Gypsum Board Drying Process: How the Multi-Deck Dryer Works

The gypsum board drying process removes the surplus mixing water from boards that have set. After cutting, wet boards are spread over the stacked roller decks of a multi-deck dryer and carried through heated zones, hottest at the entry and cooler toward the exit, for roughly 20–60 minutes. The aim is to drive off the free water without heating the gypsum crystals enough to lose their chemically bound water.

Why Gypsum Board Has to Be Dried

A gypsum board core is made by mixing stucco (calcium sulfate hemihydrate) with water. Only part of that water is needed for the chemistry. As the board sets, the hemihydrate takes up water and turns back into gypsum (calcium sulfate dihydrate), the reaction described in US EPA AP-42, section 11.16. The rest is there only to make a slurry fluid enough to spread between two sheets of paper at line speed.

Eurogypsum, the European gypsum industry association, puts the numbers for standard plasterboard at about 1.4 liters of water per m² taken up by stucco hydration and about 4 liters per m² that has to be evaporated in the dryer (Eurogypsum Water Roadmap). In other words, the dryer removes roughly three times more water than the board keeps.

That ratio fits the chemistry of our reference line. With 7.0 kg of stucco per m² on 12 mm board, the hydration reaction (CaSO₄·½H₂O + 1½ H₂O → CaSO₄·2H₂O) binds about 1.3 kg of water per m². Patent literature quotes water-to-stucco ratios of about 0.5 to 1.3 for board slurries (US 2017/0131701), and everything above the 0.19 that the reaction needs must leave through the dryer.

The board cannot go to stacking wet. Free water left in the core makes boards heavy, weak and prone to mold in the warehouse. If drying goes too far, the gypsum at the surface and edges starts to lose its bound water again, and the bond between paper and core suffers. The whole drying process is a balance between those two failures.

Where the Dryer Sits in the Gypsum Board Production Line

The dryer is one of the longest machines in the plant and sits between the wet end and the dry end. A typical sequence after the forming station is:

1. Setting belt and roller conveyor. The board hardens as it travels from the forming station to the knife.
2. Cutting knife. The continuous board is cut into wet boards of the ordered length, with a small allowance for end trimming.
3. Accelerating rollers. Cut boards are sped up to open a gap between them.
4. Wet-end transfer and turning. Boards are moved sideways off the main line and, on many lines, turned over before drying.
5. Distributor (dryer infeed). Boards are fed deck by deck into the dryer, usually two or more side by side on each deck.
6. Multi-deck dryer. Boards travel through the heated zones and an end cooling zone.
7. Dryer outfeed and dry-end transfer. Boards are unloaded deck by deck and brought back into a single line.
8. End trimming, bundling, stacking and packing. Ends are sawn square, boards are paired face to face, stacked and wrapped.

The gypsum board forming machine and setting belt upstream decide how evenly the dryer is loaded. A board with thin edges, voids or uneven thickness will dry unevenly, however well the dryer is tuned.

How a Multi-Deck Gypsum Board Dryer Works

A board dryer is a long insulated housing with several roller conveyors stacked one above the other. Each conveyor is a deck. Hot air is circulated over and under the boards by fans, heated by burners or heat exchangers, and part of the moist air is exhausted. Stacking the decks gives a long drying path on a short floor plan. A CertainTeed patent describes conveyors stacked “six, eight, or more conveyors high” to maximize residence time and evaporative capacity (US7726040).

There are two basic ways to move air past the boards, and many dryers combine them:

The heat source can be direct or indirect. In a direct-fired dryer, combustion gases from natural gas or light fuel oil mix with the circulating air. In an indirect dryer, the air is heated through a steam or thermal-oil heat exchanger (Grenzebach; US EPA AP-42, section 11.16). Direct firing puts the fuel’s heat straight into the drying air, without heat-exchanger losses. Indirect heating lets a plant use fuels whose combustion gases should not touch the board.

On our reference line (3,000,000 m² a year, 12 mm basis), the dryer is a multi-deck roller dryer with 6–7 decks, 30–40 m long, heated by burners or a hot-gas system. Large high-speed lines are much bigger: a German patent notes that in practice dryers often have ten to twelve decks and drying sections of 100 m or more (DE10146179C1).

Drying Zones and the Temperature Profile

The dryer is divided into zones, each with its own fan, heater and air temperature. The logic follows the board. When it enters, it is full of water, and evaporation keeps the board itself cool, so the air can be very hot. As the free water runs out, evaporative cooling fades and the same air would overheat the board. The air temperature therefore steps down along the dryer.

The German patent DE10146179C1 describes 3 to 7 zones in general practice and a design with two high-temperature zones and one low-temperature zone, plus pre-drying and cooling. Published figures from that patent, a newer two-stage dryer patent (US11644238) and Eurogypsum give this picture:

Eurogypsum gives a simpler summary: dryer temperatures are usually around 200 °C.

The board itself must stay much cooler than the air. US11644238 states that heating the gypsum layer above about 100 °C has to be avoided, because the dihydrate starts converting back to hemihydrate and liner adhesion weakens. In its example, core and surface both stay at about 90 °C. A 1977 USG patent gives a similar range, noting that kilns conventionally heat the board to about 170–210 °F (77–99 °C) (US4009062A).

Residence Time, Dryer Length and Deck Count

Board residence time in the dryer is typically 20 to 60 minutes (DE10146179C1). The time a given dryer provides depends on four things: its length, the number of decks, how many boards sit side by side on each deck, and the forming line speed.

Because the boards from one continuous line are shared between all the decks, each deck runs much slower than the forming line:

• Deck speed ≈ line speed ÷ (number of decks × boards side by side)
• Residence time ≈ dryer length ÷ deck speed

The gap opened between boards by the accelerating rollers makes the decks run slightly faster than this, so the real residence time is a little shorter.

Worked example (illustrative arithmetic, not a specification). Take a dryer 40 m long with 7 decks and two boards side by side, fed by a line running at 15 m/min. Each deck moves at about 15 ÷ 14 ≈ 1.07 m/min, so a board spends about 37 minutes in the dryer. The same dryer with 6 decks and 30 m length, at the same speed, gives about 24 minutes.

This simple arithmetic explains why dryer size grows so quickly with line speed. Our reference line forms at 12–15 m/min. High-speed lines run far faster; a Yoshino Gypsum patent describes lines at 60 m/min and above, with examples at 150 m/min (US10195765). To give boards the same time in the dryer at ten times the speed, the dryer needs about ten times the total deck length. That is why large plants combine more decks, wider decks carrying more boards side by side, and drying sections of 100 m or more.

Thicker boards hold more water and need more time. A line that makes 6 mm and 25 mm board on the same dryer runs very different speed and temperature settings for each.

Water Load and Energy Use in Gypsum Board Drying

Drying and calcination are the two big heat demands in a gypsum board plant. Evaporating water takes about 2.26 MJ per kg (the latent heat of water, a physical constant), so 4 kg per m² needs about 9 MJ per m² before counting the heat lost in exhaust air, the housing and the warm boards that leave the dryer.

On our 3,000,000 m² a year reference line, natural gas consumption for calcination and drying together is 1.5–2.0 million m³ a year, about 0.5–0.7 m³ per m². Much of that goes to the dryer. Three practical points follow for a new plant:

• Fuel supply sets output. If the gas supply cannot keep up, the dryer is underfed and the whole line has to slow down. Size the gas connection for the dryer at full speed on the thickest board.
• Less water in the slurry means less fuel. Every liter of mixing water above what the reaction needs is a liter the dryer must evaporate. Foaming agents, dispersants and stucco quality all change the water demand. Starch is one additive whose behavior also depends on the drying curve: it has to cook and migrate to the paper while the board is still wet.
• Heat recovery pays back on the exhaust. A dryer supplier’s brochure (Grenzebach) lists plate heat exchangers, exhaust return systems and pre-drying sections. Eurogypsum’s roadmap reports one Saint-Gobain plant that recovers condensate from dryer steam, with water use down almost 10% and energy consumption down 10.6%.

US11644238 also warns about the opposite problem: if the moist air cools below its dew point anywhere in the dryer, water condenses, corrodes the housing and marks the boards. Exhaust ducts and cold spots need attention, especially in plants that stop and start often.

Drying Faults and How Plants Control Them

Most drying faults come from the board drying unevenly, not from the dryer being too hot or too cold overall. Thin tapered edges dry first. A 1970 patent describes the classic trade-off: raising entry temperatures “frequently creates an over-dried condition in the thin tapered edges”, while lowering them to protect the edges leaves the center underdried (US3529357A). CertainTeed’s patent adds that the exposed outer edges of the outermost boards on each deck are the most likely to overdry, because the wet boards act as a heat sink and cool the air over the middle of the deck (US7726040).

Drying faults overlap with forming and formulation faults, and the board does not tell you which one it is. Our article on gypsum board production problems covers how to separate them.

Routine drying checks

1. Board weight in and out. Weigh a wet board before the dryer and the same board after it, by deck and by position across the deck. Weight loss that drifts shift by shift shows drying change before quality complaints do.
2. Edge and end inspection. Look for soft, dusty or brown edges and ends on every deck. Scratch the edge core with a fingernail or key.
3. Paper bond. Score the board and peel the paper. A good bond tears inside the paper, leaving paper fibers on the core.
4. Zone temperatures and exhaust humidity. Log them per shift, and record changes in board thickness and line speed alongside them.
5. Combined water near the surface. When overdrying is suspected, a combined-water test (ASTM C471M) on the surface layer shows whether gypsum has been converted back toward hemihydrate.

The full test routine for finished boards is in gypsum board quality testing.

What to ask a dryer supplier

• Number of decks, effective width, boards side by side and dryer length, and the residence time at each board thickness you plan to make.
• Number of zones, the air flow type in each, and how temperature is controlled per zone and per deck.
• Heat source, rated burner or heat exchanger output, and the fuel consumption at full speed on your thickest board.
• How edge overdrying is prevented: nozzle layout, edge dampers or edge-zone control.
• Heat recovery, insulation and exhaust design, including how condensation is avoided at stops.

FAQ

How long does gypsum board stay in the dryer?

Typically 20 to 60 minutes, according to German patent DE10146179C1. The real figure depends on board thickness, the dryer’s length and deck count, how many boards travel side by side, and the forming line speed. Thick board needs longer, so lines that make both thin and thick boards run different speeds and zone temperatures for each thickness.

What temperature is used to dry gypsum board?

Public sources give air inlet temperatures of about 200–300 °C in the first high-temperature zones, falling to well below 200 °C in the last drying zone (DE10146179C1). Eurogypsum summarizes dryer temperatures as usually around 200 °C. The board itself should stay below about 100 °C, because hotter gypsum starts losing its bound water (US11644238).

How much water does a gypsum board dryer remove?

Eurogypsum puts it at about 4 liters per m² for standard plasterboard, against about 1.4 liters per m² that stays in the board as chemically bound water. The exact figure depends on board thickness, density and the water-to-stucco ratio of the slurry. Reducing mixing water is one of the most direct ways to reduce dryer fuel.

Why do gypsum board edges burn or go soft in the dryer?

The tapered edges are thinner and more exposed than the center, so they lose their free water first and then heat up. The outer edges of the outermost boards on each deck are most exposed. Once the gypsum there passes about 100 °C, it starts to calcine, becomes soft and loses its bond with the paper. Edge-zone temperature control and nozzle layout address this.

How many decks does a gypsum board dryer have?

It depends on line speed and capacity. Our 3,000,000 m² a year reference line uses a 6–7 deck roller dryer, 30–40 m long. Patent literature describes dryers stacked six, eight or more decks high (US7726040), and large high-speed plants often run ten to twelve decks over 100 m or more (DE10146179C1).

Planning a Gypsum Board Production Line?

The dryer is sized from your output, board thicknesses and fuel, so those inputs come first. See how the dryer fits the whole gypsum board production line, and use our gypsum board plant setup guide to plan fuel, power and building before you compare suppliers.

Sources

• US EPA, AP-42 Fifth Edition, Volume I, Chapter 11.16: Gypsum Manufacturing — https://gaftp.epa.gov/ap42/ch11/s16/final/c11s16_1995.pdf
• Eurogypsum, Water Roadmap — https://eurogypsum.org/wp-content/uploads/2025/11/Eurogypsums-Water-Roadmap-final.pdf
• Grenzebach, Gypsum Solutions (technical brochure) — https://www.grenzebach.com/fileadmin/Grenzebach_Group/Gips/Grenzebach_Gypsum_Solutions_EN.pdf
• DE10146179C1, German patent on multi-zone board dryers (Google Patents) — https://patents.google.com/patent/DE10146179C1/en
• US11644238B2, Method and device for drying gypsum board (Google Patents) — https://patents.google.com/patent/US11644238
• US7726040B2, Apparatus and method for the uniform drying of board materials, CertainTeed Gypsum (Google Patents) — https://patents.google.com/patent/US7726040
• US3529357A, Method and apparatus for high-speed drying of gypsum board (Google Patents) — https://patents.google.com/patent/US3529357A/en
• US4009062A, Gypsum wallboard – making process and product, United States Gypsum (Google Patents) — https://patents.google.com/patent/US4009062A/en
• US10195765B2, Gypsum board manufacturing method and manufacturing device, Yoshino Gypsum (Google Patents) — https://patents.google.com/patent/US10195765
• US 2017/0131701 A1, System and method for manufacturing cementitious boards with on-line board measurement (Justia Patents) — https://patents.justia.com/patent/20170131701
• ASTM C471M-25, Standard Test Methods for Chemical Analysis of Gypsum and Gypsum Products (Metric) — https://store.astm.org/c0471m-25.html