Back to Basics – Basic Refrigeration, Part 2

By Dave Bird, Dyer Appliance Academy

Last time, we discussed some of the science of refrigeration. Now I’d like to take a look at the components of the sealed system. What are they, what do they do and how do they work together to make things cold?

The heart of any sealed system is the compressor. A compressor is an electro-mechanical vapor pump that circulates refrigerant in the sealed system. The compressor only pumps refrigerant while it is in a vapor or gaseous state. If liquid refrigerant gets into the compressor it will cause damage to the pump mechanism and ultimately compressor failure. Manufacturers go to great length to keep this from happening.

Once the compressor starts pumping, the input or “suction” side of the compressor draws in low pressure, low temperature vapor refrigerant from the evaporator. This vapor is compressed into a high pressure, high temperature vapor.

The refrigerant leaves the compressor through the discharge line and is sent to the condenser coil. The condenser coil is designed to expel heat absorbed by the refrigerant out of the sealed system into the air. This process is assisted by the condenser fan which provides constant air flow over and through the condenser coil.

Refrigerant enters the condenser coil as super-heated, high pressure gas. As it travels through the coil it cools and condenses (turns to a liquid). By the time the refrigerant leaves the compressor it is almost all liquid.

When the refrigerant leaves the condenser it flows through the filter-dryer. The purpose of the filter-dryer is to absorb any moisture as well as to remove any particulate from the refrigerant.

While the normal diameter of the tubing in a sealed system is quarter inch, the output of the filter-dryer is a very narrow diameter tubing known as capillary tube.

The internal diameter of capillary tubing is about as wide as a pin. This capillary tube (also known as the metering device) controls how much and how fast the refrigerant flows into the evaporator.

This is where the magic takes place. The pressure difference between the small diameter of the capillary tube and the much larger diameter of the evaporator coil as well as the temperature differential immediately causes the refrigerant to vaporize. This change in state causes the evaporator to get cold.

Since the evaporator is the coldest part of the sealed system, it can absorb any heat present in the air passing through its coils. The air movement is facilitated by the evaporator fan. This process is so efficient that it can remove heat from a freezer at 0 degrees Fahrenheit. By the time the refrigerant leaves the evaporator coil, it is all vapor. This vapor travels through the suction line back to the compressor and completes the refrigeration cycle.

The capillary tube and the compressor set up the pressure differentials in the sealed system. From the output side of the compressor to the output side of the capillary tube is known as the “High Side”.

It is called the high side because of high pressure and high heat it contains. On the other side, from the input side of the evaporator back through the suction line and to the compressor is known as the “Low Side”. This is because of the low pressure and low temperature of the refrigerant it contains.

Now that we know about what the sealed system is, we need to consider some other issues.

There are two systems in a modern refrigerator. They are the sealed system and the electrical system. In a refrigerator with a “no cool” problem, you must always verify proper operation of all electrical components first before considering the sealed system. There a lot more components in the electrical system and thus a greater chance for failure there.

Nine times out of ten, a no cool condition is not a sealed system problem.

Whenever I work on a refrigerator that is not cooling, I start on the fresh food side. I verify air flow and for vent obstruction. I also verify the control settings and that the light shuts off when the door closes.

Next I look at the freezer side. I listen for the evaporator fan and look for frost on the back panel of the freezer. If frost is seen, that is a good indication that there is a defrost problem. If the defrost system has failed, ice and frost build up will obstruct the air flow past the evaporator. No airflow means no cooling.

The frost pattern on the evaporator coil is an excellent trouble shooting tool for potential sealed system problems. Frost should be present in an even coating over the entire surface of the evaporator without obstructing the air flow. A partially frosted evaporator could indicate a refrigerant leak or restriction in the sealed system.

The next thing to check is the mechanical section in the back of the refrigerator. I check that the compressor is running, verify that the condenser fan is blowing and that the start relay is not short cycling (turning on and off about once every minute or so).

I’ve found that the two most common failure items in a no cool scenario is a bad defrost control or a bad start relay on the compressor.

I hope you’ve found this overview helpful and wish you good luck and success in your fault analysis. And as always, be safe.

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