Multi-circuit evaporators utilizing a thermostatic expansion valve (TXV) will have a refrigerant distributor connected to the outlet of the TXV to ensure the refrigerant leaving the TXV is fed equally into each circuit.
As the refrigerant leaves the TXV it is in a saturated state — a mixture of the liquid and vapor. The mixture is mostly liquid, approximately 75 percent liquid to 25 percent vapor by weight; however, the vapor occupies much more volume, and the two components are moving at different velocities. Because of the relationship, without a distributor, unequal amounts of refrigerant would enter each of the circuits. This would lead to some of the circuits receiving mostly liquid, possibly causing TXV hunting and floodback problems, and some of the circuits being starved for refrigerant.
To solve this issue, as the saturated refrigerant leaves the TXV, it is fed into a distributor. The distributor’s nozzle increases the velocity of the saturated refrigerant by creating a pressure drop. This causes a mixing of the liquid and vapor components, and combining this with the position of the nozzle within the distributor causes equal amounts of refrigerant to be fed into each circuit.
Sizing of the nozzle and distributor’s tubes is critical in creating the required pressure drop to increase the refrigerant’s velocity and properly feed each distributor tube. Always refer to the manufacturer's selection guideline when replacing any component of a distributor, including its distribution tubes. Properly sized distributor tubing, in both length and diameter, is also required to create the appropriate pressure drop and ensure good mixing of the refrigerant and equal flow.
When replacing a distributor, in addition to following the manufacturer's selection criteria, make sure the distributor’s tubes are all equal in length and that there are no sharp bends or kinks in the tubing. This will ensure equal flow into each circuit. A distributor should be connected directly to the outlet of the TXV; however, if needed, a short piece of straight pipe (no elbows) can be installed between the outlet of the TXV and the distributor. An externally equalized TXV will need to be used with a distributor because of the pressure drop created by the distributor. Generally, a distributor can be mounted in any position; however, best performance is usually obtained when the distributor feeds vertically upward or downward.
Distributors are typically made of brass, and since distributor tubes are typically copper (as well as the outlet fitting of the TXV), the majority of brazed distributor connections are brass to copper. For this brazed connection, you will need to use the appropriate flux and filler material. When heating the distributor body, uniformly heat prior to applying the filler metal to the joint. After brazing, allow the distributor to cool slowly. Quenching a hot distributor can crack it.
Avoid overheating brass distributor bodies. A low-temperature solder may also be used to install the distributor. This can help to reduce the possibility of overheating the distributor or TXV’s body. A combination of brazing and soldering can be helpful when connecting these components. Braze the distributor tubes to the distributor, and then use a low temperature solder to connect the distributor and TXV. This can help reduce the possibility of accidentally undoing the distributor tube joints or overheating the TXV body.
Understanding the purpose of the distributor and how to properly size and install these components can be helpful to any refrigeration technician.