Air Blowoff Systems for Industry

Canadian Air Systems Co. is a world's leading designer and manufacturer of high-penetration air-knives for a variety of industrial process applications:
 
* Liquids Blow-Off Devices 
* Fast Air Cooling
* Conveyor Belt Cleaning
* Process Air Curtains
* Conveyor Dryers

Blower and fan units used in tank, blender, and conveyor tube drying can reduce drying time by an average of 50% compared with natural air drying. They are almost always a direct-drive design in order to minimize size and cost. However, an in-line electric heater must be added, and this ultimately raises both the cost and total power consumption. The final assembly still must be compact enough to mount on a portable cart to easily service multiple mixers, and this portability requirement limits the size and drying power of these forced-air dryers.

There are other performance issues and technical drawbacks that make blower and fan units problematic for many users. They must be connected to an in-line electric heater in order to introduce hot air into the tanks, blenders, and auger conveyors. The blowers tend to have high pressure at low airflows, but low pressures at high airflows. The fan designs have low pressures at all airflow ranges and this low pressure makes it impossible to overcome air pressure resistance through the heater and throughout the tank, blender, and conveying circuit without losing all of the benefits of high air volume.

The end result is low air volumes for both the blower and fan dryer units. This causes equally low rates of air exchange throughout the whole tank, blender, and conveyor system. The net effect of such low air volumes at high air temperatures in regard to the heaters is that air temperature spikes can occur in certain areas. These become dry while other internal surfaces of the tank, blender, and conveyor system remain moist, thereby remaining at ambient temperature longer and not drying at the same rate. These temperature extremes can also cause damage to rubber glands and seals within the tank, blender, and conveyor circuit. Because electric heater elements have a typical surface temperature of 1200ºF, safety considerations increase if the electric heater coils or the heater controller has any type of malfunction.

High-speed belt drive blowers with electric motors ranging from 3 to 50 hp offer several advantages over the direct-drive blowers. They occupy the same smaller space of direct drive while having higher horsepower than the direct-drive blowers and fans. Because hot air is generated by recycling the natural blower heat, they do not require heaters. It is also possible to connect multiple tank, blender, and conveyor tube ports from one blower simultaneously, but care should be taken to make certain that the airflow is balanced to ensure that the drying time is equal.

As discussed earlier, the optimum design for drying tanks, blenders, and conveyors is by use of a pressurized air circuit in order to achieve the smallest temperature variance throughout. Since this pressure must be maintained along with an air exchange rate of 1–2/min, there must be some means of restricting exit air.

There are certainly a number of variables among the wide range of manufacturers of tanks, blenders, and auger conveyors, but the common denominators for defining an effective drying system usually prove to be very much the same.

The first step is to determine the maximum continuous air temperature to which the entire circuit can be exposed. Although there are certain systems where Teflon seals and gaskets can handle greater than 200ºF, a more common acceptable maximum is 150ºF continuous air temperature. Just as with the high-temperature solutions, operators must take care to avoid bare skin contact with metal surfaces at 150ºF. However, food safety personnel endorse the 160ºF air temperatures for the 15–30-minutes drying cycle as it supplements the sanitizing process.

Next, to minimize air temperature gradients within the entire tank, blender, and conveying system, the drying air must be pressurized at all times to between 0.8 and 1.0 psig. Air molecules mix better at these slightly compressed levels, and the “hot spots” that are common with the low-pressure blower, fan, and electric heater systems are greatly reduced.

The last piece of the drying formula, with the air temperature at 160ºF and the internal air pressures at ~1.0 psi, is to obtain a total air volume exchange rate sufficient to meet the drying cycle time objective. The more air exchanges, the faster the drying time. With a wash temperature of 150ºF, the air volume exchange rates are as follows: with one air exchange per minute, the drying cycle is approximately 30 minutes; with two air exchanges per minute, the drying time is generally 15–20 minutes.

However, if the CIP temperatures are lower than 140ºF or the target drying time is less than 15 minutes, the horsepower and size of the drying system will make a portable dryer less practical.

For additional information please refer to http://nis-co.com.

Oleg Tchetchel
Industrial Applications Engineer
Canadian Air Systems
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