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As a general rule, flow velocity should be high where heat enters the fluid and low everywhere else. Pressure drop and convection each depend largely on velocity, so keep the velocity low in everything except the block(s). I personally prefer to have tubing velocity less than four feet per second and block velocity over ten feet per second. The trick is that you need to know flow rate to calculate velocity. The only way to positively determine flow is to measure flow.

For low-cost, quiet cooling: Look for a pump rated under 150 gph with 3/8” fittings. True system flow should be approximately 25-50 gph. Tubing diameter should be 3/8”, with the flow path through the block equal to ~ 1/4” diameter. Verify block fittings and pathways accordingly.

For maximum performance: Look for a pump rated in excess of 300 gph with 1/2” fittings. Match the remaining components accordingly so that true system flow is at least 90 gph. The flow path through the water block should be roughly .06 inches^2 (equivalent to about 0.3” diameter or about 1/4” by 1/4”) to maximize convection.

Whether going for extreme quiet or extreme cooling, stick with AC-powered pumps. Their reliability-to-cost ratio is better than DC alternatives. AC pumps are also more widely available than DC pumps.

Stay tuned for the next article in our series when we discuss fluid properties. Please send comments, suggestions, and questions to

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