What is a Large Scale Calorimeter?

With the continuous improvement of people's material and cultural living standards and the continuous development of science and technology, the materials used in various fields have become increasingly abundant. However, while the abundance of materials facilitates human production and life, it also increases the risk of fire to some extent, creating hidden fire hazards for fire safety. Therefore, research on the occurrence of fires in social life is urgently needed. Traditional small-scale calorimeters aim to simulate the real combustion environment of materials, but cannot accurately reflect the actual combustion conditions in a real large-scale environment. Therefore, large-scale fire testing instruments have gradually developed.

Large-scale calorimeters can test fire conditions occurring in real environments (such as rooms, furniture, cables, automobiles, energy storage batteries, railway vehicles, etc.), and are used to measure ignition characteristics, heat release rate, mass loss rate, effective calorific value, smoke characteristics, and toxic gas analysis during the combustion process.

Large-scale calorimeter heat release rate testing devices are used to test and analyze the ignition characteristics, heat release rate, mass loss rate, effective calorific value, and gas analysis of large objects or structures placed in open-air combustion processes. This device combines direct physical combustion simulation with analysis of collected flue gas data to understand the combustion characteristics of various test subjects. Simultaneously, it can also study optimal fire extinguishing methods to reduce the risks to life and property caused by fires.

Currently, there are no international standards for 10MW large-scale calorimeter equipment. Its design and assembly mainly refer to the design principles of ISO 5660-1 and ISO 9705. The system encompasses a fire safety system, a smoke extraction and flue gas treatment system, a sample gas sampling and analysis system, an ignition system, and a software control system.

Explanation of Large Calorimeters Based on heat release rate, calorimeters are classified into several types. The most common calorimeters include cone calorimeters for testing 10x10cm objects and 1-3 MW furniture calorimeters (corner fire tests) for evaluating combustion. However, while the flammability testing method using cone calorimeters for small objects (10x10cm) achieves objectivity and fairness in test results and complies with global standard testing methods, it is not suitable for evaluating the fire resistance of large objects. Therefore, it is not recommended as a reasonable standard for fire resistance performance. To ensure fire safety, depending on its purpose and the size and fire resistance rating of buildings and large complexes (including locomotives, vehicles, warehouses, and configurations), these fire resistance ratings should be verified through extensive fire resistance testing of materials, buildings, and physical models. Such testing will reduce life and property loss in fires. In other words, it is recommended to use a combination of tests—double cone calorimeter (raw material) — corner detector/single combustion item/ISO13784-1 (micro-model) — large cone calorimeter (actual)—to determine the fire safety performance classification standards, and to control a wide range of fire safety performance tests based on combustion risk, purpose, and the specifications of equipment within the building.

Structure and Function of Large Calorimeters

1) Shielding and Piping System When designing the shielding, two main considerations are the form and size of the shield. Compared to a square shielding, a circular shielding minimizes the eddy current effects that may occur along the edges of the shielding. Therefore, the use of a circular shielding allows for adjustable height, enabling testing to be conducted according to the characteristics of the smoke plume from the combustion of the test object. Furthermore, the combustion gases are thoroughly dispersed due to the sufficiently uniform flow length provided by the piping and equipment accessories. The outer diameter of the dust collection hood is 10 cm, allowing the measurement of the heat release rate, fire load, oxygen demand, amount, concentration, temperature, flow rate, and composition of combustion gases for actual-sized combustible materials and vehicles using a calorimeter with a maximum range of 10 MW. 2) Composition of Measurement Factors The factors to be measured can be broadly categorized into flow rate/temperature, gas concentration, and smoke density. The testing instrument includes processes for control and data measurement. Differential pressure probes, thermocouples, cross-shaped gas collection devices, and smoke density meters are installed on the pipeline for measurement.

3) Flow Rate/Temperature When primarily using a single pitot tube to measure the flow rate through the pipeline, particulate matter in the smoke can clog the pipe. Therefore, using bidirectional velocity probes is reasonable. This probe estimates the flow rate by measuring the pressure difference at the two ends of an opening, as the opening shape is generally the same. A total of six bidirectional velocity probes are installed using the Log-Tehebycheff method. Since the density of the substance changes with the temperature around the probe, bidirectional shielded thermocouples are inserted for density compensation.

4) Combustion Gas Collection and Analysis To calculate the heat release rate as described above, we need to know the oxygen concentration. To determine the characteristics of the combustible gas, carbon dioxide and carbon monoxide need to be analyzed. The gas collector is made in a cross shape, not a typical O-shape. An orifice is provided on the opposite side of the airflow to prevent soot from clogging the pipeline. The condenser is used to condense the gas and remove moisture generated when the collected fuel gas is cooled to below 3 degrees Celsius. Cooling and condensing the fuel gas removes impurities without altering the gas composition, thus supplying pure fuel gas to the gas analyzer. The fuel gas undergoes further moisture removal before being supplied to the analyzer. Furthermore, a temperature control circuit is installed to maintain the fuel gas sample from ambient temperature at 150 degrees Celsius during its supply from the gas conduit to the pretreatment unit. Ash filters are installed before each gas analyzer and all collection pipes.

5) Smoke Density Smoke density is measured using a photocell and a helium-neon laser system. While halogen lamps are used as the light source, a laser is chosen due to the pipe diameter, as lasers offer better directionality and lower attenuation than halogen lamps. A beam splitter divides the laser source into two: one for reference and the other for radiation to measure smoke density. Smoke density is measured using the laser scattering method.

6) Data Processing System To calculate and record heat release rates, smoke density, and gas concentrations obtained from various detectors, a data processing system and a measurement system for operating the MFC or opening/closing various solenoid valves are independently programmed according to control procedures. The data acquisition system uses hardware from National Instruments (NI) and software from NI's LabVIEW.

Experimental Principle

Unaffected by any surrounding environmental structures, a calorimeter can be used to assess the potential contribution of a single object or a group of objects to heat release and fire spread hazard during combustion. The heat release rate throughout the combustion process is calculated using the oxygen consumption principle. Sample gas is transported to a gas analyzer after a sampling pretreatment system to test the concentrations of oxygen, carbon dioxide, and carbon monoxide. Simultaneously, parameters such as temperature, pressure difference, and optical density are measured in the pipeline measurement section. Combustion characteristic parameters such as the heat release rate, total heat release, smoke production rate, and total smoke production of the test ignition source are calculated using the oxygen consumption principle.

Operating Method

Preparation

Placement and Preheating: Place the calorimeter on a level and stable stone platform, connect the power supply, and preheat to ensure the instrument reaches a stable operating state.

Inspection: Check that all components are secure, that there is no damage to the exterior, and that the water pump inlet and outlet pipes are properly connected and free from deformation or folding.

Inject an appropriate amount of water until it overflows from the overflow port.

Calibration and Standardization

Calibrate the thermistor: Place the calorimeter containing the reference material into the calorimeter. Select the calibration option from the menu and calibrate according to the instrument instructions to ensure the accuracy of temperature measurements.

Calibrate the heat capacity: Enter the same number and perform three consecutive benzoic acid experiments. Take the average of two closest values ​​and save the result. This step is to determine the instrument's heat capacity during the measurement process for subsequent calculation of the sample's calorific value.

Sample Preparation and Placement

Weighing the sample: Use a precise balance to weigh a certain amount of sample (e.g., coal sample). The particle size is typically required to be less than 0.2 mm, and the weight between 0.9 and 1.1 g, accurate to 0.0001 g or 0.0002 g.

Loading the sample: Load the weighed sample into the oxygen bomb. First, add 10 ml of distilled water to the bomb chamber. Simultaneously, connect the ignition wire, ensuring good contact between the ignition wire and the sample, but not into the crucible to avoid short circuits.

Oxygenation and Testing

Oxygenation: Open the main valve of the oxygen cylinder and adjust the pressure reducing valve to bring the outlet pressure between 2.8 and 3.0 MPa. Place the oxygen bomb on the oxygenator positioning seat, align it with the oxygenator nozzle, and begin oxygenation. The oxygenation time should not be less than 15 seconds; 30 to 40 seconds is generally recommended.

Placing the Oxygen Bomb: After oxygenation, place the oxygen bomb into the inner chamber of the calorimeter and close the inner chamber lid.

Starting the Test: Enter the relevant sample information (such as mass, total sulfur content, hydrogen content, total moisture, analytical moisture, etc.) on the calorimeter's program interface, and then click the "Start" button. The system will automatically enter the testing state.

Testing Process and Result Processing

Observation and Recording: During the test, carefully observe the instrument's operating status and the testing progress. The test typically takes about 20 minutes, during which the instrument will automatically perform operations such as ignition, stirring, and temperature rise measurement.

Result Output: After the test, the instrument will automatically output the results. You can then choose to view the data or print it.

Post-processing: After removing the oxygen bomb and releasing the exhaust gas, open the oxygen bomb cover to check the combustion status. If necessary, the data can be recalculated or saved.

Large-scale calorimeters, as key equipment for assessing the fire characteristics of materials and structures, profoundly reflect the need for accurate simulation of real fire scenarios in their design philosophy and functionality. From the hydrodynamic optimization of the circular protective cover to the precise matching of the bidirectional velocity probe and condenser tube, and the calculation of the heat release rate based on the oxygen consumption principle, every technical detail serves to improve the reliability and practicality of the test data. Especially in the absence of internationally unified standards, this equipment provides a scientific basis for fire safety performance classification by integrating multi-system collaborative control and high-precision data acquisition. In the future, with the continuous expansion of material complexity and application scenarios, large-scale calorimeters not only need further optimization in hardware structure to adapt to larger-scale test objects, but also need to achieve breakthroughs in data analysis algorithms and standardization, thereby building a more solid technical barrier to reduce fire risks and protect life and property.