Industrial autoclave is a pressure vessel used to process parts and materials requiring exposure to high pressure and temperature. The manufacture of high-performance components from advanced composites often requires the processing of autoclaves.
Video Autoclave (industrial)
Prinsip operasi
The autoclave applies heat and pressure to the workload placed therein. Usually, there are two autoclave classes. They are pressurized with steam workload processes that can withstand water exposure, while heated gas circulation provides greater flexibility and control of the warming atmosphere.
Processing with autoclave is much more expensive than oven heating and is therefore generally used only when isostatic pressure must be applied to relatively complex formwork workloads. For smaller flat parts, hot presses offer shorter cycle times. In other applications, pressure is not required by the process but is integral to the use of steam, as the vapor temperature is directly related to the vapor pressure. Rubber vulcanization exemplifies this autoclave category.
For special requirements, such as fogging nozzles of ablative composite rocket engines and nosecone missiles, hydrocells can be used, but these require very high equipment costs and high risks in operation. Hydrochlor is pressurized with water; pressure holds water in the liquid phase despite high temperatures.
A key component of the industrial autoclave is the quick opening door; this is also an important component in the cost of autoclave construction. On the one hand, operators must be able to open and close doors quickly and easily; on the other hand, the door must meet stringent security requirements. Such are the quality of autoclave door designs that the US experiences as few as five or six failures of autoclave each year.
The autoclave design is driven by a variety of security standards, most notably the ASME Pressure Vessel Code. While most countries use ASME code, some have developed their own code. The CE standards in Europe apply to ships as well as to electrical controls, and China requires that pressure vessels comply with their domestic codes. All codes define conservative requirements intended to maximize safety. Local governments may also enforce licensing requirements related to autoclave operations.
Maps Autoclave (industrial)
Design and construction
Pressure vessel
The pressure vessel design involves the Barlow formula, which is used to calculate the required wall thickness. However, the design of complex pressure containment systems involves more than the application of this formula. For almost any pressure vessel, the ASME code specifies requirements for design and testing. Prior to delivery, the pressure vessel is tested hydrostatically at 130% of its rated pressure under the supervision of the ASME code inspector. It is filled with water, and the small pump increases the pressure to the required test value, where it is held for a specified time (30 minutes in accordance with ASME code). Inspectors check the leaks as well as evidence of deficiencies or deficiencies in welding.
The design of a small autoclave does not need to consider the possibility of drawing a vacuum inside the pressure vessel, but this assumption should not be made in larger sizes. A steam autoclave, for example, may be exposed to an internal vacuum if the steam condenses completely while the vessel remains sealed. Although external pressure can not exceed one atmosphere, it is enough to break down the ship in some cases. Thus, hardening may be necessary.
In an unusual situation, the autoclave itself may have to be square or rectangular, not round, or perhaps vertical, not horizontal. If the autoclave is unusually large, it may have to be set into excavation on the floor if there is a floor level loading, as is generally the case.
Materials
The selection of materials from which autoclave is made entirely on the application. For autoclave vapor, carbon steel is used, but the corrosion allowance is added to the calculated thickness. This accommodates the rusting that occurs with recurrent cycles of exposure to steam, water, and air. Implied in this case is the need to monitor the loss of metal and disable the ship when excessive thickness loss has occurred.
For temperatures up to 650 ° C (343 ° C), no adjustments are necessary to calculate the thickness of the vessel wall. Above this temperature, the allowable voltage is lowered. Above 750 ° F (399 ° C), high temperature alloys are used. The rated temperature, which is stamped on the ship's data plate, applies to the vessel wall itself, not to the gas circulating in the autoclave. This is relevant when internal insulation is used to circulate air or gas at temperatures beyond the vessel rating.
Although design engineers can use the material of their own choice, the normal selection is SA516 Grade 70 PVQ (Quality Pressure Ship) of carbon steel. This steel is particularly suitable for use in pressure vessels because it has an outstanding dispersion between the relatively low yield strength of 38 ksi and its tensile strength of 70 to 90 ksi. Elongation on a 2 inch (51 mm) plate is 21%. This means that the metal stretches away under excessive pressure before it fails. In the case of excessive pressure, the parts will change shape before rupture, causing gradual seal loss rather than disaster. This pressure loss then acts to relieve the critical overload on the vessel structure. This failure mode assumes the absence of significant cracks in metals that are overly stressful.
The vessel specification will include the highest and lowest temperatures in which the ships' skin is exposed. Because the steel properties change as the metal becomes colder, the ship will be stamped for MDMT -20 Â ° F unless the user requires lower. Typically, metal thickness is driven by code requirements related to visual inspection. Thinner metals can be used, by providing a recycled weld. This saves cost when metal is not SA516 but stainless steel or refractory alloy.
Doors
Of all machines, the most expensive (depending on the size of the autoclave) and the most important part of the hardware is the door that opened quickly. Must be full diameter to allow access to the workspace, sealed tightly against rated pressure at the highest skin temperature, operate easily and quickly, and adhere to the same security code that governs the rest of the vessel. Of all the safety-related issues, the most important ones are those related to door operation.
There are several types of quick open doors that are commonly used. The simplest and most primitive type of doors, bolted plates or flange covers on some sort of hinge, are no longer considered to be even minimally acceptable for autoclave production because it is anything but fast in opening and closing. For ships with a diameter of up to four feet and rated no more than 125 psi, a hinged semi-elliptical door secured with a locking t-operated locking lever works essentially as fast and as easy as the more commonly used rotating locking ring doors.. The design of this door uses a dozen or more of T-bolts attached to the door hub (see photo), the stationary section mounted on the cylindrical vessel itself, which links the matching lugs welded to the door. When the operating lever is turned over the center, the T-bolt pulls the lugs closer and closes the door using the O-ring gasket.
This type is intrinsically safe, since it opens the cam-lock under the pressure of releasing the door seal and lowering the vessel pressure. In fact, ASME code does not require an interlock or mechanical pressure suppressor on the doors. Even when opened, the cam-lock does not allow the bolt to break away from the hook lug if there is pressure on the door. Because it is mounted at a small angle to the lug, the bolt can not be swung out of the lug pulling on the door if there is a pull on it.
These doors are relatively simple and economical, suitable for smaller autoclaves. The design is limited to a diameter of four feet and about 125 psi due to the limited real estate available on the door to lock-cam and door deflection when the lock is too far away.
Another limitation is the tendency of such doors to distort if not installed properly. Although very easy in the application, if the door assembly is not installed with the correct installation, the closure problem may occur. Ignoring the reinforcement claim on the ship just behind the hub door is a good way to learn how true this is. An autoclave with this type of door has a strongly reinforced support which keeps the door aligned for the lifetime of the machine. O-ring gaskets can be replaced in less than a minute and are reasonably priced.
Autoclaves of more than four feet in diameter or those with higher pressure generally use rotary locking ring doors, also called key breech doors. This door can be designed for any size and pressure. Neither the hub nor the door itself generally spin. Hubs are welded to ships and doors move on hinges that align them with hubs when closing.
The door has toothed lugs around the circumference, with lugs lined up with the corresponding opening in the locking ring, which lights up in the hub. When closed, the door facing the hub and the O-ring gasket provide a means of sealing the internal pressure. As the locking ring alternates, it locks the door lugs forward, pressing the door against the hub. In this way, there is no sliding movement from the door opposite the O-ring gasket. Thus, the gasket can be an inexpensive O-ring, a substitute that can be made in a user's own store using ordinary O-ring stock, and that lasts a long time. The very little wear and tear on the door can be replaced, making the autoclave a long-term investment.
At doors up to eight or ten feet in diameter, swings can be manual. It is not uncommon to see an eight-foot diameter autoclave whose doors can be swung open or closed with one finger. The advantage of manual swings, apart from lower costs and higher reliability, is that there is far less risk a person getting his fingers stuck in the closing doors if he moves them by hand.
On a machine with a manual turnstile, one trivial but important detail is the stop door to prevent a clash when opened. If a large autoclave is installed on the floor slightly above, then the door, once open, swings fast to the end of its journey. Without any snubber of any kind installed, it will slowly accumulate damage and wear.
In smaller autoclaves, the rotation of the locking ring is sometimes performed by a device operated manually. On larger, pneumatic or hydraulic cylinders are used. Pneumatic cylinders often provide rotation of thorns, but sometimes they may be safer because they generally can not move the door rings easily when there is a lot of pressure on the door.
Rotating ring lock doors require interlock protection to prevent operation when there is pressure in the autoclave. It is a good practice to combine the capability of reversing the door locking rings anytime and without delay. With a few doors, it is possible to swing them close not far enough and then have a locking ring jam when trying to close.
It is important to note that the autoclave is much more dangerous when the pressure is lower than when the pressure is high. This is because the higher pressure creates a frictional force that tends to make the door rings very difficult to move. In some cases, the hydraulic cylinder has been bent rather than twisting the ring. At lower pressures, the rings can be moved, and there is enough energy stored inside the ship to ruin a person's day completely. A properly designed autoclave incorporates several additional interlocks for additional warranty. These extra interlocks are relatively inexpensive and should always be designed to be unsafe.
In testing beyond design and pressure evidence, this type of door indicates that a mild deflection caused by excess pressure moves the door and the hub facing far enough to cause the O-ring seal to fail, thereby releasing the pressure. The escaping air spreads through the locking ring, preventing injury from high-speed jets.
There are other types of doors available. This is an exclusive design that has special characteristics that are suitable for various applications. For example, some door designs are simpler than ring type locking, since there are no mitered pieces at all in the locking ring. They are easy to operate by hand or power and are intrinsically safe, such as a T-bolt configuration, where each movement of the locking mechanism from closed to open will release the pressure well before the door and hub are completely separated.
Other types of simple doors can be provided for smaller, lower-pressure autoclaves, sometimes at substantial savings in acquisition costs, but not necessarily in operating costs.
The often overlooked aspect of autoclave access is the back end. In many instances, the best autoclave is that it has two doors. In a typical industrial autoclave, there is mechanical hardware behind, including fan circulation fan. An autoclave with a back door will be charged early. However, during the service life, the cost is cheaper, especially since ease of component access inside will encourage more frequent checks. In any autoclave, as with any machine, all major and minor components must be accessible for inspection, repair and replacement. Ignore this, and the owner will eventually regret it.
Full-diameter rear door that does not open a little fast, all things considered. On larger machines, they can be mounted on a hinge, davit, or dolly assembly (see photo above) that allows to be swung or removed from the autoclave after the installation of the bolt flanges has been separated. It gives the best access possible to the works in it. It also means that important components are never buried deep inside machines where they are hard to get and thus are highly likely to be ignored until they cause problems. A medium-sized autoclave built for southern workshops to be as cheap as humanly possible does not have easy access to the circulation fan, and, when last observed by one of the manufacturer's service engineers, was making the most alarming dry-bearing noise whenever it ran.
When comparing alternative designs, consider the amount of work required for the maintenance task, as well as the degree of difficulty and risk of worker damage or injury. If, for example, sixty horsepower internally mounted fan motors should be handled by humans via a small ramp or access port, potential problems can not be avoided. In user-friendly autoclaves, the back door swings open and the motor is quickly and easily lifted with a forklift or simple spar hoist.
Interior
The internal layout varies from one autoclave to another. Some have airways at six o'clock positions that also carry rail, while others have wide floors with mechanical components underneath. The others have air ducts at the top. Typically, the autoclave uses a circular air channel that flows across the full circumference of the interior.
Annular ducts are interesting because they create the smallest intrusion into the available net workspaces in the autoclave. This reduces the diameter of the cylindrical volume by only a few inches. It also creates the greatest frictional skin pressure loss. This means that the fan should be larger for the same amount of air circulation, and that there is more heating than the motor horsepower.
If it is necessary to maintain low temperature with full circulation, this may require running cooling rather than heating. Strong air circulation under pressure generates heat by itself, and this can be significant when trying to operate at very low temperatures and high pressures. Interior furniture from autoclave can be from galvanized, aluminized, or stainless steel. Up to about 400 ° C, galvanized is economical and reliable; up to perhaps 800 ° F (427 ° C) up to 1,000 ° F (540 ° C), aluminized will be required; On top of that, one is in an exotic area.
Another problem is whether the interior shell, the workspace wall, should be removed. How heavy is this wall? The thickness of the representative metal ranges from 18 gauge (0.0478 inches) to 1/8 inch (0.125 inches). Heavier, more durable and resistant walls will become dents, too, more energy will be absorbed during heating and more and more to be released during cooling. To take a typical example, consider an autoclave with an internal diameter of 8-feet (2.4 m) and a working length of 40 feet (12 m). If the interior wall is made of 11 gauge (.1196 inches) of steel, then its weight will be more than five tons. Heating only the wall itself to a operating temperature of 300Ã, Â ° F (149Ã, Â ° C) in an hour will require about 90 kilowatts of power. At typical demand costs, that will cost about $ 2,000 (for the month) in addition to energy costs (for each cycle). Reducing the wall thickness to 18 gauge will lower this cost by about 60 percent. To save $ 13,000 per year, the average autoclave operator can live with many dents.
In some autoclaves, strange sounds come from within as they heat up and cool themselves. These sounds are caused by distortions in metal interiors because these sounds expand and contract with extreme temperature changes. The interior of the autoclave described above will grow almost an inch in length during the heating part of the cycle. Provisions must be made to eliminate these movements or they will eventually perforate the interior.
If the engine is large, it will require adequate interior flooring to support personnel running on it, as well as security devices to protect personnel inside the machine against unintentional startup.
Heating
Introducing heat into the workspace can be done in various ways. For most autoclaves, and especially those used to process composite parts or adhesive bonds of metal structures, the easiest and least expensive is initially electric heat. Resistance heaters are compact and reliable and can be conveniently placed in the air ducts in circulation. Because the thermal mass of these heaters is small, the room temperature control is appropriate and additional heaters can usually be installed at a later date without excessive interference. However, this additional installation cost may be quite significant compared to the larger autoclave. This heater is basically 100% efficient and can be mounted for any single, phase or triple voltage.
Installing more capacity than necessary extends the life of the heater by allowing it to run at lower surface temperatures and providing greater assurance to achieve the required heating rate. Increased heating capacity is generally a small fee at the starting price. It is not safe to automatically assume that every autoclave manufacturer uses a high quality, quality Incoloy tubular rod, can be individually replaced and properly supported. For economic purposes, some expect customers to receive nikrom cables strung together on ceramic insulators.
The downside of electric heat is operating costs. For a small autoclave that is operated only on a regular basis, this may not prove to be a major problem. For medium or larger autoclaves, electricity bills over the life of the machine will add up to several times the cost of all autoclaves.
For example, in Rochester, New York, the area, where electricity prices quadruple of natural gas before considers the cost of demand, an autoclave is six feet in diameter and twenty-four feet long (with interior walls of a light meter ) will cost about $ 2,000 a month in demand costs plus an average of $ 14 per hour in energy when walking. Demand, which arises as soon as the autoclave is turned on, even if only for a moment, will be equal to the purchase price of the autoclave in just a few years. Experience with electric bills shows that this will get worse in the future.
The easiest alternative to this is steam heating. This presupposes the existence of a boiler capable of producing steam at high enough pressure to reach the required temperature. The existing high-pressure steam installation is a good thing to have, and facilitates the use of steam rolls, which are simple, compact, and easy to control. The purchase price of a steam coil heating is roughly proportional to electricity heating, but operating costs are dramatically lower. If high-pressure steam is not available, consider a small special boiler for autoclaves. The cost can be very low, making this alternative almost as economical as direct gas firing from an internal heat exchanger. It also allows you to run your autoclave on natural gas, propane, butane, or fuel oil, sometimes alternately if the boiler is set for dual fuel operation. Where gas supplies are susceptible to disturbance, using a small low-pressure boilers to run an autoclave and an oven can be a lifesaver when multiple fuel combustion is included.
A small vertical boiler takes up less floor space. If local laws require licensing from high-pressure boiler operators, it is often simple to train existing plant personnel and ask them to license for single-boiler operations. Equally economical to operate is an autoclave with a gas-fired heat exchanger built into a pressure vessel. Although this presents some design limitations, it is simpler than using synthetic heat transfer fluids, and the cost is somewhat lower. The gas burner assembly is fitted to the far end or the sides of the vessel and the flame becomes a heat exchanger inside the air duct. The hot end of the replaceable tube is covered with turbulence for better heat transfer. This restores the lion's share of exhaust energy. It's simple and reliable, using ordinary natural gas, butane, propane, or other industrial gas fuel.
There are alternative configurations, including a secondary circulation loop that partially transfers from the primary air stream through an external pressurized heat exchanger. This bypass flow can also be used for cooling using an air-over heat exchanger. While the gas firing is not ready to lend itself to a small engine, it can be fitted to an autoclave with a diameter of three to four feet up. The longer the machine, the longer the heat exchanger tube and thus the more efficient it is. This heating option is cheaper than hot oil and more expensive than electricity or steam (assuming the existing boilers) to buy, but the additional costs are paid back very quickly. During the full service life, an electrically heated autoclave will be costly enough to pay four or five other comparable autoclaves. For anything but the smallest laboratory machine, gas combustion and steam heating, to explain it, the best alternative to consider.
In some circumstances, when steam is available at the plant, substantial money can be saved using live vapor injection. In this approach, all parts of the autoclave are filled with live vapor at appropriate pressure. Generally used in industrial rubber products, these can be adapted for use in composite preservation. It requires a different vacuum bag material but has the advantage of eliminating heating, duct, and circulation fans. With external insulation, there is more space available for the workload, for the size of the pressure vessel provided. Of course, this approach presumes the availability of properly assessed boilers.
In certain applications, low-pressure steam autoclaves can replace ordinary preservation ovens. The combination of vacuum consolidation, which is equivalent to about ten to fourteen psi external pressures, and steam at approximately the same measuring pressure, will give better results and warm up faster than the oven. This approach would be less suitable for materials to be brought to curing temperatures slowly, because the heat transfer vapor is fast enough compared to turbulent circulating air currents. Furthermore, since the inside of the vessel is repeatedly exposed to steam and then air, again and again, allowance should be made for corrosion of the vessel wall.
In some circumstances, external heated heaters carry synthetic thermal fluid to a temperature of 600 ° F (316 ° C) to 800 ° F (427 ° C), and a special pump passes it through a heat exchanger inside the autoclave. It has both advantages - gas or oil can be used as fuel without much attention to the space occupied in the working volume of the autoclave - and the disadvantages - the cost is very high, and can be more complicated to maintain properly. In addition, it can serve to heat and cool the autoclave by directing the heat transfer fluid through either a heater or cooling coil, as required by the process.
Considering all things, the most cost-effective heating option, during the full lifetime of the autoclave, will be a high-pressure steam boiler or gas combustion using an internal or external heat exchanger.
Cooling
Cooling at the end of the process cycle requires a tool to extract heat from the autoclave. The need for controlled cooling will depend on the work being processed. With some composite materials in thick lay-up, slow cooling prevents internal microcracking of the resin matrix resulting from the thermally induced voltage.
The cooling method used will depend on the highest temperature achieved before cooling and the level of precision to be maintained when the room temperature decreases. For low temperatures and cooling rates that can be left to vary significantly or only cooling at each level of yield from a steady coolant stream, water circulated through the coils in the airflow will be effective and inexpensive. A serpentine scaly coil is placed on a circulating fan input or adjacent to the heater array serving this purpose using the plant water as a coolant. If once-passing cooling water and cooling towers are unavailable or unacceptable, then a simple closed-loop cooling can be built into the autoclave. The precise cooling rate controlled may or may not be easy to achieve with this arrangement. In an autoclave operated at high temperatures, special precautions should be taken in cooling. Pumping cold water into a cooling coil at 800 ° F (427 ° C) will shorten the life of the coil. It also makes it difficult to control the cooling rate.
When cold water touches the heat exchange coil at 800 ° F (427 ° C), flash vapor is generated, together with a mechanical shock to the system and a considerable scale inside the coil as far as water contains dissolved solids. Steam and hot water disposal may prove difficult, and the lifetime of the cooling system may be short. This can be reduced to some extent by first cooling the coils with compressed air flow followed by water mist and compressed air. It's only slightly better than cold water by itself, and it's not much to remove flash vapor.
For temperatures up to about 800 ° F (427 ° C) and the cooling rate to be performed to cover the tolerances, synthetic heat transfer fluids will extract heat from the autoclave without phase change (ie, boiling). Passing the hot oil through an air-heating coil with controlled air flow will allow the system to modulate the heat flow out of the autoclave precisely to maintain the determined ramp rate of decline. This hot air can be disposed of wherever it will produce the best or least harmful damage. The disadvantage of using liquid heat transfer is the initial cost. This adds maybe ten percent to the autoclave price in the mid-size range installation. Life liquids are estimated to range from five to fifteen years, depending on the length of the highest temperature exposure as well as the maintenance of internal hygiene.
If the highest internal temperature will not exceed 300 Â ° F (149 Â ° C), then propylene glycol may be used as a heat transfer medium. Because these chemicals constitute food products, such as ice cream, there is no need for toxicity problems. It has approximately the same specific gravity as water, so pumping is very easy. Since there is no phase change, the coil does not build a heap. Liquid life is very good if air is kept out of the loop. Propylene glycol should be used without water dilution, and stainless steel pipes are not always mandatory. The cost of propylene glycol is not trivial, so the amount of coolant in the loop must be balanced between economic importance and heat dissipation.
It's surprisingly surprising one large autoclave customer to learn that a closed loop water cooling system is strictly regulated locally. The price tag on this bad surprise is in the five-digit range. In some places, disposing clean and cooled cooling water into a drain may be illegal. In general, not using water for cooling can have a number of real advantages.
Circulation
Unless the autoclave uses steam injection, the circulation fan carries the load to ensure uniformity of temperature throughout the workspace. Because heat flows from the source, whether the electrical resistance, the steam coil, or the ignition tube, to the circulating air flow and then into the workload, the greater the airflow turbulence, the better the heat transfer, especially with heavy and dense workloads..
The fan drive should be sized for the conditions that create the largest load on the fan, that is, the lowest temperature and the highest pressure, although the combination of these conditions is rare. Ideally, this means the fans are leaning backwards; this is more efficient than the radial impeller type and the front curve.
The purpose of air circulation or inert gas through an autoclave is to ensure effective heat transfer and temperature uniformity. Strong circulation and careful attention to where airflow actually goes is the best way to achieve this. As a rough rule, do not assume less than 300 feet per minute of average airspeed through the empty workspace of the autoclave. More than this will make heat transfer more effective.
The aircraft industry has specifications directly related to temperature uniformity. Even if the application is non-aerospace, one of these specifications may be feasible to be adopted to ensure the quality and reliability of the process.
The fan drive can be internal or external. The internal drive has a motor in the autoclave in the unheated space. Thick wall insulation keeps the heat out, and the motor is under full autoclave pressure. An external drive requires a shaft seal to carry the drive shaft through the pressure vessel wall. Internal drives are simpler, generate less floor space taken, and impose small but important cooling loads; external drives require more complex pivot drive settings and use high-pressure seals.
The choice of internal/external drive settings is often idiosyncratic. The service life of a high-pressure axle seal can be difficult to predict, and it can be assumed that the seal will be more expensive than the motor itself. For example, a 2 inch (51 mm) seal, 150 psi for a 50 hp motor will cost $ 2,000, while the motor alone is half. Generally, the combined vessel pressure, shaft diameter, and fan speed encountered in the autoclave are like making practical use of axle seals.
Access to motor space on autoclave with internal fan drive through back door or manway. The autoclave workspace is not reduced, as the pressure vessel is made a little longer to accommodate the fan drive. The hardware accessibility behind is the essence. In the end, maintenance personnel need to get it, and sudden access becomes a problem.
Although this adds to the initial price, a well-designed autoclave feature can be removed which provides easy and unrestricted access to hardware in non-heated areas. It's hard to realize how precious this is until it's suddenly necessary to remove a sixty-horsepower motorcycle weighing over half a ton through an opening that's barely big enough to skip. Some autoclaves have a circulation fan, complete with a motor, mounted on the final bell with reduced diameter. While this allows assembly to be removed easily, it also means that the fan is diameter-sized and thus less efficient.
If the impeller fan is mounted on the motor shaft or an extension of it (direct propulsion), the fan speed is limited to the motor, usually 1750 rpm, and it will most likely result in a non-optimal fan operation, given its sensitivity. fans for rotational speed. Fans are like airplane propellers; the bigger they are and the slower they change, the better they are. That's the law.
Some applications allow performing no fan circulation and warming the air at all. If the processed parts are simple enough geometric, it may be feasible to use a mold that itself is heated integrally. For example, it is feasible to make flat and single-curved aircraft landing springs on cheap aluminum molds with electric heating pads tied directly to the bottom of the mold. This eliminates the cost of motors and fans as well as air heaters and uses far less electricity than required a comparable electric autoclave. In this way, the autoclave puts pressure on consolidation alone. There are limits to this approach, such as the complexity of the mold. Sometimes, parts are heated from one side only; sometimes, the mold has the top and bottom, each equipped with a heater. While it is usually not feasible in a work shop, this type of autoclave can save a lot of money when only a small number of relatively simple parts are created.
Because circulating fan failure will have immediate and unpleasant consequences for heat exchangers or heating elements, detection of circular fan failure is essential. This can be done in several ways. First, monitor the temperature of the heating surface, either a coil or an element. If the airflow fails, it will increase abruptly, and the control system can do immediate shutdown. Secondly, install at least one and preferably two airflow sensors. Since the airflow may be at very high temperatures, this can be done with a remote mounted pressure switch connected to the high and low pressure sides of the fan through the stainless steel tube long enough to put the switch well inside the cold area of ​​the ship. This switch must be connected in series on one side for the control circuit, so that one of the openings will interfere with heating power, and in parallel on the other side so that the computer can detect which one has changed the state.
Isolation
The substantial mass of the pressure vessel provides a guarantee of pressure retention, but it represents an equally large heat sink that has to be heated and cooled in cycles during the autoclave. The steam autoclave needs to be insulated on the outside, making this heat loss unavoidable. Autoclaves that use air or other gases use thermal insulation in the interior, and this creates a one-time penalty in the vessel charge and less operating costs resulting from a somewhat larger internal volume to suppress.
Isolation, which is protected behind a metal shell, is sized to keep heat loss within an acceptable range and to keep the outside surface temperature of the vessel below which will affect worker safety. Generally, this is 120 Â ° F (49 Â ° C), with 140 Â ° F (60 Â ° C) sometimes allowed on equipment and pipes. Depending on the company's policy on energy conservation, this temperature can be set lower.
Mineral wool and fiberglass are used in autoclaves. The thickness varies with the internal temperature, ranging from a minimum of two to three inches to three to four times, the roughness of the thumb becomes one inch per hundred degrees F. Economically, the biggest effect is increasing the vessel charge. by increasing its diameter. This effectively reduces the over-determining of the thickness of the insulation.
One minor factor is ensuring that isolation can "breathe", because air flows in and out of it as pressure in the autoclave changes. In addition, metal sheets that hold the insulation require some provision for thermal expansion. Even a twenty-foot-high autoclave underwent considerable movement in temperature differences of several hundred degrees.
Pressurization
The choice of pressurized agent is driven by the process. Air is acceptable for autoclaves operating at relatively low temperatures, but may be entirely unacceptable beyond that. The combustible properties of a material often used in composite parts increase under pressure, when the partial pressure of oxygen rises. Thus, nitrogen or carbon dioxide can be used for air pressure.
Hydroclaves use water as a pressurized medium. Since the boiling point of water rises with pressure, the hydrocele can reach high temperatures without producing steam. Although simple in principle, it brings complications. Substantial pumping capacity is required, since even a small amount of water compressibility means that pressurization stores non-trivial energy. A seal that works well against air or other gases fails to work properly with very hot water. Leakage results differently in hydroclaft, when leaking water turns into steam, and this continues as long as water remains in the vessel. For this and other reasons, very few manufacturers will consider making hydroclaves, and the price of the machine reflects this.
Vacuum
Parts processed in the autoclave are often bagged with a vacuum to allow the pressure to operate isostatically on the workpiece. In its simplest form, the workload is fully contained in a loose bag made of cotton-resistant plastic that is able to withstand the temperature involved. When the vacuum is pulled, the bag is compressed by atmospheric pressure and compresses the components inside. Between parts and bags, the absorbent material provides a conduit for air evacuation and wipes the residue of the resin squeezed during the preservation process.
In the processing of composite parts autoclaves, the functionality of the vacuum bag may be where the greatest variation can be found. Some stores will leave bags under a full vacuum from lay-up to post-process removal. Others will withstand the vacuum only until the autoclave reaches full pressure. Yet others will again fill the vacuum bag with inert gas, usually nitrogen, at zero pressure.
The role played by the internal pressure of the vacuum bag can be important in the consistent production of high quality multi-layered composite components. The SAMPE paper describes the benefits of controlling the vacuum and pressure under the vacuum bag in lay-up. By following a vacuum in a bag with air pressure, cavity formation in the resin matrix is ​​suppressed, reducing microscopic defects of cracked seeds and other matrix failures. Installing this capability on an autoclave involves removal control and additional software, and, given the benefits in composite material performance, does not have an unreasonable effect on the price of the machine.
The ejector pump can be used for rapid evacuation of air within an autoclave that must be pressurized with inert gas. To remove oxygen from the interior and replace it with a pressurized agent that does not support combustion, the simplest way is to remove almost any air and then insert nitrogen or carbon dioxide. It should also be noted that the autoclave should be designed for such a vacuum service, since the vessel itself may require an actor to withstand external pressures, and ordinary access doors and road coverings are usually only rated for internal pressure alone and will not be able to withstand the external stresses resulting from interior vacuum.
Vacuum is delivered to the workpiece with individual manifold and tube equipped with quick disconnection on the interior wall. The simplest vacuum system consists of a pump and an outside gauge and a quick-disconnect port on the inside. In more complicated settings, there may be a dozen or more individual vacuum supply lines entering the engine, each to a separate QD port, with many gauges returning to the vacuum sensor being transferred to the control system, and an inert gas droplet under pressure controlled when the pump is turned off and the vacuum channel is released during the healing cycle.
There should be a proper sized vacuum receptor tank that can maintain a vacuum system if the pump fails during production runs. This will be feasible only if the vacuum pipe is virtually leak free. This is why vacuum leaks are one of the main problems in pre-acceptance testing machines. For a typical composite autoclave, a 5-to-10-cubic-foot receiver (140-280 L) may be suitable. Note that this receiver should be labeled ASME for full operating pressure of autoclave, as it is conceivable that vacuum system failure can result in much of this pressure being released to the receiver.
When some parts are processed, it may be beneficial to have a separate vacuum line for each, reducing the potential loss if one vacuum bag leaks during healing. This is easily accommodated with multiple supply tubes from a single manifold on the outside of the autoclave.
In addition, it is necessary to decide whether each line should be monitored individually and how this should be done. If there is a single vacuum sensor, whether the electronic transducer is connected to a control computer or only visually inspected sensitive gauge, determine which vacuum bag is leaking means closing each one in turn and overseeing a slight change in the pressure of the vacuum manifold. Given the limitation of airflow in the average lay-up and vacuum lines, even complete vacuum bag failure may appear little more than a small change in the vacuum level.
Placing the sensor in every vacuum channel will take care of this, but it costs a few hundred extra dollars per line. Some applications involve two vacuum lines per pocket. One is connected to the bag penetration at one end of the part being processed and the vacuum supply of the pump and manifold. Another line returns from separate pocket penetration at the end of the vacuum bag and through the back line through the vessel wall to the sensor that measures the vacuum level in the bag rather than the level on the manifold. This is preferred, as it gives an accurate indication of the real vacuum seen by the whole part.
As an option with this arrangement, if the vacuum is turned off before completion of completion, as mentioned in some applications, before the supply line is released into the atmosphere, the gauge is backfilled with nitrogen from the gas regulator without pressure, preventing the atmospheric contaminant from entering the breath/bleeder. Individual monitoring of each vacuum line does not necessarily require a measuring line, also an inert gas dewatering. However, the additional cost of providing a gauge line is not much. If vacuum bag pressure control is used, appropriate valves and regulators must be constructed.
If the process generates a substantial resin flow from the workpiece, the process specification may require a resin trap. Some materials lose a lot of very moving resins during the heating process, and these streams can sometimes run back through a vacuum pipe that may be far enough to connect critical components. Much easier to prevent this kind of damage than to fix it.
Some resins, such as polyester, secrete large amounts of volatiles during healing. This will be done through the vacuum port and sometimes cause damage to the pump. Better vacuum pumps use oil reservoirs and oil recirculation, and these volatiles can quickly convert oil into a reburious slurry. They also attack the seal of the vacuum valve and cause deposits to accumulate over time. To prevent this, it may be necessary to install the condenser on the vacuum port line. This will require cold water 35Ã, Â ° F (2Ã, Â ° C). A small chiller will add several thousand dollars to the cost of an autoclave, plus a few hundred dollars per port for condensers and separators. The stainless steel resin traps should be designed and manufactured to be easily disassembled and cleaned. Of course, they must also be fully accessible.
Controls and instrumentation
While many simple autoclave operations can remain manual, temperature controls are almost always automatic, as this is easy to do at low cost. The value of products processed in most autoclaves justifies high-level automation. The hardware and software available for industrial process automation makes a fully automated autoclave operation affordable and reliable. It is realistic to design and implement such automation without the services of outside vendors in most cases.
Temperature
As with any other parameter, the required accuracy of the temperature control depends on the process specification. The autoclave should exceed this capability with sufficient margin to prevent all possible insufficient or excessive temperatures in the workload. Too hot and parts can be damaged or undergo a thermal outing; too cold and full structural properties may not materialize. Equally important is to avoid temperature variations throughout the working volume of the autoclave. Aerospace specifications include maximum permissible variations and how to test uniformity.
Electric heating is virtually unlimited variable and thus suitable for precise temperature control, usually Ã, Â ± 1Ã, Â ° to Ã, Â ± 2Ã, Â °. Such precision can be achieved by indirectly heating the gas, but not so easily. The electric power drawn by the heating element can be controlled to a 12-bit precision by an SCR device that is driven by an analog signal from a temperature controller. The low mass of the heating element makes it responsive, and sudden and dramatic output changes - though generally unnecessary - are achievable.
The need for accuracy and precision in air temperature measurements within the autoclave places importance on the selection and implementation of sensors. Cheapest and easiest is a single thermocouple placed somewhere in the airflow. For better results at a trivial price, two or three average RTDs work better, with higher precision and fewer deviations. While RTDs will respond to sudden temperature changes less quickly than thermocouples will, this is not a problem, since sudden temperature changes in the autoclave do not occur. One can be placed in the inlet to the circulation fan; this one feels the lowest temperature, assuming that the heater is the downstream part of the fan. Others can be placed at the point where the airflow reverses the direction and starts to flow past the workload. One third can be placed close to the center. Feeling the air temperature close to the surface of the wall will usually cause boundary layer errors, or, worse, stagnation errors.
Pressure
Pressure control presents the fewest challenges. Given sufficient air or gas sources of pressure and flow capacity, the autoclave control system opens the pressurized valve and closes it once the internal pressure reaches the setpoint. Depressurization occurs when the exhaust valve is opened. In large autoclaves, silencers or exhausts may be required. The valve is on/off rather than modulated, for cost reasons.
As the temperature rises, the gas expands, pushing upward pressure. The slim valve releases the excess, maintaining the setpoint.
In some applications, the precision of direct pressure control is applied to the success of the process. For example, core materials have limited compressive strength at high temperatures; even a small excess pressure can undermine the core and damage the workload.
In a poorly designed autoclave, pressure oscillations can produce inlet and outlet chamber chitches. One way to prevent this is to use large valves to fill and dispose and small valves for pruning at and near the setpoint. Alternatively, valve modulation will avoid this phenomenon.
Vacuum
Often the most uncontrollable factor in an autoclave, a vacuum may or may not require modulation. In some cases, it is not automatic at all and involves little more than a connection to the factory vacuum system, some manual valves, and gauges. At the other extreme, the vacuum control system may be much more complex than the air temperature.
Security warranty
Security is always a concern with autoclaves. ASME code is very conservative; as a result, the pressure vessel is one of the safest, least risky, types of machines in use today. However, this does not mean that security can be taken for granted.
The ASME code requires, in addition to the highly conservative ship design and equipment, the installation of the code-labeled safety valve is set at the design pressure. This valve is safe to prevent changes to its settings and will open every time the pressure in the autoclave exceeds the design pressure. While this valve trigger will reduce the possibility of excess pressure in the vessel, it must also be able to keep the pressure source, whatever it is, from encouraging enough air, inert gas, or steam to bring pressure back to an unsafe place. level even with the safety valve open wide.
A conservatively designed autoclave has several safety valves that are each sized to be able to overcome the largest available airflow into the vessel plus no less than 30%. The valve is mounted on a manifold allowing multiple pressure vessel outlets to feed multiple safety valves, each of which can handle the entire exhaust air by itself, even if a pressurized vessel outlet is accidentally blocked by debris from an internal failure. The additional cost of the redundant safety valve is approximately one tenth of one percent of the machine price .
Air or nitrogen from a source of air pressure is not the only cause of sudden excessive pressure potential. Fire autoclave is guaranteed to raise internal pressure, and this can exceed the ability of the safety valve to vent fairly quickly. The solution is a large safety valve and broken discs, and more than one of each.
Composite components and materials used in the preservation process are often flammable, though not readily available at room temperature and atmospheric pressure. The high temperatures and pressures involved in the curing process increase the risk of combustion potential. While air is a suitable means of suppressing an autoclave preserver at 100 psi and 350 Â ° F (177 Â ° C), it may be too dangerous in processing autoclaves with potentially combustible materials at 500 psi and 700 Â ° F (371). Â ° C). The risks are also too high in very large (and expensive) healing costs. In such applications, nitrogen can be used, as both are inert (in this case will not normally support combustion) and are available. In the form of large liquid, the price is less than soft drinks. This is also a realistic alternative for special air compressors when pressure is significantly higher than the required hundred psi.
The simplest and most cost-effective security device is a broken disk. Put into a pressure vessel in fabrication, this is simply a port inside a vessel that is closed by a fine engine plate that will explode at a predetermined pressure. These plates can be made of aluminum or carbon. The discs are mounted in flanged assemblies that release the autoclave pressure to the exhaust pipe to carry the hole away from the personnel. The disks are quite cheap and can be easily replaced. The rupture disk should be used to back up the safety valve and sized to lower internal pressure as quickly as possible. Autoclave fires can release considerable energy into the air inside, resulting in sudden spike of pressure. The cracked disk is designed to release the pressure slightly above the ASME safety valve and well below the hydrostatic test; it was never called play unless there was a sudden increase in pressure beyond the capacity of the safety valve. The cost is very simple even for a pair of large pieces of discs making this a very interesting choice. The disk rupture shall be not less than twice the diameter of the inlet to or outlet of the vessel, whichever is greater.
The type of door will determine if it requires its own security device. The T-bolt door is intrinsically safe, and the ASME code does not require interlocking for it. Other types of doors do require an interlock to prevent the possibility of opening when there is ~ ½ psi or more inside the ship. Each autoclave will have this much; it's the absolute most legal. However, a wise autoclave operator should not be satisfied at least as needed. Even the best made components are not perfect, so a conservatively designed autoclave uses a backup interlock of both hardware and control software to reduce risk to the lowest level that can reasonably be achieved. For example, if the control system feels any pressure in the vessel, it prevents the door from opening the cylinder with unsafe valves, thus blocking any attempt to bypass the safeguards manually. This is in addition to the code-mandated interlocks. If desired, additional interlocks can be mounted on the T-bolt door as well.
Another security consideration is how sensors are connected. If a device is capable of failure under certain circumstances, then failure should be such that an indication of incorrect pressure is given. This is much better than false indications without pressure.
However, checking the pressure as a condition for opening the autoclave is not safe enough. Particularly with industrial size autoclaves, such as those used in the rubber industry, it is important that the autoclave be opened only after examining both pressure and temperature gauges. If the water in the autoclave has succeeded to be superheated, the pressure gauge may not indicate the presence of steam even though the temperature may be much higher than the local boiling point for water. If an autoclave is opened under these circumstances and superheated water is disrupted, a steam explosion becomes possible. This phenomenon can easily produce fatal burns for people around the explosion. Often, the victim dies only after the painful suffering that sometimes lasts for a month.
Because insurance and regulatory requirements vary from one location to another, they should be subject to discussion during the design process. If a small high-pressure steam boiler is required, it may be necessary to have an operating engineer to run it. In many places, existing employees can be trained to do this and provide a limited license to the boiler at the factory. Some parts of the world require that autoclaves be licensed or operated only after inspections have been performed and permits have been issued.
Economy
Construction
The price of an autoclave will vary greatly as a function of what has been designed and built in it. Sensitive to several factors and insensitive to others. The price will depend on the diameter and a little further on the length. Some autoclave features allow various options, and some can dramatically affect prices. In considering the acquisition of an autoclave, it is wise not to take anything for granted; see all possible options and variations. This includes whether the autoclave is the optimal way to meet process requirements. New technology in advanced composites, for example, is reducing the supremacy of the old autoclave in the industry.
While doubling the design pressure can increase the cost of autoclave by fifty percent, doubling the diameter may double or triple the cost. On the other hand, the increase in length is very cheap. Doubling the length of a 6-foot-diameter (1.8 m) autoclave may add at least five or six percent to the final price. As a rule, it's most cost-effective to think of lengths in multiples of five and ten feet. Adding one long leg to a 20-foot (6.1 m) boat is not much cheaper than adding five more legs. This is based on the customary practice of preparing pressure vessels from 5-or-10-foot-wide rolls (1.5 or 3.0 m).
Several inches of insulation should be added to the inner working diameter to obtain the vessel diameter. Placing insulation on the outside produces the smallest pressure vessel for given inner diameter, but the energy required to heat the vessel itself each cycle immediately makes this a very unattractive way to save, unless steam injection is used for heating, where the external event insulation should be used. For most machines, imagine the insulation thickness of four to six inches. On machines rated less than 300 Â ° F (149 Â ° C), this can be reduced to three inches (76 mm). For temperatures over 600 Â ° C (316 Â ° C), it should be increased to eight inches or more. This affects the diameter significantly, but trade-offs generally support additional insulation.
If price is a big problem, some changes can prevent it without releasing important capabilities, while others can be removed only at the cost of reducing the function and value of the machine materially. The price of autoclave varies spectacularly from one manufacturer to another, as well. Sometimes, they seem to change with the moon phase. It is not wise to assume that any manufacturer of autoclave or seller will charge the same price for the given configuration. In addition, the price of the machine used may vary beyond belief. It is not uncommon to find used machines that cost far above new ones, as well as rarely used autoclaves sold at auction for a penny.
When figuring out whether to buy a new autoclave or an existing machine, a potential buyer will find that the used machine may or may not be cheaper than newly built for the right specification and that they can be very hard to find in the required configuration. If the machine being used in accordance with the requirements lies in a reasonable price, be sure to check for things like door hinge wear, unmanaged modifications made by users or others (much more common than you might expect), and the availability of registration document pressures ship. To be completely safe, ask for a construction mold and then compare it with the machine itself. Uninsured additions made after insurance checks may be trivial (for example, door handles are welded after a vessel is checked), but they may allow the insurer to not allow damage claims if something bad happens.
If the U-1 form of an autoclave, issued by the constructor of a pressure vessel, is not available, or if the autoclave data plate is unreadable or may not even exist, then one may look at a homemade vessel. Know that many vessel stores will not touch vessels that have been produced elsewhere, even for the smallest modifications. Never buy an autoclave that requires work done on the vessel itself unless the agreement includes all modifications and retest and reexamination prior to acceptance of the machine. Otherwise, one can end up with the most expensive paperweights in the company's history. If in doubt, verify that the registration of the National Board of the ship is valid. Check to see if the manufacturer is still in business and whether the seal
Source of the article : Wikipedia
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