A cap tube is primarily a pressure-reducing device. When either gas or liquid is used, the reduction in pressure is linear (the pressure is reduced at the same rate for every measured length of the cap tube).

If liquid refrigerant responded the same way, it could never be used as a refrigerant control. If the inlet pressure was high, the evaporator would be flooded. If the inlet pressure was low, the evaporator would be short of refrigerant.

How Cap Tubes Modulate Flow

Since the cap tube is a refrigerant control, how does it modulate the flow rate to the head pressure? Regulating the flow rate of liquid refrigerant in a cap tube involves two principles:

1.Liquid flows faster than gas through a cap tube.

2.The colder the liquid, the faster the flow.

If the pressure-temperature of a refrigerant is at its saturated level when it enters a cap tube, the small ID of the cap tube will start to reduce the pressure. The reduced pressure causes the refrigerant to “boil” in the cap tube, and gas pockets are formed in the tube. This boiling action is similar to an evaporator, when the compressor is reducing the pressure of the refrigerant.

Now, let’s suppose that the refrigerant is subcooled by 5°F as it enters the cap tube. Another way of saying this is that the temperature of the refrigerant is 5° below its saturated level.

But what has happened to the head pressure? The head pressure is exactly the same as when the liquid was being subcooled in the condenser. But in reality, the head pressure is now above the saturation point of the subcooled liquid refrigerant.

As the subcooled liquid travels through the cap tube, its temperature remains the same, but the pressure is reduced.

Let’s say, for example, that after the liquid traveled though the cap tube for 3 ft in a 10-ft cap tube, the pressure was reduced to the new saturation level of the subcooled liquid. Any further reduction in pressure would cause the liquid refrigerant to boil and gas pockets would form. That’s exactly what happens when the liquid gets to the 4-ft mark.

The further the liquid travels, the more restriction, more bubbles, etc. We now have a 3-ft liquid length and a 7-ft bubble length because the liquid refrigerant was subcooled by 5°.

The following will illustrate how subcooling affects the flow rate of a refrigerant.


Let’s suppose that we have a cap tube system with R-12 in an 80° ambient. The condensing temperature is 110°, the head pressure is 136 pounds per sq in. gauge (psig) and the liquid temperature at the inlet of a 10-ft cap tube is 100°, which is 10° subcooled.

Now, if you look at your pressure-temperature chart, R-12 at 100°, the pressure should be 117 psig. It also means that the head pressure (136 psig) is 19 lb above its saturated point when it enters the cap tube (136 – 117 = 19).

As the 100° liquid travels through the cap tube, the temperature remains the same, but the head pressure of 136 psig is being reduced by the restriction. When the 100° liquid reaches the 6-ft mark, the 136 psig head pressure has been reduced to 117 psig, which means that the refrigerant is now at its saturation level.

As the liquid goes further into the tube, the pressure will drop to below its saturation level and bubbles will form in the cap tube. The gas pockets further reduce the flow rate causing more restriction — more reduced flow rate, etc., etc., until its outlet into the evaporator.

To sum up the above event: As the 10° subcooled liquid traveled through the 10-ft cap tube, it developed a liquid length of 6 ft and a bubble length of 4 ft, and (as an example) a flow rate of 18 psig/hr.


Now take the same system and the same cap tube, but this time the ambient went up to 90°, the head pressure is up to 157 psig with a condensing temperature of 120°. The liquid temperature at the cap tube inlet is 118° with only a 2° subcooling.

The liquid refrigerant has to travel only 3 ft where the reduced head pressure will be at its saturated level. Which means that from the 4th ft on, the reduced pressure will cause the refrigerant to boil, and we now have a 3-ft liquid length and a restrictive, 7-ft bubble length with a flow rate of 20 psig/hr.


Once again the same system and the same cap tube, but this time the ambient is down to 70°. The head pressure is at 100 psig with a condensing temperature of 90° and a liquid temperature of 70°, which is 20° subcooled.

If you look at your pressure-temperature chart, R-12 liquid at 70° should have a pressure of 70 psig. This also means that the pressure (100 psig) is 30 psig above its saturated level when it enters the cap tube.

The liquid will have to travel 8 ft before the pressure is reduced to its saturation level. This means that there is 8-ft liquid and only 2-ft bubble length to add to the restriction; the flow rate is now 16 psig/hr.


The governing factor of the flow rate through a cap tube is the inlet pressure. The modulating factor of the flow rate is the bubble length of the cap tube.

Comparing Example 1 to Example 2, where there was an increase of head pressure of 25%, the flow rate should have increased at least by the same percentage to 23 psig/hr — and flooded the evaporator.

It was the 7 ft of bubble length that added the restriction to the cap tube to allow only an 11% increase in flow rate — just enough to compensate for the additional load of the higher ambient.

Comparing Example 1 to Example 3, head pressure is reduced by 26%. If the flow rate decreased by the same percentage, the flow rate would have been 13 psig/hr. However, since the bubble length decreased to only 2 ft, the reduced restriction allowed only a 13% decrease in flow rate — just enough to keep the evaporator from starving.

Ehrens has been a long-time refrigeration expert at Sealed Unit Parts Co., Inc., 2230 Landmark Place, Allenwood, NJ 08720; 732-223-6644.

Publication date: 10/16/2000