Air-Flow Control

For air-flow control, the system may contain a control valve or damper that automatically or manually modulates system pressure drop. The dotted curves in Figure 4a on each side of the system resistance curve might represent operating extremes of the system resistance as the control valve is varied from maximum to minimum opening. These curves also intersect the fan curve at desirable operating portions of its range both for efficiency and flow control. 

Device Air flow control Initial Cost Operating Noise... 

Two-speed motors are typically used on noncondensing services where the process is not sensitive to temperature but mostly seasonal or variable throughput of fluids in the air cooler requires some degree of air flow control. This is a simple, rather inexpensive means to control air flow when volume air flow is not critical. Typical motor ratings are 1800/900 rpm, although 1800/1200 rpm types are available. 

Air control louvers or dampers, popular in the past for air flow control, are used primarily for only very low scale air flow control. Louvers are used in many winterized heat exchangers in extremely low ambient temperature locations to retain and recirculate warm air in completely enclosed heat exchangers, sometimes in very compHcated control schemes. The use of louvers on the discharge side of a fan to control air flow is inefficient and creates mechanical problems in the louvers because of the turbulence. A fan is a constant volume device, thus use of louvers to control flow is equivalent to... 

Air-Flow Control Process operating reqmrements and weather conditions are considered in determining the method of controlling air flow. The most common methods include simple on-off control, on-off step control (in the case of multiple-driver units), two-speed-motor control, variable-speed drivers, controllable fan pitch, manually or automatically adjustable louvers, and air recirculation. 

Air-flow control in some services can prevent these problems. Cocurrent flow of air and process fluid during winter may be adequate... 

Operating co.sts. Power requirements for air-cooled heat exchangers can be lower than at the summer design condition provided that an adequate means of air-flow control is used. The annual power requirement for an exchanger is a function of the means of airflow control, the exchanger seiwice, the air-temperature rise, and the approach temperature. 

Complementary accessories for ATD components of the installation which are used in conjunction with, and in several cases form an integral part of, the air terminal device for the purpose of achieving the predetermined profile or rate of flow into or from the air terminal device (e.g., air flow controllers, dampers, flow equalizers, baffles, etc.), and... 

What we can do easily is to measure the fuel gas flow rate, multiply the value by R in the so-called ratio station, and send the signal as the set point to the air flow controller. The calculation can be based on actual flow rates rather than deviation variables. 

A more sophisticated implementation is full metering control (Fig. 10.6). In this case, we send the signals from the fuel gas controller (FC in the fuel gas loop) and the air flow transmitter (FT) to the ratio controller (RC), which takes the desired flow ratio (R) as the set point. This controller calculates the proper air flow rate, which in turn becomes the set point to the air flow controller (FC in the air flow loop). If we take away the secondary flow control loops on both the fuel gas and air flow rates, what we have is called parallel positioning control. In this simpler case, of course, the performance of the furnace is subject to fluctuations in fuel and air supply lines. 

An interesting aspect of furnace control is the need to be always on the air-rich side, never on the fuel-rich side. If the furnace became filled with uncombusted fuel and then air was added, the resulting rapid combustion could blow the furnace apart. The same concern makes it important that the start-up of a furnace follow a very carefully thought-out procedure. The control system shown in Fig. 7.1 accomplishes this air-rich operation by the use of several selectors and a lag unit. When the temperature controller calls for more fuel, the air wall increase first before the fuel increases because the low selector on the fuel passes the low signal from the lag to the fuel flow controller while the high selector on the air passes the high signal to the air flow controller. The reverse operation occurs when the temperature controller calls for less fuel The fuel flow decreases first and then the air flow- decreases. 

It has been shown that during the spray-drying process the effects of nozzle orifice diameter and atomization air flow control the droplet size during atomization however, it has also been demonstrated that increasing feed concentration results in... 

Air-flow control in some services can prevent these problems. Cocurrent flow of air and process fluid during winter may be adequate to prevent problems. (Normal design has countercurrent flow of air and process fluid.) In some services when the hottest process fluid is in the bottom tubes, which are exposed to the lowest-temperature air, winterization problems may be eliminated. 

Fig. 5. A column fermentor for the monitoring of the temperature gradient in solid-state fermentation of cassaca by A.niger [28]. a Radial gradient b axial gradient. Column fermentor (volume 1 1 radius 6 cm length 35 cm) 1 jacket fermentor 2 water pump 3 humidifier 4 thermocouples 5 pressure and air flow controls 6 temperature display device 7 water bath...

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