There are few areas in a boat where so much money can be spent for so little result as refrigeration. Yet I rank refrigeration second only in importance to an enclosed loo. I have friends who would put it ahead of even that!

In this article, I'll look at the basics and examine the problems associated with operating a refrigerator on a boat. After all, we don't have too many problems operating one in a house!

ENERGY
For us to understand refrigeration, we have to delve into the basic laws of physics. Firstly, let's talk about energy.

Energy can neither be created nor destroyed - the same amount exists that the universe commenced with - but it can be changed from one form to another: mechanical energy to electrical, for example.

Energy will always try to equalise, to arrive at an equilibrium. This fact allows us to use energy to our advantage. On a cold day if we light a fire, heat is liberated from the combustion of the fuel used. This heat or, more correctly, thermal energy, is at a higher level than its surroundings and thus flows from the fire until one of two things occurs: the fire cools to the same temperature as the surroundings, or the surroundings are raised to the temperature of the fire.

This equalisation of energy is very important, as this principle is the mainspring of the universe - and also of refrigeration.

Because we are not training to build a better moon rocket, we will keep this simple. The level of energy in a substance is measured by its temperature. Various temperature scales have been developed over the years; the Celsius scale based on water has 0°C as freezing point and 100°C as boiling point. Thus a bucket of water at 5°C would have a higher level of energy than one at 4°C.

Note that this doesn't tell us anything about the amount of energy in each bucket, since that would depend entirely on the size of the buckets. This is an important principle to grasp.

WATER
Because water is one of the most common substances found on earth, a lot of the early experiments with the laws of physics were done with it. These early observations found that water exists in one of three forms: as a solid, a liquid or a gas. If the temperature of water was lowered it was found to freeze at 0°C, and was always a solid below that temperature. If it was raised, then at 100°C it became a gas, and was always a gas above that temperature. The above is always true provided the pressure on the water is one atmosphere (1488 kPa).

This change from a solid to a liquid or a liquid to a gas is called the 'change of state'. A litre of water has a mass of 1kg, and if we raise the temperature of this litre by 1°C it will require approximately 0.65 watts to do this. If we were to do the reverse, that is, cool the water by 1°C, then we would have to remove 0.65 watts of energy.

However, when we get to the point where a change of state takes place, the energy requirements are vastly different. Let's look at our litre of water again. Imagine it has been cooled to 0°C, and we now wish to turn it into a solid (ice). The amount of energy we have to remove is 93 watts, 144 times the amount needed to bring about a temperature change!

The heat we remove has not budged the temperature one iota, and is called 'latent heat'.

The other end of the scale is even more impressive. To convert 1kg of water to gas (steam) takes 627 watts of power, and when the steam converts itself back to a liquid, it will liberate this exact amount of energy. This explains why a steam burn is much more severe than a hot water scald, even though both could be at 100°C.

How is all this related to marine refrigeration? Read on...

PRESERVING FOOD
There are many ways to preserve food. One of these is to reduce the temperature to a low level. At around 4°C, most foods can be stored for at least a week without unacceptable spoilage, and they can be preserved indefinitely if deep frozen.

If we can keep our food in an enclosed chamber and lower the temperature inside to these levels, then we will have a refrigerator. Generally, the simplest way to achieve this is to use ice, and most of us do this many times a year and never think anything of it. By applying a little of the theory we have just learned, we can see some interesting things emerge.

First, let's grab a bag of ice. A standard bag is 2.5kg, which we know is 2.5lt of solid water. We also know that it will require 93 watts of power to melt 1kg of ice (latent heat), so it will require 232.5 watts to convert the bag entirely to water.

If food or drink is placed in the chamber at the temperature of 0°C, this temperature will be maintained indefinitely, since the food will not be able to supply the energy to melt the ice, as it's at the same temperature as the ice.

Well, we all know that the ice does melt, so how does this happen? Simply, the energy flows into the chamber from the outside. Remember we said that all energy will try to maintain an equilibrium, and with our small ice chest we have created an imbalance by having a temperature inside of 0°C and on the outside of 30°C (glorious Queensland!). In other words, we have a temperature difference (TD) of 30°C.

If our simple ice chest is just a cardboard box, then before long all the ice will turn to water and its temperature will rise to 30°C along with the food. Okay, the ice chest did work for a while, but it would be nice if it went for a lot longer. If only we could slow down the rate of the energy leaking into our ice chest! Well, we can.

All energy is transmitted in one of three ways, or in a combination of all three - that is, conduction, convection and radiation. The one that most concerns us here is conduction.

We can slow down the transmission of energy through the walls of the ice box by constructing them of an insulating material through which energy can only pass slowly.

There are many different types of insulating material available, two of the most common being styrofoam (styrene) and urethane. Styrofoam is the familiar white foam often used in packaging. It is light and cheap and a poor conductor of energy.

Urethane is also light, but much more rigid in structure. It's not as cheap as styrofoam, but is a better insulator.

So, with what we have learned, we can think about designing an icebox for a boat refrigerator if the cooling medium we use is ice. We will deal with more complex refrigerating systems in the following article; however, the actual insulated box will not change.

ICE CHEST DESIGN
The basic ice chest which we are going to design will need the application of a little maths before we can commence. First, let's select a size that will suit our needs. For the purpose of this exercise, let's make the box a cube 500mm x 500mm x 500mm.

The first thing we need to know is the surface area of this box, and for those who can't be bothered to work it out, it comes to 1.5sqm.

The amount of heat or energy that passes into this box is dependent upon the surface area, the TD between the inside and the outside of the box and, most importantly, the type and thickness of the insulation.

Let's have a closer look at insulation. Thermal engineers have arrived at a complex set of mathematical equations - which are beyond the scope of this article - to determine the effectiveness of different materials in resisting energy flow through them. Suffice to say, these are reduced to a factor called the 'U' factor. This U factor is expressed as watts per sqm per degree of temperature. Table One shows various figures for both styrofoam and urethane at varying thicknesses.

Now, suppose we have decided to use 50mm thick styrofoam for our ice box. We can see from the table this has a U factor of 0.72. We can now work this into a general formulae that will apply to any size box: Surface area x TD x U = watts/hr.

So, armed with this, what amount of heat will leak into our icebox per hour if the TD is 30°C? The surface area is 1.5sqm, so when this is multiplied by the TD and the U factor, the answer is 32.4 watts/hr.

If we put a standard 2.5kg bag of ice into this box, we know from our previous calculations it will need 232.5 watts to melt all the ice, and all that ice has to melt before the temperature inside the box can rise. The heat is leaking into the box at a rate of 32.4 watts per hour, so it will take 7.1 hours for the ice to melt. Good performance by any yardstick.

A few other factors come into play that any good thermal engineer will flay me alive for ignoring. But for our task the figures quoted will suffice. If we deduct 20% from them we will have a good safety margin and an ice chest that will hold for around six hours.

Some of the factors I have ignored are the number of times the box is opened (this admits more energy), the type and temperature of the produce being cooled and the design of the box itself.

PRACTICAL APPLICATIONS
Now we can start to look at the practical applications of our cabinet's design.

As usual, everything on a boat is a compromise, so we shouldn't be surprised that this also applies to the refrigerator.

If it's to be located in the galley area, it generally comes down to one of two arrangements: either under the bench with a front-opening door - very similar to a small bar fridge - or a top-loading cabinet also located under the galley bench.

Of the two, the front-opening option is by far the more user-friendly. It does have the disadvantage of losing all the cold air inside every time the door is opened, and the extra heat can add to a lot of extra cabinet load.

The top-loading style is much more thermally efficient, but, as anyone who has stayed onboard for extended periods will attest, they are a real pain to use, especially at meal preparation times when the top of the bench must be cleared before the lid can be opened. If you have plenty of bench space then this won't be much of a problem, but if this is in short supply it can make food preparation a bit of a juggling act.

If you intend to make your own cabinet and use ice or fit a refrigeration unit into it, here are a few tips that will assist you:

• Use the best insulation you can afford. As far as I am aware, this is urethane. It is better than styrofoam by roughly 25%, and an added advantage is it can be fibreglassed over (styrofoam dissolves). The minimum thickness is 25mm, but twice that is better.
• Cabinets made from stainless steel are a delight to the eye, but are expensive. Fibreglass is much cheaper and far easier to make.
• Consider making a temporary mould from laminated fibreboard. When waxed this will release quite well and produce a clean gelcoat interior. Simply making the cabinet from the rigid insulation and then glassing is easy, but the taint of the uncured resins in the fibreglass can spoil the taste of exposed food for years. I never got used to it.
• When the cabinet is finished, you should be able to immerse it in water and not get any of the insulation wet. This means it will have to be sealed both inside and out with a waterproof barrier. This is called a vapour barrier and prevents the ingress of water into the cell structure of the insulation. This is by far the biggest cause of poor operating performance in custom-made cabinets.
• Rot, rot, rot. Any sweating or condensation is pure water, which is just what the fungi that causes wood rot loves. Before installing the cabinet, rot-proof the entire area with Everdure or similar, particularly the floor area.

Front-opening cabinets have a problem in that the door seal gasket is chilled below the dew point and water will trickle off in a steady stream, especially in the tropics. Fitting a small heater strip under the gasket will solve this.

If ice is to be the cooling medium, then mould or block ice is far better than bagged ice. Unfortunately it is almost impossible to buy these days.

A friend of mine who lives on a small boat has a good idea. He uses wine casks that have been filled with water. He takes one to work every day and puts it in the freezer. When frozen, he has a block of ice that does not make a mess when it thaws.

Possibly this system is a bit primitive for today's more sophisticated boating person. For those people, my next article will look at complete refrigerating systems.