The Service Provided
The customer required 4.7 kW of cooling at controlled temperatures between 90 K and 65 K. Boiling liquid would cause problems and so the system used a high flow rate of subcooled liquid nitrogen: the heat loads would be absorbed by the temperature rise of the subcooled liquid with no vapour generation. The customer developed a concept design with Air Products and Chemicals Inc. including the following main components:

	Liquid nitrogen surge vessel
	Liquid nitrogen pump
	Subcooler
	Exhaust gas heater
	Subcooler pressure control valve
	Vacuum pump

Air Products and Chemicals Inc. came to Monroe Brothers Ltd to undertake the detailed design for the subcooler and the exhaust gas heater including the specifications for the subcooler pressure control valve and the vacuum pump. Proprietary design programs which have been developed and proven by Monroe Brothers Ltd were used. The surface area, the heat transfer and the pressure drop for the subcooler and the exhaust gas heater were accurately quantified.

The design was optimised to suit the particular manufacturing techniques of the subcontractor. The output results from the computer programs had the necessary quality control and clarity of presentation to be included directly in the design file. The capacity and component selection of the pump and the valves was also defined. The photographs show the system on completion at the manufacturers.

Above: The System on completion at the manufacturers. The large vessels for the nitrogen surge vessel and the subcooler are evident

The Benefit
By using well proven design program which have been developed and optimised for cryogenic applications, Monroe Brothers Ltd was able to provide the detail design of the heat exchangers. The system was commissioned with every component working as expected and has been operating since the beginning of 2002.

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