The function of a compressor is the same for either Refrigeration or A/C. It is really very simple.
You are essentially transferring heat from one place to another; i.e. cooler, freezer or room.
You are never adding cold…you are removing heat. Transferred by low pressure (low temp)
Refrigerant in the evaporator and moving it to the condenser where it is High Pressure High
Temperature. Usually, the fan moves the lower temp air across the super heated refrigerant
high condenser coil cooling it. This is where you feel the heated air. This is the heat removed
from the room, cooler or freezer along with the heat of compression from the compressor.
This brings me to the next part of compressor function: refrigerant flow. Starting at the
discharge line at the compressor you will notice that this is the smallest line and should be the
hottest one of the two that is connected to the compressor. The Discharge Line takes the hot
refrigerant from the compressor to the condenser where it is cooled changing it from a hot
refrigerant to a sub cooled liquid. As the liquid leaves the condenser it will usually go to a
receiver tank then to the liquid line sometimes straight to the liquid line depending on the
expansion device.
The next stage in the process is the Drier. The Drier removes the moisture and debris, if any,
from the refrigerant. All Driers have an arrow on them. The arrow should always be pointed
toward the evaporator and away from the condensing unit.
The Expansion Device follows. There are usually only 3 types: 1. Capillary type; 2. Expansion
Type; & 3. Orifice Type. Regardless of the type, they all do the same thing---Drop the Pressure
which lowers the temperature of the liquid refrigerant below the temperature of the room or
The cold liquid now enters the evaporator unit, which is located inside the room or box,
travelling through the coil where a fan usually blows warmer air across the cold refrigerant
boiling it like water on a stove. The cold liquid picks up the heat of the box and at the end of the
coil is only a super heated vapor. The super heated vapor travels to the low side of the
compressor. This line is the coolest and largest one that is connected to the compressor. Now
the compressor sucks the low pressure refrigerant vapor pushing it to a high pressure high temp
Vapor where it started its cycle all over again dissipating the heat it has removed from the


Say Super Heat to a mechanic and they will act like they know what it is but often they really
don’t understand. Here I will explain what Super Heat is, how to find it and how to set it in
simple English.
Simply, Super Heat is the temperature of the Suction Line at any given point and the suction
pressure converted to temperature (see temp / pressure chart or even the conversion temp
inside your gauges R-22, etc.). The difference is Super Heat. The Suction Line temp will be the
same or higher …It can never be colder than the Chart temp. This chart temp is also the
Evaporator Temp. Subtract the conversion chart temperature from the Suction Line
Temperature and the difference is SUPER HEAT.
So, let’s measure the suction of an A/C unit using R-22. For example purposes, let’s say it
measures 58 deg. F and we have a 65# suction pressure. Look either at the Green R-22 temp
inside the suction gauge or look at your Refrigerant Temp Pressure Chart and you will see that
65# suction is 38 deg F. The difference between the 58 deg F we measured to the 38 deg F
chart temp is Super Heat or, in this example, 20 deg F of Super Heat.
So, what does this mean? Twenty (20) degF of Super Heat at the compressor is ideal. It
means that somewhere up the suction line the Liquid Refrigerant has changed to all vapor. All
Refrigeration and A/C Compressors are designed to pump vapor not liquid. If the Super Heat is
too low, 10 deg F or less, and you will run the risk of damaging the compressor. Low Super
Heat will either thin the oil so it doesn’t do its job or it enters the cylinder and can’t get out fast
enough. Either way, rods break and you are changing out a compressor. If the Super Heat is
too high the compressor will run hot.


Rule number 1, SUPER HEAT CHANGES, so to set it properly the Room or Box needs to be at the
desired temperature. Setting this is to regulate the amount of liquid left at the end of the
Evaporator. This is done by adjusting the Expansion Valve by screwing the adjustment screw-
-- Screwing In raises the Super Heat and Out lowers it. If this is an orifice or capillary tube,
first check the box or unit for the weight of refrigerant, the amount of refrigerant and the
discharge pressure will set the Super Heat. Less refrigerant will usually raise the Super Heat.
Raising and lowering super heat cools the compressor and its oil and head temperature. Ideal is
20 deg at the compressor. Sometimes a short suction line makes this hard to do. In this
instance, get what you can.
The key point is that you must have Super Heat or a Compressor will be the next thing you

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