Water Velocities and Clean Heat Exchangers

Maintaining clean surfaces in heat exchanger tubes not only depends on proper water treatment but also on water velocity. (Here comes one of those “90% mechanical, 10% chemical” discussions!) Back in school the professors in heat transfer classes talked about the importance of turbulent flow (versus laminar flow) in the tubes for optimum heat transfer and keeping the tubes clean. This comes from proper water velocity. (Does this bring back memories of calculating Reynolds numbers?) So, how do we calculate the water velocity through a heat exchanger’s tubes?

This starts with a survey of the exchanger in question. We need to know the following:

F = Cooling water flow rate (gpm)

P = Number of passes in the waterside of the heat exchanger

D = Diameter of the tubes (inches)

N = Number of tubes

To get:

V = Velocity (ft/sec), the calculation is:

Example: A heat exchange with a flow rate (F) of 100 gpm through a heat exchanger with 2 passes (P) that has 100 tubes (N) with diameters (D) of 1 inch will have a velocity of 0.82 ft/sec:

Is that good? What velocity are we looking for? The specs on your heat exchanger should tell you that. In general, velocities over 2 ft/sec are desirable, but we are looking for that turbulent flow. How do I know we have turbulent flow? Now we are back to the reference to the Reynolds number.

To calculate the Reynolds number, we need to know the following:

V = Velocity of water (ft/s)

D = Diameter of the tubes (ft)

ρ = Density of water (use 62.4 lb/ft3)

µ = Dynamic viscosity of water (use 8.79 x 10–4 lb/(ft*s))

ν = Kinematic viscosity of water (use 1.41 x 10–5 ft2/s)

To get Re = Reynolds number, the calculation is:

Using the relationship ν = µ / ρ, we can simplify:

The lower the number, the more you have laminar flow. Since we are looking for turbulent flow, we want to have a higher number. Using the information from the velocity example above, the Re would be:

Is this good?

According to the book Principles of Unit Operations:

<2,100 — Laminar flow

>3,500 — Turbulent flow

In the example above we likely have turbulent flow, which is optimal for a heat exchanger.

What if we found out that maintenance changed out a water pump, and now we have a reduced flow rate (F) of 50 gpm through the same tubes? Velocity is slowed to 0.41 ft/sec, and the Re becomes 2,425. In this situation, the flow is approaching laminar, which would not be optimal for keeping the tubes clean, and we may start to see signs of fouling.

Now I have a little challenge for you:

A steel mill’s continuous caster’s spray cooling water system is cooled using three heat exchangers. The design is to have two heat exchangers in service and one as the backup. The foulants in this system wreak havoc on the exchangers, so the plant operations decided to put all three heat exchangers in service full time. How would this affect the performance of the heat exchangers and why? For extra points, how would you convince the operations people they should make a change? (Hint: It does not involve your calculations, which they may not believe, anyway!)