Autoclave for an AAC Plant: Size, Pressure and Curing Cycle

An autoclave for an AAC plant is a horizontal pressure vessel in which cut green cakes are cured in saturated steam. On our reference design it is φ2 × 21 m as standard, runs at 1.3–1.4 MPa (about 190–200 °C) with 7 hours at full pressure, and completes a cycle in about 11 hours. Autoclave count usually sets the real output of the plant.

Below: vessel size and loading, the cycle stage by stage, how to calculate the number of autoclaves, the door and its interlocks, and which pressure vessel code to build to.

What an AAC Autoclave Does and Why the Pressure Matters

When a cake leaves the cutting machine it is firm enough to hold its shape but still soft. Its strength comes from a hydrothermal reaction between lime and the silica in sand or fly ash, which forms calcium silicate hydrates. That reaction is slow at atmospheric pressure. In saturated steam at around 190 °C it is complete within hours, and it is what gives AAC its strength and dimensional stability.

The European AAC association describes the process as curing in high-pressure saturated steam for typically 6 to 12 hours, with the temperature reaching 190 °C at 800–1,200 kPa (EAACA). Our reference design runs a little higher, at 1.3–1.4 MPa.

Pressure and temperature are tied together in saturated steam, so one fixes the other. Suppliers do not always say whether a pressure is gauge or absolute, and the difference is about 0.1 MPa. The table uses IAPWS-IF97 steam-table values (IAPWS R7-97; the NIST Chemistry WebBook gives the same figures).

Two practical points follow. A gauge that reads 1.3 MPa while the thermometer reads 185 °C means air or condensate in the vessel. And the boiler must deliver steam above the autoclave working pressure to cover pipe and valve losses. That is why the boiler on our 100,000 m³ reference line is rated at 1.6 MPa (see our guide to the steam boiler for an AAC plant).

AAC Autoclave Size: Diameter, Length and Loading

Why the diameter is set by the cake

On a tilting six-side cutting line the cake is turned 90° before cutting and stays on its side plate from then until it leaves the autoclave. It stands 1.2 m high, 4.2 m long and 0.6 m thick. On our reference line each autoclave car carries two molds, with the two side plates side by side and the cakes standing upright. The cakes are never stacked one above the other. A φ2 m vessel has room for one 1.2 m-high cake on its car plus clearance, but not for two stacked. A larger diameter does not change the cake. It changes how much can be loaded across and above the car, so confirm the loading pattern drawing for any larger vessel.

Why the length is set by the cars

Length is a whole number of cars plus door clearance. With 4.2 m cakes, a 21 m vessel takes 4 cars. Longer vessels add cars, and the 26 m and 31 m sizes are used on larger lines where fewer, bigger autoclaves keep the autoclave bay shorter.

Single-ended or through autoclaves

A single-ended (dead-end) autoclave has a door at one end only: cars go in and come out the same way. A through autoclave has a door at each end. Green cars enter from the cutting side and cured cars leave on the separating side. Through autoclaves need more doors but allow a straight flow, which affects the building and track layout. Our AAC block plant layout guide covers this.

Thermal growth

A 21 m steel shell heated by about 180 °C grows lengthwise by roughly 45 mm (steel expands by about 12 × 10⁻⁶ per °C). So an autoclave sits on one fixed support and several sliding or rolling supports, and the steam inlet, condensate and rail connections have to allow for this movement. Check this on the foundation drawing.

The AAC Autoclave Curing Cycle, Stage by Stage

On our reference design a full cycle takes about 11 hours:

Release steam is not simply vented. On our reference line the exhaust steam from the autoclaves is piped to the pre-curing room and used there. Plants also transfer steam from an autoclave that is releasing pressure into one that is starting its rise. That practice is described in an AAC producer’s verified EPD (“once the steaming process is complete, the steam is used for other autoclave cycles”, EPD-IES-0001804) and in a published patent on steam treatment of several autoclaves (US 4,570,443).

Holding time and pressure are set by the mix design. Sand-based and fly ash-based mixes respond differently, so the cycle should be confirmed by trial batches on your raw materials, not copied from another plant.

How Many Autoclaves Does an AAC Plant Need?

Use the formula below, then check it against the supplier’s configuration:

1. Daily output = annual output ÷ operating days.
2. Molds per day = daily output ÷ m³ per mold.
3. Charges per day = molds per day ÷ molds per autoclave.
4. Cycles per autoclave per day = 24 h ÷ cycle time.
5. Autoclaves = charges per day ÷ cycles per autoclave per day.

Our reference line runs 330 days a year on three shifts. 100,000 m³ ÷ 330 days ≈ 303 m³ a day, or about 100 molds of 3.024 m³. At 8 molds per charge that is about 12.5 charges a day. One autoclave on an 11-hour cycle turns about 2.2 charges a day, so the calculation gives 5.7 autoclaves and the line has six. Six autoclaves turn about 13 charges, or about 105 molds a day, which covers the 100,000 m³ target.

The same arithmetic for other outputs, using φ2 × 21 m vessels, 8 molds per charge, an 11-hour cycle and 330 operating days:

The last row shows why larger plants move to φ2.5 or φ2.85 m vessels 26 or 31 m long: a dozen small autoclaves means a dozen doors, a long autoclave bay and a lot of track.

What this means for your plant:

• Round deliberately. Round the calculated figure up to whole autoclaves, and check what operating days and shifts the result assumes: the same autoclaves give less output on fewer days. Decide which before the layout is fixed, because the autoclave bay, the grouping tracks in front of it and the boiler are all sized around the count. A reference layout of ours has 6 grouping tracks in front of the autoclaves, so the next charge is grouped while the previous one cures.
• Allow for stoppages. Autoclaves need door seal changes and periodic statutory inspection. If the calculation leaves no margin, one autoclave out of service cuts output in direct proportion.

AAC Autoclave Doors and Safety Interlocks

An AAC autoclave door is a quick-opening (quick-actuating) closure: a dished door held by a locking ring that engages with lugs or teeth around the flange, rather than a ring of bolts. It is opened at least twice per cycle, many times a day, so the door is the most heavily used pressure part of the plant and the most dangerous if misused.

The hazard is documented. A National Board of Boiler and Pressure Vessel Inspectors paper on quick-actuating closures records a concrete block autoclave door that blew off in normal operation, killing one operator and critically injuring three, and an operator killed after opening a block-curing autoclave door while pressure remained. Its conclusion: “in almost all cases, its a combination of improper operation and poor maintenance” (National Board). In China, provincial market regulators have run inspection campaigns on quick-opening-door vessels with autoclaves listed as a priority (Shaanxi SAMR, 2021).

Good practice, reflected in the main codes and safety rules, covers:

• Interlock against pressurizing: steam cannot be admitted until the door is fully closed and the locking ring fully engaged.
• Interlock against opening: the locking ring cannot be released until the vessel is at atmospheric pressure. A manual vent or test valve on the door lets the operator confirm that no pressure is left before opening.
• Alarm: an audible or visible warning if pressure is applied with the closure not fully engaged. Singapore’s Ministry of Manpower requires this for manually operated closures (MOM safety circular, 2016).
• Pressure indication visible from the door operating position.
• No single point of failure: failure of one locking element must not let the closure open.

China’s TSG 21 lists safety interlocks among the required safety accessories of a pressure vessel, alongside safety valves (TSG 21 draft as notified to the WTO). The EU Pressure Equipment Directive requires quick-opening closures to have a device that prevents opening while pressure or temperature is hazardous (PED 2014/68/EU).

When you review the door, check:

1. The interlock works on actual vessel pressure, not just on a timer or valve position.
2. Locking-ring engagement is sensed at more than one point around the circumference.
3. The door seal type, how it is replaced, and how many spare seals are supplied.
4. A written opening procedure is posted at each door, and operators are trained on it. A bypassed interlock should be treated as a stop-work issue.

Pressure Vessel Codes, Certification and Third-Party Inspection

The code decides design pressure margins, material testing, weld inspection, the hydrostatic test and the documents you receive. On our reference lines, autoclaves are built and certified as pressure vessels under China’s GB/TSG rules. Third-party inspection or ASME U-stamp documentation can be arranged before shipment on request, at the buyer’s cost.

Points that matter to an importing buyer:

• Find out what your country accepts before ordering. Many countries register and inspect pressure vessels locally and specify which foreign codes they recognize. Registration and in-service inspection are the plant owner’s responsibility.
• Decide on ASME early. An ASME-stamped vessel has to be designed, built and inspected under the code from the start, with an inspector from an ASME-accredited inspection agency involved during fabrication. It cannot be added after the vessel is finished.
• Ask for the documents, not just the nameplate: design drawings and strength calculation, plate and forging material certificates, weld procedure and NDT reports, hydrostatic test record, safety valve certificates and the certificate issued by the inspection body.
• Order spares for the safety accessories: safety valves, pressure gauges and door seals are replaced and recalibrated on a schedule.

Installation and Operating Points Buyers Often Miss

• Foundations and supports: one fixed support, the rest sliding. Rails inside the vessel must line up with the grouping tracks and transfer car outside.
• Condensate drainage: condensate collects along the bottom of the shell. If drains block, the lower shell runs cooler than the top, which bends the vessel and can leave the bottom layer of blocks under-cured. Check the drains every cycle.
• Steam distribution: steam should enter through a distributor pipe along the length, not blow directly onto the cakes.
• Recording: a pressure and temperature recorder per autoclave makes cycle problems traceable. Cracked or soft blocks from one autoclave can be checked against its curve (see AAC block manufacturing process).
• Space for expansion: if output will grow, leave room in the autoclave bay and the steam header.

FAQ

What pressure and temperature does an AAC autoclave run at?

On our reference design, 1.3–1.4 MPa, about 190–200 °C in saturated steam, with 7 hours at full pressure. The European AAC association gives typical conditions of about 190 °C at 800–1,200 kPa. Pressure and temperature are linked in saturated steam, so a temperature well below the steam-table value for the gauge pressure points to air or condensate in the vessel.

How long is the AAC autoclave curing cycle?

About 11 hours on our reference line: 0.5 h loading and unloading, 0.5 h vacuum, 1.5 h pressure rise, 7 h holding and 1.5 h pressure release. The holding time depends on the mix and the product, so it should be confirmed by trial batches on your own raw materials before production settings are fixed.

How many molds fit in one AAC autoclave?

A φ2 × 21 m autoclave on our reference line takes 4 cars with 2 molds each, 8 molds or about 24.2 m³ of cake per charge. The two cakes on each car stand on their side plates side by side. Larger φ2.5 or φ2.85 m vessels, 26 or 31 m long, take more per charge. Ask for the loading drawing for your vessel size.

How many autoclaves does a 100,000 m³ AAC plant need?

Our 100,000 m³ a year reference line uses six φ2 × 21 m autoclaves, running 330 days a year on three shifts. The arithmetic: 100,000 m³ needs about 100 molds a day; six autoclaves with 8 molds per charge and about 2.2 cycles a day on an 11-hour cycle cure about 105 molds a day, which covers it.

Can an AAC autoclave be built to ASME?

Yes, if this is decided before design starts. The autoclaves we supply are certified under China’s GB/TSG pressure vessel rules as standard, and third-party inspection or ASME U-stamp documentation can be arranged on request at the buyer’s cost. Check first which codes your country’s pressure vessel authority accepts for registration.

Planning an AAC Block Production Line?

Autoclave count, boiler size and layout are decided together. See our AAC block production line for the full equipment configuration, and the AAC block plant setup guide for the planning sequence from raw material tests to commissioning.

Sources

• EAACA, How AAC is made: https://eaaca.org/netzero/how-aac-is-made/
• IAPWS, R7-97(2012) Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam: https://www.iapws.org/relguide/IF97-Rev.html
• NIST Chemistry WebBook, Thermophysical Properties of Fluid Systems: https://webbook.nist.gov/chemistry/fluid/
• The International EPD System, EPD-IES-0001804, Autoclaved Aerated Concrete (AAC): https://www.environdec.com/library/epd1804
• US Patent 4,570,443, Method for the steam treatment of several autoclaves and apparatus for its implementation (1986): https://patents.google.com/patent/US4570443A/en
• M.L. Snow, Quick Actuating Closures, The National Board of Boiler and Pressure Vessel Inspectors: https://www.nationalboard.org/PrintPage.aspx?pageID=205
• Ministry of Manpower Singapore, Safety Circular – Quick-Opening Closures and Pneumatic Testing (2016): https://www.iac.gov.sg/-/media/mom/documents/safety-health/circulars/2016/circular-20160125-quick-opening-closures-and-pneumatic-testing.pdf
• Shaanxi Provincial Market Supervision Administration, notice on quick-opening-door pressure vessel rectification (2021): https://snamr.shaanxi.gov.cn/__local/C/F7/B7/C4275441875A8C07BCC4365B99E_3289E2BD_21828.pdf
• TSG 21 Supervision Regulation on Safety Technology for Stationary Pressure Vessels, draft as notified to the WTO TBT Committee: https://members.wto.org/crnattachments/2015/TBT/CHN/15_3287_00_x.pdf
• ASME BPVC Section VIII, Division 1: https://www.asme.org/codes-standards/find-codes-standards/bpvc-viii-1-bpvc-section-viii-rules-construction-pressure-vessels-division-1
• European Commission, Pressure Equipment Directive 2014/68/EU: https://single-market-economy.ec.europa.eu/sectors/pressure-equipment-and-gas-appliances/pressure-equipment-sector/pressure-equipment-directive_en
• Directive 2014/68/EU, full text including Annex I points 2.3 (quick-opening closures) and 7.4 (hydrostatic test pressure), EUR-Lex: https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32014L0068