HOW TO AERATE CEMENT AND FLYASHJune 21, 2013
Aeration pads can greatly influence the feed rate of cement. By properly aerating or fluffing the cement from 94 lbs to 74 lbs per cubic foot, cement weigh up times can be drastically reduced. A good rule for a batch-man is to aerate the cement and fly-ash silo 45 minutes prior to the first batch. Then aerate each time a batch is weighed up for the rest of the day.
The more aeration the faster the flow. As cement migrates by gravity from the silo cylinder to the cone of the silo, the funnel shape + gravity pressure will slowly push the air out of the cement and make the product denser. As the density increases, the flow rate decreases. Outside forces such as yard truck traffic will vibrate the cement pushing even more air out of the cementitious material.
Aeration can be introduced back into the cement by a 4″ X 7″ rectangular pad or a 2″ nozzle. Both items transfer a low pressure / high volume of air into the cementitious product. The greater the surface area of the aeration medium, the more efficient the aeration process. Aeration pads are typically preferred over nozzles because of the greater filter area.
A typical 12 ft diameter cement silo with a 60 degree cone will require 2 rows of aeration pads. Each row will consist of 4 pads. The 4 pads are typically located at 90 degree increments (0, 90, 180, and 270 degrees). The first row is typically 24″ up from the bottom of the cone. The second row of pads should be 30″ above the lower row. The upper row of pads should be located in-line with the first row. Lining up the pads will create a aerated wedge of air that will travel up the side of the cone. The four aerated wedges will release the cement and gravity will flow the cement at approximately 2 lbs. / square inch / second. Areas of the cone that are not aerated will release as the flow of cement around it continues to move downward. The same flow can also be applied to square or rectangular cone sections. The pads should be located in the center of the plate surfaces away from the edges. Some silo geometric shapes might require more than 8 aeration pads per compartment.
Be careful not to over aerate the cement. It is possible to aerate cement to 50 lbs per cubic foot, potentially reducing your silo and weighbatcher volume by 30%.
Aeration blowers produce a high volume / low pressure / dry air as compared to compressed air is low volume / high pressure with moisture. Moisture in the compressed air is generated by the fluctuation of tank pressure throughout the day. Be careful to confirm oil from the plant oiler’s is not getting into the aeration air flow. Moisture and oil will plug and destroy the filter medium in a short time period. How the aerated air is distributed to the pads is also important. Some plant manufactures will “daisy chain” the pads in series using a 1/2″ air line. Plant air will be fed to the first pad through a T fitting and then piped onto the 2nd, 3rd and succeeding pads. By the time the 3rd pad has received it’s air, there is typically not enough air volume to get to the remaining 5 pads. A 2″ diameter X 24″ long pipe manifold is a great way to distribute the air individually to the pads. Weld eight 1/2″ couplings to the manifold and then supply each aeration pad individually. A parallel feed system will distribute the dry air equally to all of the pads.
A 4.5 Hp aeration blower ($1,400.00) will produce 125 CFM of dry air at 15 PSI. One 4″X7″ aeration pad will consume 5 cfm of air. Larger blowers are available to meet the required amount of aeration pads.
By all means, please do not use a vibrator on the cone of a silo. The vibration will pack the cement faster then the aeration pads can fluff the cement in the cone and defeat the purpose of fluffing the cement with proper aeration.