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Applied Thermal Engineering 31 (2011) 3684e3688

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Preliminary experimental study of single screw expander prototype

Wei Wang a,b,*, Yu-ting Wu a,b, Chong-fang Ma a,b, Lin-ding Liu a,b, Jian Yu a,b

aKey Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, College of Environmental and Energy Engineering, Beijing University of Technology,Beijing 100124, ChinabKey Laboratory of Heat Transfer and Energy Conversion, Beijing Municipality, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124,China

a r t i c l e i n f o

Article history:Received 23 November 2010Accepted 11 January 2011Available online 28 January 2011

Keywords:Single screwExpanderPerformance testLubrication

* Corresponding author. Key Laboratory of EnhancConservation, Ministry of Education, College of Envineering, Beijing University of Technology, Beijing 100

E-mail address: wang_wei@bjut.edu.cn (W. Wang

1359-4311/$ e see front matter Crown Copyright � 2doi:10.1016/j.applthermaleng.2011.01.019

a b s t r a c t

Efficient heat conversion of low temperature heat source is a key problem for energy saving, especially inthe fields of waste heat recovery and renewable energy utilization. At present, technical bottleneck forlow temperature thermal power is lack of suitable prime mover. As the core component of generalmachinery, single screw has many good features, including balanced loading of the main screw, lowleakage, low noise, low vibration and long working life, etc. If single screw technology is applied to thefield of expander, more efficient prime mover would be possibly obtained, compared with pistolexpander, scroll expander and twin screw expander, and so on. In order to verify the performance of theprototype, the function experiment was made. In this paper, compressed air was used as working fluidand performance test for the prototype was finished at conditions including different intake flow,different humidity, constant torque, and constant rotational speed. From the experimental data, it isshown that the power output is 5 kW, exhaust temperature is �45 �C, difference between the import andexport is about 62 �C, in the conditions of inlet pressure at 0.6 MPa and rotational speed 2850 rpm. Thetest results also show that the single screw expander has good part load characteristics. From the analysisof experimental data, we found that adiabatic efficiency of the prototype is not so high probably becauseof poor lubrication. The lubrication problem will be considered in the next work.

Crown Copyright � 2011 Published by Elsevier Ltd. All rights reserved.

1. Introduction

At present, China takes the second place in energy consumptionand the first in carbon dioxide emissions in the world. Pressure onresources and environment is increasing significantly. According tostatistics, China’s annual energy consumption in 2009 is about 3.1billion tons of standard coal [1], which has reached long-termenergy plan expecting target, whereas total energy consumption in2020 should be controlled at 3.0e3.2 billion tons of standard coal[2]. Energy efficiency admits of no delay. However, with the prac-tical work in recent years, people gradually know that the mainbottleneck of restricting improving energy efficiency is lack ofeffective technical means. The key problem is lack of small efficientprime mover, especially in the field of industrial waste heatrecovery and renewable energy utilization.

Compression and expansion, two basic processes of thermalpower conversion, are reverse processes of each other. So the

ed Heat Transfer and Energyronmental and Energy Engi-124, China.).

011 Published by Elsevier Ltd. All

history of expander is related to compressor. Since the twentiethcentury, people have been developing small expander. Due totechnical reasons, small expander efficiency is very low and smallexpander is not practical with low energy prices. In recent years,since the rapid rise in energy prices, small expanders become thefocus of attention. Meanwhile, mechanical manufacturing tech-nologies of compressor have developed quickly in recent decades,which could be realized as technological breakthroughs for smallexpander. At present, small expander becomes research hotspot inenergy field. From a practical perspective, small expanders aremainly used in three aspects of refrigeration, power generation andvehicle power. Related refrigeration research is the most popular,including expansion valve replacement, natural gas recovery andair separation, etc. Power generation is mainly applied in industrialwaste heat recovery and geothermal utilization, etc. Vehicle poweris mainly used in automobile exhaust heat recovery andcompressed air driven vehicle. Until now, different types of smallexpanders have been researched, including vane rotary expander[3e5], reciprocating expander [6e8], scroll expander [9,10], andtwin screw expander [11,12], etc. These studies show that the twinscrew expander has an obvious advantage in field of low temper-ature waste heat power generation [13].

rights reserved.

Fig. 1. The photo of single screw expander prototype.

W. Wang et al. / Applied Thermal Engineering 31 (2011) 3684e3688 3685

Single screw technology has many advantages, such as balancedloading of the main screw, long working life, high volumetric effi-ciency, high partial loading, low leakage, low noise, low vibrationand simplify configuration, etc. If single screw technology could beapplied to the field of expander, new type power equipment will beobtained. At the point of output power, single screw expander canreach 1e1000 kW range, so it overcomes the flaw that traditionalsteam and gas turbines can not be made too small. At the point ofworking fluid, high pressure gas, superheated steam, saturatedsteam, gas-liquid two phases or heat liquid all can be used as theworking fluid for single screw expander. These characteristicsenable single screw expander to be applied in many industrialfields. However, as a new type one, related research of single screwexpander is still blank nowadays, because the manufacture tech-nology of key components is very difficult.

The laboratory of authors has developed a single screwexpander prototype, shown in Fig. 1. The main task of this paper isto carry out the performance test for the prototype, obtain the basicdata of prototype performance, and provide the basis of improvingsingle screw expander.

2. Experimental systems

The experiment uses compressed air as working fluid and twofactors are considered: the one is that the air known as ideal gas iseasy to carry out experiment; the second is air can simulateindustrial pressure recovery. Experimental system is shown inFig. 2. Air compressor provided high pressure air to gas tank, which

7

6

8

4 5

3 2

1

P /

T

T/P/G

Exhaust

7. Magnetic power brake 8. Back pressure valve

4. Inlet valve 5. Single screw expander 6. Torque transducer

1. Air compressor 2. Vaccum freeze dryer 3. Gas tank

Fig. 2. Schematic diagram of experimental system.

is the stable gas source for the test. Through inlet adjusting valve,compressed air from gas tank entered into expander. Exhaust gasled to outdoors. Through a magnetic powder brake for the load, theshaft power of expander was consumed. The back pressure valvewas installed on outlet pipe to adjust the back pressure of expander.Vacuum freeze dryer was used to remove moisture in compressedair. The parameters of volume flow rate, inlet and outlet pressure,inlet and outlet temperature, rotational speed and torque, thepower, load and temperature drop performance of the prototypewere obtained.

2.1. Experimental measurement apparatus

Air compressor is Fusheng SA45A, displacement is 7.1 Nm3/min,rated discharge pressure is 1.0 MPa, and max discharge pressure is1.05 MPa. Magnetic powder brake model is FZJ-50, rated torque is500 N�m and slip power is 15 kW. Expander is manufactured byourselves and design inlet flow rate is 1.1 Nm3/min. Lubricating oilspecification is CP-4600-68.

In the experiment, some parameters are measured such astemperatures, pressures, powers and flow rates. The apparatusesare as follows:

Temperature: standard thermocouples of grade A Pt100 areused, model is WZP21, measurement range is �200 to 650 �C,accuracy is �0.15 �C.Pressure: two pressure sensors of JYB-K0-HAG are used,measurement range of the one is 0e1 MPa, another is0e0.2 MPa, accuracy of each one is �0.25%。Power: torque transducer model is CYB-803S, measurementrange of torque and rotational speed are respectively0e300 N�m and 0e6000 r/min, accuracy is �0.5%。Flow rate: vortex flowmeter is used, its model is LUGB-25, rangeis 8.5e90 m3/h, accuracy is 1.0%。

During the experiment, the temperature, pressure and flowdata are collected by Agilent 34970A data acquisition instrument.Torque, rotational speed and power data are shown on threewindows by CYB-808 intelligent torque meter and output by smallprinter.

2.2. The analysis of experimental results

According to the measured values of different parameters,mechanical efficiency, adiabatic efficiency and total efficiency ofsingle screw expander could be obtained by Eq. (2)e(6) Calculation.

Shaft power could be calculated by the measured value ofrotational speed and torque：

Pe ¼ N � n9550

(1)

The enthalpy difference between inlet and outlet of expandercould be obtained by Eq. (2) Calculation:

h1 � h2 ¼ cpðT1 � T2Þ (2)

Considering the pressure of compressed air is relatively low, andthe temperature is also close room temperature, so the working gascan be used as ideal gas. The ideal enthalpy drop was calculated byEq. (3):

Dh ¼ h1 � h2s ¼ kk� 1

RgT1

"1�

�p2p1

�k�1k

#(3)

80

2200 2400 2600 2800 3000

4.4

4.5

4.6

4.7

4.8

4.9

5.0

5.1

Power

Rotational Speed/rpm

Wk/rewo

P

-46.0

-45.5

-45.0

-44.5

-44.0

-43.5

-43.0

Outlet temperature

Out

let t

empe

ratu

re /

°C

Fig. 4. Variation of power and outlet temperature with rotational speed.

W. Wang et al. / Applied Thermal Engineering 31 (2011) 3684e36883686

Adiabatic efficiency:

hs ¼ h1 � h2h1 � h2s

(4)

Mechanical efficiency:

hm ¼ Peqmðh1 � h2Þ

(5)

Total efficiency:

h ¼ hshm (6)

where, h1, h2 are respectively the actual enthalpy of inlet and outletair of expander, kJ/kg; h2s is the ideal enthalpy of outlet air ofexpander that was calculated by Eq. (3), kJ/kg; T1, T2 are respectivelythe temperature of inlet and outlet air of expander, K; P1, P2 arerespectively the gauge pressure of inlet and outlet air of expander,Pa; cp is the specific heat of air, kJ/(kg$K); k is entropy index;Rg is gas constant, J/(kg$K); N is torque, N�m; n is rotational speed,r/min; Pe is shaft power, kW; qm is air mass flow rate, kg/s.

3. Experimental results and discussions

The purpose of this study is to inspect the actual operation of theprototype and accumulate basic data for improving it. Throughoutthe trial process, the expander runs smoothly with low noise andlittle vibration. Vibration phenomenon of the pipe and expanderoccurred only at the speed of about 1000 r/min. This rotational speedpoint may be the resonance point of testing system. Because vibra-tion problem was not the main point of this experimental study,vibration test was not carried out. In addition, single screw expanderis used in gap sealing; lubricating oil is needed to lubricate the screwand gaterotors. Considering the preliminary experiment, lubricatingoil circle was not applied to the testing system. In this study, theprototype was rotated 90� and deposited on the engine bed, lubri-cating oil was injected into the gaterotor room. In effect, this methodcanguarantee the lubricationandsealingof screwandgaterotors. Butexpander power output was influenced by the friction of lubricatingoil on gaterotor. The next stepwewill focus on the lubrication study.

3.1. 100% valve opening

The measured value of compressed air mass flow rate was504 kg/h, inlet temperature was 17 �C. Fig. 3 shows the curve ofvariation of power and torque with rotational speed in 100% valveopening. As shown in Fig. 3, the torque decreased linearly and thepower indicated parabola change with increase of the rotational

2200 2400 2600 2800 3000

4.4

4.5

4.6

4.7

4.8

4.9

5.0

5.1

Rotational Speed / rpm

Wk/re

woP

15

16

17

18

19

20

Power Torque

mN/

eu

qroT

Fig. 3. Variation of power and torque with rotational speed.

speed. Shaft power reached the maximum of 5 kW at 2850 r/minrotational speed. Fig. 4 shows the curve of variation of power andoutlet temperature with rotational speed in 100% valve opening. Asshown in Fig. 4, with increasing of the rotational speed, outlettemperature sharply decline, the lowest temperature reached�45.5 �C, the temperature difference of inlet and outlet was about62 �C. The results indicated that single screw expander has goodtemperature drop characteristics, so it has good application pros-pects on cryogenic engineering. Fig. 5 shows the curve of variationof efficiency with rotational speed in 100% valve opening. As shownin Fig. 5, adiabatic and total efficiency increased remarkably andmaintained stable within wide speed range, compared with smallvalve opening. The maximum of adiabatic efficiency and totalefficiency were 59% and 32.5% respectively. Mechanical efficiencywas slightly lower than 50% valve opening.

Furthermore, it was shown that mechanical efficiency was nothigh and it was less than 60% in full valve opening condition. Thereason for this result was not only related to the accuracy ofalignment and lubrication of expander shaft, bus also related to thelubrication and sealing of fit clearance and gear gap in internalexpander. So lubrication problem should be specially researched atthe next step.

3.2. Constant torque

The torque was set at 10.4 Nm for the experiment. Fig. 6 showsthe curve of variation of power and air flow rate with rotational

2200 2400 2600 2800 3000

10

20

30

40

50

60

70

%/ycneiciff

E

Rotational Speed / rpm

η

ηη

Fig. 5. Variation of efficiency with rotational speed.

500 1000 1500 2000 2500 3000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Power

Gas mass flow rate

Gas

mas

s fl

ow r

ate

kg/h

Rational Speed / rpm

Wk/re

woP

50

100

150

200

250

300

350

400

450

Fig. 6. Variation of power and air flow rate with rotational speed.

2 3 4 5 6 7 8 9 10 11 12 13 14 15

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Power

Torque / Nm

Pow

er /

kW

50

100

150

200

250

300

350

Gas mass flow rate

Gas

mas

s fl

ow r

ate

/ kg/

h

Fig. 8. Variation of power and air flow rate with torque.

20

30

40

50

60

70

80

90

100

Eff

icie

ncy

/ %

W. Wang et al. / Applied Thermal Engineering 31 (2011) 3684e3688 3687

speed in constant torque. As shown in Fig. 6, shaft power has goodlinear increase with increase of the rotational speed. During lowrotational speed, the increase of air mass flow rate was lower thanthe increase of rotational speed, and the situation was reversedduring high rotational speed with turning point of about1200e1300 r/min. Fig. 7 shows the curve of variation of efficiencywith rotational speed in constant torque. As shown in Fig. 7, adia-batic and mechanical efficiency were respectively sustainableincrease and decrease, and total efficiency first increased and thendecreased with increase of the rotational speed. Maximum effi-ciency point was at about 1200e1300 r/min.

2 3 4 5 6 7 8 9 10 11 12 13 14 15

0

10

Torque/ N m

Fig. 9. Variation of efficiency with torque.

3.3. Constant rotational speed

The rotational speed was set as 1500 r/min. Fig. 8 shows thecurve of variation of power and air flow ratewith torque in constantrotational speed. As shown in Fig. 8, shaft power has good linearincrease with the torque increase. During low torque, the increaseof air mass flow rate was lower than the increase of torque, and thesituationwas reversed during high torque, where turning point wasat about 8e9 N�m.

Fig. 9 is the curve of variation of efficiency with torque inconstant rotational speed. As shown in Fig. 9, adiabatic efficiencywas gradually increase, and the increase rate of it was faster thanbefore after 9 N�m. Mechanical efficiency was first increased and

500 1000 1500 2000 2500 3000

10

20

30

40

50

60

70

80

90

100

Eff

icie

ncy

/ %

Ratational Speed / rpm

Fig. 7. Variation of efficiency with rotational speed.

then decreasedwith the torque increase, turning point was at about8e9 N�m. Total efficiency didn’t has significant change.

3.4. Humidity effect

In this experiment, valve opening was full, inlet temperaturewas 17 �C, environment relative humidity was about 50%. Fig. 10shows the curve of comparison of power in the conditions ofdehumidification and pro-dehumidification. As shown in Fig. 10,powers gradua1ly increased in two conditions with the rotationalspeed increase, and the power in the condition of pro-dehumidi-fication was obviously higher than dehumidification at the samerotational speed point.

Fig. 11 shows the curve of comparison of efficiency in theconditions of dehumidification and pro-dehumidification. Asshown in Fig. 11, in the condition of dehumidification, mechanicalefficiency was slightly higher, adiabatic efficiency was significantlylower and total efficiency was lower, compared with the conditionof pro-dehumidification. After the experiment, lots of water dropswere observed at the screw and gaterotors of expander. Insummary, the expander characteristics with wet air used asworking fluid was obviously better than using dry air. It wasconsidered that the condensate in wet air improved the effect oflubrication and sealing of expander. The result provided a possibleway to improve the performance of expander. However, the anal-ysis was rough and inaccurate. Due to complex three dimensional

2000 2200 2400 2600 2800 3000

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.0

Wk/rewo

P

Ratational Speed /rpm

DehumidifiedPro-dehumidified

Fig. 10. Comparison of power in two conditions.

2000 2200 2400 2600 2800 3000

20

30

40

50

60

70

80

Eff

icie

ncy

/ %

Rotational Speed / rpm

ηm ηDm

ηs ηDsη ηD

Fig. 11. Comparison of efficiency in two conditions.

W. Wang et al. / Applied Thermal Engineering 31 (2011) 3684e36883688

flows inside of the single screw expander, the mechanism ofworking fluid phase change impacting the performance of singlescrew expander during expansion was not very clear. Generallyspeaking, working fluid phase change could increase mechanicalloss and improve sealing. Thus, author insists that the research ofworking fluid phase change at the internal expansion has veryimportant significance in improving the performance of singlescrew expander.

4. Conclusions

Through the preliminary experiment for the prototype of singlescrew expander, following conclusions can be given:

(1) Single screw expander run smoothly with low noise, butobvious vibration occurred at the speed point of 1000 r/min.

(2) In 100% valve opening, shaft power reached the maximum of5 kW at rotational speed of 2850 r/min, the maximumtemperature drop was about 62 �C, the maximum of adiabaticefficiency and total efficiency were 59% and 32.5% respectively.

(3) At torque of 10.4 Nm, air mass flow rate was graduallyincreased with the rotational speed increase, and the increaserate was first small and then big. The rotational speed point ofmaximum total efficiency existed.

(4) In rotational speed equaled 1500 r/min, air mass flow rate wasgradually increased with the torque increase, and the increaserate was first small and then big. The torque point of maximumtotal efficiency existed.

(5) The expander characteristics using wet air as working fluid wasobviously better than using dry air.

In summary, the prototype basically reached the requirementsof design; meanwhile it also has great room to improvement. Withthe continuous improvement of testing and mechanical configu-ration, single screw expander should have great prospects in wasteheat and pressure recovery, new and renewable energy utilizationand air expansion refrigeration.

Acknowledgements

The work was supported by the 973 Program’ Project (grantnumber 2011CB707202), Beijing Municipal Natural Science Foun-dation (grant number 3081002) and National Natural ScienceFoundation (51006002).

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