water level in the column and pressure over the fabric is recorded continuously at a sampling frequency of 50 Hz through an ultrasonic proximity sensor on the top of the column and a pressure sensor 5 cm over the fabric. The total drainage time is also recorded by the laser detector on the bottom of the Plexiglass column. A Labview program was written to control water filling, draining and data acquisitions.
MEASURING DRAINAGE RESISTANCE OF FIBER/FABRIC SYSTEM
When fibers deposit on the fabric, they will block the drainage hole and be embedded into the forming fabrics. The interaction between fibers and fabrics will create a resistance to the water flow. As more fibers approach the fabric, a layer of fiber mat will be formed over the fabrics. Ingmanson and coworkers [4, 5] showed that Darcy’s law and a material balance on the solids led to the following expression for the drainage velocity v(m/s) through a fibrous mat:
(1) Where, A = filtration area, m2 dV/dt = volumetric flow rate of the filtrate, m3/s -ǻPt = total pressure drop across the fibrous mate and wire, Pa µ = viscosity of the filtrate, Pa·s Rt = total drainage resistance from fiber mat and forming fabric, m-1
Fig. 2. Schematic of Gravity Sheet Former (GSF)
FABRICS To compare the drainage properties of the different types of triple layer forming fabrics and to better understand the effect of center plane resistance on the drainage property, a SSB fabric, a warp tied fabric, and a warp integrated sheet support (WISS) fabric were selected in this study. All fabrics have a plain weave in the paper side and similar air permeability. The major properties of the fabrics are shown in Table 1. Sample
Air Perm (cfm)
In the GSF drainage system, the relation between drainage velocity over the forming fabric and the drainage velocity in the water column can be shown by the following equation: (2) Where Ac = cross section area of water column, m2 h = height of water in the column, (m) At low flow rate, the pressure drop is provided by the weight of water in the Plexiglass column: (3)
Paper Side Properties Mesh x Knocking
MD yarn (mm)
CD yarn (mm)
Drainage area (%)
Fiber Support Index
Drainage Index
SSB
370
94x90
0.13
0.13
29.1
180
33.3
Warp Tied
375
95x82
0.15
0.13
25.5
173
30.8
WISS
380
81x90
0.17
0.13
24.7
174
32.4
Table 1. Characteristics of fabric samples
Where ȡ = water density (kg/m3) By combining Equation (1), (2) and (3), we can obtain: (4)
Solving differential Equation (4), we can obtain the relation between papermiddleast 21