Blowers are used to convey every conceivable small sized product. They are a means of conveying materials via pipelines and generally it is accepted that if you can get a pipe to where you want to go, you can convey material with a blower system.
Pressurized air is the conveying medium. The air is compressed by a Rootes Type positive displacement blower (see diagram on page 64). These blowers pump air by the action of rotating inter-meshing lobes within a casing. The air flow is indicated by the arrows in the diagram. The lobes do not come into contact with each other; therefore there is no need for lubrication and contamination of the conveyed product is almost impossible. Blowers are high volume, low-pressure compressors with the pressure range employed in the milling industry being 0.2 to 1 bar. Higher pressures can be achieved by two-stage blowers, that is to say, two blowers operating in series. This is sometimes necessary where very long distances, excessive elevations or very high throughputs are involved.
A blower unit comprises a number of elements besides the compressor itself. These include a motor drive, an inlet filter, silencer, pressure relief valve, outlet silencer and a non-return valve. These make up the blower unit.
The intake filter has two functions: the prevention of foreign matter from contaminating the product stream via the conveying air and the prevention of foreign matter from entering the blower and causing mechanical damage. These are two critical functions, so intake filters must be regularly cleaned and replaced, especially where the blower operates in a dusty environment, such as the flour silo. This point is so critical that it is good practice to either locate the blowers in a clean air environment or to draw the conveying air from a clean air environment.
One of the consequences of ‘dirty’ intake air is premature wear of the lobes in Rootes blowers. The wear becomes apparent as the blower struggles to maintain pressure, and thus conveying capacity, in normal operation. This is because of leakage from the high-pressure to the low-pressure side of the blowers. Wear can eventually become so bad that the blower will no longer be capable of doing the job it is supposed to do. However, long before this happens, the blower becomes an inefficient conveying tool because energy is wasted through air leakage past the blower lobes. At this point, the alternatives for the blower are refurbishment or replacement.
Silencers are used to reduce the noise created by the blower and are regarded as a necessity in today’s industrial environments. The significant noise level generated by blowers is an increasingly important consideration and should always be taken into account when planning an installation. The best advice that can be given is whenever possible to locate blowers in a blower room which does not need to be entered except for maintenance purposes. If a dedicated room cannot be allocated to this task, then blowers should be located where personnel are not exposed to the noise generated by them.
An advantage of a dedicated blower room is that the air used for conveying can be optimized. This simply means that it is easier to ensure that clean, cool air can be directed towards blower intakes. Cool air is better for conveying because moisture losses in conveying are increased with higher conveying air temperatures. This is an especially important point in tropical and arid environments where conveying air temperatures can be very high.
The pressure relief valve is a safety precaution. It is set to blow off at a slightly higher pressure than the normal conveying pressure, to prevent damage to the blower. It also provides a useful audible warning if conveying conditions have altered out of normal operating range. This can happen in the case of a choke in the conveying line. The non-return valve allows stock and air to pass one way. This, also, is protection for the blower.
The exhaust system on a blower installation is certainly as important an element as the blower itself. Inadequate exhaust can cause operational and hygiene (dust) problems. Insufficient or improperly maintained exhaust arrangements mean that the pressure losses on the line can be excessive and this can cause choke situations. If the exhaust is insufficient, positive pressure will exist at the discharge point for the blower and it can be very difficult to prevent dust leakage from the conveying system.
Maintenance of blowers includes regular checks on oil levels in the gearbox. Topping up and changing of oil should be as described in the manufacturers instruction booklet. Remember that the type of oil used here is critical because ‘splash’ and not forced lubrication is used. If the gearbox regularly needs topping up, then the leak should be investigated immediately. This is especially important if the leak is along the shaft seals, because the leaking oil will make its way along the shaft into the compression chamber of the blower. From here the oil will become entrained in the conveying air and so will contaminate the products being conveyed. In extreme circumstances, the leaking oil could even taint the products being conveyed.
Energy efficient motors should be considered in any flour milling process because of the extended running hours involved with most plants. Blowers are an ideal location for installing energy efficient motors. This is because blowers’ loadings can be made fairly constant. Thus, motors can be well matched to blower applications. A well-loaded motor can generate efficiencies in excess of 92% to 93% and give a quite attractive payback for the plant operator. A beneficial side effect of energy efficient motors is the fact that they produce significantly less heat that conventional motors and so help in the quest for cool air in the environs of the blower. Energy efficient motors are also quieter than ordinary motors because smaller fans are required to keep the motor cool.
While it has already been said that blowlines can be routed almost anywhere, it must be noted that there is a price to be paid for this flexibility. Pneumatic conveying is the most expensive and inefficient form of conveying. However, much can be done to make the conveying system as efficient as possible. This includes minimizing the number of bends in the conveying line, using radiused bends, keeping the route as short as possible and maintaining the mechanical elements of the system.
A final point regarding efficient operation of blowers is that the feed to a blower should be maintained as constant as possible. This can be achieved by controlling the discharge into the blowline via the rotary valve. Motor speed controllers are invaluable tools in achieving this objective. The operational advantages of a constant discharge include constant blowline pressure, constant load on the blower and motor, and smooth product flow. This latter point is highly important because it means that the likelihood of a choke is dramatically reduced.