High velocity in heat exchanger tubes 2

[tim02]

In normal the operation of our cross flow heat exchanger the rate of heat transfer goes up proportionally to the cold side flow and we control exit temperature that way with a PID loop program operating the cold side throttle valve. But we seem to be able to reach a condition when the flow rate reaches a certain point the rate of heat transfer starts to drop, so the throttles open more and the rate goes up more so the rate of transfer drops even more and continues into an out of control situation.
I seem to remember something from an Advanced Fluid Flow course (about a zillion years ago) where high velocity flow drastically reduces efficiency but I don't remember any details or even what the phenomenon is called.
Can someone explain to me the mechanism by which cooling water flow above a certain velocity in the cold side tubes will drastically reduce the heat transfer ability? And what solutions either in design or controls do you suggest?

 

[hacksaw]

Is the fluidizing air stable during this exercise? Some fluidized beds will be stable and others will pulsate or surge in a cyclical manner.

Does the cw supply pressure hold up during the upsets? We've had HEX's (coolers) lose control when the cw header pressure dropped as a result of other demands in the plant.
Having several coolers in parallel can lead to similar demands. We had to install restriction plates to limit excessive user demands. Mecanical stops on the valve also worked (max flow limit).

good luck,

 

[25362]

After correcting my assumptions of the tubes diameter, all Reynolds numbers appear to be beyond the critical regime.

It still seems that one bundle is stealing water from the other when CV's open. Can you find a way to check that ?

 

[hacksaw]

By the way, I agree with 25362, that cw maldistribution is taking place, whether a hydraulic imbalance or an triggered by changes on the process side.

It is a tough problem

 

[vpl]

Tim (and others) Here's my two cents:

Based on the information you've given me, I've calculated Reynolds numbers which are in the 3,500 - 38,000 range, dependent on initial temperatures and flow rates. The first number is for 50 degree water, 200 gpm flow, the latter for 90 degree water and 1200 gpm flow. So I think you may be getting some transition range instability. Have you noticed this happening more on days when the inlet water temperature is hotter? At 90 degrees and 300 gpm, I calculated a Reynolds number of 9,500, which is right at the transition point. (Over 10,000 is definitely turbulent flow). If you don't have a way to control the inlet temperature, is it possible to limit the flow to under 250 gpm? That keeps further away from the instability regime.

Basically: Re= [rho]*d[sub]i[/sub]*v/[mu] where [rho] stands for density and [mu] stands for viscosity, both of which are functions of temperature. Density varies from about 62.4 to 62.1 lb[sub]m[/sub]/ft[sup]3[/sup] while the viscosity varies from about 8.8E-4 to 5.1E-4 lb[sub]m[/sub]/ft.sec.

For the velocity, because I used a spread sheet, I first converted gpm to lb[sub]m[/sub]/hr by multiplying by the density, dividing by the conversion factor for the weight of water (7.48 lb/per gal) and multiplying by 60 minutes/hour. I then used v = m[sup]dot[/sup] divided by the density, divided by 3600 (hours back to seconds), divided by the area of the tubes. Of course, you could save a step by taking m[sup]dot[/sup] in gpm and just dividing it by 60 and the area of the tubes.

The area of the tubes is pi*d[sub]i[/sub]/4 times the number of tubes (286) divided by the number of passes (2).

Patricia Lougheed

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[tim02]

Patricia,
We are actually in the process of changing the supply system over to a newer cooling tower system that has bypass mixing capability so we can control inlet temperature where we want it. We actually use only about 600gpm total at 90F inlet temp. Usually about 2/3 in the first stage (hot end) and 1/3 in the second stage (cooler end, process control).
Also, this effect has happened at different times of the year, with water at various inlet temperatures. I think it starts with a surge of sand or at least hotter sand into the first stage that suddenly makes that stage struggle to drive the cooler sand midpoint control temperature back to its control setting. The control valve, which is a butterfly-type, starts to open more. (I know, I wanted real throttle valves with more linear characteristics but got told these have always worked in the past so why change now). It gets to the point (about 1/2 open) where further opening of the disk has very little effect on flow. At this point the second stage inlet sand temp starts to go up and that stage's throttle goes through the same thing. These sudden high flow excursions probably have as much to do with the poor throttling capability of the valve in the upper half of it's open position as much as anything else. But even now, nobody will even consider changing them to a different style. In our plant, maintenance and production don't believe any plant engineers without flying in "experts" from the OEM or consulting firms from at least 100 miles away to corroborate.