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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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19 Mar 2019
For more than 100 years, the car has simply been used as a device for transporting a driver and passengers from point A to point B at speed with minimum effort.
With the introduction of Advanced Driver Assistance Systems (ADAS) and other semi-autonomous driving technologies, a different concept of the vehicle is emerging. In the future, the car will be a media playback center, telephone, office and extension of the home’s living room which also happens to be able to convey passengers from A to B.
This is having a profound effect on the characteristics and on the sheer number of electronics systems in new vehicles and this in turn will dramatically extend the demands on the EMI shielding devices used to attenuate the radiated emissions that could affect circuits in the car. EMI shielding materials will need to perform over a wide range of frequencies, in more applications as electronic systems take over more and more aspects of the car’s driving operations, while adding as little as possible to the weight of the vehicle.
The time for OEMs to consider the options for achieving EMC in new car designs is at the start of a new design project, before the electrical and mechanical features of the vehicle’s systems have been decided. This gives design engineers the opportunity to bring considerations of EMI and shielding devices into the design process and enable optimization of the size, cost and performance of EMI shielding in the final system.Top challenges of next generation 5G networks
The first challenge for automotive design engineers is the range of frequencies that need to be attenuated will be far greater in new cars than it was in the past. Until recently, the main frequencies of interest were the AM and FM bands used by radio and frequencies below 3GHz used by Bluetooth radio and mobile phone networks.
With the future introduction of 5G mobile phone network coverage, frequency coverage of EMI shielding materials will need to be extended. A higher frequency range is not the only issue. Cars are also going to support a much greater number of wireless communications systems within the vehicle.
The second challenge is that the effectiveness of EMI shielding is likely to be more tightly specified in the future as automotive manufacturers move towards a strict view of the functional safety of the electronics systems in cars, codified in the ISO 26262 functional safety standard.
So, what does this mean for the specification of EMI shielding materials?
Parker Chomerics maintains an intensive research and development program aimed at producing new filler materials for electrically conductive elastomer products. An important goal for this research program is to produce EMI gaskets that can cover the broader frequency range of interest in autonomous vehicles, while maintaining the desirable mechanical characteristics. Parker Chomerics CHO-SEAL conductive elastomers are widely used in automotive systems and offer useful properties, including resistance to high temperatures and contaminants, and the ability to provide environmental sealing to protect circuits from the ingress of liquids.
These elastomer gaskets resist compression set, accommodate low closure force, and help control air flow. They are available in standard sheet form, extruded or custom shapes.
In addition, Parker Chomerics CHOFORM Form-In-Place automated EMI gasket material can be dispensed directly onto castings, machined metal and conductive plastic and is widely used in tightly packed electronic housings. This advanced technology allows dispensing of precise positioned gaskets in very small cross sections and can free up valuable packing space of up to 60%. CHOFORM offers excellent shielding effectiveness which exceeds 100dB between 200 MHz and 12GHz.New opportunities for weight saving
The development of autonomous and semi-autonomous vehicles is leading to a huge increase in the number of electronics modules per vehicle. This increases the scope for car makers to reduce weight by replacing conventional metal housings with lighter conductive plastic housings. While the weight saving on each module might appear small, when multiplied across the 100 or more electronics modules, the total weight saving becomes invaluable.
Parker Chomerics PREMIER™ PBT-225 is a single-pellet conductive plastic for use in automotive housings. PREMIER PBT-225 offers excellent resistance to hydrolysis when exposed to extreme temperatures and provides for easy processing and uniform filler dispersion. As a result, EMI housings made from PBT-225 offer tightly controlled electrical and mechanical performance throughout complex geometries. A weight saving of 30% is also possible when replacing an equivalent metal or aluminium housing with PBT-225.
By collaborating early in development projects with Parker Chomerics, automotive system designers can ensure that their electronic and mechanical design is optimized for shielding purposes.
Learn more about Parker Chomerics EMI shielding and thermal solutions for the automotive Industry.
This blog post was contributed by Mel French, marketing communications manager, Chomerics Division Europe.
19 Mar 2019
Honeycomb air ventilation panels are used in applications where superior electromagnetic interference (EMI) shielding must be incorporated with heat dissipation in the form of airflow. Every vent panel has a variety of design features, each providing benefits to end customers based on specific application needs. These design features can include framing, plating/painting, gasketing, and vent size control.
An often overlooked but highly important phenomenon to consider when designing EMI vent panels is that of polarity.
What this means is that honeycomb vents can have differences in shielding effectiveness, sometimes as great at 50 dB, depending on the direction of the electromagnetic waves.
For example, a basic aluminum honeycomb vent may provide shielding of 70 dB in the horizontal direction while only providing shielding of 25 dB in the vertical direction. This characteristic is due to the manufacturing process of standard aluminum honeycomb vent panels.
Basic aluminum honeycomb is created using thin ribbons of aluminum that are bonded using a non-conductive adhesive. Polarity is associated with seam leakage caused by the non-conductive bonds from cell to cell created during the manufacturing process of adhering aluminum ribbons together to make the honeycomb. While thin, this non-conductive gap is the cause of difference in shielding effectiveness (SE). It is important to note that polarity is only an issue for aluminum vents, not for steel, stainless steel, and brass honeycomb due to a different manufacturing process (steel and brass honeycomb use a welding process, eliminating the non-conductive gap). The below graph demonstrates the significant difference in shielding effectiveness in the horizontal and vertical directions.
Fortunately, there are several solutions to combat this polarity issue:Layered vents
With the addition of a second layer of honeycomb, offset at a 90-degree angle, the polarization effect can be dramatically reduced. The Chomerics term for these layered vents is Omni Cell. By rotating the second layer of honeycomb 90 degrees, RF wave interaction in both the X and Y axes are combated by the seam orientation of each layer of honeycomb. This means that while the electromagnetic waves may pass through one layer of the honeycomb, the offsetting layer will not allow them to pass through the entire vent assembly. Of note, airflow through the vent is not significantly impacted, allowing for enough heat dissipation.
While the maximum shielding effectiveness of the Omni Cell vents is nearly identical compared to that of a single layer vent, the directional consistency is instantly noticeable. There is no longer a difference in the horizontal and vertical shielding effectiveness, with the offsetting layers eliminating the polarity effect.Plating
Electroless nickel plating is an ideal plating option to combat polarity on aluminum vents. The nickel plating covers the non-conductive bonds and eliminates seam leakage between aluminum ribbons. The nickel plating electrically connects the aluminum ribbons which overcome the non-conductive adhesive. Not only does nickel plating effectively eliminate the polarity effect, it increases the durability of the vents and improves their lifespan in harsh environments.
As with Omni Cell vents, the polarization effect is eliminated with the vent exhibiting nearly no difference in shielding between horizontal and vertical testing. A properly plated vent will also increase the SE of the entire honeycomb array, creating conductive contact between every individual aluminum ribbon in the assembly. The nickel plating also improves the electrical connection of the honeycomb to the frame if the plating process is done after assembly.
Based on the above graphs, a conclusion can be made about the techniques used in eliminating the polarity effect. Since the plating process can eliminate the polarization effect AND increase the SE, this approach is most common. Omni Cell construction is effective if it meets the desired shielding effectiveness level. It is rare to see a nickel plated Omni Cell vent.
Individual project specifications such as airflow requirements, shielding performance, environmental exposure, budget and a myriad of others will drive the design process of EMI shielding honeycomb ventilation panels, but it is important to know about principles such as polarity in making final considerations.
14 Mar 2019