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.