A refrigerant distributor is a device connected to the outlet of an expansion valve when paired with a multi-circuit evaporator. The outlet of the distributor is machined to accept tubing which connects the distributor to each evaporator coil circuit.
A portion of the liquid refrigerant passing through the expansion valve normally flashes, resulting in two-phase (liquid and vapor) flow at the valve outlet. This mixture is predominately liquid by weight, but the vapor occupies most of the volume. An additional problem arises due to the fact that liquid and vapor move at different velocities. This is sometimes referred to as slip, since gravity has a greater influence on the liquid portion of the flow than the vapor portion.
If a simple header is used instead of a refrigerant distributor, circuits will not receive equal amounts of refrigerant. This is because there is a natural tendency for the liquid and vapor to separate, resulting in unequal amounts of liquid being fed to the various circuits. Gravity will pull the liquid to the lower circuits, and vapor will go to the upper circuits. This will lead to hunting from the expansion valve and possibly cause flood-back issues.
To achieve proper distribution, the liquid portion of the two-phase flow must be divided equally to each evaporator coil circuit. Two-phase refrigerant flow leaving the expansion valve enters the distributor nozzle. The nozzle increases the velocity of the two-phase flow, mixing its liquid and vapor components. Furthermore, the nozzle is positioned such that flow is focused onto the dispersion cone, equally dividing the mixture into passageways spaced evenly around the cone. The refrigerant is then transferred to each evaporator circuit through the distributor tubes.
To ensure equal refrigerant flow, it is important to size the distributor nozzle and tubes to match the system capacity as closely as possible. By doing this, the proper pressure drops and velocities are created to completely mix the liquid and vapor.
In addition to feeding equal amounts of liquid and vapor into each circuit to utilize the full capacity of the evaporator, each circuit must be equally loaded. Figure A is a schematic illustration of typical temperature conditions in the evaporator when both equal distribution and equal loading occur.
Figure B illustrates the same evaporator but with the air flow (and thus the load) less over circuit #3. The load imbalance will be indicated by low superheat at the outlet of circuit #3, and high super-heat at the outlet of circuits #1 and #2. Other symptoms are lower than normal suction pressure, reduced evaporator capacity and expansion valve hunting with possible flood-back.
Optimum distributor performance is obtained when the distributor is mounted directly to the expansion valve outlet. If the distributor cannot be mounted directly to the valve outlet, it can be connected by a piece of straight tubing. The tubing should not exceed two feet, and it should be sized to maintain high refrigerant velocities. Elbows located between the expansion valve and distributor hinder proper distribution, and are not recommended. The expansion valve should be tied to a single distributor. Multiple distributors on a single expansion valve results in poor refrigerant distribution in the evaporator coil.
A distributor can be mounted in any position. If the system operates over widely varying conditions, best performance is usually obtained when the distributor feeds vertically upward or downward, see Figure C. For applications where the distributor is not mounted directly to the expansion valve, the vertical feed arrangement is recommended.
When planning a system refrigerant conversion, it is critical to consider nozzle and tube sizing due to differing net refrigerating effects of refrigerants. For complete sizing and application information refer to Parker Sporlan Bulletin 20-10.
HVACR Tech Tip Article contributed by Jason Forshee, application engineer, Sporlan Division of Parker Hannifin
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