Pad wetting

FIG. 23 Photograph of through-hole incomplete pad wetting and incomplete fillet formation. 

A second general approach to achieve reduced wet pickup is based on the use of the vacuum slot technology (15,16). In this case the fabric may be padded conventionaUy, but is then mn against a vacuum slot (17). This vacuum slot removes a certain amount of water solution from the fabric, so that a reduced level of wet pickup is achieved prior to drying. A lower level of wet pickup is achieved on blend fabrics than it is on 100% ceUulosics using the vacuum slot technology. 

The standard conventional finishing process has been modified to suit the purposes for different fabrics and garments. Eor example, tubular knits are frequently handled using specialized equipment to control tension and to get adequate padding. Some tubular knits are subjected to wet-on-wet padding, dried and cured in large dmm dryers, and steam-treated to achieve a relaxed and nondistorted knit. 

Eabric can be cross-linked either in the dry or the wet state. If fabric is cross-linked in the dry state, smoothness returns on tumble drying. By contrast, if fabric is cross-linked in the wet state, smoothness is achieved byline-drying the fabric. This concept has been demonstrated using formaldehyde in pad—dry—cure or wet cure processes (eq. 2) (29). 

Filters for mists and droplets have more open area than those used for dry parhcles. If a filter is made of many fine, closely spaced fibers, it will become wet due to the collected liquid. Such wethng will lead to mathng of the fibers, retenhon of more liquid, and eventual blocking of the fiter. Therefore, instead of fine, closely spaced fibers, the usual wet filtrahon system is composed of either knitted wire or wire mesh packed into a pad. A looser filtrahon medium results in a filter with a lower pressure drop than that of the filters used for dry parhculates. The reported pressure drop across wire mesh mist eliminators is 1-2 cm of water at face velocihes of 5 m sec T The essenhal collechon mechanisms employed for filtrahon of droplets and mists are inertial impachon and, to a lesser extent, direct intercephon. 

Abrade Hand abrade with Scotch-Brite wet with MEK until the surface is shiny. Use fine-grit, dry Scotch-Brite - pads on high-speed grinder driven by dry. oil-free N2 until the surface is shiny. 

Many of the warmer climates have a dry atmosphere (see Figure 23.8). In such areas, considerable dry bulb temperature reduction can be gained by the adiabatic saturation cycle (Section 24.4). The apparatus draws air over a wetted pad and discharges it into the conditioned space. It is termed an evaporative or desert cooler (Figure 25.3). 

Ap = Pressure drop, in. water Apo = Pressure drop, no entrainment, in. water Api = Pressure drop due to liquid load, in. water Apr = Pressure drop, total across wet pad, in. water Qd = Dispensed phase volumetric flow rate, cu ft/sec... 

In hot, dry climates an inexpensive alternative to air conditioning is the swamp cooler. In this device water continuously wets porous pads through which fans blow the hot air. The air is cooled as the water evaporates. Use the information in Tables 6.2 and 6.3 to determine how much water must be evaporated to cool the air in a room of dimensions 4.0 m X 5.0 m X 3.0 m by 20.°C. Assume that the enthalpy of vaporization of water is the same as it is at 25°C. 

Few experiments exist which have tried to determine the temperature rise in the contact area between mbber and a hard slider. Schallamach mbbed a thermocouple over a rubber pad at different loads and obtained considerable rises in temperature. The author had a thick mbber strip glued to a tire-testing dmm and a thermocouple imbedded in a small spherical slider to which different loads could be applied. Figure 26.17 shows the temperature rise in the contact area between a dry mbber surface and the slider as function of speed and on a wet surface for two different loads [15]. On the dry surface the temperature rises rapidly with increasing speed reaching values of over 300°C. The curves can be described exactly by a simple relation... 

An empty vessel may be employed, but horizontal baffles can be used to reduce turbulence and assist the coalescence through preferential wetting of the solid surface by the disperse phase. More elaborate methods to assist the coalescence include the use of mesh pads in the vessel or the use of an electric field to promote coalescence. Chemical additives can also be used to promote coalescence. 

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