Intro to Pressure Drop

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An Introduction on Liquid Pressure Drop and How to Select a Pump

Advancements in technology continue to drive the complexity and functionality of new products. As these products increase in complexity, OEMs are adding more components to meet increasing functionality demands of their customers, which comes at additional cost in materials, manufacturing, logistical complexity, and assembly.

Aavid, Thermal Division of Boyd Corporation designs thermal systems for maximum performance at a specific flow rate. Less flow will cause the system to underperform. Flow rate is dependent upon the system's pressure drop and the pump's head pressure. This application note reviews how to determine your pressure drop and how to select a pump for your system. It also provides tips on how to minimize pressure drop.

Determining System Pressure Drop

Pressure drop is a term used to describe the differential pressure that a fluid must overcome to flow through a system. Pressure drop is a result of resistance caused by friction (shear stresses) or other forces (such as gravity) acting on a fluid. The pressure drop is exponentially proportional to the flow rate. When the flow rate doubles, the pressure drop increases by a factor of four. The pressure drop of a system is equal to the sum of each component's pressure drop within the system which includes hoses, cooling component(s), and any other sections of the system. In order to determine the system pressure drop curve, pressure drop at various flow rates needs to be calculated and plotted.

For example; if a system has a CP10 tubed cold plate attached to a 6105 copper heat exchanger with 10 feet of 3/8" tubing, add the CP10, 6105 and hosing liquid pressure drop curves together. 1-2 psi is a good assumption for standard pressure drop of 10 feet of tubing at 1-2 GPM. When the results are plotted, the graph should look similar to Figure 1.

Selecting a System Pump

In general, the flow rate provided by a pump is inversely proportional to pressure, which means that the flow rate will increase as pressure decreases (see Figure 2). In order to select a pump with appropriate head pressure, the pump curve provided by the pump manufacturer should be plotted over the system pressure drop curve. The system will operate at the intersection of the two curves. In our example, the pump will operate at 1.6 GPM and 13.5 psi (see Figure 3) because the two plotted lines intersect at this point.

If the pressure drop of the system is known for one point, the curve can be estimated by drawing a straight line from no flow and no pressure drop to the known pressure drop point. The line's intersection with the pump curve provides a good estimate of the expected flow rate. In our example, assume a system pressure drop number of 2 GPM and 18 psi is known (see Figure 4). Using this method, the estimated system flow rate is 1.5 GPM, close to the 1.6 GPM determined using the more precise method.

Minimizing Pressure Drop

In most cases minimal pressure drop through a system is desirable. Some tips on how to reduce pressure drop are:

When feasible, keep the number of 90° bends to a minimum. Like a kink in a garden hose, a sharp bend causes pressure drop.

Keep hose lengths as short as possible. Longer hose or tube lengths create greater surface area that is in contact with the fluid and causes additional fluid friction and pressure drop.

Work with large diameter hoses. Ever try to drink through a narrow coffee stirrer? The small diameter makes you work much harder than you would with a regular straw.

Where possible, use a fluid that has low viscosity. Fluid viscosity is a liquid's ability to flow (think about water as opposed to molasses). Using a liquid that has high viscosity will adversely affect the pressure drop of your system.

Quick disconnect fittings should be avoided, as they often cause unnecessary loss of pressure.

Remember the importance of pressure drop and match the pump curve to your system pressure drop curve. The pressure drop can be minimized by removing the kinks; avoiding long and thin hoses; and keeping the system on the same level. Follow these simple steps and your thermal solution will deliver the promised performance.