Pro/Forums - View Single Post - My micro-channel concept block

myv65 · 10-11-2002, 09:52 AM

OK, you are on the right basic track, but ya haven't quite arrived.

If you have a pump with an open discharge, you'll get maximum flow at what is basically zero discharge pressure.

If you add a 1" line of some length (even if the pump discharge is only 1/2") you'll add some finite resistance. Flow will drop slightly and you'll have some small, measurable discharge pressure.

If you add a 1/4" fitting, your flow will drop dramatically. Your pressure will go up dramatically. You will have gone from the "far right" end of the P/Q curve to the left region.

If you add another 1" line to the end, your resistance will go up slightly again. There won't be much change at all in flow or pressure, but some very small change will occur. Your flow will be ever so slightly lower and your pressure ever so slightly higher.

If you add another 1/4" restriction to the second line, your resistance will go up again and your flow will drop again. The changes will not be anywhere near as dramatic as that first major restriction, but they will be real. And here's the point that your earlier post missed: With both 1/4" restrictions in place, each will present precisely the same resistance and each will yield precisely the same pressure drop. Each individual pressure drop will be lower than that caused when only a single 1/4" restriction existed, but the total drop due to two tubes and two restrictions will be higher.

This is really a case where a graph might help. I suppose you can think of it this way: Pressure drop across an orifice is a function of velocity. When you have a single orifice, it will have some flow vs pressure curve. If you put two in series, that same flow vs pressure curve applies. However, since there are two, the total pressure required to maintain a fixed flow would go up a lot. Since a pump actually reduces flow as pressure goes up, we know this can't happen. What does happen is that the flow goes down (total overall pressure increases) while the pressure drop across each individual resistance goes down (flow rate drops).

10-11-2002, 09:52 AM	#80
myv65 Cooling Savant Join Date: May 2002 Location: home Posts: 365	OK, you are on the right basic track, but ya haven't quite arrived. If you have a pump with an open discharge, you'll get maximum flow at what is basically zero discharge pressure. If you add a 1" line of some length (even if the pump discharge is only 1/2") you'll add some finite resistance. Flow will drop slightly and you'll have some small, measurable discharge pressure. If you add a 1/4" fitting, your flow will drop dramatically. Your pressure will go up dramatically. You will have gone from the "far right" end of the P/Q curve to the left region. If you add another 1" line to the end, your resistance will go up slightly again. There won't be much change at all in flow or pressure, but some very small change will occur. Your flow will be ever so slightly lower and your pressure ever so slightly higher. If you add another 1/4" restriction to the second line, your resistance will go up again and your flow will drop again. The changes will not be anywhere near as dramatic as that first major restriction, but they will be real. And here's the point that your earlier post missed: With both 1/4" restrictions in place, each will present precisely the same resistance and each will yield precisely the same pressure drop. Each individual pressure drop will be lower than that caused when only a single 1/4" restriction existed, but the total drop due to two tubes and two restrictions will be higher. This is really a case where a graph might help. I suppose you can think of it this way: Pressure drop across an orifice is a function of velocity. When you have a single orifice, it will have some flow vs pressure curve. If you put two in series, that same flow vs pressure curve applies. However, since there are two, the total pressure required to maintain a fixed flow would go up a lot. Since a pump actually reduces flow as pressure goes up, we know this can't happen. What does happen is that the flow goes down (total overall pressure increases) while the pressure drop across each individual resistance goes down (flow rate drops).