Absorber Heat Transfer and Other Issues A Comparison between MICE and the Forced Flow Absorber System

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Absorber Heat Transfer and Other Issues A
Comparison between MICE and 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 MICE Absorber Issues

• The Heat transfer on the helium side is forced
  convection in the absorber case tube. The flow
  of the helium is set by the flow from the
  refrigerator.
• Heat transfer on the hydrogen side is by free
  convection. Buoyancy determines the mass flow of
  the sub-cooled hydrogen. The hydrogen mass flow
  goes up as the heat load (QB QH) to the 0.5
  power. The change in the bulk hydrogen
  temperature goes as the heat load to the 0.5
  power.
• Freezing the liquid hydrogen in the absorber is
  not allowed when there is no heat load into the
  absorber, so the helium enters the absorber body
  at 14 K.

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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 can 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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A Simplified Schematic of the MICE Absorber Heat
Loads
Window T gt 55 K
QC 20 W QR 10 W QBQH 70 W QT
100 W
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Thermal Modeling the MICE Absorber using an
Electrical Network Analogy
QT 100 W
TI gt 55 K when QR 0
QC 20 W
TR 300 K
QR lt 10 W
Tf lt 20 K
QB QH gt 70 W
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Desired
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A Comparison of the MUCOOL Forced Flow Absorber
with the MICE Free Convection Absorber
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Possible MICE Absorber Heat Exchangers
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Counter Flow Heat Exchangers versus Parallel Flow
and Mixed Heat Exchangers

• In a parallel flow heat exchanger, the coldest
  temperature of the warm stream is always higher
  than 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 mixed flow heat exchanger has all of the
  disadvantages of a parallel flow heat exchanger
  plus the heat exchange and hydrogen flow in the
  absorber is unbalanced. Heat from the outlet
  stream is shorted to the colder inlet stream.
• In situations where a change of phase occurs on
  one side of the heat exchanger, any type of heat
  exchanger works.

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Parallel Flow and Counter Flow Heat Exchangers
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An Estimate of the Pumped Hydrogen Forced Flow
Heat Exchanger U Factor
The U factor for the MICE absorber Heat Exchanger
is much lower.
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A MICE Absorber Heat Exchanger
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MICE Peak Bulk LH2 Temperature Vs 14 K Helium
Mass Flow and Heat Load into the Absorber
Inlet He T 14 K
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MICE Peak Bulk Helium Temperature Versus the Heat
Load into the Absorber
Helium Inlet T 4.3 K
Note The two-phase helium flow is at least 3.5
g/s.
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The pumped Hydrogen Forced Flow Absorber
Configuration Studied
The configuration shown was given at last weeks
telephone meeting. This weeks configuration is
not the same. The new configuration will have
improved performance.
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Forced Flow Peak Bulk Hydrogen Temperature
VsHelium and Hydrogen Mass Flow for Q 225 W
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Forced Flow Peak Bulk Hydrogen Temperature
VsHelium and Hydrogen Mass Flow for Q 375 W
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Problems with the Pump Loop

• 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 MICE absorber appears to be OK for heat loads
  to the hydrogen of up to 100 W. (30 W is
  transferred to the helium gas directly.) The
  MICE absorber appears to work with liquid helium
  in the absorber with total heat loads up to about
  45 W. (30 W goes to the two-phase helium
  directly.)
• The new MUCOOL forced flow experiment will work
  as designed. The flow experiment works because
  the mass flow in the both streams of the loop is
  larger than the optimum.
• Increasing the MUCOOL 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 improve the loop performance.
• A free convection hydrogen loop appears to be
  feasible. A free convection loop may not fit
  into the lab G solenoid.