Turndown calculation

When the throughput is turned down to below 10% of full load, axial dispersion is important and is Included in the energy equation. Figure 8 shows the transition to a 1% throughput, simulating the approach to a banked condition, with the fixed carbon-to-oxygen feed ratio changed to 3.5 at time zero. The time constants with axial dispersion are similar to the other turndown calculations. 

Electronic ratio controller. In this type of controller, a proportion of both gas and air is diverted through a bypass in which a thermistor sensor measures the flow. The air and gas flows can be compared and the ratio calculated and displayed. A ratio control valve in the air or gas supply, depending on whether the mode of operation is gas- or air-led, will automatically restore a deviation from the pre-set ratio. The electronic controller maintains ratio over a 19 1 turndown. The principle of operation is based on mass flow, so that it can be used with preheated air in recuperative systems. 

Then round off your calculated width and adjust the tube length if needed. Normally tube lengths are 16 to 50 ft. It is important here to fix the tube length with the width in order to get full coverage of equal leg squares in FA. The reason is that fans of equal blade diameters will cover the entire FA area. Remember also that a multiple number of fans is the better design for turndown and reliability. 

These checks will use all the parameters in the layout summary in Sec. 6.5.3 (Table 6.11, third trial). The minimum throughput calculations will be performed for stages 8 and 9 in the depropanizer, where the loads are lowest. Since the example requires column turndown to 60 percent of the expected design loads, the vapor and liquid loads shown in Table 6.10 for stages 8 and 9 are multiplied by 0.6 for the turndown checks. 

Calculate the maximum and minimum vapor and liquid flow rates, for the turndown ratio required. 

For the problem of Example 5.7-1, calculate the following (a) pressure drop across the top tray, (b) turndown ratio, and (c) downcomer hackup for the tray below Ihe top tray. 

If turndown is required, then mixing-time calculations should be based on the lowest flow rates (i.e. lowest velocities) to be processed and pressure-drop calculations on the highest flow rates (i.e. highest velocities). 

Discuss the applicability of valve trays. The relatively low pressure drop that can be maintained without undue loss of turndown in vacuum columns is probably the valve tray s greatest attribute. Although the accuracy of either the sieve or valve-tray calculation procedure is probably no better than 20 percent, a lower pressure drop is likely to be achieved with Venturi orifice valves than with sieve trays if a reasonable turndown ratio (say 60 percent) is required. This is of little concern at column pressures above 400 mmHg (53 kPa), when pressure drop becomes a minor consideration. 

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. 

The closer to flooding the column can operate, the greater is the column capacity. The closer to dumping the column can run, the greater is the column turndown. For best results column feed rate must be limited. As maximum permissible coltmin AP is approached, the column feed rate should not exceed that which the column can l dle (i.e., can separate properly without flooding). Similarly, as the column operation approaches dumping, feed rate should not fall below a minimum to avoid using excessive steam per povmd of feed. Both maximum and minimum feed rate may be calculated by the column model. 

---