Hi guys I feel that my Giotto could do with a little more poke in the steam wand when I make a single portion of milk it is fine but needs more umph to steam two coffees worth........has anyone successfully changed the tip or will that just make me run out of puff too early?
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It has two so you got more steam by closing off one hole? I was expecting to add holes?Originally posted by Gavisconi007 View PostHow many holes does the tip have? When I had a Giotto I blocked one hole up with a toothpick trimmed down to be flush with the end of the tip. Worked a treat.
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Erm, no.Originally posted by Gavisconi007 View PostBlock one off and the pressure out of the remaining hole will increase. Should solve your problem.
Block one hole and your *flowrate* will roughly halve (as the flow area has halved, but the pressure remains much the same). It will also possibly be slightly wetter, as the steam will spend longer in the wand.
Steaming will definitely take longer.
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I think the point is probably that 'steam pressure' is probably not quite the issue. It's a matter of how much steam is blasted into the milk in a given time (and the moisture content of that steam). So while the pressure within the steam wand may be doubled when you halve the aperture, the amount of steam exiting the wand isn't double. I may be wrong.
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The force in this equation is due to collisions between molecules and a surface. Halve the area and you also halve the number of collisions (and thus the halve the force) - no change in pressure. Not an equation that is very useful when we are dealing with a flowing fluid.Originally posted by Gavisconi007 View PostI thought Pressure=Force divided by Area
Force should be fairly constant due to boiler back pressure. The steam exits a smaller area, hence wouldn't the pressure at the tip exit point be greater given that we are dividing by a smaller (area) number?
Pretty much. The flow through a hole is proportional to its area. When you open a tap, more water come out...Originally posted by Barry O'Speedwagon View PostI think the point is probably that 'steam pressure' is probably not quite the issue. It's a matter of how much steam is blasted into the milk in a given time (and the moisture content of that steam). So while the pressure within the steam wand may be doubled when you halve the aperture, the amount of steam exiting the wand isn't double. I may be wrong.
How does this affect the pressure?
The steam pressure in the boiler depends on how densely packed together the water molecules are (and their temperature). More molecules = more frequent collisions = greater force = higher pressure. Higher temp = faster molecules = more frequent collisions with more force = higher pressure.
The pressure *in* the tip is only slightly lower than that in the boiler (due to energy lost from friction between the flowing steam and the piping). Halving the hole area will reduce the flowrate of steam, and thus slightly reduce the amount of energy that is lost = slightly less pressure drop = closer to boiler pressure (but always lower).
The pressure outside the tip is essentially always the same as the surrounding air. What people think of as higher pressure is often just greater flow or higher velocity flow - which is all we care about with respect to steam (and temperature of course).
Clear as mud?
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Mr Jack
Yes, clear as mud.
In the second last paragraph of your thesis you say that halving the area will reduce the flow rate of steam. In your last paragraph you say that what people think of as higher pressure is often just greater flow. How can there be greater flow if the flow rate has reduced?
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