Turndown ratio, defined

When conparing tray designs the turndown ratio is inqjortant because it is a measure of the flexibihty of a column in dealing with a change in flow rate. The turndown ratio is defined as the ratio of the maximum to minimum operating flow rate. For bubble cap and valve trays, the turndown ratio is about ten whereas for sieve trays it is only about three. 

This effect is not unique to fired heaters almost all process gains on aplant will vary with feed rate. Given that we tolerate 20 % variation in process gain, we can therefore tolerate 20 % variation in feed rate. Assuming a reference feed rate of 100, oiu controller will work reasonably well for feed rates between 80 and 120. The turndown ratio of a process is defined as the maximum feed rate divided by the minimum. We can see that if this value exceeds 1.5 (120/80) then the performance of almost all the controllers on the process will degrade noticeably as the minimum or maximum feed rate is approached. Fortunately most processes have turndown ratios less than 1.5, so providing the controllers are tuned for the average feed rate their performance should be acceptable. The technique used, if this is not the case, is covered in Chapter 6. 

Determining minimum required flow is even more difficult. Fortunately most wet chemical processes seldom have a turndown (maximum required flow/minimum required flow) greater than two or three to one for normal operation. Startups and shutdowns, however, often require very low flows for awhile. A suggested typical process turndown figure for ve sizing and selection is therefore five to one. Valve-flow turndown is defined here as the ratio of flow not Cp) at 95 percent lift to that at 10 percent lift. If, for a given application, calculated valve-flow turndown is less than process turndown, two or more split-ranged valves may be required. 

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