The Global Leader In Viscosity For Over 75 Years
Brookfield AMETEK

Ground Coffee

Powder Flow Application Data Sheet

USE

To make coffee

Test Equipment

  • Instrument: Powder Flow Tester (PFT)
  • Trough: 230 cc, 6-inch diameter
  • Lid Type: Vane Lid, 304 s/s, 33cc, 6-inch diameter (Flow Function)
    Wall Lid, 304 s/s, 2B finish, 6-inch diameter (Wall Friction)
  • Type of Test: Flow Function Test, Wall Friction Test
  • Temperature: Room Temperature (70-72° F)
  • Humidity: 48%

Method

A Brookfield Powder Flow Tester, equipped with Powder Flow Pro software for automated instrument control and data acquisition, was used to test this name brand ground coffee. The ground coffee was scooped into the trough, and the scraping tool was then used to evenly distribute the powder throughout the trough. After recording the sample weight and entering it into the software, a standard flow function test and then a wall friction test were run. Time required for each test was 35 minutes and 20 minutes respectively.

Parameters Measured

  • Flowability: Very Cohesive to easy flowing
  • Wall Friction: 20 to 15
  • Bulk Density: 300 kg/m (fill density) to 400 kg/m

Analysis

  • Hopper Shape: Conical
  • Arching Flow Factor: 1.40
  • Critical Arching Dimension: 59.8 mm (2.35 inches)
  • Rathole Diameter: Dependent on bin diameter

Results


Figure 1: Ground Coffee Flow Function Graph

Figure 1 shows the flowability of the ground coffee at different levels of consolidating stress. These results show that the coffee is generally easy flowing except at very low levels of consolidating stress where it begins to fall into the cohesive range (below 1 kPa).

Note: The Flow Function data is indicated by the red line. The other lines are references (or "Standard Flow Indices"), which distinguish the different types of flow behavior, ranging from "non-flowing" to "free flowing".


Figure 2: Ground Coffee Wall Friction Graph

Figure 2 represents the angles of wall friction at different levels of normal stress. Angles of wall friction represent the friction between the sliding powder and the wall of the hopper or chute at the onset of flow. In this test a stainless steel lid was used, illustrating what the friction would be like if the ground coffee was in a stainless steel hopper. At a low normal stress of about .5 kPa, the effective angle of wall friction is about 20 and goes down to about 15 at higher levels of normal stress (4.75 kPa). Wall friction angles above 30 are considered very high and less than 10 are considered very minimal.


Figure 3: Ground Coffee Bulk Density Graph

Figure 3 shows the bulk density of the material at different levels of consolidating stress. This graph tells us that the ground coffee has a fill density of about 300 kg/m and rises to about 400 kg/m at around 12 kPa of consolidating stress. In general, a free flowing powder will show very small changes in bulk density, while a cohesive or poor flowing powder will generally show a large increase in bulk density.

Conclusion

The ground coffee is a cohesive powder at low consolidation stress levels and easy flowing at consolidation stress levels above 1 kPa. This means that the ground coffee may have flowability issues as the hopper empties. Possible problems include arching (when the powder forms a cohesive bridge over the outlet) and ratholing (when the powder flows out only from the center leaving the rest of the material static against the walls). The critical arching dimension of 2.35 inches (59.8 mm) provides a conservative estimate to prevent arching from happening, provided the minimum outlet dimension of the hopper exceeds this value. The critical ratholing dimension is dependent on the diameter of the bin. The rathole diameter can be automatically calculated by Powder Flow Pro once the bin diameter is entered.