development and performance evaluation of high speed cryogenic turboexpanders … · 2018. 11....

Development and Performance Evaluation of High Speed Cryogenic Turboexpanders at BARC, India

Anindya Chakravarty

Cryo – Technology Division, BARC

C1OrB-01

Anindya Chakravarty, CrTD, BARC CEC/ICMC - 2017

Introduction

Bhabha Atomic Research Centre (BARC), Mumbai involved in the development of

cryogenic turboexpanders for helium liquefiers and refrigerators of different

capacities for departmental usage

Coefficient of performance (COP)/liquid yield of a modern helium

refrigeration/liquefaction system is largely dependent on the performance of the

cryogenic turboexpanders employed in the thermodynamic process cycle

Three different series of turboexpanders, Series A, B and C developed and

subjected to field trials

The series A and B correspond to the first expansion stage of a standard helium

refrigerator/liquefier while series C caters to the second expansion stage


Schematic of the BARC Cryogenic Turboexpander System


Major design parameters of expansion turbine series

Parameter Series A

Design

Series B

Design

Series C

Design

𝑃𝑖𝑛 (MPa) 𝑃𝑜𝑢𝑡 (MPa) 𝑇𝑖𝑛 (K) 𝑇𝑜𝑢𝑡 (K) Rotational Speed (Hz)

Mass flow rate (g/s)

Power developed (W)

Velocity ratio, U/Cs

Isentropic efficiency

Characteristic flow

1.65

1.1

70

63.3

4400

50

1820

0.65

0.65

0.044

1.2

0.65

50.09

42.5

4500

45

1824

0.67

0.7

0.046

0.649

0.195

13.56

9.58

2833

45

779

0.66

0.7

0.039

dhubdD

Dex

L 32L1L

Din Dt0

Dinter

c2

-U2

w2

U 2

2

-U3U 3

3c

3

w3

Wheel Inlet Velocity Triangle Wheel Exit Velocity Triangle

Turbine - Diffuser

Diffuser

Turbine Wheel

HP gas in

LP gas out

01

3

01

h

s

02

2s2

03

ex

P

P02

P2

P030-exP

exP3P

3s

01 - Inlet (total)

02 - Wheel inlet (total)

2 - Wheel inlet

2s - Wheel inlet (isentropic)

3 - Wheel exit

03 - Wheel exit (total)

ex - Diffuser exit

0-ex - Diffuser exit (total)

3s - Wheel exit (isentropic)


Major Design Features of the Turboexpanders

Major design features of expansion turbine series

Features Series A

Design

Series B

Design

Series C

Design

Application

Turb Impeller Size

No. of full blades

Splitter blades

Nozzle diffuser

Nozzle incidence

Brake Impeller size

1st stage, 20 K He Ref

16 mm

13

_

separate

0o

28 mm

1st stage, 4.5 K He

Ref/Liq

16 mm

8

8

combined

-30o

28 mm

2nd stage, 4.5 K He

Ref/Liq

16.5 mm

13

_

combined

0o

35.5 mm

Overall design of the IFR turbines accomplished on the lines of Balje and Kun; for

blade design, methods from Hasselgruber with inputs from Balje are adopted

Series C brake compressor impeller large size owes to lower rotor design speed and

the fact that the compressor operates in a lower ambient pressure domain

MOC of all the impellers is high strength aluminium alloy (7075T6)


Series A Turbine Impeller Series A Nozzle

Series B Turbine Impeller

(Splitter blades)Series C Turbine Impeller

Series A and B Brake

Compressor Impeller

Series C Brake

Compressor Impeller


Machining of Impeller at BARC workshop

0.5 mm Rib and Ball end mill cutters

Roundness tester (L) and Balancing machine (R) Turboexpander test laboratory


1 2 3

4 5 6 7

89

1

2

3 4

5

6

7

9

Process

Compressor

HEX-1 HEX-2

CARefrigerator

Load

1st Stage

Turboexpander

(Series A) 2nd Stage

Turboexpander

T

s

Gas purifier

8

First Heat Exchanger Second Heat Exchanger

Gas purifier

Series A

Field Trials of Turboexpanders: Series A

Cold box fed by a helium screw compressor (0.2 MPa to 1.7 MPa, 52 g/s); mass

flow measured using an orifice meter at the process compressor suction end;

control valve at the compressor discharge end controls the flow of gas into cold box

Temperature and pressure sensors provided at nozzle inlet and at diffuser exit of the

turboexpanders, accelerometers are mounted on the body of the turboexpanders;

online monitoring and recording of rotor speed and vibration done

20 K rev Brayton cycle

Refrigerator, BARC


Process compressor is switched on and run in the cold box by-pass mode till the

steady suction and discharge pressures of 0.2 MPa and 1.7 MPa are reached

Gas is slowly fed into the cold box using the manually operated control valve in

the compressor discharge line

Field Trials of Turboexpanders: Series A

As the lowest process temperature drops, so does the turbine speed, more gas is

led into the cold box piping to maintain turboexpander speeds in excess of 4 kHz

Entire process is in quasi-steady mode so that the vibration and speed values, as

are the output from temperature and pressure sensors, get registered correctly at

each step

During the trials, minimum temperatures of 14.9 K and 16.5 K are registered

without refrigeration load and with a load of 200 W respectively; about 470 W of

refrigeration load capacity achieved at 20 K


Field Trials of Turboexpanders: Series B & C

BARC 4.5 K Helium Refrigerator/Liquefier (Modified Claude Cycle)

Consists of a pre-cooler and two process turboexpanders interspaced by a multi-

stream heat exchanger

Series B and C turboexpanders correspond to the first (high pressure) and the

second (low pressure) expansion stages of the process respectively

For the experimental results presented here, pre-cooler not in operation


From process screw compressor, a max flow rate of 67 g/s, at 1.05 bar suction

pressure, is available; Discharge pressure ranges from 13 – 17 bar(g)

For measuring flow rates through the turboexpander and JT circuits, orifice plates

are installed in the piping

Temperature sensors with redundancy and pressure sensors are provided at

nozzle inlet and diffuser exit, as also at other process points of significance

Rotor speed and vibration are measured and monitored in real time and signals

(especially during transients) recorded for off-line analysis

During start-up, BSCV – 5 is throttled and the turboexpanders are started at full

pressure available from the process compressor; higher pressure deemed to be

benign for the gas bearings, to help the rotors get over the start-up transients



As the system cools down, the pressure-drop across BSCV – 5 reduces, larger

pressure head is available across the turbines and more gas flows into the circuit

However, the turboexpander speeds do not shoot up since the gas velocities do

not change much owing to an increased density of colder gas

Slight adjustment of the valve required during different modes to bring the

turboexpander speed, circuit pressure & flow parameters to optimum levels

Process parameters registered during the cooldown are regarded as quasi-steady,

since the process is sufficiently slow

Maximum liquefaction and refrigeration capacities of 32 l/hr and 190 W

respectively, at 4.8 K, are realized during the experimental runs



51.9

14.9

7.4

59.9

301.3

4.8

11.5

1

10

100

1000

Tem

pe

ratu

re (K

)

Entropy

Series B

Series C

37.8

9.2

5.5

46.0

302.1

4.86.7

1

10

100

1000

Tem

pe

ratu

re (

K)

Entropy

Series B

Series C


Major process-point temperatures, especially, those at the inlet and exit to the

Series B and C turboexpanders are marked in the figures

T-s diagrams of the helium liquefaction (L) and refrigeration (R) processes


Performance Evaluation of Turboexpander Series

For single working fluid and for high flow Reynolds numbers:

𝑓 𝑃𝑅, 𝜂, 𝜃,𝑀𝑢 = 0

01

3

01

h

s

02

2s2

03

ex

P

P02

P2

P030-exP

exP3P

3s

01 - Inlet (total)


2 - Wheel inlet


3 - Wheel exit


ex - Diffuser exit



Common practice is to present isentropic efficiency, 𝜂, in relation to the isentropicvelocity ratio, 𝑈/𝐶𝑠, which is a combination of stage pressure ratio, 𝑃𝑅 and rotornon-dimensional speed

𝑈 = 𝜔.𝐷

2

𝐶𝑠 = 2. ∆ℎ0𝑠Τ1 2

∆ℎ0𝑠 = ℎ𝑖𝑛 − ℎ𝑜𝑢𝑡_𝑖𝑠𝑒𝑛


Performance Evaluation of Turboexpander Series

Turbine isentropic efficiency, 𝜂, is computed as follows:

𝜂 =ℎ𝑖𝑛 − ℎ𝑜𝑢𝑡

ℎ𝑖𝑛 − ℎ𝑜𝑢𝑡_𝑖𝑠𝑒𝑛

ℎ𝑖𝑛 = ℎ 𝑃𝑖𝑛, 𝑇𝑖𝑛

ℎ𝑜𝑢𝑡 = ℎ 𝑃𝑜𝑢𝑡, 𝑇𝑜𝑢𝑡

ℎ𝑜𝑢𝑡_𝑖𝑠𝑒𝑛 = ℎ 𝑃𝑜𝑢𝑡, 𝑠𝑖𝑛

01

3

01

h

s

02

2s2

03

ex

P

P02

P2

P030-exP

exP3P

3s

01 - Inlet (total)


2 - Wheel inlet


3 - Wheel exit


ex - Diffuser exit



Non-dimensional mass flow parameter (characteristic flow) 𝜃, is defined as:

𝜃 =ሶ𝑚

𝜌1 𝑎1𝜋 Τ𝐷2 4

State point properties are computed using HEPAK® software


0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.25 0.35 0.45 0.55 0.65 0.75

Isen

tro

pic

Eff

icie

ncy

,

Isentropic Velocity Ratio, U/Cs

Series A

Series B

Series C

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.035 0.04 0.045 0.05 0.055 0.06

Isen

tro

pic

Eff

icie

ncy

,

Characteristic Flow

Series A

Series B

Series C

Comparison of major design and operational parameters of expansion turbine series

Parameter Series A Series B Series C

Design Operation* Design Operation* Design Operation*

𝑃𝑖𝑛 (MPa) 1.65 1.285 1.2 1.013 0.649 0.576 𝑃𝑜𝑢𝑡 (MPa) 1.1 0.759 0.65 0.492 0.195 0.174 𝑇𝑖𝑛 (K) 70 67.81 50.09 46.00 13.56 14.45 𝑇𝑜𝑢𝑡 (K) 63.3 59.74 42.5 37.76 9.58 10.47 Rotational Speed (Hz) 4400 4295 4500 4447 2833 2826

Mass flow rate (g/s) 50 48.1 45 46.7 45 41.5

Power developed (W) 1820 2084 1824 2044 779 744

Velocity ratio, U/Cs 0.65 0.58 0.67 0.64 0.66 0.63

Isentropic efficiency 0.65 0.63 0.7 0.72 0.7 0.67

Characteristic flow 0.044 0.054 0.046 0.055 0.039 0.043 *Best Efficiency Point (BEP).


Turbine isentropic efficiency rises with 𝑈/𝐶𝑠 ratio for all the turbine series

Efficiencies computed for Series B are a notch higher than those of Series A for

same 𝑈/𝐶𝑠 ratios which may be attributed to the design modifications

Series C efficiency found to be lowest, which may be attributed to larger operating

(and design) pressure ratio than what is normal for IFR turbines

Efficiency of Series B and C peaks out at around 𝑈/𝐶𝑠 ratio of 0.63 – 0.65

Since it is not possible to reach the design 𝑈/𝐶𝑠 ratio for Series A duringoperation, its efficiency shows an upward trend without any sign of peaking

Discussion of Results

All the turbines exhibit best efficiencies at characteristic mass flows much higher

than design

Plot indicates possibility of even larger swallowing capacity of the turbines A and B

For higher efficiencies (steady state), Series C is also quite unaffected by the

characteristic flow


Successful field trials of BARC turboexpander series, exhibiting isentropic

efficiency of around 70% (Series B)

However, from the analysis of experimental data, it is evident that more runs are

required to understand the off-design characteristics, especially, for the high-

pressure ratio Series C turbines

Higher swallowing capacity of the turbines also needs to be investigated

thoroughly

In order to develop turbines with even higher efficiencies, it may be necessary in

future to review the currently employed design methodology at BARC

Conclusion

The authors would like to thank Bhabha Atomic Research Centre (BARC),

Trombay for supporting the work. The inputs and efforts of all Cryo-Technology

(CrTD), BARC technical personnel during turboexpander development and helium

liquefaction/refrigeration plant field trials, are highly appreciated.

Acknowledgement


References

[1] Cretegny D, Schönfeld H, Decker L and Löhlein K 2004 Efficiency improvement of small

gas bearing turbines – impact on standard helium liquefier performance Advances in

Cryogenic Engineering 49 272 – 78

[2] Chakravarty A and Singh T 2011 High speed miniature cryogenic turboexpander impellers

at BARC Indian Journal of Cryogenics 36 1 – 9

[3] Menon R et al. 2012 High speed cryogenic turboexpander rotor for stable operation up to

4.5 kHz rotational speed Indian Journal of Cryogenics 37 40 – 45

[4] Ansari N A et al. 2017 Development of helium refrigeration/liquefaction system at BARC,

India IOP Conf. Series: Materials Science and Engineering 171 1 – 8

[5] Balje O E 1981 Turbomachines (USA: John Wiley and Sons)

[6] Kun N C and Sentz R N 1985 High efficiency expansion turbines in air separation and

liquefaction plants Int. Conf. of Production and Purification of Coal Gas and Separation of Air,

Beijing, China 1 – 21

[7] Hasselgrüber H 1958 Stromungsgerechte gestaltung der laufrader von

radialkompressoren mit axialem laufradeintrict (in German) Konstruction 10 22

[8] Balje O E 1970 Loss and flow path studies on centrifugal compressors Part - II Trans.

ASME J. Eng. Power 70 287 – 300

[9] Whitfield A and Baines N C 1990 Design of radial turbomachines (England: Longman

Scientific & Technical)

[10] Baines N C and Sieverding C H 1992 Radial Turbines (Belgium: von Karman Institute for

Fluid Dynamics)


Thank You

development and performance evaluation of high speed cryogenic turboexpanders … · 2018. 11....

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