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Why Replace the Orifice Tube / Expansion Valve?

Expansion Valves:

The expansion valve's place in the system is at the evaporator inlet.  Like any other valve, its job is to control flow; in this case the amount of refrigerant entering the evaporator.  Since system operating conditions vary (sometimes high cooling demand, sometimes low cooling demand) it is necessary to be able to adjust the amount of refrigerant entering the evaporator.  For any given operating condition, if we were to allow too much refrigerant to enter the evaporator, it would get too cold, and the moisture collected on it could freeze.  This would not allow the hot interior air to pass through its fins, and the refrigerant flowing inside the evaporator would not be able to absorb the heat from the air.  This would eventually bring cooling to a halt.  If we were to allow too little refrigerant to enter the evaporator, there may not be enough to properly absorb the interior heat, which would also result in inadequate, or no cooling.  this process of varying refrigerant flow based on system cooling demand is referred to as "metering" the refrigerant into the evaporator.

So how does the expansion valve know how much refrigerant to meter into the evaporator, and how does it do it?  First the "how it does it", and its quite simple.  Expansion valves contain a movable rod which travels up and down inside the valve.  As the rod moves up and down, it can open and close the passage inside the valve that serves as the flow path for the refrigerant.  The valve does not have to be fully opened or fully closed at any given time.  Its position can vary, or modulate, between the fully opened and fully closed positions.  Because of this, it can very accurately meter the precise amount of refrigerant needed to meet any given cooling demand.

This internal passage inside the TXV is much smaller than that of the refrigerant flow pipe that delivers the refrigerant to it.  Because of this, as the refrigerant flows through this passage, its pressure drops, and it becomes the low-pressure liquid we referenced earlier.  So as you can see, the expansion valve also serves as a "dividing line" between the high and low pressure sections of the system.

Now the "How does it know how much" part...

This is based on evaporator's outlet temperature.  The warmer the evaporator is, the more refrigerant flow needed, and vice-versa.  The expansion vale has a temperature sensing device called a sensing bulb.  The sensing bulb measures temperature at the evaporator's outlet and sends a signal to the movable rod inside the expansion valve.  The signal corresponds to the amount of refrigerant needed, the rod moves to the proper position, and the correct amount of refrigerant enters the evaporator.

An Expansion valve could somewhat be likened to the thermostat in an engine cooling system:

   The thermostat controls the flow of coolant from the engine to the radiator based on cooling system temperature.

   The Expansion valve controls the flow of refrigerant entering the evaporator based on evaporator temperature, or A/C System load/cooling demand.

Orifice tubes are used in systems that don't use expansion valves.  Like and expansion valve, the orifice tube is used to control the amount of refrigerant entering the evaporator.  The diameter of the orifice tube is similarly small to that of the passage inside the expansion valve, but there is one big difference between orifice tubes and expansion valves.  An orifice tube is a simple fixed device with no moving parts.  It cannot vary the amount of refrigerant flowing into the evaporator the way an expansion valve can.  So in systems that use orifice tubes, some additional method of refrigerant flow control must be employed.  Two of the most popular methods used to do this are turning the compressor on and off at appropriate times, or cycling it, or installing a valve inside the compressor that can actually cause an adjustment to the pumping capacity of the compressor.

Orifice Tubes also serve as a dividing line between the high and low pressure sections of the system.

Things that can go wrong with expansion valves and orifice tubes:

Expansion valves may fail in these three ways:

   Clogging or Blockage

   Sticking either open or closed, or partially open or closed.

   Loss of proper metering ability due to wear, or an internal failure.

Of course, any of the above conditions could cause the wrong amount of refrigerant to be metered into the evaporator, which could lead to improper system operation.  Any of these problems will require replacement of the expansion valve.

An orifice tube is a very simple component with no moving parts.  About the only thing that ever goes wrong with them is clogging from debris, which always requires orifice tube replacement.