Weigh batching hoppers are used for weighing dry products to dispense accurate amounts as required by downstream processes (mixer/packaging, etc.).
In this blog, we will cover everything you need to know about weigh batching hoppers:
- What are Weigh Batching and Weigh Hoppers?
- Product Dosing Methods
- Weigh Batching Systems
- Weigh Hopper (Scale Hopper) Components
- Weigh Hopper Calibration
What are weight batching and weigh hoppers?
The process of weighing the product in the hopper and discharging the required weight is known as weigh batching and the hopper in which the product is weighed is called “weigh hopper” or “scale hopper.”
Most of the powders/solids processing facilities require an accurate and known weight of the product for making a blended product or a packaged product. This process is generally conducted manually and is very time-consuming.
A weigh batching hopper along with a handling system can help simplify and expedite this process.
Many powder handling systems require the dosing of products and the preparation of material batches for ingredient formulations.
Typical applications are for food mixes, soups and ﬂavorings, pre-mixed baking recipes, infant formulas, health and nutritional supplements, breakfast cereals, drinks, sauces, confectionery, pharmaceuticals, and many others.
Product Dosing Methods
There are 2 techniques of dosing products in any process operation:
- Volumetric Dosing
- Gravimetric Dosing or Weigh Batching Systems
Volumetric Dosing System
Volumetric dosing systems use a rotary valve or screw conveyor running at a constant speed to discharge powder from a storage hopper at a consistent rate (volume per unit time – e.g. liters/min.)
Frequently, the hopper includes devices to condition the bulk material to ensure an even bulk density by using vibration, agitation, massaging or other means to eliminate air pockets and maintain flow.
Accuracy is dependent on the consistent bulk density of the powder and the ability to fill the device.
Gravimetric Dosing System (Weigh Batching System)
Gravimetric feeding systems generally use a volumetric feeder associated with a weighing system to control the discharge of powder from a storage hopper at a constant weight per unit time – eg. lb/min. The weigh signal is used via a feedback control system to constantly monitor the actual feed rate and compensate for variations in bulk density, porosity, etc.
Weighing systems may include platform scales, load cells, weigh hoppers, and other devices to provide optimum performance as required by design requirements.
(Note: There is also a continuous weigh batching system possible for the process that requires continuous feeding and material weighing at the same time.)
Weigh Batching Systems
Weigh Batching Systems can be subdivided into 2 categories:
- Loss in Weight Batching
- Gain in Weight Batching
Loss-in-Weight Batching Systems (LIW)
Loss-in-Weight Batching Systems measure the weight of the material as it is discharged from the weighing hopper and can also be used with various feeding and conveying devices including pneumatic, augers, rotary feeders, and others.
The main benefit of loss in weight batching is that multiple products can be simultaneously weighed and discharged (eg.. a mixer).
The accuracy is good but slightly lower than the gain in the weight system.
Batch Weight (LB) Formula
Empty vessel weight (0 lb) when tared on load cell) + powder in the hopper (lb) – product remaining in the hopper (lb)
Example: Flour dough mixer requiring 500lbs flour dispensed from a flour storage
Each Mixing Cycle of a liquid dough mixer would need a batch of 500lb of dry flour, flour is available in a storage hopper or ‘Day Bin’ or a storage silo which holds a large amount of flour (10,000 LB-, 50,000 LB or more).
The hopper is supported on the load cell weighing system and used to dispense the exact amount required, 500 LB in this case.
Once the 500 LB is taken out from the silo, the batching process is stopped.
Assume that the design specifications are as follows:
Empty hopper weight: 5000 LB
Flour weight: 10,000LB
Load Cell Weigh System: 20,000LB.
System Weight Accuracy: 0.1% ➔ (0.1 x 20,000/100) = 20LB
Batch Transfer Weight = 10,000LB-9,500LB = 500 LBs, but has accuracy of +/-20 LBs
Gain-in-Weight Batching Systems (GIW)
Gain-in-Weight Batching Systems measure the weight of the material as it arrives at the receiving hopper and can be used together with a wide variety of feeding and conveying systems including pneumatic, augers, rotary/vibratory feeders, and others.
The main benefit of gain-in-weight batching is that it is very accurate (subject to the load cell accuracy) as the exact weight is added to the weight hopper.
Batch weight (lb) formula:
Empty vessel weight (0 lb) when tared on load cell) + powder added (lb)
Example: Flour dough mixer requiring 500lbs flour dispensed into a 500lb scale hopper
Each mixing cycle of a liquid dough mixer would need a batch of 500lb of dry flour.
Flour is available in a storage hopper or ‘Day Bin’ or a storage silo which holds a large amount of flour (10,000 LB-, 50,000 LB or more).
The flour is transported into a batch scale hopper (or a weigh hopper) which is supported on a load cell weighing system and used to add the exact amount required, 500 LB in this case.
Once the 500 LB is added to the weigh hopper, the batching process is stopped.
Assume that the design specifications are.
Empty hopper weight: 500 LB
Flour weight: 500 LB
Load Cell Weigh System: 2,000 LB.
System Weight Accuracy: 0.2% ➔ (0.2 x 2,000/100) = 4 LB
Batch Transfer weight = 500 LBs, but has accuracy of +/-4 LBs
Note: System weigh accuracies in LIW and GIW also depend on metering device used and additional factors such as material in flight, instrument response times etc.
Weigh Hopper (Scale Hopper) Components
Weigh Hopper Body
A typical weigh hopper volume is designed to hold the Specified Amount Of Product
The Weight Measurement is done by load cells.
Tension Load Cell (Measures the Weight By Suspending The Hopper): Generally ONE for small hopper and more for large hopper.
Compression Load Cells (Measures the Weight At Supports): Generally THREE OR FOUR.
Weight Transmitter (Or Load Cell Transmitter):
The transmitter collects signals from the load cells and converts them into weight in LBs based on the calibration.
(If there is more than one load cell, the load cell signals or summed up in a junction box and a single output is sent to the weigh transmitter).
The Transmitter communicates with the system automation system to control the batch weight, batch time, and batch accuracy.
Each weigh batch process is unique and designs depend on:
- Type of material · Minimum, average, and maximum flow rate
- Bulk density · Average and maximum particle size
- Moisture content
- Material temperature
- Angle of repose
- Feed device and discharge devices
- Load cell and accuracy of load cell
- Space limitations and mounting constraints
- Environmental conditions: dusty, wash down, sanitary, or corrosive.
Weigh Hopper Calibration
- It is important to understand that a weigh hopper is only as reliable as the stand calibrated. Weigh hoppers are calibrated using static test weigh calibration or material tests.
- The material test is the most accurate calibration method because it uses known actual operating conditions.
- The known weight of the material can be obtained by weighing the test load on a platform-type scale. The material can be weighed before or after that, the weighing device used for the test load is accurate and calibrated by a certified agency.
To see weigh hopper calibration in action, watch the video below:
Pneu-Con provides a range of volumetric and Gravimetric Powder Feeding Systems to guarantee high accuracy for critical applications such as:
- Dosing Systems
- Minor / Major Ingredient Preparation Systems
- Pre-Mix Systems
- Recipe Handling Systems
- Blending Systems
- Packing Systems
Contact sales today to learn more about the customizable systems to meet your conveying needs.