Painting by Immersion and by Compressed Air: A Practical Handbook

CHAPTER V.

After failing to achieve satisfactory results with such simple apparatus when applying viscous paints, etc., further experiments were carried out with a somewhat different type, commonly known as the "kettle" type of sprayer. The principle employed was to employ compressed air, supplied, in this case, not from a hand pump but some form of power compressor, and at a pressure varying from 20 to 50 lbs. per square inch.

The air was delivered through a nozzle controlled by a convenient plunger or trigger type of valve, and impinged at an angle across a similar nozzle communicating with the paint receptacle of the sprayer. The action of the air impinging on or across the paint nozzle created a vacuum in the paint tube, and thereby sucked up the paint from the receptacle and gave a fan-shaped spray. Up to a point these experiments proved satisfactory, but still did not overcome the difficulty of successfully dealing with intricate shapes, small articles, and particularly in applying heavy paints with fair percentages of varnish embodied therein.

The Concentric Form of Spray.

Attention was therefore directed to the concentric jet form of sprayer, and this is now almost universally employed where painting, enamelling, etc., is carried out on a large scale.

The advantages of the latter type are many, and in addition to being designed, in most cases, to produce the greatest effect with the least possible consumption of air, they are also capable of applying the paint in very finely divided particles to the smallest work, such as buttons, imitation jewellery, small electrical camera parts, or of applying silky coats to such large work as motor bodies, constructional iron work, large tinplate work, agricultural machinery, domestic gas apparatus, etc. at a speed approximating to four to ten times the speed of handwork, and giving a far superior finish, in many cases with less coats.

Many people who contemplate the subject of painting with a spray think only of a smother of paint being discharged from a nozzle. They are not aware that the colour can be better controlled in a suitably constructed spray than is possible with a hog's hair brush.

The flow of paint is stopped or started instantly, and the amount of paint delivered is at all times under perfect control, so that a quantity corresponding to that delivered by a ¼-inch brush or pencil can be increased to the quantity distributed by a 4-inch brush in a single stroke. In other words, you have a tool which is the equivalent of half a dozen brushes ranging from ¼ inch to 4 inch in width.

A little practice is, of course, necessary to master the instrument, but nothing like the practice which is required for successful painting with hog's hair.

The principal care is to put on the paint until the beads of paint coalesce. Too much would make the paint run, too little would not leave the surface covered. When properly done the surface is superior to other painting, as the hairs of the brush are not dragged through it to disturb the evenness of the surface.

The air pressure required for painting varies with the consistency of the paint, its viscosity as well as its thickness. Some liquids have a quality of stringiness or hanging together which require a higher pressure to break up.

Thin lacquers and varnishes may be sprayed with 18 or 20 lbs. to the square inch, and from that to about 50 lbs. will cover most classes of paint.

The lowest pressure at which a paint breaks up is the best pressure to use, as high pressures have a tendency to make more dust and put fine particles of paint in the air, where they are not wanted.

The volume of air has also to be considered. This varies with the size of the aperture through which it exhausts and to some extent with the pressure—for a small nozzle such as is used for lacquers and thin liquids, say, one cubic foot of free air per minute, and up to 3 cubic feet for oil paints.

When it is stated that a pistol sprayer consumes say two cubic ft. of free air per minute, it means when working almost continuously, but as the periods of actual work do not represent more than two-thirds of the actual number of working hours, there is a margin left. Nevertheless, it would be unsafe to state a lower figure, for in some cases where convenient feeding of the work to the operator is arranged, the consumption of air is continuous.

It should also be remembered that the air is used for other purposes, i.e., a small amount is used to provide a pressure feed of colour to the instrument in many cases, and where a heater is employed, a small amount of air is bypassed through the pistol to keep this warm when spraying is momentarily stopped. Although the practice of heating the air slightly increases the ultimate efficiency of a given compressor, yet it is unwise to count too much upon this fact.

In submitting the following figures as to air consumption, the Airostyle pistol has been taken as a type:—

For work with a tip and needle of 1 m/m. dia., 1 cub. ft. per minute; 1½ m/m. dia., 1¾ cub. ft.; 2 m/m. dia., 2½ cub. ft.; 2½ m/m. dia., 3 cub. ft.; 2½-3 m/m. dia., 3½-4 cub. ft.; 4½ m/m. dia., 5 cub. ft.

These consumptions naturally vary slightly with the adjustment of the nozzle of the pistol, but they are from actual tests and so may be taken as authoritative.

CHAPTER VI.

First, therefore, comes the Aerograph Co.'s Spray Painter. This instrument is of the concentric jet type, and was one of the first of the kind made in Europe. It is capable of excellent work. In Fig. 35 is shown one form of the handpiece. A flexible tube for compressed air connects with the nipple A. The finger lever F controls both the air valve and a colour valve in the nozzle N, so that little or much colour may be allowed to pass out into the current of escaping air.

The colour may be supplied under pressure, through a flexible tube from a large paint pot, and pass through the tube P to the nozzle N, or in smaller quantities from a cup C, which is attachable at B, and when so attached cuts off the supply through the tube P. This cup is useful when small quantities of colour are wanted with frequent changes. A ball joint permits work to be done in either an upward or downward direction.

In Fig. 36 is illustrated a pistol pattern of the Aerograph, which has a large side cup which can be sprayed at any angle, being adjusted to facilitate spraying in an upward, downward or horizontal position. Thus work can be done by it on table, wall or ceiling. The cup referred to contains the paint or other liquid to be sprayed, and a series of cups may be provided for a quick change of colour, if desired, as will be readily understood.

In cases where electric power is available, the electric motor outfit shown in Fig. 37 will be found very serviceable. It may be described as a "spray painter" outfit complete, and it includes a 1 h.p. motor mounted on a trolley ready for use, which can be employed for many different classes of work. In Fig. 38 it is shown being used for spraying paint on a gas holder. It would appear that this outfit might be employed for ship bottom painting, provided that the necessary scaffolding or its equivalent were available.

Going to the other extreme, a form of Aerograph, suitable for very delicate work, is shown in Fig. 39. This may be used for photograph retouching, pottery, and small decorative work generally—in fact, it is suitable for any work requiring delicacy of work.

In another chapter the subject of artistic work done by means of compressed air is considered at some length, and a number of specimens of work executed by this process are given.

The Aeron.

The system of spraying is manufactured by the De Vilbiss Manufacturing Co., Toledo, Ohio, U.S.A., and 71, Newman Street, London, W.

The method consists, briefly, in spraying stain, shellac, varnish, or enamel, as the case may be, on the work by the means of compressed air, under a pressure varying from 30 to 80 pounds per square inch.

The necessary outfit comprises the Aeron sprayer itself—which is made in two styles and several sizes—the air compressor and receiver, an air transformer set, for regulating the pressure and purifying the air supply, a steel Fumexer, in which the work is done, and an exhaust fan to disperse the vapours or fumes resulting from the atomising of the finishing material. Besides these necessary appliances, turn-tables are used where advisable to facilitate the handling of the work while the "aeroning" is being done.

The two styles of Aerons are shown in the illustrations, each connected with an air transformer set to which the air supply pipe is attached. In one style of Aeron the finishing material is carried in a pint or quart capacity cup forming a part of the Aeron itself. The other machine takes its supply from a five-gallon container suspended above the nozzle. The work is placed in the Fumexer on the turn-table, which can be tilted at any convenient angle and revolved by hand, and is coated on the top, sides, and front, with the drawers in place. Drawers may also be finished inside by this process.

A full coat can be applied to a vertical surface as well as to one in a horizontal position. There are two reasons why. It is impossible to put on a perfectly uniform coat with a brush, especially on a vertical surface, and runs and sags are caused by the heavier portions of the coat dropping down over the lighter coated spots. With the Aeron a uniform coat of varnish is applied, and it is a fact that as the coat is the same over the entire surface it will not run, even if it is as heavy as a brush coat, which would show sags. The second reason is that an Aeron coat sets somewhat quicker than a brush coat, owing to a slight evaporation of the solvent caused by the action of the compressed air. Again, since the coat is uniform it sets uniformly. For the same reason the final drying is hastened to some degree.

In brushing flat horizontal surfaces where a heavy coat is applied it is almost impossible to avoid fatty edges, and in brushing carved or ornamental relief work the low spots are sure to collect more varnish than the high places. Corners and edges in panel work are also causes of difficulty in this respect. In using the Aeron the surface varnished is covered uniformly and evenly, whether flat, panelled or carved, with one sweep of the machine at an even speed. Consequently, heavy spots, runs and fatty edges are entirely avoided.

In applying finishes with the Aeron a slight reduction of the material with turpentine or other solvents is logical; with some materials, necessary. Reducing is logical on account of the slight evaporation of solvent previously mentioned. In other words, in order to have the Aeron application of the same consistency after reaching the surface as it is when brushed, a reduction of from 5 to 10 per cent. would be required. This is not always desirable by any means, especially in the case of the heavy gloss coat spoken of. Whenever a free-flowing coach varnish is used, reducing is not necessary. Conditions in different plants govern this matter to a great extent.

Rubbing varnishes are usually reduced slightly, as a full-bodied, heavy coat is not required, also because a rubbing varnish does not flow as readily as a coach varnish. On account of this evenness of the coat, an Aeron coat of rubbing varnish is somewhat more easily and quickly rubbed either by hand or by machine.

Undercoat varnishes are reduced to some extent for Aeron work for similar reasons. They are also more easily sanded than when brushed.

Pigments, primers or first coaters are Aeroned to better advantage if the pigment content is less than for brushing. It is also sometimes desirable and expedient to use a more finely ground pigment.

Flat finishes are applied to great advantage, and, because of greater uniformity of surface, more closely resemble a rubbed finish than when brushed.

This spray offers perhaps greater advantages in the application of shellacs than with other materials. It is a matter of common knowledge that shellac is very difficult to brush, while, on the other hand, it sprays easily. Instead of cutting the gum 4 or 5 pounds to the gallon of alcohol, as for brushing, the material is used in the proportion of 2½ to 3 pounds of gum to the gallon. The sprayed coat of shellac is perfectly smooth and uniform, and requires practically no sanding compared with the work necessary to sand a brushed coat.

The Aeron is simple and easy to operate, and a week's work is sufficient to make an efficient and expert operator of the average man.

The work is clean, and, what is more, is healthful and sanitary, as all vapours and fumes are removed from the finishing room by the exhaust fan used with each outfit.

Cleaning the Aeron is accomplished by spraying a solvent through the nozzle instead of the finishing material, and the whole machine may be placed in a can of thinner overnight if desired. It is necessary to clean the Aeron but once a day if it is used more or less continuously. The whole operation of cleaning may be performed in a minute or two.

There is usually some loss in finishing material when the Aeron is used, though this is hardly noticeable except on small work, when it may amount to 15 or 20 per cent. In many cases there is no waste, in others an actual saving—where coats are eliminated by the use of the machine. On an average, however, there is some slight loss—largely of the solvent used in reducing the material. In any event, however, the waste is offset many times by the saving effected in time and labour, without taking into consideration the saving in floor space, the greater ease in handling the work, the better quality of the work done, the advantage to the workmen, and, finally, the general all-round convenience of the Aeron system.

Description of the Illustrations.

We will now give a description of the illustrations of the different parts of the Aeron. Fig. 40 shows the attached cup straight barrel Aeron, which can be fitted with either metal or glass cups. Metal cup is the standard equipment.

Attached cup Aerons can be furnished with four sizes of nozzles: F—.047" dia., E—.070" dia., D—.081" dia., and C—.094" dia. These nozzles are selected according to the nature of the work that is to be done.

Fig. 41 shows an attached cup angle barrel Aeron, a type of advantage in spraying work lying in a horizontal position.

In some classes of work, where the liquid to be sprayed is apt to settle, an agitator is required, such as is shown in Fig. 42.

In Fig. 43 is shown an attached cup Aeron with type V double nozzle spray head. This type is especially adapted for the finishing of large surfaces, such as automobile bodies, case furniture, etc.

The V spray head shown separately in the above illustration is interchangeable with the single nozzle spray head, and can be attached to any standard Aeron.

Type G Aeron is illustrated in Fig. 44. The nozzle is ·027 inch in diameter This Aeron can also be furnished with other sizes of nozzles to special order.

In Fig. 45 are shown various attachments used with this form of sprayer. At the top left-hand corner is a cup holder and screws, and on the right, metal or glass cups. Beneath are cleaning pail and length of pressure tubing with connections.

Fig. 46 shows the construction of the Aeron, the parts being disassembled for ease of cleaning.

We now come to the type of Aerons which take their supply of material from a five-gallon tank placed above the level of the nozzle, the fluid flowing down by gravity. This arrangement offers all the advantages of the pressure feed tank type with none of its disadvantages. It is shown in Figs. 47 and 48.

The next type is termed the M Aeron with V-A spray head and is illustrated in Fig. 47.

There is the same advantage in the use of this Aeron as with the former type in that the supply of material is taken from a container placed overhead, the fluid flowing down by gravity. The slightest pull upon the trigger releases the material instantly—the varying of the pressure adjusts the flow automatically. As with the type L there is but one adjustment.

This type Aeron is built for large work. It produces a "wide spray," similar to the V spray head, which covers large surfaces with remarkable rapidity.

The nozzle can be turned so that a full, fan-like spray is projected horizontally, vertically, or in any intermediate position.

A container with agitator and hose for types L and M Aerons is shown in Fig. 50.

It is drawn from one sheet of steel and is heavily tinned to prevent rust.

The standard and block and tackle (Fig. 49) are furnished for holding the container above the level of the Aeron. The block and tackle, which is of the safety, self-locking type, is recommended where it is possible to obtain a ceiling fastening. The standard (Fig. 51) can be used if the block and tackle is impracticable.

The air transformer set (Fig. 52) is for the purpose of regulating and purifying the compressed air, and is a necessity with each Aeron installed. The air duster (D) is a valuable addition to the equipment for removing loose dust or dirt from the work before finishing.

The auto-filter (S) is for removing dust, grit, and oil from the compressed air. The auto-regulator and gauge (R) makes possible the regulating of the air pressure between 5 and 80 pounds, simply by turning a thumb-screw. The auto-condenser (C) is for separating moisture from the compressed air.

Full details of the air compressor and other special appliances used in connection with the Aeron will be found elsewhere.

The Airostyle.

Next comes the Airostyle and Lithos, Ltd., with their "Airostyle" type Record, also a concentric jet-type, an illustration of which is given in Fig. 55 and a section of which we give also in Fig. 54. This type is largely used for small, medium or large work, and is considered to be extremely sensitive and very fast in operation. It has a gunmetal casting for the main body, and all wearing parts are of steel, tempered or case hardened.

It is supplied either with or without pressure fed accessories, although the makers are strong advocates of pressure feeds for most work, and with which they have met considerable success.

In addition to the Record Pistol this firm makes other types, and in order must be mentioned the type "Ultra" for use where frequent changes of colour may be necessary, and where no continuous colour feed is required. This type is specially adapted for stencilling in oil colours, and is illustrated in Fig. 56.

Another type about to be introduced is shown in Fig. 53, for small and medium work, simplicity of construction and ease of adjustment being specially claimed for this type. This type is called the Airostyle Pistol "M."

Yet other type of Airostyle may be mentioned for smaller work, namely, the "Stencil and Universal Lustre and Photo," which is illustrated in Fig. 53.

These types are mainly employed for decorative work, and give a very wide selection, but save that they are employed in process work and for textile goods, they do not enter into the scope of this book.

As the pistol type is used for all general painting and japanning, the adjustment of this may be advantageously explained in greater detail, and referring to Fig. 54 the instructions are as follows:—

If a wide spray is required the nozzle 8 is unscrewed to a small extent and then locked by means of a small ring 8a, but if a narrow spray is wished for, the nozzle 8 is screwed on farther and locked in position. On all-round work one position serves, that is, a position in which the recessed point of the nozzle coincides with the end of the tip 9, which just allows the needle 5 to project through it. If it should happen that a leakage of colour occurs through 9 when spraying is suspended, this may be obviated by giving an increased tension to spring 15 which may be obtained by screwing in the spring box 16 slightly. It may happen that some foreign matter becomes lodged in 9, in which case the pistol from the colour tube must be first disconnected, the nose 4 removed and the parts brushed or swilled with turps or spirit. This having been effected, the nose 4 will be replaced, care being taken that the washer 3 beds down on its seating in the body of the pistol.

It is well to provide for any possible contingencies which may arise, and it may, therefore, be remarked that if any leakage of air occurs through the air valve 19, it will probably be caused by some grit to be found on the face of air valve washer 20, and to remove this the handle can be taken off by unscrewing the milled nut 25 and the valve body 23 in order to get to the air valve. The replacement of the parts is simple.

The adjustment of the pistol for working is made as follows:—First release the screw 14 on cross head 13, and while the instrument is connected up with the air supply, slightly withdraw the trigger 12, allowing a small amount of air to pass through the nozzle. While maintaining the trigger in this position, move the cross head along the needle until it just bears against the cam 10, and in this position clamp the same by screwing home the clamping screw 14.

Another important point is to see that the lubrication of the compressor is carefully watched; the sight feed lubricator on top holds sufficient lubricant for 10 working days and is capable of adjustment by means of a needle valve. It can, therefore, be easily set. The main bearings are ring oiling and only need occasional attention. The fan bearings are ball bearings and only need attention once in six months.

The Eureka Spraying Machine.

An inspection of Figs. 58 and 59 will show that this machine is of entirely different construction to those already described. The advantages claimed are the simplicity in construction, the absence of complicated parts which might get out of order, and the fact that the machines cannot clog in use. The operation is as follows: The feed pipe from air tank should be connected up with a high-pressure hose to the sprayer and a fluid nozzle is then adjusted so that the tip of the nozzle is in alignment with the centre of the air nozzle. This adjustment varies according to the thickness of material. When using thin liquids the fluid nozzle should be slightly below the centre of the air nozzle. When ready for spraying the valve is pressed, and this releases the air, which blows through the air nozzle, across the top of fluid nozzle and draws up the material from the inside, spraying the fluid on to the work. The shape of the spray is that of a fan. The air pressure required varies from 20 to 50 lbs. according to the viscosity of the material being dealt with. As a rule the best results are obtained when the machine is held from 4 to 6 in. away from the work. The Fredk. Crane Chemical Co., Armoury Close, Bordesley Green, Birmingham, are the agents for these machines, and it is also manufactured in the United States. The remarks given under the head of "Exhaust," "Air Compressor," etc., will apply to this machine also. It may be observed that the cup holding the metal can be very quickly changed, so that a variety of colours may be sprayed one after the other with very little trouble. The price of the machine (which may be provided with an agitator if required) is low.

"The Invincible."

These sprayers are designed on an excellent "universal movement" principle and are particularly well constructed throughout. They further claim distinction on the score that every section is an independent unit of standardized pattern, and as spare parts are always available, replacements and renewals can be made instantly with the minimum of trouble and cost.

They are made in several types to suit varying classes of work, ranging from the fine lining and shading of small objects required by artists, etc., to commercial painting on the scale practised by motor car manufacturers and other large users; and important points in their favour are the special facilities provided for quick cleaning after use, their completely enclosed mechanism, which prevents the penetration of any dirt, colour or liquid to working parts, and the adjustment of the needle so that no pressure or damage can occur at any time to the colour tip, even if trigger is allowed to snap forward.

Automatic control is also provided for work requiring a given volume of colour, it being only necessary to turn a regulator to ensure a constant flow of any desired density.

The "Invincible" Pistol Sprayer type "E" is illustrated in section (Fig. 60). The body is gun metal with working parts of finest steel, highly polished where necessary to ensure smooth working and minimum of wear. The instruments have a handsomely nickelled finish.

Working.—When connector (4) is attached to colour cup or extension cock for paint pot and connector (18) affixed to air-line, the air brush is ready to commence work.

Dusting.—No separate or special dusting attachment is necessary with the "Invincible" apparatus, as a slight pull on trigger (20) carries back steel sleeve (7), depressing steel ball (15) and bringing the air valve (19) into "open" position. This allows a stream of air to pass forward through air channel (21) to nozzle (1) and the projection of this upon surface to be treated removes all dust.

Spraying.—A continuation of the trigger pull carries back the spring buffer (8) and opens the control barrel (11) of the needle valve (3) allowing colour to flow through colour tip (2), at the end of which it meets the vacuum created by the air stream, and is then (atomized) broken up into thousands of minute particles which coalesce upon meeting their object, and form a perfectly level, smooth and even surface.

Control.—The trigger control is very sensitive, giving a spraying range from a fine line to the fullest flush of the instrument's capacity. For automatic control it is only necessary to adjust regulator (No. 14).

Cleaning.—By unscrewing union nut (5) the complete fore-part may be removed for cleaning without interfering with any other part of the mechanism, therefore the operation of cleaning is always quick and thorough, even when sticky, heavy liquids have been used.

The movement for the above sprayer applies to all "Invincible" sprayers. Type "A" for artists, miniature tinting, black and white process work, etc. Type "B" for Christmas and show card work, etc. Type "C" for ceramic work, posters and so on.

A cheaper series is also made which, whilst lacking some of the refinements of the standard models, are reliable, and soundly made instruments.

The patentees and manufacturers of "Invincible" Air Brushes are the Air Brush Manufacturing Co., Ltd., Pneumatic Works, 13, Arlington Street, Rosebery Avenue, London, E.C., who also provide all accessories for complete spraying installation, such as air-compressors, air-receivers, air valves, reducing valves, condensers and clarifiers, pressure paint pots and paint pot stands, exhaust benches, fans, turn-tables, automatic cut-outs, motors, etc., of which complete details will be sent upon request.

The Midland Sprayer.

From what has already been said, it will be obvious that the most important part of a spraying plant is the correct construction of the actual sprayer or spraying pistol. The Midland Fan Co., Ltd., of 46, Aston Road, Birmingham, have placed on the market a sprayer which is shown in Fig. 65, and this, we learn, has given a great deal of satisfaction. Its principal claim to superiority is the simplicity of construction, for one reason because the sprayer may be used by those who have but little knowledge of mechanics. The construction permits of easy cleaning, which is of vital importance, and for this purpose the needle can be removed by a turn of the fingers and replaced with equal ease. The illustration shows the larger pistol, and it may be mentioned that smaller sizes are made for finer work.

The Paasche Air Brush.

This spraying machine is manufactured by the Paasche Air Brush Co., 9, South Clinton Street, Chicago, in various sizes, for spraying by compression air liquids of all kinds. The above illustration shows what is known as the "Three in One" quick action model "S" brush, and is recommended for use with heavy material. Instead of the paint pot being at the top an underslung jar with aluminium cover as shown on the lower part of Fig. 67 may be used. This figure shows the same air brush or sprayer with one gallon container; in either case the spraying of the colour or material is quickly done. The flow of the liquid is entirely independent of the air circulation. An agitator is provided to keep the material well stirred, and should be used with all liquids which have a tendency to settle, such as bronzes, enamels, paints, etc. The coupling or taper stem of the flexible metal tube or bottles are easily detachable, and one material after the other can be used without stopping. If the underslung jar or the gravity pot be used no agitator is necessary.

This instrument is made in much larger sizes. The general form of apparatus is shown in Fig. 68, the special finishing hood with exhaust fan being in this case employed, while a paint container is suspended on an adjustable stand. In this case a chair is shown being painted.

A somewhat novel apparatus for separating oil and water is that shown in Fig. 69. This is constructed to obviate the trouble which is sometimes caused by the oil and water in the air main mixing. It is connected at the end of the air main as close to the air regulator or air outlet to the brush as convenient. Oil gets into the air main through the piston of the compressor, and will in time saturate the walls of the air pipes. The condensation of water which is due to changes of temperature makes the iron pipes sweat, and a considerable amount of water is accumulated in this manner. The simple appliance shown in the illustration will remove this.

Fig. 70 shows the Paasche Automatic electric controller which is used for automatic starting and stopping motor driven compressor outfits where from 1/8th to 1 horse-power motors are used.

CHAPTER VII.

The compressors must be designed to give an absolutely pure supply of air free from oil or grit, and the air-main must be so arranged as to avoid any trouble due to condensation. This is overcome, first, by using, at least for high-class work, a horizontal air compressor, water cooled, with a sensitive sight-feed needle valve lubricator to the cylinder, capable of delicate adjustment, so as to avoid excess of lubrication. Needless to say, the machining of the cylinder, piston, etc., of the compressor must be of the highest class to ensure satisfactory running under such conditions. Then a suitable air filter must be fitted to the intake of the compressor. This filter should be of large diameter, and have a gauze screen and wad of cotton wool.

The air valves should be so arranged that they may be removed without breaking the water joint, and be so arranged that they may be taken out and replaced in a few seconds.

The main tank must be provided with safety valve drain cock and pressure gauge, and the air main, of not less than ¾ in. diameter barrel, preferably steam barrel, must be arranged to travel in a downward direction from the main tank towards an end station tank, which is a duplicate in miniature of the main tank or receiver. All branches must be taken upwards off the main. Such an arrangement, if care is taken that the end station tank has its relief valve set to blow off before the main tank safety valve acts, ensures that all moisture is collected in the end station tank.

The Airostyle and Lithos, Ltd., claim to be the originators of such a system, and it is here explained in print for the first time.

All experiments with a view to scrubbing or purifying the compressed air of moisture, oils, etc., on the air main side of the main tank, other than the above simple expedient, have proved more or less failures, either from want of appreciation of the increased capillary attraction set up in the tanks and air main, or from the difficulty of satisfactorily and frequently cleaning out such scrubber as may be inserted, and the incidental difficulty created in the prevention of dust being carried out of the scrubber throughout the system.

It need hardly be said that ample air must be provided in order to have sufficient pressure always available for the full number of operators employed, and this point is one which cannot be too strongly emphasized, for in too many cases firms have installed small compressors and have been misled as to their maximum capacity, mainly through over anxiety on the part of the salesman to secure his order for apparatus, he apparently having been afraid to ask a high figure for a really efficient compressor.