Turndown Devices

Turndown devices are most often control valves (not shutoff valves) or dampers. The best valve turndown characteristic is usually accomplished with adjustable port valves or with characterized globe-type valves. Butterfly valves usually have very poor characteristics (not straight-line), but their characteristic curves can sometimes be improved by undersizing or selecting reduced port models. 

Speed controls on blowers (VFDs variable frequency drives) are becoming more acceptably priced so that they can now accomplish a net saving over the old energy-wasteful method of controlling input by throttling flows with valves. 

Example 6.3 If a 30-hp blower is operated at an average of 70% of its rated volume for 50 weeks per year, how much energy could be saved by using VFD  

From the fan laws, p. 200 of reference 51, flow is proportional to rpm, but power required is proportional to rpm, so when hpi = 30 hp rating, 

A blower with VFD can take care of modulating the air flow, but the flow of fuel must still be reduced by a throttling valve in the fuel line, sometimes by a regulator, which is a form of globe-type control valve. This leads to a brief review of air/fiiel ratio control systems. 

---

Furnace pressure or draft is normally controlled by a damper in the stack, thus choking off the outfiow of gases and pressurizing the furnace. (See sec. 6.6.3.) If negative furnace pressure is needed, use a speed control on an induced draft fan, a pressure (volume) control on an eductor jet, or a barometric damper. (See sec. 6.7.1 on Turndown Devices.)... 

A compressor is typically a specially designed device, and comes with far less surplus capacity than other process components. As a result compressors merit great care in specification of flow, inlet pressure, and discharge pressure. Similarly, the control system and equipment need to be carefully matched to provide turndown with maximum efficiency. 

Atomizers The common need to disperse a liquid into a gas has spawned a large variety of mechanical devices. The different designs emphasize different advantages such as freedom from plugging, pattern of spray, small droplet size, uniformity of spray, high turndown ratio, and low power consumption. 

After any maintenance work involving replacement or adjustment of components it is essential that the plant be re-commissioned in a safe way, including dry mns with the gas turned off to ensure that flame failure devices operate correctly and pilot turndown tests to ensure that if the pilot can energize the flame detector then it can also smoothly ignite the main burner. 

In general, the power industry desires the following of a scrubbing device in a flue gas-cleaning system operational reliability, a minimum of four-to-one turndown ratio, low pressure drop characteristics, high absorption efficiency potential, good alkali utilization, acceptable mist elimination, and future process adaptability. 

Often, a question is asked about the design turndown ratio of a device. It is practical to express this as a ratio of the design loading to the loading at the point of efficiency drop-off, whether represented by physical weeping or dumping. Fignre 12.35 represents this lower limit. 

Reference to Fig. 5.7-5 shows thei at low vapor rales there is a decline in efficiency, and this must be taken into account in determining the turndown imio" (ratio of maximum allowable rale to minimum allowable rale) of the tray device. It has been found that a reasonable prediction of the minimum rale can be mede with the use or Fig. 5.7-13. This chart was developed earlier by Fair13 on the basis of visual tests and was later confirmed by Zanelli and Del Bianco. 4 In practice, it predicts the poini of minimum turndown 1 as denoted by Fig. 5.7-5. The pressure loss in fotraing a gas bubble mey be estimated from the dimensional relationship... 

The main consideration for introducing reflux or intermediate feed into a packed tower is adequately distributing the incoming stream to the packing. Unlike most tray columns, packed towers are sensitive to distribution. Maldistribution is detrimental to packing efficiency and turndown. The main devices that set the quality of distribution in a packed column are the top (or reflux) distributor, the intermediate feed distributor, the redistributor, and sometimes the vapor distributor. Adequate hydraulics in the inlet area is also important failure to achieve this can affect distributor performance and can also cause premature flooding. 

Cannot be used for up to 25 % turnup capacit) avoid the use of inertial devices for up to 25 % turndown capacity. 

Another form of redundancy is the excess capacity often built into plants with more than one processing line. If two sets of drying towers are used, for example, the capacity of each may be more than 50% of the plant design capacity. The primary justification for multiple trains usually is to enhance turndown capability or to allow continued operation even when one train must be shut down. The same approach may be taken with the cooling section. Given the sizes of chlorine gas headers, valves would be expensive and probably ineffective as shutoff devices. Many plants therefore use U- or J-shaped traps in the piping. These can be filled with water or acid to stop the gas flow and to isolate equipment for maintenance. 

Karbate Separator 1. High efficiency 2. Good turndown characteristics 3> Vary low pressure drop-lass than one Inch water 1. Only available In Karbate 2. Liquid loading limited 3. Somewhat difficult to clean I). A trap device Is needed for 1 Iquid col lection... 

It is designed for columns with >800 mm and suitable for systems prone to contamination. The device is rektively umdfreted by fluctuations of the liquid flow rate. The turndown ratio of the construction is usually 2.5 1. 

AS. 1 Representative sampling is more difficult to document and verify when a portable sampler is used. The flow sensing device is usually Umited in accuracy and turndown. Stream conditioning is usually limited to piping elements and flow velocity. The sampler controller data logging is usually limited. Special precautions and operating procedures with additional record keeping by the operator can overcome these limitations. 

Contactor controls (see Fig. H-36) are generally simpler and less costly tiian thyristor controls. However, being on-off devices, they do not offer the high level of outlet temperature control and turndown capability available from thyristors. Contactor control is most frequently used for tank heating applications. 

---