Input and output restrictions are often the cause of slow recovery. In order to truly achieve fast recovery speeds, you need to have full flow on both ends of the recovery machine.
Input Restrictions
The most common input restrictions are valve cores and core depressors, which block up to 90% of all flow. During recovery, they act as expansion devices and turn any liquid refrigerant into vapor resulting in slow recovery & hot tanks. Have you ever noticed frost forming around components or hose fittings? This is typically a sign that a restriction is present and can make it easier to pinpoint restrictions in the setup and troubleshoot them. However, the frosting is also a sure sign that ambient heat is being absorbed at that point and making its way into the tank. Have you ever encountered hot tanks during recovery (especially common with R410A)? A hot recovery tank is a sure sign that input restrictions are present.
Note on frosting components: It is normal to see some light frosting on compressor crankcases & filter dryers during recovery. Since refrigerant & oil mix together during operation the refrigerant will also attempt to escape it’s bond with the oil during recovery...which is a similar action to when liquid refrigerant moves through a restriction. This is most common on these components, as this is where the majority of the oil sits in the system.
Service Tips: Use a valve core removal tool to remove valve cores in access valves, and remove any core depressors from the ends of hoses (they are not needed if no valve core is present).
Input restriction Examples
Access Valve Cores
Core Depressors
How to use a Valve Core Removal Tool
Output Restrictions
These restrictions are typically easier to diagnose and solve than input restrictions. The valves on a recovery tank can be a source of restriction if they are not fully opened. Even the port that you connect the output hose to can be a restriction. Recovery tanks have a dip tube that runs all the way to the bottom of the tank on the liquid port; while this is great for charging, it’s not ideal for recovery due to its small diameter. Simply connecting to the vapor port on the recovery tank can help open up the flow. Have you ever had a recovery machine that’s noisy (or ‘knocking’) during recovery? That noise is typically generated due to the machine pumping refrigerant faster than it can empty into the tank.
Service Tips: Ensure full flow on the output side by fully opening the valves on the recovery tank and connecting to the vapor port.
Use 3/8" hoses for recovery
Regardless of where it’s connected, the ‘standard’ 1/4” hose is also highly restrictive in nature. In fact, 1/4” hoses were originally intended only for charging and taking pressure readings because they are restrictive - the small volume means that less refrigerant is lost. However, in a recovery application they just slow everything down. During recovery, the primary concern is throughput and speed, which is why it’s recommended to use 3/8” hoses on every connection.
Service Tip:Since 3/8” hoses have a larger interior diameter, using as short as possible (4ft recommended) output hose - from the recovery machine to the tank - ensures you will meet the EPA’s de minimis requirements.
Evacuation is often one of the most time consuming processes that a technician will encounter on a job, and speeds are largely determined by the available flow to the vacuum pump. Without adequate flow, even the most powerful vacuum pump will struggle to evacuate a system.
Flow
Flow in a vacuum operates quite a bit different than it does in a pressurized environment. As such, the diameter of the hoses and other restrictions have a dramatic effect on the overall speed of an evacuation. Every restriction that’s present will limit the overall flow (throughput) of the setup, and as a result even a large 8CFM pump can be limited to 0.2CFM by something as simple as a valve core left in an access port.
Conductance
Conductance is the measure of how effectively fluids (air, moisture) are transported through a medium, and can be expressed in Cubic Feet per Minute (CFM). Throughput for an evacuation is controlled largely by the conductance of the connecting hoses, and to a lesser extent the fittings on the ends of the hoses. Throughput is also affected by the size of the piping within the system and several other factors, but since hoses are one of the few easily controlled variables they should be a high priority for a technician. It’s important to understand that (due to how flow in a vacuum works) as the hose diameter increases, the flow (conductance) increases exponentially. What this means is that a 1/2” hose has about 16x more flow than a 1/4” hose, despite only being 2x physically larger!
Flow Restriction Examples
Access Valve Cores
Core Depressors
Hose Diameter Comparison (1/4", 3/8", 1/2")
How to use a Valve Core Removal Tool
Keys to Full Flow in Evacuation
A technician should be concerned with maximizing flow to the vacuum pump if they are interested in pulling a fast vacuum. Not only can it ensure that the job is done sooner (and you can move onto the next job... or go home for the day), but it can also allow the technician more time to verify a complete evacuation and reduce the chance for a call back or warranty claim from the customer.
If a system has traditional access ports with valve cores, it’s critical to remove them for every evacuation otherwise the vacuum pump will be limited to 0.2CFM regardless of size. With valve cores removed, a core depressor has no purpose so be sure to remove any that are present as they block an additional 50% of the hose fitting.
Larger diameter hoses allow more flow to the vacuum pump, it’s recommended to connect as many 1/2“ hoses as possible for the fastest evacuation. In real world terms a standard 1/4” hose has only about 1 CFM of potential flow, a 3/8” hose has around 2 CFM, and a 1/2” hose has 3CFM of potential flow. While this may not sound like a major difference on paper, the difference in the field is dramatic and can make the difference of hours or even days on a job.
Service Tips: Utilize Valve Core Removal Tools to remove all valve cores from the system access ports. Connect as many 1/2” hoses as possible directly to the vacuum pump in order to maximize flow and speed up evacuations.
The equipment used for evacuation should be dedicated for that purpose, and as such should also be designed for the task. Equipment that is designed for pressure (ie. charging & recovery) is not necessarily designed for use in vacuum and may leak. Always check to make sure that the equipment being used is vacuum-rated and designed for evacuation; this includes valve core tools, hoses, and any other adapters used in the setup.
It’s common to use a traditional gauge set manifold during evacuation, however it is not necessary and will bottleneck the entire evacuation to 1CFM or less. The ideal setup in any evacuation is to connect hoses directly from the system to the vacuum pump. If a vacuum pump requires more connection ports (ie. for a ductless system with 3 ports) it may be necessary to use an adapter to add more ports to the vacuum pump; in the form of a vacuum ‘manifold’ or y-fitting. It’s always recommended to connect as many 1/2” hoses as the system (& vacuum pump) will support.
Clean oil is the other major influence on evacuation speed next to hose diameter. Once vacuum pump oil has become fully saturated, the evacuation will grind to a halt as a vacuum pump relies on the sealing properties of oil in order to pull to a deep vacuum. Learn more here.
Service Tips: Always use vacuum-rated equipment for evacuation to reduce the possibility of leaks & slow evacuation. Connect hoses directly to the vacuum pump for the fastest & most efficient setup. Monitor the condition of the vacuum pump oil throughout the evacuation, and change it as needed.
