All elements of a marine refrigeration system are important for efficient operation. For example, this top-loading refrigeration box is well-insulated and has twin gaskets.
Several components are mentioned in the accompanying description of the refrigeration process. Each of these components, and a few that were omitted for simplicity, serves a critical role in a properly operating, reliable marine refrigeration system. Compressors The heart of any refrigeration system is the compressor, and much like a human heart, it is a pump: receiving, pressurizing and pumping vaporized refrigerant. Compressors are similar to internal combustion engines in that they are efficient compressors of gases. Therefore, much like an engine, a refrigeration compressor is unable to pump liquid refrigerant through its cylinders. Liquid is an incompressible medium, and thus, if it were to enter the cylinder of a compressor, it would rapidly damage the compressor, perhaps irreparably.
Several different types of compressors are used in marine refrigeration systems. The most common is the hermetic style. This is similar to the type of compressor that operates domestic refrigerators and freezers. Its compact, quiet design and low cost make it particularly attractive in this role, and the longevity of these compressors is legendary, some of which have operated for decades.
The reliability of this compressor stems from its hermetic, or closed, design. The compressor and the electric motor that operates it are sealed in a steel container. This minimizes refrigerant leaks and contamination of the system. Longevity is further enhanced by the oil bath in which these components sit. Airtight and permanent lubrication makes for an especially long-lived unit.
The weakness of hermetic compressors is their limited power output, and as such, they lack the muscle of some other types of compressors that are capable of pulling down a refrigerator or freezer rapidly. They are designed for continuous operation, which means they always need power available in order to maintain the temperature of a box.
While the actual power they draw may be comparatively low, between four and eight amps DC at 12 volts, they need it almost constantly. Ideally, the duty cycle of these systems is 50 percent, which means they run half the time. This ratio is, unfortunately, rarely achieved, particularly in warm weather with an under-insulated box. Do the math: if the system were to run ideally, that is 50 percent of the time, that’s between 48 and 96 amp-hours a day. Throw in regular opening and closing of the box, higher cabin temp and inferior insulation, and these numbers can easily double.
Hermetic compressors are also available in 120-VAC varieties for use when shore or generator power is available. The same limitations of duty cycle and power requirements apply, however, for vessels that frequently run generators or are dockside; this system is sometimes used as a supplement to another type of compressor, or as a redundancy feature.
The other type of compressor commonly found aboard cruising vessels is open, or belt-driven. These compressors are invariably of much greater horsepower than their hermetic brethren, and as such, the work they are capable of doing is substantially greater. Open compressors are powered either directly or indirectly, through a belt, from a motor. Those powered by the vessel’s own propulsion engine or generator (these are typically the most powerful, up to about 2 hp), invariably use a belt. Open compressors powered by electric motors (usually limited to about 1/2 hp), either AC or DC, are either driven by a belt or directly via a shock-absorbing coupling.
The strength of an open compressor is, as mentioned, its capability for pumping a great deal of refrigerant very quickly. Engine-driven compressors are commonly installed aboard voyaging vessels, belted to the main engine, usually using an electromagnetic clutch-engaging mechanism so the compressor would not turn continuously. These compressors are the most economical approach for open, high-capacity holding-plate (more on this term later) refrigeration.
The primary drawback of engine-driven compressors is they can only be used when the engine is running, or conversely, you must run the engine for them to work. This means unattended operation is infeasible, and unless an auxiliary electric refrigeration compressor is plumbed, the engine has to be run regularly (how often depends on the efficiency of the system, usually anywhere from twice a day to every other day), even when dockside. For this reason, most of these systems are installed with either a DC or an AC hermetic compressor, for use when shore power or another source of power is available.
Other than expense, there are few drawbacks to the electrically driven DC open compressor. When coupled with a properly sized holding plate and well-insulated box, these compressors may need to be run from one half to one hour per refrigeration session. While the typical system may draw 30 or 40 amps while it’s running, the net daily amp-hour usage is considerably lower than the hermetic system.
Controversy abounds within the voyaging community and among refrigeration equipment manufacturers regarding which system is best suited to a vessel’s needs. When making this decision for yourself, keep in mind that in spite of the attributes of the open compressor system, it costs more to install and requires an integrated electrical system that is capable of supporting its ampere needs. Thus, this system should not be seen as standalone. It should be installed with a properly sized battery bank, high-output alternator and multistage regulator (although to an extent, this goes for hermetic units as well). This could easily double or triple the cost of the system. For the ultimate in long-range, efficient voyaging refrigeration, however, the open compressor/holding-plate system does, in my opinion, offer the best alternative.
The time required to replace the 30 to 60 amp-hours used by an open DC compressor on a daily basis clearly represents less engine run time than would be required to replace amp-hours used by a hermetic compressor.
Condenser
The condenser may be either air- or water-cooled, and its primary function is to assist in the conversion of gas refrigerant to liquid refrigerant. In the air-cooled version, the refrigerant passes through what resembles a miniature automotive radiator. Air is blown through the radiator by means of an electric fan, removing heat from the refrigerant. Because of their limited capacity for heat removal, and resultant inefficiency, air-cooled units are used almost exclusively with hermetic compressors. The efficiency of any hermetic compressor can, by the way, be improved by adding a water-cooled condenser. Some hermetic units, such as those offered by Frigoboat/Veco have passive keel-cooling condensers, which require no raw-water pumps.
Water-cooled condensers, on the other hand, resemble, either in form or function, marine-engine heat exchangers. Seawater is pumped through one chamber, either with an electric pump or, in the case of engine-driven systems, with the engine’s own raw-water pump (all water-cooled units, whether they use a dedicated electric pump or the engine’s own raw-water circuit, must have a proper seawater strainer). Refrigerant is pumped through the other chamber by the system’s own compressor. The resulting intermingling of the two media, hot refrigerant and the comparatively cool seawater, causes the refrigerant to condense into a liquid. The seawater is then dumped overboard.
The efficiency of both of these types of condensers falls as air or water temperature rises, such as that found in the tropics or in shallow bays and harbors of temperate regions. Air-cooled systems will benefit from ducting in cool air from outside the boat, as opposed to pulling typically warmer cabin or engine-room air. Ducting discharge air will help keep the cabin temperature, and those of its occupants, down as well.
The strength of an open compressor is, as mentioned, its capability for pumping a great deal of refrigerant very quickly. Engine-driven compressors are commonly installed aboard voyaging vessels, belted to the main engine, usually using an electromagnetic clutch-engaging mechanism so the compressor would not turn continuously. These compressors are the most economical approach for open, high-capacity holding-plate (more on this term later) refrigeration.
The primary drawback of engine-driven compressors is they can only be used when the engine is running, or conversely, you must run the engine for them to work. This means unattended operation is infeasible, and unless an auxiliary electric refrigeration compressor is plumbed, the engine has to be run regularly (how often depends on the efficiency of the system, usually anywhere from twice a day to every other day), even when dockside. For this reason, most of these systems are installed with either a DC or an AC hermetic compressor, for use when shore power or another source of power is available.
Other than expense, there are few drawbacks to the electrically driven DC open compressor. When coupled with a properly sized holding plate and well-insulated box, these compressors may need to be run from one half to one hour per refrigeration session. While the typical system may draw 30 or 40 amps while it's running, the net daily amp-hour usage is considerably lower than the hermetic system.
Controversy abounds within the voyaging community and among refrigeration equipment manufacturers regarding which system is best suited to a vessel's needs. When making this decision for yourself, keep in mind that in spite of the attributes of the open compressor system, it costs more to install and requires an integrated electrical system that is capable of supporting its ampere needs. Thus, this system should not be seen as standalone. It should be installed with a properly sized battery bank, high-output alternator and multistage regulator (although to an extent, this goes for hermetic units as well). This could easily double or triple the cost of the system. For the ultimate in long-range, efficient voyaging refrigeration, however, the open compressor/holding-plate system does, in my opinion, offer the best alternative.
The time required to replace the 30 to 60 amp-hours used by an open DC compressor on a daily basis clearly represents less engine run time than would be required to replace amp-hours used by a hermetic compressor.
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Condenser
The condenser may be either air- or water-cooled, and its primary function is to assist in the conversion of gas refrigerant to liquid refrigerant. In the air-cooled version, the refrigerant passes through what resembles a miniature automotive radiator. Air is blown through the radiator by means of an electric fan, removing heat from the refrigerant. Because of their limited capacity for heat removal, and resultant inefficiency, air-cooled units are used almost exclusively with hermetic compressors. The efficiency of any hermetic compressor can, by the way, be improved by adding a water-cooled condenser. Some hermetic units, such as those offered by Frigoboat/Veco have passive keel-cooling condensers, which require no raw-water pumps.
Water-cooled condensers, on the other hand, resemble, either in form or function, marine-engine heat exchangers. Seawater is pumped through one chamber, either with an electric pump or, in the case of engine-driven systems, with the engine's own raw-water pump (all water-cooled units, whether they use a dedicated electric pump or the engine's own raw-water circuit, must have a proper seawater strainer). Refrigerant is pumped through the other chamber by the system's own compressor. The resulting intermingling of the two media, hot refrigerant and the comparatively cool seawater, causes the refrigerant to condense into a liquid. The seawater is then dumped overboard.
The efficiency of both of these types of condensers falls as air or water temperature rises, such as that found in the tropics or in shallow bays and harbors of temperate regions. Air-cooled systems will benefit from ducting in cool air from outside the boat, as opposed to pulling typically warmer cabin or engine-room air. Ducting discharge air will help keep the cabin temperature, and those of its occupants, down as well.
Filter-dryer-receiver
This combination unit, sometimes serving all three functions in one canister, protects the compressor from damaged caused by debris within the system. Just as your engine is equipped with fuel, air and oil filters, the refrigeration must be protected from contamination within the refrigerant lines, thus the inclusion of a filter.
Water within the refrigerant loop will also cause damage, leading to icing, which may prevent the system from operating efficiently or lead to corrosion. The dryer absorbs and holds water in suspension so it cannot circulate within the system.
The receiver accepts liquid flowing from the condenser and supplies it to the expansion valve, previously referred to as the orifice. The receiver can serve two purposes. It ensures that there is enough refrigerant in the system so that the expansion valve is always supplied with liquid. It also can be designed to hold the entire refrigerant charge of the system in the event the remainder of the system must be opened for service or repair. The liquid refrigerant can be pumped into the receiver and then isolated with service valves. It is typically located between the condenser and the evaporator.
As mentioned in the description of the refrigeration process, the expansion valve, which may also be a simple capillary tube, is a restriction through which liquid refrigerant passes. Once on the downstream side of the valve, the refrigerant expands and boils. Expansion valves are metering devices that are usually controlled through a temperature-sensing bulb, similar to a thermostat. They may be adjusted by a trained refrigeration technician in order to achieve peak efficiency for a given system.
Evaporator
The evaporator, where the business of "making cold" takes place, is the next stop for refrigerant. The process has already been described: refrigerant boils off here, taking heat with it. The primary types of evaporators available are direct and indirect. Direct evaporators, sometimes called evaporator coil systems, are thin, shell-like devices that are usually coupled with hermetic compressors. They are often oval shaped, creating a small freezer compartment within their enclosed area. These evaporators are referred to as direct because the cooled refrigerant within acts directly upon its surroundings, the refrigerant cools the shell plate, which in turn cools the box.
Holding-plate, or indirect, evaporators work a bit differently. They are designed to work more like a rechargeable block of ice. The expanding refrigerant still passes through this plate; however, it contains a second cavity around the evaporator tubing, which is filled with a eutectic or antifreeze solution. This solution is in turn frozen by the refrigerant.
Once the refrigeration system is turned off, the frozen solution continues to absorb heat from the box until it thaws.
Holding plates are typically used in conjunction with open compressors. A lot of refrigerant must be pumped through a holding plate in order to freeze the eutectic or other solution, and thus, high-capacity compressors are best suited to this task.
Holding plates work best, whether for refrigeration or freezer applications, when mounted near the top of the box and with a stand-off from the box side. High box location prevents exterior frosting, which can ruin food in the fridge, while maximizing movement of air over the plate (air circulation within a box is desirable; some installations now include ultra-low-draw muffin fans to improve air movement). A stand-off of a quarter of an inch or more will also increase the surface area of the plate, which improves efficiency further still.
The final component in the refrigeration circuit is the accumulator. This is nothing more than a reservoir for liquid refrigerant, installed on the compressor's suction or intake line, it prevents liquid from being drawn into the compressor. As mentioned, refrigerant entering the compressor as a liquid, also called slugging, can have disastrous results. Thus, an accumulator unit is excellent addition to a refrigeration system because it is a simple and inexpensive way of preventing this type of damage from occurring to one of the more expensive components in the system: the compressor.
While they may not all be readily visible to the casual observer, most of the system components mentioned here are part of both closed or hermetic and open refrigeration systems. Hermetic components are typically lighter when compared to the elements of an open compressor system, and these items may be mounted compactly on the unit's single "skid" pad. Other hermetic systems forgo some of these items. However, nearly every open compressor refrigeration system will or should contain all of these parts and components.
Marine refrigeration systems have been around for several decades, and thus, their strengths and weaknesses are well understood. If you are preparing to purchase a new system, do your homework. This is possible since copious amounts of literature are available from various equipment manufacturers, both in print and on the Internet.
Contributing Editor Steve C. D'Antonio is a freelance writer and the boatyard manager of Zimmerman Marine in Mathews, Va.
For more information on refrigeration and eutectic solutions, go to www.OceanNavigator.com and click the Web Extras button.
