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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
Thomas Jefferson National Accelerator Facility Page 2
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
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 3
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
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 4
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
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 5
NASA JSC Test Results
5
Same type of result
for Jlab 12 GeV
4.5K cold box to
20% load
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 6
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
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 7
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
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 8 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
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 9
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
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 10
Bypassing Flow
Around
Cold Compressors
for Reducing Load
Flow
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 11
Quality of Energy, Ideal
11
Note energy difference to re-cool
helium from 30K to 2K if bypassing
cold compressors with 4K refrigerator
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 12
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
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 13
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….
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 14
4 vs 5 Stages of
Cold Compressors
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 15
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
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 16
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
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 17
MOVED FROM 4 TO 5 STAGES FOR COLD COMPRESSORS
Page 17
Early CHL-1 Design
Internals Before, 4 stages Internals After,
5 stages
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 18
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
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 19
Combination of Warm Helium Screw
Compressor and Cold Compressors
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 20
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
Operated by the Southeastern Universities Research Association for the U.S. Department of Energy, DOE Contract DE=AC05-06OR23177
Thomas Jefferson National Accelerator Facility Page 21
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