Guidelines for design, installation and testing of timber seasoning kilns (Compartment type with cross-forced air circulation)
1974 Edition

Overview of Scope & Essential Specifications

• Scope: Covers design calculations for RCC roofs, chambers, and foundations adhering to standard civil engineering codes without special formulas.
• Kiln Testing Requirements:
  • Follow drying schedules to reach target moisture content.
  • Reverse airflow every 6 hours.
  • Keep daily moisture content records for 18 matched samples.
  • Plot drying curves and analyze drying rates across stack positions.
  • Ensure uniform drying rates and final moisture content among samples.

RCC Roof Design Guidance:

• Use IS 456 for RCC design.
• Load factors include dead, live, and environmental loads.
• Flexural design formula: [ M_u \leq 0.87 f_y A_{st} (d - \frac{a}{2}) ]

Drying Test Process Flowchart:

flowchart TD
    A[Load Test Sample] --> B[Dry to Target Moisture]
    B --> C[Reverse Airflow Every 6 Hours]
    C --> D[Record Moisture Daily]
    D --> E[Generate Drying Curves]
    E --> F[Calculate Drying Rates]
    F --> G[Compare Across Samples]
    G --> H{Uniform Drying Achieved?}
    H -- Yes --> I[Accept Process]
    H -- No --> J[Modify Drying Parameters]

Contact Address: Manak Bhavan, 9 Bahadur Shah Zafar Marg, New Delhi - 110002 for official clarifications.

Minimum Operational Standards for Compartment Kilns with Cross-Forced Air Circulation

• Applicability: Only compartment kilns with cross-forced air circulation; excludes tunnel and natural circulation kilns.
• Design Essentials:
  • Cross-flow forced air circulation.
  • Effective baffling for uniform air distribution.
  • Adequate heating and humidification.
  • Well-insulated chamber to prevent heat loss.
  • Venting to manage moisture and air exchange.

Performance Benchmarks:

Testing Summary:

• Measure temperature and humidity at various points.
• Confirm airflow velocity and pattern with anemometers.
• Monitor moisture reduction in timber samples.
• Ensure parameters meet set tolerances.

Air Circulation System Diagram:

flowchart LR
    A[Heating Unit] --> B[Air Circulation Fan]
    B --> C[Cross-Forced Airflow]
    C --> D[Timber Compartments]
    D --> E[Exhaust and Ventilation]
    E --> F[Humidity Control]
    F --> B

Note: Refer to annexures of IS 7315 or seasoning handbooks for detailed heat and moisture transfer formulas.

Key Aspects of Kiln Chamber Construction per IS 7315

Chamber Materials & Construction Types:

• Acceptable materials include brick, concrete blocks, asbestos cement boards, metals, and wood.
• Two main types:
  • Masonry construction.
  • Panel construction, including prefabricated units.

Masonry Details:

• Walls: Preferably brick with a minimum thickness of 30 cm.
• Concrete walls (if used) should incorporate cinder aggregate to reduce heat loss.
• Roof: Reinforced cement concrete slab, minimum 10 cm thick, with horizontal underside and pitched top for drainage.
• Floor: Concrete slab, minimum thickness 10 cm.

Panel Construction:

• Materials: Steel or aluminium double panels, wood sheathing over wooden framing, asbestos-cement boards.
• Insulate the gap between double panels with material providing thermal resistance equal to or exceeding a 30 cm brick wall.

Summary Table:

Thermal Insulation:

• Panel insulation must equal or surpass the thermal resistance of a 30 cm brick wall.
• Typical brick conductivity: 0.6 - 1.0 W/m·K.

flowchart LR
    A[Kiln Chamber] --> B[Masonry Construction]
    A --> C[Panel Construction]
    B --> D[Brick Walls (≥30 cm)]
    B --> E[RCC Roof (≥10 cm)]
    B --> F[Concrete Floor (≥10 cm)]
    C --> G[Double Panels]
    G --> H[Insulating Material Between Panels]
    H --> I[Equivalent to 30 cm Brick Wall]

Materials and Vapor Protection in Kiln Chambers According to IS 7315

Construction Classifications:

• Masonry: brick, concrete blocks, asbestos cement boards, metal, and wood.
• Panel: double layers of steel/aluminium, wood sheathing on frames, asbestos-cement boards on wood/metal frames.

Thermal Insulation Criteria:

• Insulating material filling the panel gap must meet or exceed the thermal resistance of a 30 cm thick brick wall.
• Thermal conductivity values:

  Material	Thermal Conductivity (W/m·K)	Thickness for Equivalent R-value
  Brick (30 cm)	0.6	~0.5 m²K/W
  Insulating Material	0.04 - 0.06	Varies with thickness

Vapor Barrier and Coatings:

• Interior of masonry walls and roofs coated with asphaltic or vapor-resistant paint, reapplied periodically.
• Hollow concrete slabs/tiles require asphalt-impregnated felt membranes as vapor barriers.
• Exterior coatings must allow vapor permeability to facilitate drying.

Kiln Door Construction:

• Double panel doors with insulation.
• Interior facing made of aluminium sheet (avoid steel).
• Wood or asbestos doors lined with aluminium or asphalted felt vapor barriers.
• Doors equipped with heat-resistant gaskets and tight fastenings to reduce heat and vapor loss.
• Prefer one-piece door construction rather than double-flap designs.

Summary Table:

flowchart TD
    A[Kiln Chamber] --> B[Masonry Construction]
    A --> C[Panel Construction]
    B --> D[Vapor-Resistant Coating]
    C --> E[Double Panel with Insulation]
    E --> F[Vapor Barrier]
    F --> G[Kiln Doors]
    G --> H[Aluminium Interior]
    G --> I[Heat-Resistant Gaskets]

Kiln Door Specifications per IS 7315

Materials and Assembly:

• Doors consist of double panels formed from sheet metal, wood, or asbestos boards mounted on steel or wooden frames.
• Interior surfaces must be lined with aluminium sheet to prevent corrosion; steel is not permitted inside.
• Wood or asbestos doors should incorporate an aluminium lining or an asphalted felt vapor barrier to inhibit moisture penetration.
• One-piece door designs are preferred over double-flap types to minimize heat and vapor loss.
• Doors must be sealed with heat-resistant gaskets and securely fastened.

Insulation:

• The space between double panels must be filled with insulating material providing thermal resistance equal to or greater than a 30 cm brick wall.

Inspection Doors:

• Small inspection openings are required on both sides of the timber stack.
• Latches must be operable from inside and outside.
• Adequate lighting should be provided.

Structural Dimensions:

• Walls: Brickwork with thickness ≥ 30 cm.
• Roof: RCC slabs with thickness ≥ 10 cm, pitched for drainage.
• Floor: Concrete slab with thickness ≥ 10 cm.

Thermal Insulation Comparison:

flowchart LR
    A[Kiln Door] --> B[Double Panels]
    B --> C[Outer Panel: Steel/Wood]
    B --> D[Inner Panel: Aluminium Sheet]
    B --> E[Insulating Material Filling]
    A --> F[Heat-Resistant Gaskets & Secure Fastening]
    A --> G[One-Piece Door Design Preferred]
    A --> H[Vapor Barrier for Wood/Asbestos Doors]

Guidelines for Timber Loading as per IS 7315

• Loading Techniques:

  • Manual stacking inside the kiln is suitable only when drying duration significantly exceeds stacking time or during frequent idle periods.
  • Mechanical loading using trucks or trolleys is preferred for operational efficiency. Kiln trucks running on tracks allow outside stacking, saving time and enabling full kiln utilization.

• Stacking Capacity:

  • Defined by volume of timber with thickness 2.5 cm and crossers 20 mm thick.
  • Stack length corresponds to the maximum timber length to be processed.

• Kiln Dimensioning:

  • Determined based on maximum timber stack size including crossers.
  • Must accommodate full timber length, height, and width for optimal loading.

Volume Calculation Formula:

[Stack\ Capacity = Length \times Width \times Height]

Where:

• Length = maximum timber length (m)
• Width = stack width (m)
• Height = stack height (m)

Use timber thickness = 2.5 cm and crosser thickness = 20 mm for volume estimations.

Loading Process Diagram:

flowchart LR
    A[Stacking Timber Outside Kiln]
    B[Kiln Trucks / Trolleys]
    C[Trackways Leading into Kiln]
    D[Transfer Mechanism for Trucks]
    E[Loaded Trolleys Positioned Inside Kiln]

    A --> B --> C --> E
    D --> B

Note: RCC roof and chamber structural design must follow standard civil engineering practices.

Stacking Volume and Kiln Dimensioning as per IS 7315

Highlights:

• Stacking Capacity:

  • Volume based on stacking 2.5 cm thick timber with 20 mm thick crossers.

• Stack Dimensions:

  • Length equals the maximum timber length.
  • Width and height depend on timber quantity, fan placement, and single or double row stacking.

• Stacking Details:

  • Use square-edged planks with no gaps between layers.
  • Edges and ends must remain vertical.
  • Gaps under trolleys or in stack length/height require baffling.
  • Prepare 18 matched kiln samples, evenly distributed on both sides.

Stacking Volume Calculation:

[V = L \times W \times H \times \frac{t}{t + c}]

Where:

• L, W, H = length, width, height of stack (m)
• t = timber thickness = 0.025 m
• c = crosser thickness = 0.020 m

Stack Dimension Summary:

flowchart LR
    A[Max Timber Length] --> B[Stack Length]
    C[Quantity & Fan Location] --> D[Stack Width & Height]
    B & D --> E[Kiln Chamber Dimensions]
    E --> F[Stacking Volume Calculation]

Fan Requirements and Installation for Internal Fan Kilns (IS 7315)

Fan Design and Selection:

• Fans must be fully reversible.
• Capacity, quantity, and placement should satisfy air velocity standards.
• Large diameter propeller fans at low RPM are preferred for power efficiency and uniform airflow.
• Aluminium fans are favored; mild steel with corrosion-resistant coatings also acceptable.

Fan Mounting:

• Fans mounted on vertical partitions:
  • Overhead or underground kilns use a central vertical partition dividing the plenum.
  • Side-mounted kilns have a vertical partition on one side from floor to ceiling.

Ventilation Arrangement:

• Two rows of vents/chimneys on kiln roof for intake and exhaust.
• Fresh air supplied through ducts below floor level.
• Airflow direction reversal managed by switching high and low pressure vents.
• Vent locations align with vertical plenum gaps.

Fan and Ventilation Setup Table:

flowchart LR
    A[Cross-shaft Fan] --> B[Vertical Partition]
    B --> C[Timber Stack]
    C --> D{Airflow Direction}
    D -->|Forward| E[Roof Chimneys (Exhaust)]
    D -->|Forward| F[Fresh Air Intake Ducts (Below Floor)]
    D -->|Reverse| F
    D -->|Reverse| E

Heating Equipment and Steam Coil Standards per IS 7315

Materials and Dimensions:

• Coils constructed from black steam pipe, Class C.
• Pipe runs: minimum 25 mm nominal bore.
• Headers: minimum 75 mm nominal bore.
• Pipes may be plain or finned; fins must be firmly attached by sweating, pressing, welding, or winding.
• Aluminium fins require bonding agents to enhance heat transfer.
• Coils must pass a hydraulic pressure test at 10.5 kg/cm² before installation.

Steam Trap Requirements:

• Steam traps installed to drain condensate, preventing accumulation.
• For kiln stacks of 14 m³ timber, traps should handle 180 kg condensate per hour.
• Multiple coils:
  • For simultaneous operation, each coil has a dedicated trap.
  • For alternate operation, a single trap with stop valves on each coil is acceptable.
• Traps should be located below and near coils for effective drainage.
• Bypass lines with stop valves and strainers must be provided.

Heating Performance:

• Pipe sizing and coil layout must enable raising kiln temperature from ambient to operating level within approximately 12 hours under full green timber load.
• Steam supply details provided in Clause 2.2.

Steam Coil Pipe Size Table:

Steam Coil Drainage Diagram:

flowchart LR
    Coil -->|Steam| SteamTrap[Steam Trap]
    SteamTrap -->|Condensate Drain| DrainPipe
    SteamTrap -.->|Bypass Valve| BypassLine[Bypass Line with Stop Valve]
    Strainer --> SteamTrap
    BypassLine --> Coil

Humidification Techniques in Timber Kilns (IS 7315)

Furnace-Heated Kilns:

• Steam generated at atmospheric pressure using an auxiliary flue gas coil submerged in a water vessel, located inside or outside the kiln.
• Water sprays delivered via nozzles (pump pressure) or spinning discs (tap pressure) to create fine mist enhancing vaporization.
• Feed water pre-heating recommended for improved vaporization.
• Facilities for draining water settled on floors and walls must be provided.
• Steam sprays must comply with specified design standards.

Electrically-Heated Kilns:

• Steam generated electrically at atmospheric pressure in an external water vessel.
• Methods include electrode heating or fully immersed resistance heating elements.
• Maintain constant water level to keep elements submerged.
• Pre-treatment of feed water necessary to prevent scale buildup.

Ventilation for Humidity Control:

• Vent openings sized and spaced to reduce relative humidity to approximately 35% within 15 minutes after ceasing humidification, even under full green timber load.

Humidification Summary Table:

flowchart LR
    A[Water Vessel] -->|Steam| B[Kiln Atmosphere]
    B --> C{Humidification Method}
    C --> D[Nozzle Spray (Pump Pressure)]
    C --> E[Spinning Disc Spray (Tap Pressure)]
    C --> F[Steam Spray (Auxiliary System)]
    A --> G[Heating Method]
    G --> H[Flue Gas Coil (Furnace Kiln)]
    G --> I[Electrode/Resistance Heating (Electric Kiln)]

Ventilation System and Vent Specifications in Kilns (IS 7315)

Vent Placement and Quantity:

• Exhaust vents/chimneys located on the high-pressure side of the fan.
• Fresh air intakes situated on the low-pressure side.
• Uniform venting is critical for effective airflow.
• Cross-shaft internal fan kilns use two rows of roof vents for intake and exhaust, alternating based on airflow direction.
• Overhead/underground fan arrangements align vents and ducts with vertical plenum gaps flanking the timber stack.
• Side-mounted fans have vents and ducts positioned alongside the fan and the gap between partition and stack.

Vent Design Features:

• Vents equipped with adjustable dampers or lids for airflow control.
• Fresh air ducts, if present, run beneath the kiln floor with regularly spaced openings.
• Chimneys and vents extend through the kiln roof and false ceiling into plenum spaces.

Airflow and Plenum Dimensions:

• Air velocities through stack layers and plenum widths must comply with specified air speed requirements.

Ventilation Formula:

• Airflow rate (Q) = Vent area (A) × Air velocity (V)

[ Q = A \times V ]

• Ensure velocity matches drying requirements, commonly between 1.5 to 3 m/s in ducts.

Vent Arrangement Diagram:

flowchart LR
    subgraph Kiln Roof
    ExhaustVents["Exhaust Vents (High Pressure Side)"]
    IntakeVents["Intake Vents (Low Pressure Side)"]
    end
    subgraph Kiln Floor
    FreshAirDucts["Fresh-Air Intake Ducts (Below Floor)"]
    end
    Fan["Internal Cross-Shaft Fan"]
    ExhaustVents -- Air Out --> Fan
    Fan -- Air In --> IntakeVents
    FreshAirDucts -- Supply Air --> IntakeVents

Automatic Temperature and Ventilation Controls as per IS 7315

Steam-Heated Kilns:

• Steam supply regulated by diaphragm valves (compressed air operated) or motorized electric valves.
• Controlled through dry- and wet-bulb temperature indicators/recorders (mercury or vapor-filled extension tube types).
• Temperature sensors actuate mechanical air supply controls or electrical switches that operate valves or solenoids.
• Automatic valves positioned between main steam supply and heating coils/steam spray lines.
• Pressure reducing valves limit maximum steam pressure but do not maintain minimum pressure.
• Manual valves and pressure gauges installed for backup control.
• Roof vents automatically operated by compressed air or electric motors linked to thermostats.

Furnace-Heated Kilns:

• Fuel feed controlled by diaphragm, motorized, or solenoid valves with thermostatic feedback.
• Roof vents and flue gas dampers automatically adjusted via compressed air or electric actuators.
• Water spray humidification controlled by valves and magnetic switches connected to thermostats.
• Waste-wood fuel feed regulated by motorized hopper feed with thermostatic control.

Automatic Control Logic Diagram:

flowchart TD
    TempSensor[Dry & Wet Bulb Sensor]
    TempSensor -->|Mechanical| AirSupplyControl[Compressed Air to Diaphragm Valve]
    TempSensor -->|Electrical| ElectricSwitch[Electrical Switching]
    ElectricSwitch --> MotorizedValve[Motorized Steam Valve]
    ElectricSwitch --> SolenoidValve[3-Way Solenoid Valve]
    MotorizedValve --> SteamCoils[Heating Coils]
    DiaphragmValve --> SteamCoils
    TempSensor --> RoofVentControl[Roof Vent Controller]
    RoofVentControl -->|Air or Electric| RoofVents

Instrumentation Requirements and Monitoring Practices in Kilns (IS 7315)

Temperature Measurement:

• Use mercury-in-glass thermometers for visual indication.
• Prefer extension-tube recording thermometers with copper bulbs and capillaries for continuous monitoring.
• Temperature range: 20–100°C.
• Bulbs are placed mid-depth within the timber stack for representative readings.
• Capillaries armored to prevent corrosion.
• Dual thermometers (wet-bulb and dry-bulb) installed to simultaneously measure temperature and humidity.

Control Equipment:

• Manual control for fundamental kiln operations.
• Automatic thermostatic control using temperature instruments to regulate valves, vents, and drying conditions.

Instrumentation Flow Diagram:

graph LR
A[Temperature Bulb Sensor] -->|Capillary| B[Pressure Element]
B --> C[Indicator / Recorder]
C --> D[Motorized Control Valve]
D --> E[Heat and Humidity Systems]

Specifications for Temperature and Humidity Instruments (IS 7315)

Instrument Types:

• Visual/indicating mercury-in-glass thermometers.
• Recording extension-tube thermometers with mercury, gas, or vapor-filled bulbs connected to pressure-sensitive elements.
• Recording instruments preferred for continuous data logging and automatic control.

Instrumentation Requirements:

• Each kiln chamber must have at least one dry- and wet-bulb temperature recorder.
• At least two hand-held mercury-in-glass hygro-meters for spot checks and reversed circulation monitoring.
• If recorders are unavailable, two dry- and wet-bulb distant indicating instruments plus two hand hygro-meters are acceptable.
• Temperature range: 20–100°C.
• Bulbs and capillaries must be corrosion-resistant and sufficiently long for mid-stack placement.
• Reading accuracy for dry bulb must be within ±2.5°C.

Instrument Setup Diagram:

flowchart LR
    A[Kiln Chamber] --> B[Dry & Wet Bulb Recorder]
    A --> C[Hand Hygrometers (2 units)]
    B --> D[Extension-Tube Thermometer]
    C --> E[Mercury-in-Glass Hygrometer]
    B --> F[Corrosion Protection & Proper Capillary Length]

Laboratory Tools and Procedures for Moisture Testing in Timber (IS 7315)

Equipment:

• Motor-driven fine-cutting handsaw (30 cm wheel) preferred for sample preparation.
• Beranger scales with 10 kg capacity for weighing bulk samples.
• Physical balance with 250 g capacity and 5 mg sensitivity for precise weighing.
• Thermostatically controlled ventilated drying oven capable of maintaining 105°C ± 2°C for drying.
• Optional electrical moisture meters providing quick moisture content readings between 8% and 25%.

Testing Procedures:

• Follow kiln drying schedules with air circulation reversed every 6 hours.
• Record daily moisture content for 18 matched kiln samples.
• Plot moisture content against drying time to generate drying curves.
• Calculate drying rates for identical moisture levels across different stack positions.
• Ensure no significant variation in drying rates or final moisture content.

Equipment Summary Table:

Testing Workflow Diagram:

flowchart TD
    A[Sample Preparation] --> B[Fine-cutting Saw]
    B --> C[Weighing on Scales]
    C --> D[Drying in Oven]
    D --> E[Moisture Content Measurement]
    E --> F[Plot Drying Curves & Calculate Rates]

Essential Testing Equipment and Kiln Performance Requirements (IS 7315)

Equipment:

• Motor-driven fine-cutting handsaw with 30 cm wheel for test sample preparation.
• Beranger scales (10 kg capacity) and physical balance (250 g capacity, 5 mg sensitivity) for weighing.
• Thermostatically controlled ventilated drying oven with an operating range up to 105°C ± 2°C.
• Electrical moisture meters for rapid moisture content assessment (8–25%).

Performance Requirements:

• Kiln designs vary in chamber materials, air circulation, heating, humidification, venting, and baffling.
• Kiln must reach operating temperature from ambient within about 13 hours under full green timber load.
• Functional tests verify compliance with minimum performance standards.

Equipment Summary Table:

Kiln Testing Flow Diagram:

flowchart LR
    A[Green Timber Load] --> B[Kiln Chamber]
    B --> C{Heating System}
    C --> D[Forced Air Circulation]
    C --> E[Thermal Circulation]
    B --> F[Humidity Control]
    B --> G[Temperature Monitoring]
    G --> H[Target: Operating Temp in ~13 hrs]
    B --> I[Sampling for Moisture Content]
    I --> J[Sawing Equipment]
    I --> K[Weighing Scales]
    I --> L[Drying Oven]
    I --> M[Moisture Meter]