First of all this isn’t really a Frequently Asked Questions (FAQ) on refrigeration but rather a collection of comments that were made to us by various industry people on the topic of marine refrigeration. We’ve put them into a FAQ format as it seemed the best format available. We can’t say these are definitive opinions but rather “common wisdom” or “best practice” within the industry. There is a companion page Refrigeration Research which is our research in refrigeration alternatives.
- How thick is it worth having the insulation?
There’s not much return on having polyurethane insulation over 100 mm thick. A general rule of thumb is 1mm of insulation per litre of cooled capacity.
- What effect does doubling my insulation thickness have?
Increasing polyurethane insulation thickness from 50mm to 100mm reduces heat loss by a further 25% to 35% (one technical support group claimed the saving would be on the order of 50% but the others claimed that to be overly optimistic).
- How much power do I save by reducing the volume of the cooled space?
Reducing the box volume by 50% would not reduce the power consumption appreciably. Reducing the box volume from say 250 litres to 50 litres would reduce energy requirements by about 1/3.
- Are there other tricks to reducing my power consumption?
Yes, in general keep the fridge as full as possible. If you have bottles of water filling unused volume they will prevent warm air from filling the fridge when it is opened. Once they are cooled to temperature they reduce heat loss. Leaving the space empty allows a maximum of air to enter the cooled volume each time the fridge is opened thereby increasing the cooling load and therefore increasing the power required. Also if possible, load the fridge with previously cooled items (i.e. take them from another fridge on shore) which means the other machine used the energy to cool them and your fridge needs only maintain that temperature.
- Is it better to use a more efficient (and more expensive) compressor to cool a given volume or a less efficient (and usually cheaper) compressor?
In general terms, using a less efficient compressor and evaporator versus a more efficient compressor and evaporator consumes roughly the same amount of power as the more efficient configuration uses more power but for a lesser amount of time. This is based on the assumption that both compressors are rated sufficiently highly to cool the actual volume in question.
- If I have limited space in which to put insulation around a box of fixed size, how should I prioritize placing the insulation?
Insulate the bottom, sides and top in that order.
- How do I decide if I want a compressor mechanically driven by the engine or an electric compressor?
A compressor on the engine is more efficient if the engine is running already (e.g. a power boat). For a sailing boat that rarely uses the engine this may be a limitation as often the engine will have to be run twice a day. The engine mounted compressor also will cost more to purchase than the electric compressor but on the other hand will not need the battery reserve that the electric compressor will need.
- How do I decide if I want an air cooled, water cooled or keel cooled? What are the pros and cons?
The first fact is that water will take on more heat than air and is therefore more efficient thermally. Using water to cool will require a though hull fitting which is a long term integrity and maintenance issue regardless of why it is introduced into the hull. Using a pump based water cooling also implies that electrical current will be used to move the water. Using a bilge cooled or stand pipe cooling removes this current requirement as the natural motion of the water is utilized to remove the heat – it was suggested that this would require 20% less power than an air cooled unit. Some persons we talked to stated that they generally used water based pump on bigger boats where multiple fridges are used or on steel or ferro-concrete boats where you don’t want a brass hull fitting. They also felt that engine driven compressors were only practical on larger boats.
- If I go for water cooled can the water be too cold to cool efficiently?
One of the technical support groups claimed that it is possible to overcool the gas. They stated that a water cooled unit is less efficient in cold water (less than 12 degrees C) as the reduced coolant pressure inhibits the coolant from taking on heat easily. Consequently they did not recommend water cooled units for yacht whose main cruising was in northerly waters.
- What materials can I get to fabricate my own box?
A variety of products exist:
– A stainless steel box can be fabricated to measure by most marine welders. Stainless steel is quite acceptable for food use and highly durable but is a poor thermal insulator and would have to be layered with polyurethane.
– Isotherm (now Indel Webasto) makes a plastic faced polyurethane 50mm panel which is available through the chandleries in Scandinavia. It is a good thermal insulator which they recommend joining with SikaFlex 291 or Bostic polyurethane glue
– Tek-Tanks in England will construct a box out of food grade high density polyethylene which is the milky-white coloured material that chopping boards and kitchen counter protective covers are made out of. Being food quality, bacteria can not lodge on it and it is cleaned with warm water and a mild detergent. It is a good thermal insulator but they still recommend covering it with polyurethane insulation to increase the insulation properties.
– Technautic in the US makes a high vacuum panel which has amazing claims. One concern mentioned to us was the panels will fail completely if punctured. In the “Offshore Cruising Encyclopedia” Steve Dashew suggests encasing the panels in polyurethane foam to both protect them and provide some backup insulation value if a puncture does occur.We never found a supplier for Technautic in Europe or Canada.
– On material costs, slab polyurethane foam is cheaper than expanding, so it is most cost effective to use slab polyurethane and cut it to shape, then fill the corner and remaining gaps with expanding polyurethane foam. Also we were recommended to use 2 part expanding polyurethane foam, not the builders one part foam which is cheaper but apparently not as good a thermal insulator.
– So far we have not found anyone selling the myriad of rubber gasket material etc. that are required to build a box from scratch.
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