An Overview of Sub-atmospheric Cold Compressor … Overview of Sub-atmospheric Cold Compressor...

Thomas Jefferson National Accelerator Facility

Operated by the Southeastern Universities Research Association for the U.S. Department of Energy Contract DE-AC05-06OR23177

An Overview of Sub-atmospheric Cold Compressor Turndown Design

Methods

November 10, 2014

Dana Arenius

Thomas Jefferson National Laboratory

On Behalf of the Cryogenics Engineering Department

Newport News, VA, USA

Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177


What do we mean by turndown?

• Generally Speaking…

• Reduction of sub atmospheric refrigeration load

• Have to be careful….many believe this is means utility turndown

• When does load reduction happen?

• If the plant has been designed with positive margin

– Unknown final actual refrigeration loads

– Includes future upgrades

– Operational variance of loads (ex: beam energy)

• Oops!....the refrigeration load is far less than we expected

– Natural tendency to believe actual loads go up, not down

– Better to have too much than too little available capacity, assumption

that although a problem, not as big



As Plant Operators Ourselves

We want…..and work to achieve (using new technology

and lessons learned from project to project)

— Low cost capital plant equipment cost

— High reliability

— High efficiency regardless of % of plant load

— High plant availability

— Low helium gas loss

— Low utility and manpower cost

When we add design margin to our loads and cryoplant calculated performance,

we are saying we will most likely be operating at some partial load

So our operation needs to be efficient at some unknown turn down load



Not knowing for sure what the loads will be…..

• All refrigeration capacity comes from the 4.5K refrigerator

system, none from 2K portion of the system

—Need to have wide, stable, and operational cost effective

variability of 4.5K cold box technology (includes warm

helium compressor performance), done……. • Demonstrated retaining constant high efficiency, continuous variability all the

way down to 20% of design load for 4K

—Future large scale cold compressor systems are placing

constraints on matching 4.5K cold box technical capability as

to maximum future plant size/number and/or turndown

efficiency with load reduction – Multiple 4.5K plants & cold compressors trains coupled with load

segmentation, real $ challenge on total project cost

– Parallel cold compressor trains development on single 4.5K cold box, need to

demonstrate



NASA JSC Test Results

5

Same type of result

for Jlab 12 GeV

4.5K cold box to

20% load



Latest SRF Q Technology Development

Doubling of current SRF cavity Qs development directly

affects number of size of future plants for sub atmospheric

applications (so far, target is ~1/2 load for current cold

compressor capacity technology)

— May help in curbing how much the cryogenic

technology development must achieve in the near future

as it pertains to SRF applications

—But “high Q” development still has many challenges

ahead



GANNI CYCLE COMPRESSOR CONFIGURATION

250-350 kPa

• MP and HP compressors compress 4.5K cold box turbine

bypass flows

• LP compressor stage compress loads that must operate at

100 kPa (i.e. cold compressor discharge, etc.)

• There is no attempt to normally control suction, inter stage,

or discharge pressures of the MP and HP stages with bypass

valves. Pressures are allowed to occur naturally 2.5-3.5

pressure ratio with pressures based on system gas charge

into or out of the system.

• System charge pressure is controlled by liquid level within a

dewar used to sub cool the supercritical 4.5K leaving the 4.5K

cold box. Decreasing level (increase charge)/Increasing level

(decrease charge)

• 4.5K cold box turbine inlet valves are allowed to remain

fully open, refrigeration is controlled by varying HP and MP

differential pressure, variable turbine brake control keeps

turbine specific speed efficiency at design

• With small variations of refrigeration load, system is self

regulating based on amount of refrigeration return flow

Turbines

Cold

Compressors


Thomas Jefferson National Accelerator Facility 8

In the floating pressure process, the compressor suction and discharge vary, but

at a constant pressure ratio (pr). This is opposed to traditional cycles that attempt

to force the controls to the design T-s condition by adding heat, bypassing expander

flow, bypassing compressor flow or the throttle expander inlet valve.

Floating Pressure T-S Diagram



For Larger Sub Atmospheric Systems Three Major Designs

—100% serial connected cold compressors compressing

from load to 1 atmosphere (4 or 5 stages), variable

speed for turndown

—Combination of sub atmospheric inlet screw

compressors with 3 or 4 stages of cold compressors,

variable speed of cold compressors for turndown

— 100% serial connected cold compressors with flow

bypass or turndown, 4 Stages



Bypassing Flow

Around

Cold Compressors

for Reducing Load

Flow



Quality of Energy, Ideal

11

Note energy difference to re-cool

helium from 30K to 2K if bypassing

cold compressors with 4K refrigerator



What we should be taking away from previous slide…….

• Even for ideal W/W COP, from a cold compressor train of

view (4K inlet, 30K outlet)….

— Most of the energy required is in the cooling of the gram of

helium from 30 to 4K (i.e. the load on the 4.5K cold box)

— If we bypass flow around the cold compressors to reduce

load flow and re-cool the bypass with 4.5K refrigeration to

inlet temperature conditions then we are not substantially

reducing the load on the 4.5K cold box, i.e. 4.5K cold box

continues to run at full refrigeration capacity at one operating

point

—Smaller plants may not be a problem, larger plants with high

margins may present higher operating cost and maintenance



Cold Compressor Bypassing

• Pros

— One 4.5K and cold compressor operating point

— Should be able to turn down the load flow substantially

to 1/3

• Cons

— Have to look at exergy of the system with consideration

of size/margin of the plant at sub atmospheric

temperature…higher costs of operation at reduced

refrigeration loads….



4 vs 5 Stages of

Cold Compressors



1999 JLab 5 Stage CC Development

• 5 Stage design, 1999

• Elimination of previous

CC inter-stage P&T

/Speed control

• Venturi discharge flow

measurement

• Locked speed ratios

after pump down of

CC1-CC3 to CC4 speed

• CC4 speed controlled to

maintain discharge flow

• CC5 speed is fixed

pressure delta P control

• CM heat control under

tunnel return pressure

• Menu based auto speed

ratio data base pump

down



WHY 5 vs 4 STAGES OF COLD COMPRESSORS

Pr

Fr

Adding 5th stage

cold compressor

lowers Pr for every

stage & widens flow

turn down range for

every stage

Reduced Flow Rate

(CC Speed Control)

Red

uce

d P

ress

ure

Rati

o

DID NOT AFFECT PRESSURE STABILITY

(+/- 100 MICROBAR, SLOW RESPONSE)

• NO NEED TO BYPASS CC TRAIN

FLOW FOR FLOW TURN DOWN

WHICH USES MORE 4.5K

REFRIGERATION FROM PLANT

Remember the earlier COP Power Graph

WIDEN FLOW

TURN DOWN

RANGE

120 kPa

30K

2.7 kPa

Cold Compressor Wheel

Diameter Design

Boundary



MOVED FROM 4 TO 5 STAGES FOR COLD COMPRESSORS

Page 17

Early CHL-1 Design

Internals Before, 4 stages Internals After,

5 stages



5 Stage CC Design Development & Performance

• Tested 250 g/s CC train with sample flow reductions at each

pressure

• Time prevented lower testing

• Upper flow rate limited by

4.5K cold box max capacity

Page 18

120

130

140

150

160

170

180

190

200

210

220

230

240

250

260

0.020 0.022 0.024 0.026 0.028 0.030 0.032 0.034 0.036 0.038 0.040

Flo

w (

g/s

)

Linac Pressure (bar) Figure-5.2.0b

Flow

Reference:

CEC 2001, Madison ,Wi

Advances in Cryogenic Engineering

Volume 47A, Pages 288-304

“Design, Fabrication, Commissioning, and

Testing of a 250 g/s, 2-K Helium Cold

Compressor System”

V. Ganni, D.M.Arenius,B.S. Bevins, W.C.

Chronis, J.D.Creel, J.D.Wilson Jr.

Large turndown range

without heat or bypass flow



Combination of Warm Helium Screw

Compressor and Cold Compressors



Sub Atmospheric Warm Compressor and Cold Compressors

• The fourth stage of a full compression ratio 4 Stage CC

system is replaced by a warm helium screw compressor

operating at a suction pressure ~.35-.59 bar

• Flow turn down to 1/3 capcity has been achieved but

utility turn down with load is unknown

• Sub atmospheric load pressure regulation appears to be

higher than other methods, must be considered for SRF

applications for tuner tracking performance

• May have higher capital equipment costs due to

additional compressor system and expanded helium

guard system



Thank You for Your Kind

Attention

May I Answer Your Questions ?

21

I gratefully acknowledge the contributions of the JLab

cryogenic engineers, designers, technicians, and partner

labs which made this technology possible

An Overview of Sub-atmospheric Cold Compressor … Overview of Sub-atmospheric Cold Compressor...

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