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Gas Moving Device

Usually, centrifugal fans are used to move gases to and through a wet scrubber. Exceptions are the use of eductors, wherein the motive power is provided by the pump pressure and the momentum of the liquid through the eductor. Some other systems exhibit a process pressure high enough to move gases through the scrubber.

24.2.4.4.1 Fans

Industrial centrifugal fans are typically of radial blade designs where solids are present and backward inclined or radial tip designs where only gas is treated. Fiberglass-reinforced plastic fans sometimes have special wheels, given the centrifugal reaction forces on the wheel.

Fan problems are usually related to balancing (vibration) and alignment. Fans that constantly go out of balance are usually being impacted by particulate. The source of the particulate must be controlled, or if that is not possible, the fan wheel type must be better matched to the application. Often, a periodic fan wash using an inlet nozzle is required. The discharge stack or duct, however, must be designed to accommodate the spray that is generated. With the advent of field laser alignment technology, fans can be set up within exacting alignment tolerances. Laser alignment once per year as a minimum is suggested for any fan developing more than 30 inches w.c. static pressure.

Some fans have an adjustment for the gap between the inlet bell (gas inlet) and the wheel. This is sometimes called the "wheel face to cheek" dimension. On high static pressure fans, as the fan runs up to speed, the fan inlet can be drawn slightly into the fan wheel. The gap allows this movement (along with thermal expansion). If the gap is too large, however, the wheel can start to "windmill" and not develop the proper static pressure. With wind milling, some of the gas moved by the fan simply short-circuits to the gas inlet. This gap should be measured upon installation and reset every time the wheel is cleaned or removed.

On belt-driven fans, the inboard bearing usually is a tapered roller bearing or otherwise has a thrust bearing. The outboard bearing is typically a floating bearing to allow thermal expansion. Too often, when the bearings are changed, the order is reversed. A floating inboard bearing allows the wheel to move toward the fan inlet and can sometimes cause contact as the bearing wears. The result is noise and possibly wheel failure. The bearings must be professionally installed.

A more common problem, at least at startup, is incorrect fan rotation. This can occur during a change of the motor control center or starter. A fan can move air even if it is rotating backward. Changing one of the fan leads corrects the rotation on three-phase motors.

The fan vendor's literature, though usually quite sparse in content, should be followed for fan servicing. Particular attention should be paid to the lubrication program.

Fans usually have housing drains, particularly when the fan is applied to a wet environment. A common occurrence is that the plug for the drain is missing, thus allowing outside air to enter. If the fan is handling a wet gas stream, the fan drain should be piped to a sealed drain, that is, through a trap so air cannot enter. Depending upon its location on the fan housing and downstream gas flow resistance, the drain may be under positive or negative pressure versus atmosphere. The trap can be quite high on many systems. One way to check is to make a threaded replacement plug with a tap to a pressure gauge. Install the plug and read the pressure when the fan is moving cold air (cold static) if possible. The seal leg should be at least 10% higher than this reading in inches of water. If you cannot get a reading, use the fan performance curves, and measure the outlet static pressure. The drain is usually at least at the outlet static pressure.

24.2.4.4.2 Eductors

The key element of an eductor is the nozzle. If the nozzle does not produce the proper spray pattern or maintain the correct flow rate, the draft produced, and the flow produced can be insufficient. Causes of reduced gas flow rate usually are the result of low liquid rates to the eductor. Reductions in draft are typically caused by poor spray patterns from the nozzle, that is, a partially plugged nozzle. In either event, the nozzle should be inspected and cleaned or replaced if low flows or low drafts are experienced.

Another potential problem is wet/dry line buildup just ahead of the restriction in the eductor (throat). The wet/dry line buildup occurs at the point where the spray meets the converging section of the eductor or at a point just inside the beginning of the throat. If the gas stream contains some particulate or the liquid contains dissolved or suspended solids, the solids can migrate to the wall of the scrubber and stick. This layer can build up to the point where a gas flow restriction is created, reducing the gas flow through the eductor. The remedy is to clean the converging section. If this problem is chronic, the converging or "approach" section can be made fully wetted by adding tangential liquid inlets that swirl the liquid much like the action in a dentist's bowl. The added liquid, however, may reduce the momentum effect of the spray liquid. It is best to contact the eductor vendor regarding any modifications.

Throttling the exhaust can also reduce the flow through an eductor. If excessive backpressure is created by downstream equipment, it can reduce the flow through the eductor. Sometimes increasing the liquid flow rate through the eductor can compensate for downstream resistance. Pressure losses downstream of 4-6 inches w.c. should be overcome by most eductors. If the gas flow is low but the eductor is working well, look at the equipment or ductwork located downstream of the eductor to see if excessive restrictions (such as an overfull tank or closed damper) could have occurred.

Gas Discharge Device

Exhaust stacks are the most common discharge devices. They look so simple. They can be a major problem source if they are not intelligently designed. With a wet scrubber, the gases are saturated or should be nearly saturated with water vapor as they leave the stack. Exceptions are those cases where an exceedingly high static fan is used, or a gas reheat system precedes the stack. The heat of compression of a high static pressure fan can heat the stack gases about 4°F for every 10 inches w.c. of fan static pressure. Heated gas can hold more moisture than the colder gas. Though the gas enters the fan saturated, it can leave below saturation.

Since the gases are near saturation, any cooling of the gases can cause condensation. If the stack gas velocity is excessive, the condensation that is formed can go up the stack and be carried out. Worse yet, these droplets can accumulate particulate and be caught in the sampling train only to be counted as gas-borne particulate.

As a result, many wet scrubber stacks are limited to vertical gas velocities of less than 40 ft/s. Experience has shown that condensation entrainment can rise in a stack at or above this velocity. Stack velocities of 35 ft/s are normal design figures.

If a stack test is failed by high front-half particulate and the testing firm notes excessive probe wash and water accumulation, you could have a stack problem. Check the stack velocity. If it is over 40 ft/s, you likely have an ascending wall entrainment type problem. This can be determined by making a small test probe from a piece of pipe (see Figure 24.4). The slot in the probe is oriented so that water rising in the stack near the wall is diverted into the slot of the probe and drains out (watch out, this water is likely to be hot on systems attached to high-temperature sources). If the probe is turned so the slot faces upward, it will catch water descending the stack wall. Descending wall liquid is usually normal and does not cause a problem unless the stack lacks a drain or has some internal obstructions that the liquid can hit and be splashed into the gas stream.

FIGURE 24.4

Method for checking stack entrainment.

If the probe, slot up, is pushed into the stack farther (away from the wall), few if any droplets should drain out. If the probe slot is turned so that it faces downward, it should catch liquid spraying upward if the gas velocity is too high or there is entrainment from some upstream device. This is called ascending center entrainment.

If you detect ascending center entrainment, you likely have more than one problem. If you also measured ascending wall entrainment, your stack is likely too small, and you have some sort of upstream entrainment problem. If you did not measure ascending wall entrainment, the stack is likely to be of sufficiently low velocity, but you have another entrainment producer.

A common error is to discharge the fan exhaust directly into the stack base. If the high velocity fan exhaust hits the condensate that drains down the stack, a spray could result that sends the condensate up and out of the stack as entrainment. The fan discharge, sometimes called an evase (E-vah-say), should extend into the stack and be equipped with a brim (that looks like a horse collar) that moves the descending liquid around the inlet. The stack should also have a suitably sealed drain. The drain should be sized to handle about 3%-5% of the recycle liquid of the scrubber if possible. This would allow recovery in case a gross entrainment accident occurs.

 
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