 2006Available online 22 December 2006

Abstract

During the last several years, the power density and thermal density of telecommunication equipments have been increased. The opti-mum control of the PCB surface temperature is very important for obtaining high performance and operation reliability of telecommu-nication equipments. In this study, the cooling characteristics of telecommunication equipments were measured and analyzed as afunction of the equipments’ heat density. In addition, the performance of a novel hybrid refrigeration system for telecommunicationequipment rooms was measured at various operating conditions. The PCB surface temperature ranged from 35 to 60 �C, which was rel-atively higher than the air temperature due to heat trapping and improper air distribution. The hybrid refrigeration system operated inthe vapor compression cooling mode at high outdoor temperatures, but in the secondary fluid cooling mode at low outdoor tempera-tures. The outdoor temperature for the mode switch was approximately 8.3 �C. The COP of the hybrid refrigeration system was signif-icantly enhanced at low outdoor temperatures as compared with the conventional vapor compression system due to no operation of thecompressor.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Hybrid refrigeration system; Cooling performance; Heat density

1. Introduction

Electronic and telecommunication industries are strivingto develop compact, high power density components. Theequipments installed in telecommunication equipmentrooms consist of rack mounted units with chips on thePCB (printed circuit board) module. Advances in telecom-munication technology have increased the performance ofthese components with respect to their power density andheat dissipation rate per unit area [1–3]. A proper heat dis-sipation method from the PCB module can ensure the reli-able operation of electronic components. The maximumallowable temperature for the proper operation of an elec-tronic component is approximately 75 �C. Therefore, the

1359-4311/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.applthermaleng.2006.12.004

* Corresponding author. Tel.: +82 2 3290 3366; fax: +82 2 921 5439.E-mail address: yongckim@korea.ac.kr (Y. Kim).

optimum surface temperature must be maintained toobtain thermal reliability and high performance of elec-tronic components [4–7].

Schmidt and Shaukatullah [1] analyzed various aspectsof cooling methods for telecommunication equipmentrooms by experiments and numerical analysis of coolingschemes, energy saving schemes, and other related areas.Hayama and Nakao [8] investigated the air flow systemsfor high heat density telecommunication equipment roomsto determine the factors affecting the surrounding condi-tions of equipments and the ambient temperature of electri-cal components. Hayama and Nakao [8] investigated theeffects of air flow direction on the cooling performance oftelecommunication equipment. Lock et al. [9] measuredthe heat transfer and pressure drop characteristics in thePCB module. Leung et al. [10] numerically analyzed theheat transfer characteristics of a horizontal PCB assemblysubjected to fully-developed laminar-flow convection.

Table 1Specifications of air cooling system for the unit rack

Item Specification

Full unit Structure Fans/fins/baseSize 420 · 330 · 140 mm3

Fins Type/pitch Flat plate type/4 mmSize 34 · 305 · 1 mm3

Fan 1,3 (inlet and exit) Manufacturer NidecModels no. TA 450DC B33534-33AInput (Nidec 24 V/0.45 A)

Fan 2 (inlet) Manufacturer NidecModels no. TA 350DC M34261-16Input (Nidec 24 V/0.28 A)

Heat source Heated area 280 · 200 mm2

J. Choi et al. / Applied Thermal Engineering 27 (2007) 2026–2032 2027

Tuckerman and Pease [11] introduced the cooling conceptof a micro-channel heat sink. The performance of liquidcooling heat exchangers has been extensively investigated[12–14].

Most previous works on the cooling of telecommunica-tion equipments and rooms focused on the characteristicsof micro heat transfer in packaging and on the effects ofair flow direction. However, the effects of air temperatureentering the evaporator on the performance of a vaporcompression type air cooling system for telecommunicationequipment rooms have been rarely studied. In addition,there have been only few studies on the performance of ahybrid refrigeration system. In this study, the operatingcharacteristics of telecommunication equipments withrespect to heat density and operating conditions were mea-sured and analyzed. In addition, the performance of ahybrid refrigeration system, which is an energy efficientcooling system for telecommunication equipment rooms,was measured at various operating conditions.

Psychrometric chamber

Unit rack

GPIB

Power supplier

Power meter

Telecommunication equipment

Datalogger .

Air inAir out

Fig. 2. Schematic of experimental setup.

2. Experimental setup and test procedure

2.1. Test setup for the unit rack

As shown in Fig. 1, the unit rack of telecommunicationequipment consists of the PCB board, fans, and fin-plateheat exchangers. Table 1 shows the specifications of theair cooling unit used in the unit rack. In this study, thecooling characteristics of the unit rack were measured byvarying the power input and inlet air temperature. Basi-cally, the test setup was designed to have the same config-uration as the unit rack used in the field. However, the PCBboard was replaced by a silicon rubber heater to control theheat input to the unit rack. The size of the silicon rubberheater was the same as that of the PCB board.

As shown in Fig. 2, the test setup was installed in a psy-chrometric chamber to so that the inlet air temperature andheat rejection from the unit rack could be easily controlled.The psychrometric chamber was equipped with a refrigera-

0

PCB (Heat source)

Fin plate heat exchanger

Intake fanExhaust fan

Fig. 1. Schematic of unit rack for telecommunication equipment.

Table 2Test conditions for the unit rack

Parameter Test conditions

Inlet air temperature (�C) 11, 15, 19, 23, 25, 27Power input (W) 200–550Fan voltage (V) 24, 28

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tor, electric heater, and humidifier. The power input to thesilicon rubber heater was adjusted by a power supplier andmeasured by a power meter with an uncertainty of ±0.01%of full scale. The surface temperature of the silicon rubberheater and the air temperatures at the inlet and outlet ofthe unit rack were measured by using T-type thermocou-ples with uncertainties of ±0.1 �C.

As shown in Table 2, the tests for the unit rack wereconducted by varying the power input to the heater, inletair temperature, and fan speed. The inlet air temperaturewas varied from 11 to 27 �C. The power input to the heaterwas varied from 200 to 550 W at the fan voltages of 24 and28 V.

2.2. Test setup for the hybrid refrigeration system

The telecommunication equipment is installed in a cab-inet with heavy insulation, which is called telecommunica-tion equipment room. The telecommunication equipmentroom is often cooled by using typical air-conditionersdesigned for residential use. However, the air-conditionerhas to be operated even in winter because of the large heatflux generated from the equipments. The operation of theair-conditioner in winter season causes additional energyloss due to frequent on/off operations. In addition, itmay reduce the reliability of the compressor. Therefore,in this study, a hybrid refrigeration system was designedto overcome these problems.

As shown in Fig. 3, the hybrid refrigeration system con-sists of an indoor unit and outdoor unit. The indoor unitincludes a compressor, condenser (plate heat exchanger),capillary tube, and dual function fin-tube heat exchanger.The outdoor unit consists of a heat rejection heat exchan-ger, pump, and reservoir. The hybrid system operates intwo modes according to outdoor temperature. At high out-door temperatures, the hybrid system operates in the vaporcompression cooling mode, which is called ‘‘mode 1’’ inthis paper. The ethylene-glycol solution supplied by thepump enters the condenser and then returns back to theoutdoor unit. The condenser rejects the heat to the ethyl-ene-glycol solution, and the dual function heat exchangerabsorbs the heat generated from the telecommunicationequipment. At low outdoor temperatures, the hybrid sys-tem operates in the secondary fluid cooling mode, whichis called ‘‘mode 2’’ in this paper, with no operation of thecompressor. In mode 2, the ethylene-glycol solution entersthe dual function heat exchanger and then returns back tothe outdoor unit, rejecting the heat generated from the tele-communication equipment by the heat exchange betweenthe air and the solution.

As shown in Fig. 4, the dual function fin-tube heatexchanger was designed to work with the refrigerant andethylene-glycol solution. The refrigerant and the ethylene-glycol solution pass through the dual function fin-tube heatexchanger in mode 1 and mode 2, respectively. Therefore,the dual function heat exchanger has four piping systemsbecause the refrigerant and ethylene-glycol solution flowloops can not be shared. The hybrid refrigeration systemcan save energy significantly in mode 2 because its com-pressor does not operate.

The performance of the hybrid refrigeration system wasmeasured at various outdoor and indoor temperatures. Formode 1, the outdoor temperature was varied to 16.7, 27,and 35 �C, while the indoor temperature was maintainedat 21 and 27 �C. For mode 2, the outdoor temperaturewas varied to 5, 8.3, and 16.7 �C, while the indoor temper-ature was maintained at 21 and 27 �C. The relative humid-ities of the outdoor and indoor chambers were 50% at alltest conditions. Dry bulb (DB) and wet bulb (WB) temper-atures were maintained within ±0.15 �C and ±0.1 �C of theset point, respectively.

The cooling capacity of the hybrid refrigeration systemwas determined by using the air enthalpy method basedon the ASHRAE Standard [16]. The cooling capacity wasestimated by using the air flow rate and enthalpy differenceacross the indoor unit. The air flow rate was measured inthe airflow chamber with a nozzle and a differential pres-sure transducer according to ANSI/AMCA 210 [17]. Table3 shows the sensors used in this study and their uncertain-ties. The data were recorded every 5 s and averaged over aperiod of 30 min at steady state.

3. Experimental results and discussion

3.1. Performance of the unit rack

The PCB surface temperatures and the inlet and outletair temperatures of the unit rack for the telecommunicationequipment were measured for a month at actual operatingconditions. The maximum inlet and outlet air temperaturesin the rack were 25.4 �C and 30.4 �C, respectively. The sur-face temperature of the PCB module was not uniform,ranging from 35 to 60 �C. The heat trap and improperair distribution in the unit rack increased the temperatureat a certain location. However, this unit rack operatedwithin the maximum allowable temperature range for elec-tronic equipments [15].

The performance of the unit rack was tested by replac-ing the PCB module with a silicon rubber heater, whichcan control the heat load to the unit rack. The standardheat load was approximately 293 W, which was determinedfrom the field data. Fig. 5 shows the effects of heat load onthe overall heat transfer coefficient that was calculated byusing the method suggested by Lee et al. [2]. The heattransfer coefficient increased with the rise of the fan speed,but the maximum surface temperatures at both fan speedswere very similar, as shown in Fig. 6. The similarity may be

Outdoor chamber

Mass flowmeter

Psychrometric calorimeter

Air flow chamber

Compressor

Dual function HX

EXP

Mass flowmeter

Plate HX(Cond)

Indoor chamber

Heat rejection unit

E/G

Overflowline

TT

T

TT

PT

Refillline

E/G solution

Refrigerant

FilterDryerPT

T

Fig. 3. Schematic of hybrid refrigeration system.

Fig. 4. Schematic of dual function heat exchanger.

Table 3Specifications and uncertainties of sensors

Parameter Uncertainty Full scale

Temperature(T-type thermocouple)

±0.1 �C �270–400 �C

Pressure transducer ±0.2% of full scale 3447 kPaMass flow meter

(Coriolis meter)±0.2% of reading 5 kg/min

Power meter ±0.01% of full scale 20 kW

150 200 250 300 350 400 450 500 550 600

176

177

178

179

180

181

182

183

184

185

Indoor air condition : 25 oC DB, 50% RH

Heat input (W)

Hea

t tra

nsfe

r co

effic

ient

(W

/m2

o C)

Fan input (V)2428

Fig. 5. Heat transfer coefficient according to heat input.

J. Choi et al. / Applied Thermal Engineering 27 (2007) 2026–2032 2029

due to the flow stagnation at a hot spot inside the unit rack.When the heat load increased from 293 to 400 W by 37%,the maximum surface temperature was higher than 75 �C,which is the maximum allowable temperature for electronicequipments [15]. As given in Figs. 5 and 6, the indoor airtemperatures entering into the unit rack were maintainedat 25 �C.

Fig. 7 shows the variations of the maximum and averagetemperatures of the unit rack as a function of the indoor airtemperature. Both temperatures decreased with the reduc-tion of the indoor air temperature at all heat loads. Theaverage surface temperature was lower than the maximumallowable temperature of 75 �C at all indoor air tempera-tures. The maximum surface temperature was also accept-able for operation of the equipments at the standard heatinput of 293 W. However, thermal problem may occur at

150 200 250 300 350 400 450 500 550 60040

50

60

70

80

90

100

110

Indoor air condition : 25 oC DB, 50% RHMax

imum

sur

face

tem

pera

ture

(o C

)

Heat input (Watt)

Fan input (V) 24 28

Fig. 6. Maximum surface temperature according to heat input.

10 15 20 25 3040

50

60

70

80

90

100

Fan input = 24 V

Sur

face

tem

pera

ture

(o C

)

Indoor air temperature (oC)

Max Ave Heat input(Watt) 293 400

Fig. 7. Maximum and average surface temperatures according to indoorair temperature.

16 18 20 22 24 26 28 30 32 34 36 38 406.0

6.5

7.0

7.5

8.0

8.5

9.0

Mode 1

Capacity Power Indoor air temperature 27 oC 21 oC

Outdoor temperature (oC)

Cap

acity

(kW

)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Pow

er consumption (kW

)

Fig. 8. Cooling capacity and power consumption of the hybrid refriger-ation system with outdoor temperature in mode 1.

2200

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the heat input of 400 W when the indoor air temperatureincreases beyond 20 �C.

15 20 25 30 35 40400

600

800

1000

1200

1400

1600

1800

2000

Mode 1Eva

pora

ting

or c

onde

nsin

g pr

essu

re (

kPa)

Outdoor temperature (oC)

Pressure (kPa) Indoor air temperatureEvaporating Condensing

27 21

Fig. 9. Evaporating and condensing pressure with outdoor temperature inmode 1.

3.2. Performance of the hybrid refrigeration system

The cooling capacity of the unit rack can be enhancedby increasing the size of the heat exchanger or the flow rateof the coolant. However, the both methods have disadvan-tages with respect to the compactness of the cooling unitand control of the maximum surface temperature. Themaximum surface temperature of the telecommunicationequipments can be properly adjusted by reducing theindoor air temperature. Since the telecommunicationequipments dissipate large heat flux to the telecommunica-tion equipment room, the refrigeration system should beoperated even in the winter season to allow reliable opera-tion of the equipments. Therefore, the refrigeration systemused for all seasons should be optimally redesigned toincrease its capacity, performance, and reliability. In this

study, a new hybrid refrigeration system was designedand then tested.

Fig. 8 shows the cooling capacity and power consump-tion of the hybrid refrigeration system in mode 1 withrespect to the outdoor temperature. The cooling capacitydecreased with the increase of the outdoor temperature.As the secondary fluid temperature entering the condenserincreased, the condensing pressure increased more than theevaporating pressure did (Fig. 9). In addition, the massflow rate passing through the capillary tube linearlyincreased due to the rise of the condensing pressure, whichcan increase the cooling capacity. However, the decrease ofthe temperature difference between the refrigerant andthe secondary fluid in the evaporator affected the coolingcapacity more than the increase in the mass flow did.Therefore, the cooling capacity decreased with the increaseof the outdoor temperature [18]. The power consumptionincreased with the increase of the outdoor temperaturebecause both the mass flow rate and the pressure difference

4 6 8 10 12 14 16 180

2

4

6

8

10

Mode 2

CO

P

Outdoor temperature (°C)

Indoor air temperature 27 °C 21 °C

Fig. 12. COP of the hybrid refrigeration system with outdoor temperaturein mode 2.

J. Choi et al. / Applied Thermal Engineering 27 (2007) 2026–2032 2031

between the evaporator and condenser increased with theoutdoor temperature. As shown in Fig. 10, the COPdecreased with the increase of the outdoor temperaturedue to the reduction of cooling capacity and the increaseof power consumption. These trends are very similar tothe data reported by Domanski and Didion [18].

Fig. 11 shows the capacity and power consumption ofthe hybrid refrigeration system in mode 2, which is acti-vated at low outdoor temperatures. The cooling capacityincreased with the decrease of outdoor temperature. Whenthe outdoor temperature decreased, the secondary fluidtemperature entering the dual function heat exchanger alsodecreased, which enhanced the heat transfer rate betweenthe ethylene-glycol solution and air. Generally, the ratedcooling capacity of an air-conditioner is set at an outdoortemperature of 35 �C and an indoor temperature of 27 �Caccording to ASHRAE Standard 116 [16]. The coolingcapacity at the outdoor temperature of 8.3 �C in mode 2was similar to the rated capacity in mode 1 at the standard

15 20 25 30 35 402.0

2.5

3.0

3.5

4.0

Mode 1

CO

P

Outdoor temperature (oC)

Indoor air temperature 27 oC 21 oC

Fig. 10. COP of the hybrid refrigeration system with outdoor temperaturein mode 1.

4 6 8 10 12 14 16 180

2

4

6

8

10

Mode 2

Capacity Power Indoor air temperature 27 oC 21 oC

Outdoor temperature (oC)

Cap

acity

(kW

)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Pow

er consumption (kW

)

Fig. 11. Cooling capacity and power consumption of the hybrid refrig-eration system with outdoor temperature in mode 2.

test conditions. Therefore, the outdoor temperature for themode switch was set at 8.3 �C. In addition, the decreasingslope of cooling capacity according to outdoor temperaturein mode 2 was higher than that in mode 1.

Fig. 12 shows the COP of the hybrid refrigeration sys-tem in mode 2. The COP of the hybrid system in mode 2was significantly higher than that in mode 1 due to no oper-ation of compressor. When the indoor temperature was setat 27 �C, the COP varied from 2.6 to 3.5 and from 3.2 to9.0 in mode 1 and mode 2, respectively, with a variationof outdoor temperature. Nakao et al. [5] reported thatthe averaged COP of the conventional cooling system fortelecommunication equipment room was approximately2.8 for all year round. Therefore, the averaged COP ofthe hybrid system is higher than that of the conventionalvapor compression system. Especially, the hybrid refriger-ation system becomes more effective at lower outdoortemperatures.

4. Conclusions

The cooling characteristics of telecommunication equip-ments were measured and analyzed as a function of heatdensity and operating conditions. The performance of thehybrid refrigeration system for telecommunication equip-ment rooms was also measured at various operating condi-tions. The maximum inlet and outlet air temperatures ofthe unit rack were 25.4 �C and 30.4 �C, respectively. ThePCB surface temperature ranged from 35 to 60 �C, whichwas relatively higher than the air temperature due to theheat trap and improper air distribution. The maximum sur-face temperature should be properly controlled to obtainthermal reliability in telecommunication equipments. Thehybrid refrigeration system was designed to achieve highperformance and system reliability at low outdoor temper-atures. At high outdoor temperatures, the hybrid refrigera-tion system operates in the vapor compression cooling

2032 J. Choi et al. / Applied Thermal Engineering 27 (2007) 2026–2032

mode, but at low outdoor temperatures, it switches to thesecondary fluid cooling mode with no operation of thecompressor. The outdoor temperature for the mode switchwas approximately 8.3 �C, which was determined based onthe rated cooling capacity in mode 1. The COP of thehybrid refrigeration system was significantly enhanced atlow outdoor temperatures as compared with the conven-tional vapor compression system due to no operation ofthe compressor.

Acknowledgements

This work was supported by the Korea Research Founda-tion Grant funded by the Korea Government (MOEHRD)(KRF-2004-042-D00023).

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