TechniSolve Software

Software for the air conditioning, ventilation and refrigeration industry

Capillaries are commonly found on small systems

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The capillary tube is the simplest type of expansion device you could use.

ph diagramEssentially, it is a very small bore tube that is selected such that the frictional pressure drop equals the required pressure drop between the condensing and evaporating pressures. 

Since there are already some fittings in the liquid line and at the evaporator there is a distributor, tails and the evaporator itself, these pressure drops must be included.

Going from 3 to 4 is complicated by the change in refrigerant quality.  We solve the problem by a step-by-step simulation where at each step, the incremental length to achieve a temperature difference of 1°C is calculated.

At each step, the following equations are solved:

  1. Mass conservation
  2. Energy conservation
  3. Momentum conservation


capillary tube programAt the end of the simulation, the incremental lengths are summed to give the total capillary length.  Of course, this is just an estimate since it is based on theory only but it does give a good starting guess.  The final selection is by cut and try.

An interesting feature of the calculation is the emergence of the choked flow concept.  As you reduce the suction pressure, you will eventually reach a point when a further increase in mass flow is impossible.  This appears as a negative incremental length, clearly an impossible situation.  If this occurs, we terminate the calculation and report the suction temperature when the calculation was stopped.  The best way out of this is to increase the tube diameter.  The idea is to find the smallest possible tube without getting choked flow.  This would result in the shortest tube length.