Page 1

HELIUM REFRIGERATION SYSTEMS

for

SUPERCONDUCTING ACCELERATORS

V. Ganni

Jefferson Lab

Page 2

What is an “Optimal” Refrigeration System?

Typical request from an experimenter is

“Give me an optimal cryogenic system”

What is an optimum system? Does it result in the:

1) Minimum operating cost

2) Minimum capital cost

3) Minimum maintenance cost

4) Maximum system capacity

5) Maximum availability of the system

A combination of some or all the above

or, some other factors?

Page 3

What is an “Optimal” System (cont.)

Optimize

Compressor

System

(Vendor)

Optimize

Cold Box

Design

(Vendor)

Maximum

Efficiency, Reliability,

Low Maintenance

(Operations)

Design of

Loads

(Experimenter) Minimum

Capital

Cost

(Construction)

• One’s viewpoint can be based only on their role and focus within a project

• Easy to believe that one’s goals are mutually exclusive of others

• Many believe that maximum system efficiency occurs only at one set of fixed

operating conditions

Page 4

Primary Parameters

The cryo plant design is set by the :

1. Operating Temperature / Pressure

2. Refrigeration Capacity (Isothermal/ Isobar)

3. Liquefaction Capacity

4. Different modes of operation

Page 5

Important Factors to be Considered in Cryo Plant Design

1. Capacity requirements (2-K, 4.5 K, shield loads)

2. Capacity requirements to achieve pump down

3. System commissioning requirements

4. System operational and maintenance requirements

5. Different modes of operation (e.g., 2-K, 4.5 K, etc.)

6. Provisions for reconditioning the CM components

7. Part load operations

8. System stability

9. System availability

10. System efficiency

11. System simplicity

12. Operational vs. design pressure of various circuits

Page 6

2-K Refrigeration System

General sub-atmospheric helium refrigeration system

Page 7

CHL-II Compressors & Oil Removal

Compressor and oil removal systems are designed by JLab and

produced by industry to JLab drawings

Compressor System Oil Removal

Page 8

CHL-II 4.5K Cold Box(s)

The cold box process is designed to JLab Ganni Cycle -

Floating Pressure Technology and produced by industry

Page 9

CHL-II 2K Cold Box

2-K cold box is designed by JLab with cold compressors

and 4-2K heat exchanger supplied from industry.

Control system and pump down philosophy were

developed by JLab and used for last 20+ years

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Refrigeration Efficiency

Inverse coefficient of performance vs. 4.5 K refrigerator efficiency

Page 11

Floating Pressure Process – Ganni Cycle

• Maintains high cryo plant operational efficiencies at full and reduced plant

capacities and for refrigeration, liquefaction and mixed mode cycles

• Adopts non-interference control philosophy (except to protect equipment),

using only a few key process parameters

• Does not require prerequisite knowledge of the design TS conditions

• Does not require user manual intervention

• Does not throttle turbines to force them to operate at TS conditions

• Adjusts system charge automatically to meet required mode (liquefaction to

refrigeration) and load conditions, both efficiently and stably

• Does not require additional hardware not otherwise present in traditional

systems

• Can be implemented on existing designs as easily as new designs.

• Utilizes a minimum number of control elements and process variables

• Is not affected by imprecise instrument accuracy or poor calibration

Page 12

Both the expander and compressor are essentially constant volume flow devices, so for a given mass charge they set their own inlet pressures, thus,

•Compressor establishes the suction pressure

•Expander establishes the discharge pressure

With these, the gas charge establishes the system mass flow rate

If left unconstrained, these two devices establish

Essentially constant pressure ratio and,

Essentially constant Carnot efficiency

For a given gas charge

Floating Pressure Process

Page 13

Floating Pressure Process

Entropy (s) [J/g-K]

Nat

ura

l lo

g o

f T

emper

ature

, ln

(T)

Compressor

Expander Load

ln(TSR/TSS)

TSR

TSS = Tx,o

Tx,i

Tx,o

ln(Tx,r)=

ln(Tx,i/Tx,o)

R·ln(pr)

R·ln(pr)

Load: DsL

R·ln(pr)

Upon decreasing load,

cycle shifts to the right,

maintaining same ‘size’,

mass flow decreases

proportionally

Page 14

Observations of Floating Pressure Process TS Diagram:

• Y-axis is the natural logarithm of temperature

• Between any two arbitrary points ‘1’ and ‘2’,

• So, at constant temperature (isotherms),

• At constant pressure (isobars),

• Slope of isobars is equal the specific heat at const. pressure ( )

• Keeping Ds the same at any given temperature,

regardless of the mass flow, ensures a constant efficiency

2 1 2 1 2 1( ) ( / ) ( / )ps s s C n T T n p pD ( ) ( )p r rs C n T n pD

( )p rs C n pD

( )p rs C n TD

pC

Floating Pressure Process

Page 15

Floating Pressure Process

As the “Claude Cycle” is essentially a constant pressure process

and, the “Sterling Cycle” is a constant volume process

the “Floating Pressure Cycle” is a constant pressure ratio process

That maintains essentially constant Carnot efficiency over a very wide operating range

(100% to ~ 30% of maximum capacity in practical systems)

,2,2

,1 ,1

1 Constant

hh v Cr

x pl l

Tp Qp

p C T

ConstantL Lcarnot

C CW w

D

Page 16 Page 16

Warm Screw Compressor Efficiencies

Typical 1st & 2nd stage RS compressor isothermal efficiencies

Optimum pr ≈ 3 to 4

1st Stage Compressors 2nd Stage Compressors

TRADITIONAL

CYCLE pr RANGE

Licensing Agreement

Jlab has licensed the Ganni Floating Pressure Helium Process Cycle technology to

Linde Cryogenics,

Division of Linde Process Plants, Inc. and Linde Kryotechnik AG

for world wide commercialization.

Page 18 Page 18

Some Historical Reasons Given (for the last 20+years) to Stay Status Quo:

“We have done this before….” (i.e., and it works ‘great’ or at least good

enough…why change?)

Industry,

An increase in system efficiency comes with,

• “Increase in capital cost”

• “Reduced availability”

• “High risk to the basic program”

Users,

• “T-S design is the optimum, force the system close to it”

• “You should not change system operation from the basic design and/or the

operation method”

• “Cryogenics is not the experiment”

• “The cryo system is running fine. Don’t change it”

• “Scale the new system from an existing one”

• “Requires re-training of the operators”

And many, many more !!!

Page 19 Page 19

Some Results So Far

Page 20 Page 20

NASA-JSC/JLab Collaboration

James Webb Telescope

Replaces Hubble

~1 million miles out

Telescope Mockup at the National Mall, D.C.

Floating Pressure Technology For Telescope Testing

• Environmental Space Simulation Chamber-A

The existing 3.5 kW 20 K cryogenic system is converted to JLab’s Floating

Pressure Technology. Improved temperature stability from 2.5 K to 0.25 K and efficiency (follows)

• New 20K, 13 kW refrigerator design is based on the Floating

Pressure Cycle

Page 21 Page 21

NASA JSC – 20 K Refrigerator

Performance

– Essentially constant efficiency down to ~1/3 of max. 20 K load!

Note (1)

Note

(2

)

Page 22 Page 22

Brookhaven Reports on Operational Results

Brookhaven RHIC Cryogenic Staff

Brookhaven Reports Its Program Results Brookhaven TODAY, Feb 11, 2008

http://www.bnl.gov/today/story.asp?ITEM_NO=544

“Jefferson Lab technology saving BNL $1.5M in electricity alone for typical 30

week experiment”

“ Seen increased reliability, stability,

and efficiency”

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JLab-CHL-I Helium Refrigeration System

This cryogenic plant supports operation of the Continuous Electron Beam Accelerator

Facility (CEBAF) cryomodules in the tunnel. The accelerator power is adjustable from 500

MeV to 6 GeV but the original cryogenic plant was designed to operate only at one design

capacity consuming more than 6 MW of electrical power. Through the years the

Cryogenics Group has completed several phases of technological improvement and

increased the plants operational envelope to allow its capacity to be varied to better match

the cryogenic load. The operational envelope now allows the plants power consumption

to be varied from 4.5 MW up to 6 MW in conjunction with the CEBAF accelerator

requirements.

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JLab CHL-2

– New 12 GeV CHL-2 compressor system provided the helium at the required temperature, pressure and flow rate for the 12 GeV CHL-2 cold box commissioning in all the design modes

– 12 GeV cold box system demonstrated its ability to meet all modes of operation at the guaranteed capacities with high efficiency, including the warm-up and cool-down utility modes.

• Preliminary results suggest, for Mode-1 (max. capacity)

CHL-2 CHL-2 CHL-1

Expected Tested Actual

Total load exergy 1.27 MW 1.27 MW 1.27 MW

Total input Power 4.1 MW 3.8 MW 6.2 MW

Cold box efficiency 62% 59%

Compressor system efficiency 47% 51%

Total system efficiency 29% 29% 19%

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• Re-design of LN pre-cooler for CHl-II cold box by the

manufacturer met the RFP design goals and established the new

standard for LN use

• Full realization of the Ganni Cycle – Floating Pressure process

has been successfully demonstrated in the JLab 12 GeV cold box

and compressor system

This allows a very wide range of operation (19.5 to 6.5

bar supply to the cold box) with high efficiency (nearly

flat down to ~1/3 of max. capacity)

• Similar results from implementation of this process on the

NASA-JSC 20-K refrigeration system for the James Webb project

• Additionally, this process is being used for all the follow up

projects like the MSU-FRIB and LCLS and is anticipated to be

used in other projects in the future

Conclusions