An average 12-yard dry concrete batch plant has a typical theoretical production rate of 200 yards per hour. However, a batch plant can only produce as fast as the mixer truck will take the product. Depending on the truck, a typical mixer truck will charge at a rate of 165 to 185 yards per hour. The weigh up time for a dry batch plant is 1/3 of the overall batch time.
During the weigh up time the mixer truck will be idle under the plant until the start of the discharge of material. By incorporating discharge hoppers, the weigh batchers can be cleared sooner allowing the next batch to start while the current truck is being charged.
3 types of discharge hoppers:
Surge Hoppers are typically 3 to 4 yards in capacity and allow the aggregate on the transfer conveyor to collect if the mixer truck charge rate is exceeded. As the truck hopper begins to choke, incoming aggregate is surged into the surge hopper, and relieved into the mixer truck by gravity as the mixer pulls the material into the drum. By clearing the aggregate weight batcher faster, the next batch can be started sooner, increasing overall plant production.
Holding Hoppers are 12-yard capacity and should have separate hoppers for aggregate and cement. The individual weigh batchers weigh up and discharge into the individual holding hoppers. The holding hoppers are on separate scales which provide control of the discharge flow rate into the mixer truck. Mixer trucks are charged by the holding hoppers instead of directly from the weigh batchers. These hoppers allow the next batch to start and completely weigh up while the current batch is being discharged into the truck.
Gob Hoppers are used with central mix plants and allow the mixer to discharge the finished product into a hopper above the mixer truck. The truck is then fed the mixed material through the gob hopper versus directly from the mixer. This allows the plant to batch the next load into the mixer and begin the mixing process while the current batch is being discharged into the truck.
Dust Collectors, Central versus Individual, Compare the 2 types
Two types of dust collection systems are available for concrete batch plants, central and individual. Central systems utilize ducting and one central unit that collects at all dust points of the batch plant; silos, cement batcher, truck collection hood for dry batch plants, and mixer for central mix batch plants. Individual systems vent each silo, cement batcher, mixer, and truck collection hood separately with individual units.
Determining the best option for you requires accounting for all dust collection points and their CFM requirements. It is always a wise practice to overestimate the CFM to account for loss of efficiency over time. Below is a list of average CFM requirements and Pros and Cons of each system.
Silo CFM Truck Blower: 850 CFM
Silo 60HP Blower: 1,250 CFM
Silo 75 HP Blower: 1,800 CFM
12 Yard Mixer: 5,000 CFM
Truck Collection Hood: 7,000 CFM
Cement Batcher: 750 CFM
Pros and Cons of Central Dust Collector System:
PRO – Central maintenance point
PRO – Does not require climbing to silo tops
PRO – Only need one cartridge or bag size
CON – Increased cost and requires ducting
CON – Requires damper valves
Pros and Cons of Individual Dust Collection System:
PRO – Less cost than central systems
PRO – Individual collected material will not be cross contaminated
PRO – Reclaim same material directly back into silo
CON – May require multiple cartridge and bag sizes
CON – Multiple collectors to maintenance at different locations
How to save time filling cement silo, the 2 factors
Two main factors come into play when reducing cement silo filling times, dust collection and blower CFM. Cement silos can be round or rectangular and are typically filled by a 4-inch or 5-inch fill pipe. High volume batch plants will often have two fill pipes per cement compartment.
The cement, slag, or flyash is transferred by a pneumatic blower, the blower size will determine the amount of cement that will transfer into the silo per minute. Although blower size is a key factor to speed, dust collection plays just as important of a role. If the silo is unable to expel the air being charged into the silo to move the material, the load time per minute will be drastically reduced.
Dust collection should be appropriately sized to handle the CFM of the blower being used to transfer the material. It is important to know the manufacturer’s specifications before purchasing a new dust collector or a new blower. Buying a larger blower with higher CFM than your existing dust collector can handle will not increase loading speed.
850 CFM – Typical truck mounted blower
1,250 CFM – Typical 60HP blower
1,800 CFM – Typical 75HP blower
Under sizing the silo dust collector will not only reduce loading speed, but may lead to bigger problems. Typical cement silo is capable of handling 3 PSI of pressure. If you over pressurize the silo and the emergency pop-off valve fails, the silo could get damaged or even split the silo.