Which mechanism is preferable for reciprocating compressors
Compressors and Nitrox systems
1 Compressors and Nitrox systems Compressor accessories and selection of Nitrox systems
2 Compressors and Nitrox Systems Scuba Publications Daniela Goldstein Jan Oldenhuizing All rights reserved. This work including all its parts is protected by copyright. Any use outside the limits of the copyright law without the consent of the publisher is not permitted and is punishable. This applies in particular to reproductions, translations and storage and processing in electronic systems. The reproduction of common names, trade names, trademarks, etc. in this work, even without special identification, does not justify the assumption that such names are to be regarded as free within the meaning of trademark and trademark protection legislation and can therefore be used by everyone. With thanks to Lenhardt & Wagner GmbH for the provided pictures and to Aerotechnica Coltri S.p.A. for a compressor block for recordings.
3 Table of Contents Compressors ... 3 Accessories & Selection ... 24 Nitrox Systems ... 37 Index ... 49 Introduction All divers at a higher level should have knowledge of compressor handling and Nitrox systems. But there are almost no publications on this subject. This book explains all the important concepts and principles for anyone who would like or need to know more about this topic. Compressors (and the attached Nitrox systems) can be seen as the heart of a dive center. If the compressor stops working, all diving activities will stop. Even if you do not operate a compressor yourself, knowing about it can improve your own safety during your diving activities. Clean air and the correct nitrox mixture are important for the safety of every dive. Knowing the equipment used to fill the tanks will help you identify the signs of a potential problem during the filling process. In addition to the security aspects, this topic is also interesting. In order to better understand the working principles, repair procedures and setting procedures for compressors are explained again and again throughout the book. It is NOT the intention to train you to repair or maintain compressors. All maintenance and servicing must be carried out by a trained repair technician from the relevant manufacturer. Only these technicians have the right tools and spare parts, are informed about any changes and recalls and have the manuals and protocols for the individual components of the system. The unauthorized manipulation of compressors and Nitrox systems leads to the loss of the guarantee and shifts the responsibility of the manufacturer to the person who manipulated the system. Just reading this book will not give you a complete understanding of the matter. This book was written to be used in conjunction with a course on Compressors and Nitrox Systems. The instructor will help you understand all the aspects covered in this book and show you practical examples.
4 compressors Compressors are part of a filling system. This chapter is limited to the compressor block and the parts directly connected to it. The motor, the filling console, the accumulator and other parts are covered in the next chapter. First, the most common types of compressors and their most important properties are explained. Next, the individual parts of the compressor are discussed with their functions and purposes. It will become clear that it is not possible to increase the air pressure to 200 or 300 bar without this having consequences for the humidity and temperature. Many parts of the compressor block are necessary to prevent humidity and temperature complications. Page 3
5 Rotation and Axial Movement In an internal combustion engine, a timed explosion moves a piston (the gray part in the drawing) up and down. The resulting axial force is transmitted to the crankshaft (the green part in the drawing) via the connecting rod (the blue part in the drawing). The connecting rod is mounted (away from the center of rotation) so that axial movement of the piston causes rotation. This rotation is passed on through several mechanisms and thus rotates e.g. the wheels of a car. A compressor has the same parts, but in the end it works exactly the opposite. The rotation is provided by an internal combustion or electric motor. The rotation is used to drive the crankshaft. The piston is moved up and down by the connecting rod and thus changes the volume in the cylinder again and again. When the piston is pushed up, the volume of the cylinder decreases and the pressure increases. Thanks to valves, the compressed air is pressed out of the cylinder in the right direction. A second valve allows new air to flow in for the next compression cycle. Divers want their cylinders to be filled to a pressure of 200 or 300 bar. To achieve this pressure with a single cylinder, a piston would have to move between the lowest and highest positions by a factor of 200 to 300 to change the volume in the cylinder. While it would be theoretically possible, it usually doesn't work that way. Most compressors in diving centers have 3 or 4 cylinders, also called stages. Air enters the compressor through the first stage, where it is compressed to medium pressure. This compressed air is then in turn compressed to a higher mean pressure. A pressure of 200 or 300 bar is only achieved in the last stage of the compressor. This could give the impression that the air is being compressed in certain steps. The compressed air is passed on from one stage to the next after it has been compressed to a new medium pressure. That's not the case. You should think of it as an ongoing process. A valve allows air to flow into the cylinder while the piston is moving down and closes again when the piston is moving up. At that moment another valve opens and allows air to flow into the next cylinder. The valve closes again when the piston begins to move downwards. T, W or X The most commonly used compressors for diving belong to the types T, W or X. There are also other types, but they are less common. Once you understand how the most common models work, you can apply this knowledge to other types as well. page 4
6 The name T, W or X refers to the position of the cylinders. In a T-type compressor, the first stage is on top of the compressor and the second and third stages are horizontal on the sides. A W compressor also has three stages, the only difference being that the second and third stages are mounted at a slightly upward angle. W compressors are larger than T compressors. X compressors have four cylinders that are mounted in the shape of an X. T-compressors have a 90 degree angle between the first stage at the top and the two horizontally mounted cylinders on the sides. T compressors are intended for personal use. They are small and can take up to 20 minutes to fill a bottle. Divers diving in remote areas use this type of compressor to fill their own cylinders. The main problem with these compressors is lubrication. The compressor does not have an oil pump or other efficient mechanism to distribute the oil during filling. The lubrication is achieved by a centrifugal pin which is mounted on the crankshaft and which scoops oil from the oil pan and throws it upwards into the interior of the compressor. For this primitive lubrication mechanism to work, the crankshaft must rotate very quickly. It is normal for these models to make up to revolutions per minute. At this speed, the compressor heats up quickly. Because of this problem, combined with poor lubrication, most T-compressors should be turned off every half hour to cool down before filling another bottle. W compressors also have three cylinders. But they are equipped with a better system for oil distribution. They turn only half as fast as T-compressors. The lubrication is achieved either by an oil pump or by a mechanism that lubricates the moving parts in the machine with the piston movement. Most W compressors can run all day without a break for cooling. Thus, they can be used commercially. The average compressor of this type can fill a bottle in 8 to 10 minutes. T- page 5
7 compressors are mostly portable and weigh less than 50 kg. W compressors are heavier and weigh more than 100 kg. The X-type compressor has four cylinders arranged in an X shape. They can compress more air in a shorter time and are used in larger dive shops. The air enters the compressor through the first stage, which can be easily recognized by the air filter that is mounted on it. The size of the air filter is an indication of how much air can be compressed by the compressor. The larger the air filter, the faster the compressor can fill bottles. The second tier is on the opposite side of the first tier. The third stage faces down on the other side of the compressor and the fourth and last stage is opposite to the third stage. Four cylinders have several advantages. The compression (from ambient pressure to 200 or 300 bar) now takes place in four steps (as opposed to three). In a three-stage compressor, typical compression steps are as follows: 1 bar 7 bar 48 bar 220/330 bar In a four-stage compressor the steps could be as follows: 1 bar 3.5 bar 17 bar 67 bar 220/330 bar The pressure load between two adjacent stages is halved So with a four-stage compressor, compared to a three-stage compressor. This leads to a significantly lower level of noise pollution and also reduces the wear and tear on valves and other moving parts. Another important aspect is that the pressure ratio between two adjacent stages results in a lower temperature rise in the air. The temperature at which the air is compressed in a compressor is an important aspect that we will discuss in detail later. The pressure up to which the bottles can be filled is not determined by the compressor. Almost all compressors can fill bottles with 200 or 300 bar. A compressor can compress air to a much higher pressure and must therefore be equipped with a safety valve (ultimate pressure safety valve) to prevent the compressor from destroying itself. It is the attached ultimate pressure safety valve that decides the pressure up to which a diving cylinder can be filled. Due to the important function of the final pressure safety valve, this is sealed by the manufacturer. If this valve malfunctions, it will not be repaired on site, but replaced. In many cases you can send the broken valve to the manufacturer for exchange. A compressor can therefore be used as required for 200 or 300 bar. This is not the case with various other pieces of equipment in the system. The filter housing, lines, filling console, storage bottles and filling hoses must all withstand the pressure at which the compressor operates. Although it would be possible to install a 300 bar safety valve in a 200 bar compressor, you should not do this without consulting the manufacturer. page 6
8 Temperature and Humidity You cannot increase the pressure of a gas without increasing the temperature. As we saw earlier, this problem is greater in a three-stage compressor than in a four-stage one. However, the increase in temperature is an important consideration for all compressors that fill cylinders to 200 or 300 bar. We'll discuss other considerations later. But before we start with the functions of compressors and their individual parts, we have to deal with the influence of temperature on humidity. 100% humidity at sea level degrees Celsius Humidity in grams per kg of air for 100% humidity grams grams grams grams grams grams grams The amount of water in the air is expressed as humidity in percent. 100% humidity means that the maximum amount of water is in the air. If the amount of water exceeds 100%, the excess condenses and becomes liquid (water). But water is not compressible. If water condenses in the compressor, then there is a risk to the compressor. If the (upward) movement of the piston is blocked by water then the piston itself, the connected linkage or the crankshaft can be damaged. It is therefore very important to keep the humidity in the compressor below 100%. The humidity is hardly dependent on the density of the air at all, but on the temperature. The amount of water in the air, which corresponds to a value of 100% humidity, is not influenced by the density of the air, but will change extremely with a change in temperature (see table). You should be familiar with this concept by now. If you put a cold drink on the table on a warm summer's day, the water in the air will begin to condense on the outside of the glass. The air near the glass is cooled by the cold drink. At the lower temperature, the humidity in the air now exceeds 100%. The same concept also applies to regulators. Many divers choose a metal or carbon second stage because warm exhaled air is cooled on the colder inside of the second stage, some of the moisture condenses and is inhaled again with the next breath. Divers with a metal or carbon second stage (good heat conductors) are therefore less affected by a dry mouth during the dive. Two opposite processes take place in a compressor. The air is compressed. The same amount of moisture is now in a smaller volume and the humidity rises. At the same time, the temperature rises, which in turn reduces the humidity. For this reason, no problems with humidity in the first and second stages of the compressor should be expected. Let's assume that we have a three-stage compressor and want to fill bottles with an ambient temperature of 10 C and a humidity of 50%. As we have seen before, the first stage in such a compressor reduces the volume by a factor of 7. Thus, the humidity would rise to 350% and lead to condensation. At the same time the temperature rises. Air at 10 C and a humidity of 50% would correspond to water of 3.88 grams per kilogram of air (7.76 divided by 2). If the air temperature were to rise to 50 C, then the 3.88 grams of water (at the same pressure) would now lower the humidity below 5% (see table). Thanks to the rise in temperature, the humidity drops (air with less than 5% humidity which is compressed by a factor of 7 results in a humidity value of less than 35% and that in turn is lower than the humidity in the surrounding area - Page 7
9 air). In reality the temperature rise is much higher than in the given example. All of the calculations in this chapter are simplified to draw your attention to the problem. The values and calculations are not suitable for real problems. As the air moves through the next stages of the compressor, the volume continues to decrease, but the temperature rise is only slight. The advantage of the temperature rise, which counteracts the reduction in volume, is an advantage that only applies to the first and second stage of the compressor. Before the air of the second stage can flow further into the next stages, water must be removed from the air. If this is not done, the compressor can be irreparably damaged. In the next chapters we will explain the different parts of a compressor in detail. Temperature, safety, but above all humidity are the reasons why a compressor consists of more parts that are necessary to compress the air to a higher pressure. Decreasing volume and piston shape In a car engine, all cylinders and pistons are the same size and shape. But by now it should already be clear that this is not possible in a compressor. As the air moves from one stage to the next, the space available for the air must be reduced. If the next cylinder were the same size as the previous one, the already compressed air would expand again to ambient pressure. The maximum volume of the next cylinder (piston in the lowest position) should more or less correspond to the size of the minimum volume (piston in the uppermost position) of the cylinder from which it received the compressed air at medium pressure. The size of the cylinders varies greatly. The fourth stage piston (left) is just the light part in the middle. Compare this diameter with the first step on the right in the picture. The smaller volume of the other cylinders in each case requires smaller and smaller piston diameters.This creates a problem in the design of the compressor. The rotation of the crankshaft results in a right-left oscillation at the connection of the connecting rod, while this has to convert the rotation into an axial movement of the piston. The diameter of the cylinder must allow this oscillation of the connecting rod, which limits the diameter to a minimum. As the diameter of the piston decreases, it becomes increasingly difficult to provide the space required for the oscillation of the connecting rod. To solve this problem, the compressor is equipped with differently shaped pistons. Some of these pistons are unique to compressors and are not found in other machines. In the beginning, the diameter allows for a normal piston like the one in cars and page 8
10 other engines finds. Depending on the size of the compressor, the diameter of the second stage may still allow for a normal piston, but this is rarely the case in T or W compressors. The free-flight piston is an elegant solution for this. You can think of a free-flight piston as a graduated piston that has been intentionally broken by the manufacturer. The free-flight piston is pushed upwards by a driver piston. The pressure in the cylinder, above the free-flight piston, causes the downward movement. Although the drive piston and the free-flight piston are not connected, they are in contact during all up and down movements, provided that the pressure in the cylinder is high enough to push the piston down. The first variant is a stepped piston (in the photo on the left with a connecting rod). This is a piston that has a smaller diameter at the top than at the bottom. However, this is only possible up to a certain diameter. The upper part of the piston can hardly move sideways. The piston must seal with the cylinder wall to minimize the amount of air lost towards the oil pan. However, the lower part of the piston is affected by the vibration of the crankshaft and so we have stress on the metal part, where the diameter is reduced. Imagine you bend a metal wire over and over in the same place. At some point it will break. Should the diameter of the smaller part of the piston become so thin that the force in the compressor can bend it, then the piston will break just like the wire. If the compressor has just started, the pressure required to push the piston down is not available. The air that flows into the first stage needs a few cycles before it (at high pressure) reaches the last stage. The stage with the free-flight piston. This creates the typical noise when a compressor is started tak-a-tak-tak-atak-rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr. Before the compressor runs smoothly, there is a period of time when the driver piston hits the free-flight piston during the upward movement. This happens because the free-flight piston does not move down with the driver piston. When the sound of the compressor changes, the moment has come and page 9
11 has built up enough pressure above the free-flight piston to effect permanent contact with the driver piston. The seal between the piston and the cylinder is made with piston rings. Three or more piston rings around the piston hold a film of oil in place. The combination of piston rings and oil film prevents high pressure from escaping from the cylinder. Small amounts of air (maybe 5 to 10%) can escape towards the oil pan, but most of the air remains in the circuit and flows from one stage to the next. Not all free-flight pistons are equipped with piston rings (normal pistons and stepped pistons do, however). If a free-floating piston has no piston rings, another system must ensure that the oil (which is required to lubricate the moving parts) is not forced into the oil pan. A good solution for this is an oil nozzle. This nozzle receives the oil from the oil pump and holds it back until the oil pressure reaches approx. 60 bar. Now the pressure is high enough to push away a spring that releases a passage on the side of the free-flight piston (this mechanism is reminiscent of the first stage of a piston-controlled regulator). The injected oil has a pressure between the highest pressure in the last stage of the compressor (200/300 bar) and a pressure at which the air flows into the stage. As a result, the injected oil moves up and down repeatedly before being replaced with new oil. Inlet and outlet valves The valves that allow air to flow in and out of the cylinder are located in the cylinder head. The inlet valve is on the inside and the outlet valve on the outside (on most compressors). Inlet / inside and outlet / outside. The size of the valves differs depending on whether it is the first stage of the compressor or a later stage (and smaller). A narrow, weak spring presses a metal valve against the metal seat. Even the slightest piston movement is enough to overcome the spring force. When the piston moves down, the inlet valve opens practically at the same time. The same thing happens with the exhaust valve when the piston moves up. One valve is always open while the other is closed, depending on whether the piston is moving up or down. In the drawing you can see the same parts as in the picture above. A spring presses the valve (orange part) against the seat. All other parts play a role in relation to the seal. The seat is pressed against a copper washer (the blue part) by the four arms of the spring holder. The spring holder is held in place by a nut, which in turn is sealed on the cylinder head by a washer. A screw through the top of the nut allows additional tension to be applied to the spring retainer to hold the seat firmly against the lower washer. When the screw is tightened, an additional washer is placed and everything is sealed airtight with a nut. As the piston moves up, the increasing pressure pushes the valve away from the seat and air can escape to the side and flow into the next cylinder. Page 10
12 If an exhaust valve is broken, then in most cases the seat has eaten its way through the valve. Both the valve and the washer (the two parts on the right in the picture) must be replaced. Not all brands have exhaust valves that can be dismantled into individual parts. In some, the exhaust valve is a single piece in which all other parts are assembled and sealed. If an exhaust valve is not working properly, then in most cases the seat has eaten its way through the small valve. The picture on the left shows a damaged one next to a new exhaust valve. If the discharge valve can be disassembled, then it is usually sufficient to replace the small valve and washer and the compressor is repaired. If the outlet valve is a one-off, it must be completely replaced. The cost of an exhaust valve is offset by simply replacing it. You saw that the cylinders in a compressor (and therefore the cylinder heads) are attached at a certain angle. To change an exhaust valve, the cylinder head must be in a horizontal position. Therefore, you will need to remove the cylinder head from the cylinder and reinstall it when the exhaust valve is repaired. If the outlet valve is a one-off, then it can be changed at any angle, which makes the process easier and faster. In most compressors, the small inlet and outlet valves of the last stage (the cylinder with the free-flight piston) are unique pieces (picture on the right). If the last stage of the compressor fails, the entire valve must be replaced. The construction of the inlet valve is much simpler. The reason for this is its position in the cylinder head. The pressure in the cylinder presses the seat against its copper washer. A complex mechanism is therefore not necessary for this purpose. In the picture you can see the parts of an inlet valve of a second stage with the special tool for assembly. The washer is placed first, followed by the seat with the sharp edge pointing in the direction of the small valve that comes next. The spring is placed in the housing and screwed on with the screw in the cylinder head to fix all parts in their position. The screw must be screwed flat into the housing of the cylinder head, otherwise the valve has moved and the process must be repeated. The special tool on the left is needed to remove and reassemble the inlet valve. The size of the different cylinders and therefore the cylinder heads varies. For this reason, the available space for the valves is also different. Above you saw a small valve on the last stage of a compressor. The first stage has larger valves. In older compressors, that would just be an enlarged version of the valves we saw before. Newer compressors are equipped with a different technology. A laminated plate contains 2 passages, one where the air flows in and one where the air flows out. The valves page 11
13 are identical but mounted in opposite directions. The laminated plate with the valves is attached between the cylinder and the cylinder head. In the picture you can see a laminated plate (right) of a modern compressor with inlet and outlet valve of the first stage. It is mounted between the cylinder head (left) and the cylinder. The seal is achieved with a cylinder head gasket. Older compressors have a discharge valve on the last stage that is screwed into the cylinder head. The pressure that flows through the valve is very high and there have been several malfunctions with this type of valve. Today most compressors for the last stage have a double cylinder head. The first contains the valves and the second (of the same diameter) is mounted on top to hold the exhaust valve in place. A screw through the 2nd cylinder head exerts additional pressure on the valve to ensure the seal. It is basically the same function that we have already seen with the 2nd stage exhaust valve. Pressure holding valve The pressure holding valve is essential for the correct functioning of the compressor. As already discussed, the free-flight piston depends on the pressure in the last stage in order to stay in contact with the driver piston. We also discussed that the exhaust valve opens the moment the piston moves upwards. If an empty bottle were connected to the compressor, then the last stage would not be able to build up the pressure needed to push the piston down again, as all compressed air would immediately flow into the empty bottle. The compressor would now hit the free-flight piston again and again after starting (it would continue to make the typical tak a tak a tak sound). Pressure holding valves prevent this. A spring-loaded valve requires a pressure that can overcome the spring force before the air from the compressor can flow into the bottles to be filled. The spring force can e.g. Be 120 bar. The air in the compressor must reach a pressure high enough to push the small ball away against the spring force before the air passage to the bottles is open and they can be filled (the yellow arrow in the drawing). The pressure holding valve also performs two other desired functions. After use, it maintains the pressure in the compressor and prevents moisture from entering the compressor. This in turn prevents corrosion in the compressor. Some users bleed the compressor after use, but this is not recommended. page 12
14 Equipping with a pressure retention valve has another important function. The valve is not installed immediately after the last stage, but after the chemical filter. When a filled bottle is unscrewed from the filling hose, the filling hose must be vented. With the venting, the pressure drops back to ambient pressure and the vented volume becomes part of the work done during the next filling process. If a pressure control valve is installed after the chemical filter, it prevents the chemical filter from being vented together with the filling hose. This reduces the volume that has to be pressurized in the next cycle. The above discussed does not apply if filling hoses are provided with a vent valve, which allows partial venting of the filling hose connected to the bottle (this is almost always the case with larger compressors). In this case, too, it is important that the pressure control valve is located after the filter. This ensures that only a small volume of air flows through the filter (air at 100 bar takes up only 1/100 of the space than the same amount of air at 1 bar. The smaller volume flows more slowly through the filter, which in a longer and Better contact between the filter material (contents) and the air results (and therefore better filtration). Intercooler and separator (moisture) The rotation of the compressor is used to blow air at ambient temperature over the compressor. Even in the warmest climates the ambient air is a lot cooler than the temperature of a rotating compressor. For a compressor, the ambient temperature is cool, even if it is hot for you. Compressors still have a maximum working temperature (e.g. 40 C). The air circulation helps convection cooling of metal parts. Convection means, that air in contact with a warm surface flows off as it heats up and colder air flows in t. Convection happens naturally because warm air is lighter than cold air. However, this is accelerated when air is blown over the compressor. Cooling is necessary to prevent the temperature in the compressor from becoming too high during the compression steps and also to ensure adequate separation of moisture in the compressed air. As the air flows from one cylinder to the next, it is passed through a cooling coil to reduce the temperature before the next compression step. The diameter of the cooling spiral is large between the second and the third stage, but becomes smaller in the subsequent stages because the air is more compressed and therefore takes up a smaller volume. The required length of the cooling coil is determined by the design of the machine. It should be long enough to reach a temperature below 100 C for the separators to function properly (we will discuss this later). The air humidity must condense in the separators, but water with a temperature higher than 100 C will remain gaseous (steam). There is no separator between the first and second stage of the compressor. You can find the reason for this in the section on temperature and humidity. A three-stage compressor has two ab- Page 13
15 divider. One between the second and third level and one after the third level. A four-stage compressor has three separators. Separators are always located directly after the cooling spiral. The air must now be cooled to the point where it reaches around 100 C so that the separators can function properly. There are three types of separators. The first is called a nozzle or spray separator. The air enters the separator through a tube and is sprayed against the walls. The water condenses on the cold walls and drips down, while the dry air flows through a tube that is in the top of the separator into the next stage of the compressor. A hole in the bottom of the separator is fitted with a valve that can be opened periodically to drain the condensed water. The water is pressed through the hole in the bottom. This type of separator has a disadvantage. The air is always blown to the same place. Although metal is a good conductor of heat, it takes a while for the heat to be evenly distributed. The second type of separator is the vortex. Basically it works in the same way as the nozzle separator, only now the air is sprayed around everywhere and not just on a certain point. This enables a cooler temperature for the separator wall. Sometimes the cooling of the separator walls cannot withstand the heat inside. In this case a solution has to be found that is independent of the effectiveness of the cooling of the outside (walls) of the separator. If an increase in pressure increases the temperature, then the opposite is also true. If a pressure drop is provoked, a cooling effect can occur and therefore also a separation of water. This is achieved by means of a sintered filter that throttles the air flowing through it. The air has to press its way through the filter, which in turn leads to a higher pressure. When the air reaches the outside of the filter, the pressure drops sharply. As the pressure drops, so does the temperature and the separation of moisture can begin. The moisture collects in the lower part of the separator and must be emptied regularly. The compressor user manual will specify the exact intervals, such as: B. every half hour. If bleeding is forgotten and the separator fills completely, the water can be sucked into the next cylinder, where it can irreparably damage the compressor.The same damage could also occur if the suction pipe that brings the ambient air to the compressor falls into the water while the compressor is running. Water cannot be compressed and should liquid enter the cylinder, the piston cannot move all the way up. The connecting rod will either force the crankshaft down or the cylinder head up. In both cases, the damage to the compressor is significant. For this reason, many single-sided compressors are 14
16 installation that automatically vent the separators. The separator is automatically vented by means of an electronic solenoid valve. A timer opens the valve at set intervals. In some cases each separator has its own timer, in others, the venting of all subsequent separators is initiated by the pressure drop from the previous separator. When the first solenoid valve is opened by the electronic timer, the liquid is drained through the bottom of the trap (just like you blow out your mask). During this process, the pressure in the separator drops. The mechanical solution to take advantage of this pressure drop (if not all separators are equipped with their own solenoid valve) works with small pistons that block the drain as long as the pressure is high enough to keep the piston in the closed position. The pressure of the first separator acts on the piston that closes the drain of the second separator. When the pressure drops, the piston moves up and opens the way to drain the second separator. Should there be a third separator (as is the case with a four-stage compressor), then the pressure drop in the second separator is used in the same way to initiate the process in the third separator. There is no need to periodically empty the filter (as it does not collect moisture). The valve on the filter is used for venting when the filter material is changed. Note, however, that in some cases a separator and filter are combined in one housing. Safety valves A compressor is equipped with safety valves for each stage in which the pressure increases. These valves are not only used for safety, but are also an important reference for the compressor technician. For example: After the first stage in a three-stage compressor, a safety valve is attached that is activated (opened) at a pressure of 8 bar. This valve blows off. The first stage increases the ambient pressure to 7 bar, which is not enough to start the safety valve. Thus there is a likelihood that the air from the second compression step will find its way back towards the first step. The technician would then (first) check the second stage inlet valve for its seal. Although there are no requirements for safety valve placement, they are in the same location on many models. On the cylinder head of the second stage you can often find the first stage safety valve (on the side where the first stage air enters the second stage). The safety valve for the second stage is located on the separator between the second and third stage. In a four-stage compressor, the same applies to the third-stage safety valve (on the separator between the third and fourth stages). The final pressure safety valve has no standardized space, but is often combined with the final separator. Visually, the safety valves can be very different (compare the model on the right in the picture with the final pressure safety valve on page 4). The descriptions above will help you identify the safety valves on a compressor. Page 15
17 Lubrication The oil pan (the part where the crankshaft rotates) is partially filled with oil. This oil needs to be distributed to the moving parts of the compressor to reduce friction and help with cooling. There are several ways to achieve this. Very small compressors (T-shape) use a sling pin that is mounted on the crankshaft. We have already explained that this option requires a high speed of the crankshaft and therefore the compressor can only be used for a short time. Other options must ensure both a lower speed and a reliable distribution of the oil to the most important parts of the compressor. The first stage of a compressor is lubricated by deliberately contaminating the ambient air that flows into the first stage with oil. This is achieved by sucking in air from the oil sump, some of which is mixed with oil vapor. A liquid like oil can only migrate from high to low pressure. Since the pressure in the cylinders is higher than the pressure in the oil pan, you cannot expect the oil to migrate up between the cylinder and the piston. It must come from above or be injected under pressure that is higher than the pressure in the cylinder when the piston is at the bottom. Lubrication from above is the easiest, but it is inconvenient. In order to guarantee the quality of breathing air after the air has been compressed, a complete removal (filtering procedures we will discuss later) of the oil residues must be guaranteed. Two-stacks use a similar technique. Oil is added to the gasoline and thus lubricates the piston from above. We mentioned earlier that there is a certain pressure loss due to air permeability on the piston ring in the oil pan. That would raise the pressure in the oil pan. Air with oil would be forced out of the pan and smear the outside of the compressor with oil. The inlet of the oil pan directs the air enriched with oil to the first stage. So lubrication and keeping the outside of the compressor clean go hand in hand. As already said, there is no separator between the first and second stage. Part of the oil vapor will reach the second stage unhindered and lubricate it from above, just like the first stage. The separator after the second stage will remove some of the oil from the air flowing through the compressor. The traces of oil mix with the water in the separator, which explains the milky-white color when emptying. The white, cloudy color of the excreted water indicates that the compressor is working properly. If the water is brown, then this is an indicator that the compressor needs too much oil and if the water is clear, that the compressor is running dry (without lubrication). Note that the color of the drained water, once it stands, will change over time. The color should be checked the moment the separators are emptied. The separation of moisture and oil vapor is only partial. Separators are not intended to produce completely dry and oil-free air. This means that oil vapor also reaches the third stage and for some models it could be sufficient for lubrication. In most cases this is not the case and additional lubrication of the later stages is necessary. Sometimes the movement of the piston is used to move oil through a hose that connects the oil pan to the cylinder. That would not be lubrication from above (the oil could not be transported against the high pressure in the cylinder). The maximum that could be achieved is that the oil in the cylinder- page 16
18 that would be transported high enough to be picked up by the piston rings. The main purpose of such lubrication is not to reduce the friction between the cylinder and the piston, but to reduce the friction between the piston and the moving parts. Some compressors use an oil pump. The oil is pumped to the last stage of the compressor where it is delivered to the oil pressure regulator. Such an oil pressure regulator is similar to the first stage of a regulator. The passage of the oil into the cylinder is blocked by a piston that is held in place by a strong spring. The oil pump keeps pumping oil to the oil pressure regulator, which in turn increases the pressure. When the pressure is high enough to compress the spring in the oil pressure regulator, the piston is moved upwards and the oil can be injected between the (free flight) piston and the cylinder. When the oil pressure is between the maximum (piston in the upper position) and the minimum (piston in the lower position) pressure in the last stage, most of the oil will remain in place for effective lubrication of the last stage of the compressor. The oil pressure regulator is set to such a pressure. The oil pump is activated (driven) by the crankshaft. On a three stage compressor, this could be a round piece that protrudes from one side of the crankshaft. The oil pump is equipped with a small wheel that is pressed down every time the crankshaft rolls over it. Since the oil pump is installed below the level of the oil, oil is pumped to the oil pressure regulator. A one-way valve prevents the oil from flowing back into the oil pan when the spring in the oil pump pushes the small wheel back into its starting position and it is ready for the next pumping process. The design of an X compressor requires that the motion of the crankshaft be used indirectly. The oil pump must be completely under oil. Sometimes a chain transfers the rotating motion of the crankshaft to another axis. This is only there to activate the oil pump. The free-flight piston in compressors with an oil pump and a built-in oil pressure regulator do not have piston rings in most cases. The piston is precision work and allows minimal passage for the high pressure oil to slowly travel up and down. You should always remember the most important consequence when lubricating a compressor. The air is purposely polluted to allow top lubrication. Since the separators only remove some of the moisture and oil, the air must be filtered before it meets the requirements for breathing air quality. It also means that compressors must be effectively cooled. In some countries there are requirements that the design of a compressor must ensure that the air in the cylinders does not get warmer than 160 C. If the air (with oil vapor) got too hot, this mixture could ignite itself and form carbon monoxide and carbon dioxide. That would not only be a problem for the request- Page 17
19 represent changes in the air quality, but would also be risky for divers, as carbon monoxide is a poisonous gas. Air quality standard To be recognized as breathing air quality, air must meet strict standards. These requirements can vary from country to country. However, they usually indicate a minimum content of oxygen and a maximum content of oil (hydrocarbons), carbon monoxide, carbon dioxide and moisture. All divers experience the requirements for the water content. As a result, you get a dry mouth while diving. However, drying the air is necessary to avoid corrosion in the diving cylinders. Countries in which steel bottles are mostly used require a lower moisture content than countries in which aluminum bottles are used. There are also different requirements regarding the oil content (hydrocarbons) in the air. For breathing, a relatively high oil content might be acceptable. However, if the air is mixed with pure oxygen, the higher oil content could pose a risk of spontaneous combustion (and thus the formation of carbon monoxide). These considerations have led (in some countries) to different requirements for breathing air and oxygen-compatible air. Norms% O2 CO2 ml / m 3 DIN 3188 EN CGA E-Grade Modified CGA E-Grade Tec%% 21% - +/- 2% 20.0%% 20.0%% CO ml / m 3 H2O mg / m 3 Oil mg / m,, Depending on use 67 Depending on use For European compressors, DIN (Deutsche Industrie Norm) 3188 was previously used as a reference. You can still find this standard in the manuals of older compressors, e.g. as a basis for the frequency of the filter change. Other European countries did not have their own standards and adopted the German requirements. That changed with the introduction of the EN European standard for air. Leading European manufacturers consider air to be oxygen-compatible if it complies with the EN standard. So there is only one standard for filling scuba tanks with air as well as for Nitrox systems. The situation is different in the USA. The standard for breathing air quality allows a relatively high oil content (10 times more than the European standard). This has raised concerns about using air to fill Nitrox. These concerns were discussed at the 1993 Tec conference. The participating organizations agreed that the standard for air used in Nitrox systems would have to be modified. The result of this agreement was a modified E-grade (E-grade is the CGA Compressed Gas Association's expression for breathing air quality). The analysis of the air quality can provide some information about the technical condition of the compressor. A lower oxygen content combined with an increased carbon monoxide and carbon dioxide content indicates a self-ignition in the compressor he dies (although the loss of oxygen through the combustion may be too little to be detected). A high level of carbon monoxide and carbon dioxide can also mean that the intake manifold is too close to the 5 0.1 page 18
20 exhaust of an internal combustion engine is located. It could even be the motor used to rotate the compressor. A high level of moisture and oil means that the compressor filter is not working properly. This can be due to the filter material that was not changed in time, but it can also indicate that the air is flowing through the filter too quickly (if the pressure is too low) due to a problem with the pressure control valve. Filter Filtering begins before the air is drawn into the first stage of the compressor. Attached to the first stage you will find an air filter, similar to the air filter on a car. The larger the filter, the more air can flow through (and be filtered). Most compressors with large filters have a high filling speed. the air filter. Often the air is already filtered in front of the air filter. A corrugated hose is attached to the air filter, which directs air from a cleaner and / or cooler environment to the inlet of the compressor. In such a case, a pre-filter is attached to catch flies, mosquitoes, leaves and other contaminants so that they do not get in. The corrugated tube that carries air from another location to the inlet of the compressor should not be too long. If a longer hose is to be connected, a different one than that supplied with the compressor, it should have an appropriately large diameter in order to keep the air resistance low. The installation of pre-filters, corrugated hoses and air filters requires some attention from the user. If one of the parts creates too much air resistance for the air flowing through, the compressor will try to draw in air from somewhere else where the resistance is lower. This could very well happen through the oil pan (and therefore too much air pollution with oil vapor). As we discussed earlier, some air is purposely withdrawn from the oil pan. For the most part, this is the air that has seeped past the piston rings (e.g. 5% 10% of the total air volume). If a considerably larger amount of air is extracted from the oil pan, this will result in a larger amount of oil in the compressor. This in turn increases the risk of spontaneous combustion (and therefore the formation of carbon monoxide) and the filters are saturated much faster than would normally be the case (before the normal change). A good indication of an inlet drag problem is the color of the water exiting through the separators. In the picture you can see the housing of an air filter. The air always flows through the filter in the same place. Many compressor users have developed the habit of using the same filter four times. When a new filter is installed, mark which side the air inlet is on and the date. According to the information in the manual of the compressor, turn the filter by 90, so that the air page 19
21 can now flow through an unused part of the filter. This procedure is carried out until the filter has reached its starting position again and has thus been used four times. Using the inlet filter in this way prevents an increase in air resistance. Cleaning the pre-filter and making sure the corrugated tube is free of kinks and other blockages will do the rest. Later we will deal with nitrox system considerations and the location of the pre-filter. Similar considerations should be made to avoid the resistance of the air inlet to the compressor. Between the stages and immediately after the last stage of the compressor, the air is mechanically improved using the separators (as already discussed). After the last separator, chemical filters improve the air quality to such an extent that the standards for breathing air are met. If the compressor is used in accordance with the instructions for use, the manufacturer guarantees the air quality.The intervals between filter changes are set so that the manufacturer can be sure that adequate filtering of the air is guaranteed. Compressor users keep a logbook to prove that the manufacturer's recommendations have been followed. In this, maintenance such as Filter change recorded. This is also the reason why compressors are equipped with an electronic counter so that you know the operating hours of the compressor. Three substances are used in a filter: molecular sieve, activated carbon and hopcalite. The molecular sieve only binds the moisture. The small spheres have a structure in which the water molecules bind. The air is dried mainly to protect the tanks from corrosion, but also to ensure that the activated carbon is working properly. Activated carbon removes the hydrocarbons. This includes both oil and other combustible materials, but also aromatic hydrocarbons, which have a ring-shaped structure. The name aromatic hydrocarbons comes from the variety of odors they can produce. The activated charcoal not only removes oil, but also neutralizes odors. The Hopcalite works as a catalyst and releases an additional O (oxygen atom), with which carbon monoxide (CO) can bind and thus CO 2 (carbon dioxide) becomes from CO. Although high levels of carbon dioxide are a problem, it's not as toxic as carbon monoxide. Hopcalite is not systematically inserted into every filter. The use of Hopcalite is not widespread. However, activated carbon and molecular sieves are used in almost all compressors. The filter housing must be able to withstand the ultimate pressure of the compressor (200 or 300 bar) and can therefore be compared to a diving cylinder. The filter material is filled into a cartridge and then placed in a filter housing. Disposable filters are made from plastic or aluminum. Filter cartridges that are filled by the user are mostly made of steel. Each manufacturer has its own philosophy when it comes to filter cartridges. Some require the use of prefabricated cartridges and others recommend that the user fill the filters with filter material himself. Page 20
22 There are several ways to insert the cartridge into the filter housing. One method provides additional security because no bottles can be filled if a filter cartridge is not installed. To ensure this, there are two O-rings (red in the drawing) at the end of the filter, which ensure the path to normal air passage. When the filter is in place, an O-ring at the top and bottom prevents air from escaping through the safety drain. However, if no filter is installed, the air will take the path of least resistance and all compressed air will escape. Many other filters have threads with which they are screwed into the cover of the filter housing. The used filter can thus be easily removed. This is not always the case with cartridges that are sealed with O-rings at the bottom of the housing. The type of cartridges screwed into the lid do not have a safety drain. Regardless of the type, the filter housing must be vented before it can be opened and a new cartridge inserted. Due to its placement between the compressor and the pressure holding valve, the filter is always under pressure. To vent the filter and to check the venting, the filter has its own vent valve and pressure gauge. In any case, filter cartridges have felt discs between the different filter materials and in other places. One reason for using the felt disks is to prevent filter material from being lost through the perforated plates and another reason that the different filter materials do not mix. The main reason, however, is to avoid channeling. Channeling means that the air flowing through the filter forms a channel with the least resistance. All of the incoming air takes the same path and only part of the filter material comes into contact with the air in order to clean it. Since the calculation of the working time of a filter is based on the entire filter material, a channel formation according to the standards for breathing air quality would lead to a reduced air quality. Page 21
23 The Parts Associated With the discussion of the filters, we have covered all the standard parts of a compressor. We'll discuss some (optional) additions later, but now it's time to relate all of the individual parts. All parts can be seen in the order in which they are found in a compressor in the following illustrations. Page 22
24 The compressor parts on the previous page Number Name Function 1 Pre-filter A pre-filter is used when a corrugated hose draws in air further away from the compressor. It is only intended to keep leaves, mosquitoes and other things out so they don't get into the compressor's air filter. 2 Air filter The air filter prevents dust and other particles from entering the first stage of the compressor. The filter requires some attention. If the filter is clogged, the compressor can no longer function properly. 3 Inlet valve All cylinders have an inlet valve. To allow air to flow in, the valve must be open when the piston moves down. The inlet valve must close again when the piston moves up. 4 Exhaust valve The exhaust valve (on all cylinders) must be open while the piston is moving upwards so that the air can flow to the next stage. The exhaust valve must close when the piston moves down to prevent air from flowing back into the previous cylinder. 5 Normal piston The same type of piston as in internal combustion engines. 6 stepped piston A piston with a different diameter to provide enough space for the oscillating movement of the connecting rod due to the small diameter of the cylinder. 7 Driver piston The piston that pushes the free-flight piston upwards. The downward movement generates the pressure in the last stage and is dependent on the correct function of the pressure holding valve. 8 Free-flight piston The free-flight piston causes the typical noise of the compressor when it is started. 9 Medium pressure safety valve 10 Final pressure safety valve Each stage in the compressor has its own safety valve. Each valve has its specific location and is adapted to the pressure at which the cylinder is working. The final pressure safety valve decides whether a compressor can fill 200 or 300 bar cylinders. Its location can vary, but most of the time it is placed on top of the final separator. 11 Intermediate separator There is usually no separator between the first and second stage. A separator is required after each subsequent stage in order to prevent the condensation of water in the cylinders. 12 Final separator The separator after the last stage is not intended to prevent condensation, but to facilitate the work of the chemical filter by removing moisture and oil vapor from the air. 13 Cooling spirals The air must be cooled after each compression step. Separators can only work properly up to 100 C, which is also the reason why the cooling spirals are located in front of the separator. 14 Chemical filter There are standards for clean air. However, separators only remove part of the moisture and oil vapor. The chemical filter is a necessary addition for compressors that fill scuba tanks. 15 Venting the chemical filter The chemical filter is trapped between the outlet valve of the last stage and the pressure holding valve. After filling, the remaining pressure in the filter cannot escape. In order to be able to change the filter, it has to be vented beforehand and therefore it has its own vent. 16 Pressure holding valve The valve that holds back the air until enough pressure has been built up so that the free-flight piston can function properly and to ensure that the air flows slowly enough through the filter. 17 Filling console Not dealt with yet. Page 23
25 Accessories & selection This chapter is the logical continuation of the knowledge acquired about the compressor block. Mainly it concerns parts such as memory, filling consoles and others that have not yet been addressed at all. In some cases, however, additions are also explained (such as automatic filter monitoring) that are often used but are not part of the standard equipment of a compressor. The chapter concludes with the procedures for filling scuba tanks and the considerations for choosing the right compressor for a business or personal use .. page 24
26 Electronic filter monitoring The activated carbon and the molecular sieve in a filter must be changed at specified intervals. The service life of a filter can be defined by the operating hours of the compressor or by the number of bottles filled. The compressor can run under different conditions. In order for the air from the compressor to meet the standards for breathing air quality, intervals relating to the worst case must be observed. Electronic filter control allows the real circumstances under which a compressor is running to be taken into account. In many cases, this means that the filter can be used longer as the conditions are favorable. The investment in electronic filter monitoring pays for itself relatively quickly. Furthermore, it provides a guarantee of good air quality because the opposite is also true. If the compressor turns under very unfavorable conditions, the electronic filter monitoring system will indicate faster than usual to change the filter material. In some cases the electronic filter control can be connected to the motor of the compressor. If a filter change has to be made, the system can prevent the engine from starting. This is particularly advantageous when filling takes place at a different location than the compressor is and / or when many different people operate the compressor. Due to the noise from the compressor, some owners choose a more distant location where no customers are served. A distance of 30 meters between the compressor and the filling console is not uncommon. Electronic functions that switch off the compressor when it is not working properly can help avoid problems with such configurations. A typical case where a greater distance is found between the compressor and the filling console are outside filling stations. In some countries these are very popular. It enables divers to fill their bottles whenever they want. There are systems where the diver has a key to the box and others have a coin or credit card system. You won't find these options in all countries. In some countries, compressor users must have special training, which cannot be checked at external filling stations. Compressor cooling So that the intercoolers and the separators can function properly, the compressor must not get too hot. It is a common norm that the air in the compressor stays below 160 C to avoid problems with oil fumes in the cylinders. To achieve this, a relatively low ambient temperature is necessary and this leads to a model-specific maximum operating temperature
27 temperature. Above this outside temperature, the manufacturer is not sure whether the temperature inside the compressor will remain within the normal range. The ambient temperature is not the only factor. The compressor must be located in a place where sufficient cool air is supplied or, even better, sufficient removal of the air heated by the compressor is guaranteed. The room in which the compressor rotates must be sufficiently large and it should not be too close to the wall. Since warm air rises and cold air stays below, it would be best to draw in cold air close to the floor. The air that heats up during the cooling process of the compressor can then escape upwards. Any mixing of cooler air to the compressor and warmer air that is supposed to escape should be avoided. In some compressor rooms there is a hole in the wall where cooler air comes in from the outside, while a ventilation system ensures that the heated air can escape upwards and the two air flows do not mix. Diving centers in colder areas often direct the heated air from the compressor into the equipment room and use it to dry the rental equipment. Sometimes the last cooling coil of the compressor is equipped with fins. Thus, the surface of the cooling coils that comes into contact with the cold air is enlarged and the cooling is more efficient. The length of the cooling coil can thus be reduced by two or three times, and correct cooling is still ensured. The lamellar cooling coils require some consideration. The lamellas enlarge the surface that is in contact with the air. If these are covered by dust, dirt or oil, then exactly the opposite happens. Covered fins isolate the cooling coil instead of making the cooling more efficient. If a compressor is equipped with this type of cooling fins, then the fins must be brushed off over and over again. An old toothbrush is suitable for this. The Motor A compressor needs an external source of rotation. This can be a motor that is already turning and to which the compressor is connected, e.g. the unit on a boat. But in most cases a compressor has its own motor. This is either an electric motor or an internal combustion engine. The choice of motor depends on considerations such as susceptibility to moisture, the presence of a reliable power source, noise reduction, and mobility. If there are no restrictions, an electric motor is the best choice. These engines make less noise than internal combustion engines, require less maintenance and do not produce any toxic fumes (carbon monoxide) that could enter the compressor inlet. There are some limitations, see page 26
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