Industrial furnaces are used for various heating processes – from low to very high temperatures – in industries such as steel, ceramics, refractories, glass and many more.
Industrial furnaces are used to melt metals for casting or heat materials for change of shape (rolling and forging) or change properties (heat treatment). In the case of a combustion-type furnace, depending on the kind of combustion, furnaces can be categorized as oil-fired, coal-fired or gas-fired.
Industrial furnaces are designed considering various important parameters, including:
- Heating substrate
- Processes used (e.g., baking, drying, heat treatment, melting, smelting, etc.)
- Temperature ranging from 572-5432°F (300-3000°C) or higher, depending on the application
- Fuel used (gaseous or liquid)
Industrial furnaces should be operated with optimum fuel consumption and lowest maintenance, whereas the design of the furnace should allow for maximum heat transfer at a defined material feed rate.
Optimum performance of the furnace can be ensured by two factors:
- The heat-transfer rate to the material with fine control of fuel-to-air ratio
- Minimum heat loss for a defined mass with perfect insulation design
Modern industrial furnaces need optimum performance with the best possible thermal efficiency. To achieve this performance and efficiency, the latest process instrumentation is used. Flow measurement serves as the primary burner control for process efficiency with minimum maintenance and a wide measuring range for air-to-fuel ratio control.
Fig. 1. Schematic for flow measurement in an industrial furnace
Characteristics of an Efficient Furnace
A furnace should be designed in such a way that as much material as possible can be heated to a uniform temperature in a given time with the least possible amount of fuel consumption and labor.
To achieve this, the following parameters need to be considered:
- Determine whether heat or kilocalorie needs to be provided to the material
- Sufficient heat liberation within the furnace to heat the stock and overcome all heat losses
- Efficient heat transfer from the furnace gases to the surface of the heating stock
- Equalization of the temperature within the stock
- Reduction of heat losses from the furnace to the very minimum
Fig. 2. Stoichiometric point
Furnace Energy Supply
Since the products of combustion (i.e., flue gases) will directly contact the stock or material, the type of fuel chosen is of great importance. For example, some materials will not tolerate sulfur in the fuel. Also, the use of solid fuels will generate particulate matter, which will interfere with the stock placed inside the furnace. Therefore, the vast majority of furnaces use liquid fuel, gaseous fuel or electricity as energy input.
Nonferrous melting utilizes oil as fuel. Furnace oil is also the major fuel used for reheating and heat treatment of materials. Light diesel oil (LDO) is used in furnaces where the presence of sulfur is undesirable.
The key to efficient furnace operation lies in incomplete combustion of fuel with minimum excess air. Currently, natural gas is the most preferable, clean and safe energy source for furnaces due to its cost-effectiveness and achieving high efficiency up to 90-98% compared to oil or solid fuel-fired furnaces, which are in the 70-80% range.
Fig. 3. Thermal mass flow meters, without and with safety head
Fig. 4. Typical flow velocity vs. pressure drop
Current Flow Measurement Challenges in a Furnace
Burner-control design plays a vital role in generating a controlled flame temperature used to heat the furnace inner chamber. The flame is usually generated by igniting a mixture of fuel and combustion air, normally preheated in a recovery system. The higher the temperature of the preheated combustion air, the lower the energy required to heat the furnace chamber. This suggests that accurate combustion airflow and fuel-flow rate are of utmost importance for controlled flame at optimum air-to-fuel ratio.
Furnaces generally operate at constant pressure at the inlet of the burner. However, this doesn’t ac-count for the accuracy of flow rates needed for optimum combustion, which results in energy losses. Control is typically based on flame temperature. It is not attuned to the varying material heat load in the furnace.
Generally, furnace operators do not use airflow measurement. The airflow rates are assumed constant based on blower design parameters, such as pressure and power readings, which prove to be highly inaccurate. Many operators measure velocity from time to time with some portable anemometers at the suction of blowers. Some use orifice-type flow meters for air measurement. These meters generate a permanent pressure drop and provide inaccurate measurements.
Furnace fuel-gas or liquid-fuel flow rates are measured in furnaces at main headers and not in single burners due to high cost implications. Most furnace operators don’t measure it but calculate it per burner design parameters such as kilocalories. Some use volumetric flow meters in the case of gas and liquid light diesel or furnace oil. Therefore, precise control of combustion air and fuel flow rate will optimize furnace performance at varying loads and save a huge amount of energy.
The challenges faced in flow measurement for furnaces include:
- High pressure drop loses energy
- Non-versatile for all gas furnace designs
- Lower sensitivity and poor resolution
- Lower accuracy
- High maintenance
- Effects due to dust or dirt
Effective and Versatile Solutions for Air and Fuel Flow Measurement
New flow-measurement technology has emerged after more than 20 years of continuous development. The calorimetric (thermal-dispersion) technique overcomes almost all of the referenced challenges and is economical for the latest industrial furnace designs.
Insertion and fixed-style thermal mass (calorimetric) flow sensors satisfy all the needs for all types of flow-measurement requirements, whether it is blower air, burner fuel gas (natural gas) and exhaust low-temperature flue gas with same technology.
Calorimetric Flow Measurement
Calorimetric flow measurement works on the physical principle of heat dissipation from a heated element to the ambient medium (e.g., air or gases). This is affected by the velocity, density (temperature and pressure) and the characteristic of the medium. The amount of energy needed to maintain temperature difference ∆T is a function of the mass-flow rate.
Gas flows through two RTD Pt-100s: one reference (Tref) and one heater (Th). The temperature difference (over-temperature) ∆T between the reference sensor (medium temperature) and the heater sensor is controlled constantly. As per King’s Law, the higher the mass-flow rate, the higher the cooling effect of the heater sensor. Therefore, higher power is required to maintain the differential temperature constant, and the heater power is proportional to the gas mass-flow rate.
Thermal mass-flow measurement solutions provide:
- Mass-flow measurement independent of pressure and temperature variations
- Sensor probes can be made in SS-316Ti, Hastelloy C276 or any suitable metal
- Pipe diameter from 15 mm and above
- Designed to withstand up to 400°C temperature
- A high turndown ratio of better than 100:1 means highly sensitive
- Help in leakage detection
- Designed to work from low pressure to high pressure up to 16 bar or more
- No pressure drop directly saves energy
- No change in pipe diameter
- Better accuracy (up to ±1.5%) and repeatability
- No moving parts, so it can work in dirty, wet and harsh environments
- Easy cleaning of sensors; optionally with automatic air purging
- Easy installation against orifice-plate flow meter
- No periodic maintenance needed
- Cost-effective compared to other flow-sensor technologies
Fig. 5. Constant temperature anemometry (CTA)
Calorimetric (thermal-mass) flow measurement is a versatile solution for industrial furnaces. It per-forms well for almost all gas furnace applications with no pressure drop and high turndown ratio, and it is programmable to various gas options and pipe diameters with high resolution compared to currently available flow sensors in the market.
For more information: Author Manish Patel is Director of Leomi Instruments Pvt. Ltd. He can be reached at firstname.lastname@example.org.
Leomi’s technical team is continuously working on research for various mass flow measurement solutions, keeping in mind the process conditions of different industries. For more information, contact Leomi at www.leomi.in, and follow the company on its LinkedIn page.
All graphics provided by the author.