At the bottom an excerpt from a really good article on cooling systems on why a surge tank, in addition to an overflow tank, is important:
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Note the importance of coolant level. For this reason I'm planning on adding 1/4" bungs on the front for an optional coolant level sight tube. Also note the importance of deaeration.
This tank from Radium has a nice sight tube and a separate deaeration chamber.
https://www.radiumauto.com/Coolant-Expansion-Tank-P261.aspx
I have found some 1/4", nickel-plated brass, 90-degree, push-to-lock connectors that are rated to 250 degrees. I am now looking for an appropriate tube. They apparently modeled air flow and cast an optimal shape so I'm not sure if an amateur version is worth doing or not. That said, custom fuel swirl tanks are made all of the time by merely making the inlet tangential to the top of the tank. This could be easily implemented by welding an aluminum tube inside the expansion tank.
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As can be seen, the surge tank is an important and very useful component of the system. Following are a few more details on the use of a surge tank:
One of the prime reasons for mounting a pressurized surge tank in the cooling system is the flexibility it gives in the mounting location of the rad cap. Because it is the pressure relief valve of the system, the rad cap:
1.Must always be located at the highest point of the cooling system - otherwise it will be impossible to get a complete fill of coolant and air will be trapped in the system. Also, when the rad cap is the highest point in the system, steam and air will naturally migrate to the area just below the cap. In the event the cap vents due to excessive pressure, the steam and air will be purged first.
2.Should be on the low pressure side of the system - otherwise the high pressure created by the water pump running at high RPM can tend to unseat the cap and blow coolant out, leading to overheating.
3.Should be located in an area of low coolant velocity so that the any steam or air can separate from the coolant, even at high RPM.
The surge tank provides the ideal environment for satisfying all three of these requirements, and provides a low velocity, low-pressure environment for de-aeration of the coolant.
When plumbing a surge tank:
• The bottom of the tank is connected to the inlet side of the water pump with a 1/2" or 3/4" line. In the systems shown above the heater return port is used for this purpose.
•A 1/4" to 3/8" vent or "bleed" line from the side of the surge tank is connected to the engine's steam ports (if it has them) or from the highest point of the low pressure side of the radiator. That is - if connected to the radiator, the bleed line to the surge tank must originate at the top of the radiator tank that doesn't have the normal radiator inlet from the water pump. This is because coolant velocity and pressure are high at that location which would force high velocity, high pressure coolant through the line to the surge tank, defeating the purpose of the surge tank's low pressure, low velocity environment for deaeration. The bleed line allows continual circulation of some coolant through the surge tank.
•The surge tank must be large enough to allow the air to separate as the coolant flows through it. Air in the system will then migrate to the area just below the radiator cap, again so that it will be forced out first if system pressure exceeds the radiator cap's rating.
•The surge tank should be filled to a level just below the inlet ports, as shown in the system diagram above.
The continual de-aeration that a surge tank provides can be a huge benefit to your overall cooling system. We already discussed all the bad things that happen when steam or air are trapped in the cooling system. In addition to those, consider that 2% air in the system results in 8% less heat transfer, but 4% air results in a whopping 38% less!! The continual de-aeration of the coolant may be enough advantage to allow you to run a smaller, easier-to-fit radiator with a surge tank than the size you would have to run with only an overflow tank, for instance.
If your engine doesn't have steam ports, and the rad is lower than the top of the engine, the bleed line to the surge tank must come from the highest point on the engine because this is where steam and air will naturally gravitate and get trapped. A fitting on the water pump in the same passage as the outlet to the radiator can be a reasonable compromise.
If you add a surge tank to a system that already has a rad cap on the radiator, you need to permanently seal the radiator rad cap location, or at least install on the rad a cap with a rating significantly higher than the surge tank cap will have, so that the radiator mounted cap will not open before the surge tank cap.