Pumps come in many kinds: diaphragm, plunger, gear, centrifugal and others. The ones used for our purposes can be divided into 2 types: those through which liquid cannot pass under any conditions when switched off, and those through which liquid can flow by gravity.
The second type includes various centrifugal pumps with an impeller.
How to tell? Blow into the inlet pipe; if air passes freely, it is this type.
About using this type of pump.
Which factors have a significant influence in use.
- The first factor is capacity. If the manufacturer states that the pump delivers 600 l/h and lifts water to a height of 5 meters, this does not mean that the pump will move 600 liters to a height of 5 meters in an hour. It is either 600 liters at the water intake level, or 5 meters of height and however much it manages to push out (if it pushes anything at all).
- The second factor is PWM. It determines not only the volume the pump moves, but also the height to which a column of water can be lifted.
- The third factor is a parasitic one. That is, the uncontrolled "gravity flow" that may occur in the system.
Let us look at organizing cooling with this type of pump, using a specific setup that our colleague uses and that has been working for more than a year under Samovar control.
The temperature delta is 1-2 degrees at a holding temperature of 60°C at the outlet.
A container (3-5 liters, as long as the fittings and the pump fit) with a float valve and a submersible aquarium pump is raised to the height of the dephlegmator.
It is desirable that the water level in the container, held by the float, be at the level of the lower pipe of the dephlegmator. A deviation of 10-15 cm up or down is not critical.
 AirAir enters through this tube to eliminate uncontrolled "gravity flow". In essence it is a TCA (atmospheric vent tube) for water
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 TeeProvides the "jet break"
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 Float valveKeeps a constant water level in the container.
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 Pump
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When using a shell-and-tube dephlegmator.
The pump feeds the lower inlet of the after-cooler, and from the after-cooler outlet the water goes to the lower inlet of the dephlegmator. The hot water exits from the upper outlet of the dephlegmator.
At the dephlegmator outlet, install a temperature sensor and make a "jet break".
To do this, install a tee on the discharge tube and raise the tube from it upward. The main thing is that the tee is above the liquid level of the container with the pump.
What this arrangement achieves.
The pump does not need to "know" how high the water goes; it starts providing exactly a water flow, rather than churning the water inside itself with the impeller while trying to build up pressure to lift the water up.
PWM really controls the water flow.
The "jet break" completely eliminates uncontrolled gravity flow.
Note:
"Jet break" is very important. In testing, without a jet break and with the pump switched off (that is, cooling only by gravity flow through the pump), at 1800 watts of applied power the outlet water temperature did not rise above 40°.
When a Dimroth condenser is used as the dephlegmator.
The pump feeds the lower inlet of the after-cooler, and from the after-cooler outlet the water goes to the upper inlet of the dephlegmator. The hot water exits from the lower outlet of the dephlegmator.
That is, the water flows against the steam.
At the lower outlet of the dephlegmator, install a temperature sensor, and preferably route the hot water discharge tube first up to the upper inlet of the dephlegmator and then down to the drain. After the discharge tube goes down, make a "jet break" as well. To do this, install a tee on the discharge tube and raise the "TCA for water" tube from it vertically upward. Make sure that the tee is not below the liquid level of the container with the pump.
The problems that occur with centrifugal pumps hardly concern owners of diaphragm pumps.
The only problem that occurred was very cold water and a capricious motor. At a set power of 15% the pump stopped, and at 20% the outlet temperature was too low.
By the way, this can happen if you use a pump rated for a higher capacity.
The way out is to make a return discharge right at the pump outlet. A tee is installed on the pump outlet pipe and a fixed discharge through a jet (a calibrated hole) is made into the same container. This way you can raise the minimum power value, while in fact less cooling liquid will go to cooling.
The controlled discharge principle can be applied to any pumps.
