Real World Numbers

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Absolute vs. Gage Pressure

In a moment, I’ll discuss some real world flow numbers. Before that, there’s one more concept related to pressure that needs explanation. This concept is gage pressure versus absolute pressure. Gage pressure is simply the pressure measured in one region using a different region as a zero-reference. Most commonly, pressure gets measured against atmospheric pressure. Absolute pressure is a pressure referenced to a pure vacuum where there is truly no pressure.

When we are concerned with things like cavitation, we need to know absolute pressures. The boiling point of water depends upon the absolute pressure. Water boils at 100°C at one atmosphere of pressure. Water boils at much lower temperatures as the pressure gets reduced. Given the right (or rather WRONG!) conditions, pressures within a pump can reach the point where the water boils at room temperature. This was explained earlier as cavitation.

When we are concerned with leakage, we need to know gage pressures. The difference in pressure from the fluid inside our tubing to the air in our room is what matters. Here’s where an interesting point comes into the discussion. Since the pressure entering the pump in a closed-loop system is often less than atmospheric pressure, we are as likely to draw air into our system as we are to leak fluid out of it. Herein lies another common cause of cavitation in closed-loop systems.

There’s just one more point to make regarding closed-loop systems. Closed-loop systems are inherently more prone to leakage. The reason why is that fluids expand and contract as they are heated and cooled. Since our system is sealed, expansion of the fluid causes the tubing to stretch. This stretching raises the pressure everywhere within the system. The relative changes from one point to another remain precisely as graphed previously, but the entire curve shifts upward in absolute pressure. This added pressure may cause fittings to leak once things get warm.

Real World Flow Numbers

So let’s take a look at some real world flow numbers provided by fellow AMDMB member SCompRacer (Rich). Rich has been kind enough to perform some extensive flow rate testing using a single pump with various combinations for line sizes, radiators, and water blocks. This pump has a nominal flow rating of about 320 gph. As Rich’s numbers testify, the true flow through a system is much lower than this.

These tests cover the effects of line size, fitting size, and a water block. The tubing size is 1/2” or 3/8”. The radiator is a 6” X 6” X 2” with 1/2” fittings or 11” X 5” X 2” with 5/16” fittings. The water block is an unnamed commercial block with 1/2” fittings. Here are the various test conditions and flow.

Test Number Tubing ID Radiator Water Block Flow (gph)
1 1/2” 6” X 6” X 2”Yes94
2 1/2” 6” X 6” X 2”No120
3 1/2” 11” X 5” X 2”No66
4 1/2” 11” X 5” X 2”Yes60
5 3/8” 6” X 6” X 2”No81.8
6 3/8” 6” X 6” X 2”Yes63.6
7 3/8” 11” X 5” X 2”Yes54.5

In particular, let’s make note of a few things. In the “complete system” using pump, tubing, water block, and radiator we can see the effect of switching from 1/2” to 3/8” tubing. The flow drops from 94 gph to 63.6 gph when using the 6” X 6” X 2” radiator. In this instance, the tubing is a primary source of flow resistance in the system. Contrast this to the alternate radiator that uses 5/16” fittings. In this case, the flow only drops from 60 gph to 54.5 gph. In this second example, the 5/16” fittings provide the primary flow resistance. Careful selection of all components is necessary to avoid unintentionally limiting your flow.

This same effect shows in the test with and without the water block using 1/2” tubing. With the 6” X 6” X 2” radiator, the flow changes from 120 gph to 94 gph. This substantial change indicates that the tubing is no longer the main restriction. With larger tubing, the water block has become the largest contributor to flow resistance. With the alternate radiator, however, flow only drops from 66 gph to 60 gph. Once more the effect of the fittings stands out in the total flow numbers.

Whether or not a particular item significantly impacts overall flow depends on all the other items present.

If we look at the flow area of 1/2” tubing, 3/8” tubing, and 5/16” fittings we’ll quickly see what’s happening. Here’s a chart showing area and flow velocity for 60, 90, and 120 gph.

Tubing/Fitting Area (in^2) V at 60 gph (fps) V at 90 gph (fps) V at 120 gph (fps)
5/16″ 0.077
3/8″ 0.110 2.904.365.81
1/2″ 0.196 1.632.453.27

In general, the smaller the smallest flow path, the lower the flow rate will be. In addition, the smaller the smallest flow path, the faster the velocity will be through that restriction.

In general, the faster the fluid must flow in your system, the lower the pump efficiency will be. This is why one of the most important points is properly sizing your components to match your selected pump or vice versa. Regions of high water velocity should be limited to the water block. Everything else should have a lower velocity.

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