Cap Tubes and Flow Facts – Short #171

In this short podcast, Bryan goes over some information about cap tubes (capillary tubes) and flow facts.

Cap tubes are metering devices; they're long tubes with small diameters, and their flow rates are dictated by the tubing diameter size and tube length. Pistons and TXVs are some of the most common metering devices in residential HVAC, and flow restriction doesn't just happen at the metering device; distributors also contribute to the pressure drop and act like small capillary tubes in addition to the metering device. Older units, simple refrigerators, and window units are more likely to have capillary tubes as metering devices, as cap tubes are an easy and versatile use of small-gauge tubing.

The diameter is the primary factor that influences the flow rate, and length is usually secondary. However, longer tubes cause the fluids to encounter more resistance (in the form of friction) as they flow from one end of the tube to the other; the longer the tube, the lower the flow rate. Longer tubes also cause the fluid velocity to decrease more than a short tube.

When you have long runs of small-diameter tubing, you can replace a few sections with larger-diameter tubing to improve the flow rate. Sometimes, the ends of cap tubes are in hard-to-reach places, so replacing middle sections with larger-diameter tubing will still help decrease the static pressure and friction in the tubing.

Oil traps and risers may be smaller than other areas of tubing, and they have a larger pressure drop and more friction associated with them. (However, the smaller tubing also increases the fluid velocity.) The same principle applies to 1/4″ vs. larger-diameter vacuum hoses.

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