Title: Heat Transfer and Other Issues Concerning the Forced Flow Absorber System
1Heat Transfer and Other Issues Concerning the
Forced FlowAbsorber System
- Michael A. Green
- Lawrence Berkeley National Laboratory
- Berkeley CA 94720, USA
- MUCOOL Workshop Meeting
- Fermilab, Batavia IL, USA
- 22 February 2003
2A 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.
3Desired
4A Comparison of the MUCOOL Forced Flow Absorber
with the MICE Free Convection Absorber
5Counter 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.
6Parallel Flow and Counter Flow Heat Exchangers
7Estimate of the Heat Exchanger U Factor
8Peak Bulk Hydrogen Temperature versusHelium and
Hydrogen Mass Flow for Q 225 W
9Peak Bulk Hydrogen Temperature versusHelium and
Hydrogen Mass Flow for Q 375 W
10What are the Problems?
- 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.
11A 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.
12Can 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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14Some 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.