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System Characteristics
Chapter 5 Page 4

Evaporators and Frost Patterns

Visual evidence of refrigerant in the system can be done when looking at the evaporator in the freezer. As shown in the image on the right a frost pattern appears as the refrigerant in the system boils. Earlier an example of water boiling was shown in chapter 2, but the term boiling can be at any temperature depending on the medium you are trying to boil.

As explained in the refrigerant chapter, one of the ideal characteristics of a refrigerant is a low boiling point. 

Lets break this down as the refrigerant flows into the evaporator it enters in a liquid form from the "metering device" capillary tube. The air inside the freezer although it is considered cold to us at 0 degrees it still contains heat. The refrigerant inside the evaporator absorbs the heat and starts

frosted evap.png

changing the refrigerant into a vapor. This lowers the temperature of the air.  Refrigerant R134a is running at 0 PSIG on the low pressure side of the system and at a temperature of -15 degrees. This causes any moisture or humidity in the air to freeze and appear as a white snow when it passes over the tubing.

To make this test run the refrigerator with the evaporator completely covered. This is necessary for the fan to circulate all of the air evenly across the coil. after 5 minutes of operation remove the evaporator cover and observe the frost pattern. An evaporator that has a frost pattern completely covering the evaporator the compressor is operating and most likeley fully charged. An evaporator that is partially frosted can mean a few different things. 

1.  Unit has a refrigerant leak and therefore the evaporator does not have enough refrigerant - Once the refrigerant completely evaporates it no longer absorbs heat. This is evident by some of the evaporator coils not having a frost pattern.

2.  The unit has a restriction - Usually in the capillary tube or drier filter. The frost pattern on the evaporator looks similar to a system with a leak.

Follow the link to take the Chapter 5 exam - TMM Sealed System Chapter 5 Test 

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