Selaed System
Chapter 1 Page 1
The Sealed System
The Basic Refrigeration System
A full grasp of these principles are essential to understanding how and why cooling occurs in a refrigeration system.
• Everything has heat.
• Heat is measured in quantity (BTUs) and intensity (temperature).
• Heat always moves from an area of higher intensity to an area of lower intensity.
• Latent heat is "hidden" heat.
• In an enclosed container, there is a relationship between the pressure and temperature of a liquid.
• A great deal of latent heat must be absorbed in order to change the state of a liquid into a vapor.
• A vapor must give up its latent heat before it can re-condense back into a liquid.
• Lowering the pressure over a liquid reduces its boiling (evaporation) point.
• Increasing the pressure over a liquid raises its condensation point.
• Refrigerants are chemicals that have extremely low boiling and condensing points.
Refrigeration Made Simple
Refrigeration is the process by which a refrigerant is made to evaporate at very low pressures and temperatures, thereby absorbing and trapping heat. The heat laden vapor is then moved to another area where, under pressure, the vapor re-condenses back into a liquid, thereby releasing the trapped heat.
Yes, refrigeration is that simple.
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A refrigerator, ice machine, dehumidifier, and air conditioner all have different jobs to do, but they are all similar. Their similarities are the 4 main components that make up a a basic sealed system. Those parts are compressor, evaporator, condenser, and metering device “Capillary Tube”.
The most important thing for you to understand is the only moving part in the whole system is the compressor. The rest of the system is just tubing. The question is, how do we manipulate pressures to cause the refrigerant to boil in one area, move the refrigerant to a second area, and then cause the refrigerant to condense?
Let's look at a typical refrigeration system and explore each of its components. The refrigeration system is made of 6 major components, each having a specific job to perform during the refrigeration cycle.
They are:
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Evaporator- The coil inside the freezer where refrigerant absorbs heat.
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Condenser- The coil outside the refrigerator (in the back or underneath) where refrigerant gives up its heat.
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Compressor- A piston pump that pushes refrigerant from the Evaporator to the Condenser
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Capillary tube- A length of very small tubing that restricts the flow of refrigerant from the condenser to the evaporator.
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Heat exchanger- A section of tubing that allows refrigerant to give up some its heat prior to entering evaporator.
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Drier- a device that traps and holds any moisture that might be present in the system.
All these components are welded together at the factory to create a sealed system. As the name implies, the system is completely enclosed, and the refrigerant charge should never leak out.
Evaporator
The evaporator coil is typically made from aluminum and is installed in the freezer compartment. The main purpose of this coil is to absorb heat from the freezer air, thereby reducing the temperature of the stored food.
Over the years, several coil designs have been used.
Tube and Fin
Most modern refrigerators utilize a tube and fin evaporator. This design must be used in conjunction with an evaporator fan motor to move the freezer air across the coil. Warm air is pushed past the fins and gives up its heat to the cooler metal fins. The heat then travels through the fin into the tubing where the refrigerant can absorb it.
Tube and fin evaporators also require a means of removing any frost that forms on the coil. Keep in mind that any moisture in the air deposits on the very cold fin surface and freezes. If enough frost forms on the fins, air flow will be restricted. Moreover, frost acts as an insulator and prevents transfer of heat from the air to the coil. If the frost is not removed from time to time, cooling capacity suffers. In extreme frost conditions, cooling will cease altogether. For this reason, manufacturers install a heater of some type in or underneath the evaporator to defrost the coil at predetermined intervals.
General Electric has their own version of the fin and tube designed evaporator. All evaporator perform the same function within the refrigeration system. How they absorb heat and their efficiency in how they do it is what makes each evaporator unique.
Their fins are not in straight lines like the other fin and tube evaporators and has many more aluminum fins which help is absorb heat.
Shelf Evaporator
Shelf evaporators are found exclusively on upright freezer models. In this evaporator design, the coil is incorporated into the shelf system of the freezer. The customer lays food on the shelf and heat transfers directly to the tubing in the shelf. Shelf style evaporators must be manually defrosted by the customer.
Roll Bond Evaporator
Roll bond evaporators are found in older manual defrost refrigerators and most dorm sized imports. To form the evaporator, two pieces of aluminum with embedded channels are sandwiched together to form a large cooling area. Refrigerant travels though the channels and absorbs heat. This type of evaporator needs to be manually defrosted.
Cold Wall Evaporator
Cold wall evaporators are used on chest Evaporator freezers. The refrigerant tubing is imbedded in the cabinet foam and staked to the inside cabinet liner which is formed from painted steel. The metal acts as the medium by which heat is transferred from the food to the refrigerant tubing. As with shelf style and roll bond evaporators, the customer must manually defrost these evaporators.