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Defrost Methodology for Free Piston Cycle Engines

IP.com Disclosure Number: IPCOM000011350D
Publication Date: 2003-Feb-13
Document File: 2 page(s) / 30K

Publishing Venue

The IP.com Prior Art Database

Abstract

A temperature sensor is placed directly on the cold end of the Stirling. A Stirling system in combination with a heat exchanger is very is different from a Rankine system. Even in a variable speed Rankine cycle system which runs constantly, you can’t simply infer the heat exchanger temperature by measuring the “cold-end” temperature because there is no equivalent “cold-end” end. Under normal operating conditions, the temperature of the Stirling’s cold end would be in an expected range of 0C to -10C for a thermosiphon heat exchanger. When the evaporator of the heat exchanger frosts over, its performance drops. With decreased performance, a larger T is required to maintain the desired refrigeration lift. This means that the cold end of the Stirling becomes significantly colder. When the temperature of the Stirling cold end drops below for example -30C, it can be inferred that the thermosiphon evaporator is frosted over and a defrost is required. (A thermosiphon heat exchanger is given here as an example, but this system would work with other types of heat exchangers.)

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The problem to be solved

Frosting (frozen condensate) of a heat exchanger that is in direct or indirect contact with the cold end of a Stirling engine, blocks air flow and eventually prevents heat transfer from occurring.

Background

In standard Rankine cycle refrigeration systems, there are regular off cycles during normal operation.� Defrosting typically occurs during the off cycles.� The normal operation of a Stirling is very different, because a Stirling modulates at exactly the refrigeration capacity required, it is typically constantly running.� Therefore a Stirling has no “off cycles” during which to defrost.� Therefore a defrost method is needed for a Stirling system.�

“Dumb” timed defrost systems are known in the refrigeration industry.� With a “dumb” defrost, the defrost cycle is regularly initiated whether or not it is needed.� This invention pertains to a “smart” defrost.� In this “smart” defrost system, the defrost cycle is initiated only when a heat exchanger freeze-up occurs.� �

How the invention works

A temperature sensor is placed directly on the cold end of the Stirling.� A Stirling system in combination with a heat exchanger is very is different from a Rankine system.� Even in a variable speed Rankine cycle system which runs constantly, you can’t simply infer the heat exchanger temperature by measuring the “cold-end” temperature because there is no equivalent “cold-end” end.�

Under normal operating conditions, the temperature of the Stirling’s cold end would be in an expected range of 0°C to -10°C for a thermosiphon heat exchanger.� When th...