Download MUCOOl_Absorber_Feb._03

Heat Transfer and Other Issues
Concerning the Forced Flow
Absorber System
Michael A. Green
Lawrence Berkeley National Laboratory
Berkeley CA 94720, USA
MUCOOL Workshop Meeting
Fermilab, Batavia IL, USA
22 February 2003
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A Summary of Forced Flow Absorber Issues
• The Heat transfer in the forced flow absorber heat
exchanger between the helium gas and the sub-cooled
hydrogen in the absorber flow circuit is marginally OK.
• The position of the heat exchanger with respect to the
absorber and the hydrogen pump is of concern.
• The condensation of liquid hydrogen into the absorber
circuit will be a key operational issue.
• One can circulate the liquid hydrogen through the
absorber by natural convection. One should be able to
remove up to 1000 W of heat from the absorber using
natural convection.
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Desired
3
A Comparison of the MUCOOL Forced Flow
Absorber with the MICE Free Convection Absorber
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Counter Flow Heat Exchangers versus
Parallel Flow Heat Exchangers
• In a parallel flow heat exchanger, the coldest temperature
of the warm stream is always higher the warmest
temperature of the cold stream. This restriction does not
apply for a counter flow heat exchanger.
• For a given heat exchanger U factor and heat exchanger
area, a counter flow heat exchanger will nearly always
have the lowest log mean temperature difference. A
counter flow heat exchanger is always more efficient for
small temperature differences across the exchanger.
• Counter flow heat exchangers are widely used in cryogenic
refrigeration systems.
• In situations where a change of phase occurs on one side of
the heat exchanger, either type of exchanger works well.
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Parallel Flow and Counter Flow Heat Exchangers
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Estimate of the Heat Exchanger U Factor
hc = 0.023 Re
0.8
0.33
Pr
kf
DH
Turbulent Forced Flow in a Pipe
hc1 = 1161 to 1747 W m -2 K-1
for Helium 15 g/s to 25 g/s
hc2 = 179 to 2720 W m -2 K -1
for Hydrogen 15 g/s to 450 g/s
kw
= 208000 W m -2 K-1
tw
for Copper pipe t w = 0.7 mm @ 17 K
15 g/s
20 g/s
25 g/s
He Flow
15 g/s
100 g/s
450 g/s
155
159
162
478
523
555
812
948
1058
H2 flow
based on fluid
properties @ 17K
U factor (W m -2 K-1 )
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Peak Bulk Hydrogen Temperature versus
Helium and Hydrogen Mass Flow for Q =225 W
22
Q = 225 W
A = 0.267 m^2
Highest Bulk Hydrogen Temperature (K)
21
20
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Hydrogen Flow
15 g/s
50 g/s
100 g/s
200 g/s
450 g/s
18
17
16
15
14
15.0
17.5
20.0
22.5
14 K Helium Circuit Mass Flow (g/s)
25.0
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Peak Bulk Hydrogen Temperature versus
Helium and Hydrogen Mass Flow for Q = 375 W
22
Q = 375 W
A = 0.267 m^2
Highest Bulk Hydrogen Temperature (K)
21
20
19
Hydrogen Flow
15 g/s
50 g/s
100 g/s
200 g/s
450 g/s
18
17
16
15
14
15.0
17.5
20.0
14 K Helium Mass Flow (g/s)
22.5
25.0
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What are the Problems?
H2 Gas
Hydrogen
Pump
He In
He Out
Absorber
Heat Exchanger
A = 0.267 m^2
• The heat exchanger area
is too small. Increasing
heat exchanger area will
reduce the log mean
temperature difference
and improve efficiency.
• The pump flows against
buoyancy forces.
• The heat exchanger will
flood as hydrogen is
condensed into the pump
loop. As result, hydrogen
condensation will slow to
a snails pace.
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A Better Pump Loop Solution
• The heat exchanger area
is increased a factor of
three. As a result, the
system is more efficient.
• The pump and the heat
exchanger are oriented
to use buoyancy forces to
help hydrogen flow.
• The top of the heat
exchanger is above the
liquid level. The heat
exchanger is an efficient
hydrogen condenser.
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Can a Free Convection Loop be used?
• Circulation of the hydrogen using free convection should
be seriously considered. Preliminary calculations suggest
that up to 1000 kW can be removed from the absorber
using a free convection loop.
• The hydrogen flow through the absorber is proportional to
the square root of the heat removed. The bulk hydrogen
temperature rise is proportional to the square root of the
heat removed.
• The heat exchanger must be vertical with the hydrogen
flowing in the downward direction. The helium will flow
in the upward direction. The top of the heat exchanger
should be above the hydrogen liquid level.
• It is not clear if a free convection hydrogen flow loop will
fit in the lab G solenoid.
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Some Concluding Comments
• The MUCOOL forced flow experiment will probably work
as designed. Filling the pump loop may take a lot of time.
The flow experiment works because the mass flow in the
both streams of the loop is larger than the optimum (up to
ten times larger for the hydrogen).
• Increasing the pump loop heat exchanger area will improve
the pump loop heat transfer efficiency at lower hydrogen
mass flows. Correct orientation of the pump and heat
exchanger should also improve the loop performance.
• The top of the heat exchanger should be above the liquid
hydrogen surface, to improve condensation efficiency.
• A free convection hydrogen loop appears to be feasible. A
free convection loop may not fit into the lab G solenoid.
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