Rather than metal-to-metal contact as in traditional bearings, some applications - particularly very high applications, use compressed air to provide a 'cushion' on which to support a rotating shaft. Any possibility of condensation must be avoided, as this would cause failure of the bearing: hence the need for dry air, and the need for a moisture meter. Usual moisture:<-10°C.
(Or Dry Boxes) are enclosures with - usually - two access holes fitted with Neoprene gloves so that operations can be carried out in the protected environment inside the box. These operations vary widely and include the handling of dangerous or hygroscopic chemicals, assembly of transistors and other semiconductor devices, and numerous research applications. In many of these applications it is essential that the atmosphere inside the glove box is dry. The atmosphere is often air - but it may be nitrogen or any one of several other gases depending upon the application. The best way of installing the moisture monitor is to fit the sensor in the top of the box (where the moisture level will be at its highest - as water vapour rises), with a loose length of cable inside so that the sensor can be moved around to look for wet areas - caused by uneven flow of the purge air or gas. The alternative is to fit the sensor in the exhaust line, when it will give an average reading of the box moisture. Usual moisture: generally drier than -60°C.
Wave guides are tubes that are used in place of cables for the power connection to radar and microwave telecommunication dishes. The tube is tuned to the frequency of the power and, because of the high voltages involved the tube must be continuously purged with dry air. All the European early-warning radar stations are equipped with Shaw Meters. Usual moisture: <-60°C.
(Polyethyleneterapthalate) P.E.T. is a plastic developed specifically for making bottles for fizzy drinks such as Coca Cola. With an external coating to improve its porosity to oxygen it is also used for beer. It has become very popular as it reduces weight and hence transport costs, and the bottles are regarded as disposable. Its only real problem is that it is extremely hygroscopic - so hygroscopic that it is inevitably too wet to use (producing opaque or porous bottles) when it reached the drink manufacturer. The first part of the bottle making process is therefore to dry the P.E.T. by passing warm dry air through it prior to the actual moulding process. There is no equipment which will measure the moisture content of the plastic, but our instruments are used in hundreds world-wide to check that the dryer producing the dry air is working correctly. Usual moisture: <-40°C.
Date marking (sell-by or consume-by) of foods is increasingly important. There are many ways of making such marks on food packs, and one of the latest is with a laser beam. A high-power beam of laser light shines through a stencil and burns the date onto the surface of the packet. The advantage is the speed - up to 25 packs per second. The laser is switched on and off at high speed by a special device which relies on a flow of dry purge air to operate correctly. The switch costs about £600 and is damaged if the air is too wet: hence the Shaw Moisture Meter. Usual moisture: <-50°C.
As can be seen from the Black list , Chlorine is one of the few gases which attack the sensor, even at very low concentration. However, many hundreds - if not thousands - of Shaw instruments are used to measure the moisture content of the compressed air used to pressurise and purge tanks and piping used for the storage and transporting of chlorine liquid. This air must be dry, or it will become highly corrosive when mixed with the chlorine vapour, and attack the materials of the pipes and tanks. Usual moisture: <-50°C.
Dry compressed air is used in many systems for the transport of powdered or granular materials, such as soap or detergent powder, hops and grain. The air must be reasonably dry or the material will absorb moisture from it, and then will not flow properly, with the system becoming blocked. Usual moisture: <-40°C.
VESSEL AND PIPE DRYING
Large vessels - for boilers, transformers, storage tanks, etc. - are usually tested hydraulically for leaks and pressure security. Following this they must be dried to prevent rusting, and this is usually carried out by passing warm dry air through them (or, sometimes, nitrogen). During the first part of the drying process the exhaust air from the vessel is, of course, very wet, and no measurement is needed. Towards the end of the drying, which may take many days, it is necessary to measure the moisture in the exhaust. This may be done with an in-line instrument with remote sensor located in the outlet port of the vessel, or with the Shaw Automatic Dewpoint Meter by simply putting a length of flexible pipe into the outlet, so that some of the air passes through the instrument head assembly. Usual moisture: <-20°C.
This is another application - see Chlorine Padding Air - where our instruments are used in an application which is potentially hazardous to the sensor. Ozone is increasingly being used instead of chlorine to purify the water in swimming pools: the pool water is much more pleasant, and does not irritate the eyes. The ozone is simply bubbled into the re-circulating water. Because the ozone is a very powerful oxidising agent, and contact with moisture before it actually enters the water will result in severe corrosion of the gas generating plant, the ozone is blown into the water with dry air. Usual moisture: <-50°C.
All applications requiring a supply of dry air obviously need a compressor and dryer. There are three types of dryer in common use:
1. Refrigeration dryers. These simply cool the air, separating the resulting condensation. They are only capable of producing a dewpoint temperature of about +3°C at a typical operating pressure of 7 bars (equivalent to about -20°C dewpoint at atmospheric pressure) and so are only suitable for installations where no part of the system will ever be below freezing point.
2. Pressure-Swing dryers. These have two chambers filled with desiccant which are alternately in-line, and drying the air flow, or being regenerated. The changeover cycle is quite short - usually no more than 5 minutes or so - so that the desiccant is only absorbing a small amount of water vapour at the operating pressure which is usually about 7 bars. About 10 to 15% of the dried air is reduced to atmospheric pressure and passes through the chamber to be regenerated: because the pressure is now 7 times lower, and the purge air is very dry, the desiccant is regenerated. Depending on the design, these dryers may produce air with a pressure dewpoint of about -40°C or better than -70°C. The hidden running cost is the purge air, which means that the compressor must be 10 to 15% larger than would otherwise be needed.
3. Heat regenerated dryers. Like the pressure-swing type, these have two chambers filled with desiccant. In this design the cycle time is more usually 4 to 8 hours, resulting in the desiccant being heavily loaded with moisture. A small proportion of the dried air, or atmospheric air is used to purge the chamber under regeneration, while heaters raise the temperature of the desiccant to 250°C or more to drive out the moisture. All types of air (or gas) dryer have one thing in common: they need a moisture meter! A dryer without a moisture meter is like a car without a speedometer - there is simply no way for the user to know if it is operating correctly, and it is usually far too late when problems show up in the factory. A simple monitor and alarm is sufficient for the refrigeration dryer: the pressure swing and heat regenerated types can both have their operating cycle controlled by a dewpoint monitor with substantial cost savings being made - often saving the cost of the instrument in a few months. Usual moisture: depends on dryer type.
INSTRUMENT AIR SUPPLIES
Used for operation of pneumatic instruments in control rooms and similar situations. Excess moisture can result in condensation or ice formation, and the loss of control of the instrument readings leading, at worst to shut-down of the complete plant. Usual moisture: <-40°C Dewpoint.
PNEUMATIC TOOL SUPPLIES
Pneumatic tools are widely used in assembly work. They rely on a supply of clean dry air. Excess moisture can lead to a high rate of tool wear and failure. Usual moisture: <-40°C.
High quality paint spraying, particularly of motor vehicles, relies on clean dry air. Excess moisture will result in a poor surface finish, leading to expensive re-spraying. Usual moisture: <-40°C.
The simulators used for training aircrew have oxygen systems as in the real aircraft they represent. Because they are operating on the ground, and for safety reasons, air is used instead of oxygen but this makes no difference to the application.To keep the storage cylinders small, the gas is stored at high pressure: if the moisture content is too high, ice may form in the small orifice of the pressure regulator and cut off the supply. Usual moisture: <-50°C.
BREATHING AIR FOR FIRE-FIGHTERS
This application is very similar to the aircraft simulator application previously described: to avoid the risk of ice blocking the regulator orifice, the moisture content must be low. Usual moisture -50°C or drier. (N.B. This does not apply to breathing air for diving - the water in which the system is immersed acts as a huge heat-sink, and prevents ice forming.)
HOSPITAL OPERATING THEATRES*
Dry, sterile air is used widely in hospital operating theatres. The main uses are in pneumatically driven operating tools - where they are preferred to electrically operated tools because they are easily sterilised. Usual moisture: <-40°C. * Separate report available.
Yes, the Shaw Moisture Meters are used in banks! Many types of sorting and handling machinery are operated by compressed air, and cheque sorting machines in the major clearing banks are just one example of a system which relies on a continuous supply of clean dry air. Usual moisture: <-40°C.
Explosive materials are subject to batch testing to ensure efficiency and safety. The moisture in the atmosphere surrounding the explosive can have a profound effect on the resulting explosion - the charge either failing completely, or the force of the explosion being much greater than expected. In the quality control testing therefore the atmosphere's moisture must be controlled. Usual moisture: -10 to +20°C.