Chapter Fifteen - Refrigeration Basics

Readers my be wondering if ol' Sharkey has developed a case of brain barnacles. Refrigeration? I thought these articles were about renewable energy. Providing power to preserve perishable provisions can be the biggest consumer of energy in a home or bus based electrical system. Even if you are using an icebox to keep food cold, you are still consuming energy, and paying for the privilege. This series of articles will help you get the most from your refrigeration system, whatever it's type.

To begin, let's consider the common icebox or insulated cooler, supplied with a block of ice to keep the food cold. The process of refrigerating with ice is a lot more complex than you probably thought. The ice doesn't keep the food cold simply because it is cold itself, but because the ice melts.

To understand why, it is necessary to know a bit about thermodynamics (just a little bit, OK?). Consider a pound (.45Kg) of ice, with a temperature of, say, zero degrees Fahrenheit (-18°C). This ice will absorb one BTU (British Thermal Unit) of energy for every one degree it's temperature rises, up to the melting point of 32° (0°C). So, as this ice warms up by absorbing heat from your food, it will gain 32 BTU's. So far so good, but an astounding thing happens when the ice changes states, from solid to liquid, it absorbs 144 BTU while rising only one degree F! Once all the ice is melted, the melt water reverts to again absorbing one BTU per degree F of temperature rise.

From this we can see that the melting of ice is 144 times more efficient at cooling food than either solid ice or the cold water left after the ice is gone. In fact, if the ice didn't melt, it wouldn't do a very good job of cooling your grub. A 25 pound (11.34Kg) solid block of ice would absorb 3,600 BTU's of heat while melting, or the equivalent of more than one kilowatt hour of electrical power.

In this example, we considered ice to be frozen water, but in reality, it could have been any one of many substances. The laws of thermodynamics apply to the changing of states of all matter, solid to liquid, liquid to vapor. Obviously, we need to choose materials that have a melting point close to the temperature that we wish to cool to. Attempting to refrigerate food by melting candle wax, or lead would be an exercise in futility. Water very conveniently changes states very near the optimum temperature for retarding spoilage in food.

One of the more efficient ways of keeping things very cold is 'dry ice', which is frozen carbon dioxide, with a melting point of approximately -100°F (-78.5°C). Because it has such a low melting temperature, dry ice can keep foods like ice cream quite solidly frozen. It is also a bit dicey to work with, as it can quickly freeze the tissues of your skin while being handled.

Where better thermal efficiency is desired, compounds called eutectic solutions are used in place of water ice. Basically, these are anti-freeze solutions of brine or glycol that have a melting point in the range of -60 to 32°F (-50 to 0°C). In the States, one commercial product known as 'Blue Ice' is very popular as a substitute in ice chests and picnic coolers. The sealed packages are refrozen and reused repeatedly. Eutectic solutions have the advantage of being able to absorb more BTU's per volume than plain water, and will last longer while giving lower cooling temperatures.

In the next article, we'll investigate basic mechanical refrigeration in several forms.