Spray compression

In conventional air spraying, compressed air and paint are supplied to the spray gun, which atomize and transport the paint to the article being coated where they are deposited forming a uniform film. The air pressure used in this operation is critical and it should be kept at the minimum required amount, to atomize and deposit the paint onto the substrate. 

As various types of devices for nasal administration, such as unit dose containers, metered dose sprays, compressed air nebulizers, and so on, are becoming available, formulation research should be conducted while considering a dosing device. 

For paint spraying, compressed air line respirators are commonly used in a special light version - so-called light duty air line respirators (see Fig. 6.41). Depending on the overall leakage, they are divided into 3 classes. Respective maximum allowable concentrations are given in Table 6.31. These respiratory hoods should not be used for work other than paint spraying. 

Compressed Ga.s PropeIIa.nts. The compressed gas propellants, so named because they are gaseous in conventional aerosol containers, are nontoxic, nonflammable, low in cost, and very inert. When used in aerosols, however, the pressure in the container drops as the contents are depleted. Although the problem is lessened when the contents are materials in which the propellant is somewhat soluble, this pressure drop may cause changes in the rate and characteristics of the aerosol spray. A compressed gas aerosol system is illustrated in Figure 3. 

Compressed gas systems were originally developed simply to provide a means of expelling a product from its container when the valve was depressed. SemisoHd products such as a cream, ointment, or caulking compound are dispensed as such. A Hquid concentrate and a compressed gas propellant (Fig. 3) produce a spray when a mechanical breakup actuator is used. Nitrogen, insoluble in most materials, is generally used as the propellant. 

When gases that are somewhat soluble in a Hquid concentrate are used, both concentrate and dissolved gas are expeUed. The dissolved gas then tends to escape into the atmosphere, dispersing the Hquid into fine particles. The pressure within the container decreases as the product is dispersed because the volume occupied by the gas increases. Some of the gas then comes out of solution, partially restoring the original pressure. This type of soluble compressed gas system has been used for whipped creams and toppings and is ideal for use with antistick cooking oil sprays. It is also used for household and cosmetic products either where hydrocarbon propeUants cannot be used or where hydrocarbons are undesirable. 

In one extractor (FMC Inc.), the fmit is located between two cups having sharp-edged metal tubes at their base. The upper cup descends and the many fingers on each cup mesh to express the juice as the tubes cut holes in the top and bottom of the fmit. On further compression, the rag, seeds, and juice sacs are compressed into the bottom tube between the two plugs of peel. A piston moves up inside the bottom tube forcing the juice through perforations in the tube wall. A simultaneous water spray washes the peel oil expressed during extraction away from the peel as an oil—water emulsion the peel oil is recovered separately from the emulsion. 

The availabihty of spray-dried lactose, microcrystaUine cellulose, and other excipients allows for the use of granular rather than powdered phases. This eliminates some of the problems of particle segregation according to size (demixing) and even flow to the die. Direct compression eventually may be the preferred method of tablet preparation. 

In wetted-wall units, the walls of a tall circular, slightly tapered combustion chamber are protected by a high volume curtain of cooled acid flowing down inside the wall. Phosphoms is atomized by compressed air or steam into the top of the chamber and burned in additional combustion air suppHed by a forced or induced draft fan. Wetted-waU. plants use 25—50% excess combustion air to reduce the tail-gas volume, resulting in flame temperatures in excess of 2000°C. The combustion chamber maybe refractory lined or made of stainless steel. Acid sprays at the bottom of the chamber or in a subsequent, separate spraying chamber complete the hydration of phosphoms pentoxide. The sprays also cool the gas stream to below 100°C, thereby minimising corrosion to the mist-collecting equipment (typically type 316 stainless steel). 

Newer high velocity thermal spray coating processes produce coatings in compression rather than tension because of the shot peening effect of the supersonic particles on impact. This has permitted coating as thick as 12,500 p.m without delamination as compared to older processes limited to 1,250 p.m. The reduced residence time of particles at temperature minimises decomposition of carbides present in conventional d-c plasma. This improves wear and hardness (qv) properties. 

Arc wire utilizes two continuously fed 1.6-mm dia intersecting wires with a d-c arc maintained between the wire tips as they meet. Compressed gas (usually air) strips the molten metal from the tips and forms a directional spray stream. This process is widely used to spray most metals. Arc wire is the most economical process because of the wire feedstock. Moreover, it utilizes - 10% of the thermal energy of the other spray processes (0.4 vs 6.6 kWh/kg using stainless steel) because of the direct arc heating of the wire tips. 

In contrast to other polymers the resistance to water permeation is low due to the hydrolysis of the poly(vinyl acetate) (163,164). Ethylene copolymers have been developed which have improved water resistance and waterproofness. The polymer can be used in the latex form or in a spray-dried form which can be preblended in with the cement (qv) in the proper proportion. The compressive and tensile strength of concrete is improved by addition of PVAc emulsions to the water before mixing. A polymer-soHds-to-total-soHds ratio of ca 10 90 is best. The emulsions also aid adhesion between new and old concrete when patching or resurfacing. 

In the Diesel engine, a higher compression ratio can be achieved, up to about 18, before knock presents a problem, and the fuel is injected into the combustion chamber near tire end of the compression stroke as a spray. The... 

Injection moulding - Elastomers Compression moulding - Composites Hand/spray lay-up - Composites... 

Another hazard of compressed air is that it contains dust (organic and inorganic), water, and traces of hydrocarbons, which if they are not removed can cause excessive wear of tools or contamination of products. Morris writes, Those who use air for pneumatic tools or even paint spray seem to have an inbuilt resistance to any idea that the quality of their compressed air is of any serious consequence. The fact that it transmits concentrated quantities of abrasive particles and water into the finely machined orifices and cylinders of their tools seems to pass them by [12]. 

Teflon AF can be extruded and injection or compression molded It can be formed by spm casting or applied from solution by spray coatmg or direct apphea-tion [5] Its extremely low refractive index and its solubihty make it highly useful as a coating for the core of optical fibers... 

Air and natural gas are often used as a drilling fluid with no additives placed in the injected stream of compressed fluid. This type of drilling is also often referred to as dusting because great dust clouds are created around the drill rig when no formation water was present. However, modern air and gas drilling operations utilize a spray at the end of the blooey line to control the dust ejected from the well. Figure 4-185 shows a typical site plan for air drilling operations. 

Few plant operators need to be told of the problems caused by water in compressed air. They are most apparent to those who operate pneumatic tools, rock drills, automatic pneumatic powered machinery, paint and other sprays, sandblasting equipment, and pneumatic controls. However, almost all applications, particularly of 100-psig power, could benefit from the elimination of water carryover. The principal problems might be summarized as ... 

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