I read with interest Steve D’Antonio’s recent article on refrigeration (State of cool Issue 138, May/June 2004). The comparisons drawn between hermetic and open compressor designs, however, were incomplete and did not reflect more modern hermetic designs, which offer much-improved effectiveness.
Having lived with engine-driven holding-plate systems and, recently, a modern hermetic compressor-based system, there are a few important details that were not considered.
Starting in the box, modern hermetic systems, such as supplied by Frigoboat, provide dramatically increased surface area when compared to some hermetic and most holding-plate systems. Heat transfer is further improved by the thin-wall aluminum construction of the evaporator plate and the much lower operating temperatures compared to holding-plate systems. As an example, 0° holding plate will generally be used for freezing conditions within the box, typically holding temperatures 10° to 15° warmer throughout the thaw cycle. The flat evaporator will operate around minus 20°, providing improved heat exchange.
The most significant feature of a modern hermetic compressor system is the use of passive condensers. These devices use cupro-nickel coils mounted to the outside of the hull for effective heat exchange. The true beauty of this system is there is no water pumped into the boat. No additional DC power is required to run pumps or a fan on air-cooled models. Other advantages include no chance of a cooling water hose blowing off and flooding or sinking your boat if unattended. Reduced maintenance is realized through no need for cleaning fouled heat exchangers or prefilters, all the while enjoying the increased efficiency of water cooling. This greatly enhances the safety of unattended operation away from the dock.
The compressors themselves can be placed outside the living space or within, as they are almost inaudible during operation. As the article pointed out, hermetic compressors are inherently reliable, typically delivering decades of service. For this reason, a reasonable spare-parts kit is limited to electronic compressor modules and perhaps a thermostat. No belts, no brushes, no solenoid valves typically associated with the high-capacity holding-plate systems are required.
From a system perspective, an independent freezer and refrigerator can be configured. This allows the selective activation of either system delivering redundancy and extended energy efficiency, should you choose not to operate the freezer system. With digital thermostats, one can easily run the freezer box as additional refrigerator space by appropriately setting the box temperature.
As in all marine systems, efficiency comes not only from the system and components but also from factors like insulation and air sealing. I have used a hermetic system produced by Frigoboat for the past 18 months and have been very pleased with the ease of installation, quality of components, reliability, performance and energy efficiency. This system takes a significant step toward an ideal refrigeration system and should be considered when fitting a refrigeration system supported by a DC platform.
Rob Baker has sailed from the Canadian Maritimes to Martinique. He recently replaced a Passport 40 with a semicustom Perry-designed Passport 470. He lives off Spa Creek in Annapolis, Md.
Steve D’Antonio replies:
Thank you for your comments on the refrigeration piece. I have no argument on the beauty and simplicity of modern hermetic compressors, such as those offered by Frigoboat. As the manager of a custom-boatbuilding and refit yard, I’ve used these systems, as well as every other type of marine refrigeration system, on several occasions and have also been impressed with their performance. The benefit of the keel-cooler type of condenser is an added value to be sure, reducing the number of parts required as well as the overall complexity of the system, while improving reliability. These keel coolers are, however, not as efficient as a conventional tube-type heat exchanger, although most folks are willing to accept the reduced efficiency in return for added reliability.
The bottom line on hermetics, however, is that amp-hour for amp-hour (or engine running time), they are less efficient when compared with open compressors, be those 12-volt or engine-driven models. Additionally, an evaporator plate has nowhere near the BTU capabilities of a properly frozen eutectic solution found within a holding plate. Compared side by side in identical applications, the open compressor will require fewer amp-hours or engine run time to replace amp-hours expended. Thus, if efficiency is the arbiter, open compressors are preferable. Having said that, I believe hermetics, particularly the modern Danfoss multispeed units used by Frigoboat and other manufacturers, are ideal for many refrigeration applications and, apparently, you agree.
Contributing Editor Steve C. D’Antonio is the manager of Zimmerman Marine in Cardinal, Va.