Michael A. Green Lawrence Berkeley National Laboratory
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1 Heat Transfer and Other Issues Concerning the Forced Flow Absorber System Michael A. GreenLawrence Berkeley National LaboratoryBerkeley CA 94720, USAMUCOOL Workshop MeetingFermilab, Batavia IL, USA22 February 2003
2 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.
4 A Comparison of the MUCOOL Forced Flow Absorber with the MICE Free Convection Absorber
5 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.
8 Peak Bulk Hydrogen Temperature versus Helium and Hydrogen Mass Flow for Q =225 W
9 Peak Bulk Hydrogen Temperature versus Helium and Hydrogen Mass Flow for Q = 375 W
10 What 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.
11 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.
12 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.
14 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.