As we know just when we turn on the machine, pump will be activated and the pump will fill the boiler and the hx tube. The pump will stop filling the boiler just when the water touch the water sensor level in boiler. What about in hx tube? How the machine stop filling water in hx tube?

On the HX machines with which I'm familiar, the pump does not fill the HX tube at startup, only the boiler. The HX tube on most HX machines needs to be purged (flushed) before pulling a shot'; the flow from the pump is diverted to the brew path (HX tube) by a valve. In an e61, this happens when the lever is fully raised, engaging the pump switch. In some other machines, this happens when a solenoid button is pressed.

This diagram and video may help from Newbie Introduction to Espresso - Heat Exchanger vs. Double Boiler Espresso Machines:



You could think of a heat exchanger as a small, special-purpose brew boiler, i.e., it's always "full" and the volume of water that enters equals the volume that exits.

If the hx is always full, then there is no water circulation from hx tube to group head then back to hx tube (thermosyphon). Am I right?

The cooler water sinks, driving the thermosiphon.

Air in the tubing can cause "thermosiphon stall" when the force of the heavier water in not enough to push more through.

nichikumaat wrote:If the hx is always full, then there is no water circulation from hx tube to group head then back to hx tube (thermosyphon). Am I right?

Not really. At idle, the water cools in the group causing it to gently return to the bottom of the HX. This displaces hotter water in the HX to return to the boiler via the top tube. This is often i referred to as a thermo-syphon, although I prefer the term thermal circulation, as there is no siphoning involved. The water gently circulates from the HX to the group and back.

Over time, the group gets hotter until it reaches equilibrium, which for reasons I think are not important for this discussion, is higher than the ideal brew temperature. This is why most HX machines need to be flushed before brewing.

Newer designs, such as the MaraX have 'tuned' the HX loop to minimize the need for flushing. Generally, any HX can be so tuned by inserting a jet in the loop. There are rather complicated discussions on this which you can find here on H-B.

Incidentally, when one brews with an HX, the water is drawn via both the top and the bottom tube and the circulation effect stops.

Sometimes, the circulation does not happen, which is called a stall. Generally, this can be overcome by doing a brief flush.

Thermosiphon (or thermosyphon) is a method of passive heat exchange, based on natural convection, which circulates a fluid without the necessity of a mechanical pump. ... Its purpose is to simplify the transfer of liquid or gas while avoiding the cost and complexity of a conventional pump.

From Wikipedia.

Water in the heat exchanger tube is heated by conduction from the hot water and steam in the boiler. Heated water becomes less dense and so it is just a bit lighter and it rises (heat rises- think hot air balloon). The hot water rises and goes to the top of the heat exchanger and into the upper thermosyphon pipe. It gets to the brewhead and there it transfers its heat energy to that mass of meta which heats the group. The water, losing its heat energy, cools and becomes a bit heavier and "sinks." This cooler water moves downward into the lower thermosyphon pipe and moves back to the heat exchanger. As this convection current continues the water moves in this circular path, picking up more heat energy each time it passes through the heat exchanger and losing it when it reaches the brewgroup. This flow is indicated by the white arrows.

Copied from this site: http://www.espressomyespresso.com/stall.html

Nunas wrote:Over time, the group gets hotter until it reaches equilibrium, which for reasons I think are not important for this discussion, is higher than the ideal brew temperature. This is why most HX machines need to be flushed before brewing.

Just to make things more confusing, the heat exchanger runs above brew temperature, but the grouphead idles below brew temperature; the so-called "HX cooling flush" is actually a warming flush. It'd be more accurate to call it a brew water temperature equalization flush.

Good points! Equilibrium was a bad choice of words. Perhaps stabilization would be more like it. My point is, eventually, the group reaches a more-or-less stable temperature, which varies between machines, dependent upon the design of the HX loop. As for the flush, again, point taken. The main issue isn't really the group temperature at stabilization. It's that the water in the part of the HX loop inside the boiler reaches superheat (above boiling). So, regardless of the group's temperature, when the brew lever/switch is thrown, regardless of the group's temperature, this water comes out as steam until cooler water from the reservoir comes in and the brewing water (for a short while) comes out at the desired temperature. And that, in my opinion, is the beauty and art of the HX machine. Once one gets to know a particular machine, one knows how long to flush, how long recovery takes and so forth. It can be done with a group head thermometer, or it can be a zen thing .