Aircon Refrigerant Distributor: Why a Coil Cools Unevenly

A refrigerant distributor is a small brass fitting with one inlet and several outlets. Sporlan defines it as a device connected to the outlet of a thermostatic expansion valve. Its outlet end is machined to accept tubing running to each circuit of the evaporator coil.

Its function fits in one line: the distributor equally divides refrigerant flow from the expansion valve into each circuit of a multi-circuit evaporator coil.

Not every indoor coil carries one, since a single refrigerant path has nothing to divide. Daikin Applied's evaporator coil manual supplies single or multiple distributors depending on the circuits required. Its interlaced coils use two distributors, each feeding every other tube in the first row.

The parts are few: a body, a nozzle, a retainer ring, and the tubes. On most models the nozzle lifts out. Daikin's installation steps have the technician inspect the distributor and confirm the nozzle is in place, held by a retaining ring or formed into the body.

Where it sits matters. A multi-split can also carry a branch box, a different machine with a different job. The branch box sits in the pipe run upstream of any indoor unit. The distributor sits at the coil, downstream of the expansion valve that meters it.

Downstream of the valve is a precise position. The valve decides how much refrigerant enters; the distributor decides where it goes once inside. Confusing the two sends a technician back to the metering device while the split goes unexamined.

The refrigerant arriving at the distributor is already part vapour, because some liquid flashes as it passes the expansion valve. What reaches the fitting is a two-phase mixture, and the two phases do not behave alike.

Lopsided proportions decide the whole problem. Sporlan works a case for R-22 with liquid entering the valve at 110°F and a 45°F evaporating temperature. Liquid is 77% of the flow by weight but only 7% by volume. Vapour is the rest, at 23% by weight and 93% by volume.

Nearly all the cooling is carried by the liquid. Nearly all the volume is vapour. A split that divides the volume evenly can still hand one circuit most of the liquid.

Speed makes it worse. Liquid and vapour move at different velocities, an effect the bulletin calls slip, attributed to gravity. Gravity has a greater influence on the liquid portion of the flow.

A plain header fails, predictably: the lower circuits of the evaporator receive the most liquid, which can set the valve hunting and send liquid back toward the compressor. The upper circuits are starved at the same moment, reducing the effective evaporator surface.

The design problem is not a better-shaped header but stopping the phases separating at all. The bulletin sets two requirements: mix the liquid and vapour, then hold that mixture homogeneous until equal portions are divided into each circuit.

The nozzle does both jobs. It raises the two-phase flow's velocity, mixing liquid and vapour, and aims the flow at a dispersion cone, which divides the mixture into evenly spaced passageways. High velocity, Sporlan writes, is the key to the distributor's success.

Pressure drop is the price of that velocity, and it is deliberate. Drop across the nozzle focuses the flow and provides mixing; drop across the tubes helps balance flow into each passageway. The bulletin adds that this drop does not reduce system capacity.

Tube length is specified, and the reason is resistance: for each circuit to offer equal resistance, Sporlan says, tube lengths must match. The tubes also have to be bent with care, because sharp bends and kinks reduce cross-sectional area and raise resistance.

The second condition sits outside the fitting. Since the distributor disperses equal amounts to each circuit, the bulletin requires the heat load on all circuits to be the same. Where it is not, the effect matches poor distribution: circuits with the greatest load starve and those with the least are overfed.

That clause matters for buildings more than for equipment. A filter blocked across half its face, or a duct that dumps air unevenly onto the coil, breaks the equal-load assumption while every pipe stays perfect. The fitting does its job and the coil still runs uneven.

Position is specified as well. Optimum performance comes from mounting the distributor directly on the valve outlet. Where that cannot be done, Sporlan allows a straight length of tube or pipe under two feet, sized to keep velocity up. Elbows between valve and distributor are ruled out.

Orientation is a preference there, not a prohibition. The distributor can be positioned in any direction. Best performance usually comes from feeding vertically upward or downward, which Sporlan recommends where the fitting is not mounted straight onto the valve.

Sizing happens on a drawing board, long before anyone owns the machine. Tube size is chosen by dividing total load by circuit count and reading a rating table; the nozzle against total load. Ratings assume a 30 inch tube length and a 10 psi drop across the tubes.

The valve in front has to be chosen knowing the fitting is there. Because the distributor takes pressure drop, Sporlan calls for an externally equalised expansion valve, with the drop estimated before the valve is picked. Adding or changing a distributor therefore changes the valve ahead of it.

One more rule catches out shared systems. Each distributor must be fed by its own expansion valve. If one valve feeds two distributors, Sporlan states, the flow will not divide evenly between them.

Blanking off a circuit is discouraged for the same reason the tubes must match. Plugging outlets ordinarily results in poor distribution; where circuits genuinely have to be plugged, the bulletin says it should be done symmetrically.

A distributor fault can hide behind readings that look ordinary. Researchers at NIST describe the standard arrangement: most evaporators use an inlet expansion valve with a flow distributor to control overall superheat at the evaporator exit manifold.

Exit manifold is the operative phrase. The circuits recombine before anything measures them. Whatever the instrument reads at the suction line describes the mixture, which carries no record of what each stream brought.

The coil only works as designed when every circuit agrees with that single figure. The paper's condition: superheat at individual circuit exits should match the desired overall superheat in the exit manifold. Nothing at the suction line reports whether they match.

An experiment puts a number on that blind spot. Testing a three-circuit finned-tube evaporator, the NIST team found maldistribution cutting capacity by as much as 30%, with overall superheat held at its 5.6°C target throughout.

One test case shows the mechanism in miniature. With the middle and bottom circuits at 16.7°C of superheat, holding the overall figure at 5.6°C meant overfeeding the top circuit, which flooded. Part of the coil starved, part flooded, and the composite reading sat on target.

There is a second reason the number holds while the coil does not. Circuits that finish boiling early spend the rest of their run carrying superheated vapour, which moves heat poorly next to a two-phase mixture.

The authors close with a caution that makes the field picture worse. Their rig held evaporator exit pressure constant. A real system answers lost capacity with a drop in evaporator pressure, so in service the disparity will be greater.

None of this makes superheat and subcooling useless. It makes a single averaged figure insufficient on a multi-circuit coil. The reading rules out a whole class of faults; it stays silent about which circuits were carrying the load when it was taken.

Distributor faults are built in, not worn in. Nothing inside the fitting has a service life or a cleaning interval. The NIST paper names bends or blockages in the distributor tubes, or a non-optimal design. Each is something a person did.

Brazing is the usual field origin. Sporlan instructs that tubes soldered into the coil be pushed in far enough to prevent coil plugging from excess solder. Its key line: a leak check will not reveal a plugged circuit, and the coil passes its pressure test with one circuit carrying nothing.

Heat damages the fitting in a quieter way too. Overheating a brass body drives zinc towards the surface, the bulletin warns, leaving passageways for leakage. Large bodies are prone to stress cracking, and quenching a hot distributor can crack it outright. A leak at a distributor often records how it was fitted.

The nozzle is the other made fault. It is interchangeable by design, which allows a wrong one to be fitted. Sporlan is blunt that the orifice number does not imply a capacity rating, and asks for loading between 50% and 200% of rating; below 50%, distribution can suffer.

Absence is worth checking first. Daikin's coil instructions have the installer confirm the nozzle is present, and note a hot gas bypass kit requires it moved into that assembly.

What a household can see is a pattern, and the pattern is worth recording. Frost may form as a band across one part of the coil while the rest stays clear. A photograph showing where the cold stops carries more diagnostic weight than a description of how cold it felt.

The question that moves this forward is method. Sporlan's instruction is that each circuit be checked individually with a probe wire or air jet, which detects a restricted circuit. Asking which circuits were tested separately, and what each one gave, separates a coil that was measured from one that was looked at.

One further check costs nothing to request. On a distributor with a removable nozzle, removing it exposes restrictions in the tubing or body directly, or a probe wire can find them. Where a coil has been opened and the cooling came back uneven, that inspection belongs before more refrigerant is discussed.