Marine Refrigeration Basics

This introduction is all about heat at different temperatures above absolute zero (-460 degrees F) rather than in terms of hot and cold. References will be made to BTUs, (British Thermal Unit), a unit of heat measurement defined as the amount of heat required to raise or lower one pound of water one-degree Fahrenheit. It is important to remember that heat travels in one direction only: from a warm, high temperature area to a lower temperature area. We don’t “make cold,” we simply remove heat from one area and transfer it to somewhere else.

The Compression System

There are several types of refrigeration systems and this introduction deals with a Compression System, best described as being two halves. One half will be on the discharge side of the compressor & is called the “High Side” while the other is on the suction side & is called the “Low Side”.

The compressor, either electrically or mechanically driven, compresses a special gas (refrigerant), from a low pressure (typically 0-20 psi) to a high pressure (typically 100-150 psi). In doing so it raises its temperature considerably. This hot, high pressure gas is fed by tubing or hoses to a Condenser where it is cooled and condenses into a liquid.

The condenser can be cooled by air, water or “keel cooled,” which is the most efficient method. The cool, high pressure liquid is fed to an Evaporator which is inside the area to be refrigerated. The evaporator can be in one of several forms. At the inlet to the evaporator is a Metering Device. This is simply a restriction which separates the high and low sides of the system. The metering device can be either a mechanical device called an Expansion Valve or simply a long, thin tube with a very small bore, called a Capillary Tube (or Cap Tube). When the liquid refrigerant reaches the other side of the metering device, it is now on the low side of the system, and is exposed to the suction effect of the compressor and rapidly drops in pressure. This sudden drop in pressure causes the liquid refrigerant to boil and evaporate back to a gas. It does this at a very low temperature. From the evaporator, the low pressure gas is fed back to the compressor where the process starts again.

Heat: How a boiling liquid cools things down

There are two types of heat, Sensible Heat and Latent Heat. Imagine a container with one pound of water in it. If we add one BTU of heat, the water temperature will rise one degree Fahrenheit. This is sensible heat.

If we keep adding BTUs, the water temperature will keep rising until it reaches 212 degrees. At this point, the water will start to boil and evaporate. The temperature will not rise until 960 BTUs have been added and all the water has evaporated. This is Latent Heat of Evaporation and is the heat required to change a liquid to a vapor or vice-versa. The temperature does not change while a change of state is taking place.

An example of latent heat of evaporation is water being used to douse a fire. As soon as the water hits the burning object it turns into steam, taking a massive amount of heat energy from the fire, lowering its temperature below the flash point.

Now back to our container with one pound of water. If instead of adding sensible heat to our one pound of water we remove it, one BTU at a time, the temperature will fall one degree Fahrenheit at a time until we reach 32 degrees, when the liquid will start to freeze & turn into ice. It will now stay at 32 degrees until we have removed 144 BTUs and all the water has frozen. This is called the Latent Heat of Fusion.

This same principle of latent heat of evaporation is used in the Frigoboat compression refrigeration system. This is achieved by using a environmentally friendly Refrigerant which boils at a very low temperature and absorbs large amounts of heat in the process. Nearly all small systems now use the R134a refrigerant, which when exposed to atmospheric pressure evaporates rapidly. For this reason it is kept in enclosed containers or inside a refrigeration system where it exists in both liquid and vapor forms.

The System Overview

When the compressor starts, it pumps up the pressure of the R134a gas, as well as heating it. From the discharge side of the compressor it is piped to the condenser and is air, water or keel cooled. The condenser then cools down the gas below its dew point and it changes state into a liquid, still at a high pressure. It is now fed to the metering device and evaporator. When exiting the metering device, it is exposed to the suction side of the compressor and is at a low pressure. Now inside the evaporator, it boils rapidly and evaporates back to a gas at a very low temperature, typically around – 26 degrees F. This change of state absorbs vast amounts of sensible heat from the evaporator which in turn removes heat from the insulated box, thereby lowering its temperature.

The size, type, and settings of the components and the amount of refrigerant in the system are critical in ensuring that all of the refrigerant reverts back to a gas as soon as it leaves the evaporator and before it returns to the suction side of the compressor.

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