Flow measurement plays a significant role in industrial furnaces, which are used to melt metals for casting or heat materials for a change of shape (forging) or a change of properties (heat treatment). Unfortunately, there is no common mass flow sensor technology that suits all industrial gas furnace applications.

Types and Classification of Furnaces

Based on the method of generating heat, furnaces are broadly classified as combustion (using fuels) and electric. In the case of a combustion-type furnace, depending on the kind of combustion, furnaces can be categorized s oil-fired, coal-fired or gas-fired.

Based on the mode of charging of material, furnaces can be classified as intermittent (batch or periodical) or continuous. Based on the mode of waste-heat recovery, furnaces can be categorized as recuperative or regenerative.

Industrial furnaces are designed considering important parameters such as heating substrate, capacity, process (drying, heat treatment, melting, etc.), temperature (ranging from 300-3000°C or higher) and 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.

Modern Industrial furnaces need to operate at peak performance with the best possible thermal efficiency. To achieve both of these variables, process instrumentation is used. The latest development is a flow-measurement sensor that serves the main purpose for process efficiency in Industrial furnaces with minimum maintenance and a wide measuring range for air-fuel ratio control.

Thermal mass flow meters, without and with safety head

Characteristics of an Efficient Furnace

Furnaces should be designed in such a way that as much material as possible can be heated to a uni-form temperature, in a given time, with the least possible fuel consumption and labor. To do so, the following parameters need to be considered:

• Determine 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 heating stock
• Equalization of the temperature within the stock
• Reduction of heat losses from the furnace to the minimum possible extent

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 the energy input. Nonferrous melting, however, utilizes oil as fuel. Light diesel oil (LDO) is used in furnaces where the presence of sulfur is undesirable.

The key to efficient furnace operation lies in complete combustion of fuel with minimum excess air. Natural gas (LNG or PNG) is currently the most preferred clean and safe energy source for furnaces due to its cost-effectiveness and its ability to achieve efficiency up to 90-98% compared to oil and solid fuel-fired furnaces, which are in the 70-80% range.

Typical flow velocity vs. pressure drop

Current Flow Metering Technology

• Flow metering technology currently being used has its limitations for use with industrial furnaces. Here are two examples.
• Differential-pressure-based orifice or averaging pitot flowmeter for air, fuel gas and exhaust flue-gas flow measurement
• Volumetric measurement, dependent on pressure and temperature variation, needs to compensate for accurate mass flow rate
• Higher permanent pressure drop with low static pressure blower used for furnaces
• Narrow flow measurement range; around 4:1 turndown ratio
• Low sensitivity to change in furnace pressure and temperature
• Low accuracy
• Clogging of pressure ports due to dust accumulation

Turbine flowmeter or rotary piston displacement flowmeter for fuel

•    Volumetric flow measurement depends on pressure and temperature for mass flow rate

•    Moving parts needs periodic maintenance

•    Higher permanent pressure drop

•    Flow measurement turndown ratio around 15:1

•    Low sensitivity to change in furnace pressure and temperature

•    Low accuracy > ±1% to 2% of reading or better depending on manufacturer

Latest Flow Sensor Development for Air, Fuel Gas and Exhaust Flue-Gas Flow Measurement

Industrial gas furnaces are preferred due to their overall heating efficiency and cost effectiveness. So, the development of a new versatile mass flow sensor is of the utmost importance considering the fac-tors needed to overcome the challenges of old flow technologies and the desire to improve overall process efficiency.

Air-to-fuel ratio is the most important parameter to be precisely controlled for the best performance in achieving optimum heat mass transfer rate.

The important factors to consider when designing a flow and temperature sensor for industrial furnaces include: smart sensor with robust design, versatility for use in all gas furnace applications, minimal pressure drop, high sensitivity and resolution, high accuracy and repeatability, no maintenance and easy cleaning (if needed).

An insertion and fixed-style thermal mass (calorimetric) flow sensor satisfies the needs of all flow-measurement requirements for industrial gas furnaces, whether it is blower air, burner fuel gas or exhaust low-temperature flue gas. It works on the physical principle of heat dissipation from a heated element to the ambient medium. This is affected by the velocity, density (temperature and pressure) and characteristic of the medium. The amount of needed energy to maintain temperature difference ∆T is a function of the mass flow rate.

The temperature difference (over-temperature) ∆t between the reference sensor (medium tempera-ture) and the heater sensor is controlled constant. As per King’s Law, the higher the mass flow rate, the higher the cooling effect of the heater sensor and the higher the power is required to maintain the differential temperature constant. Therefore, the heater power is proportional to the gas mass flow rate (Fig. 5).

The advantages of a thermal mass flow sensor for industrial gas furnace applications include:

• Direct mass flow measurement independent of pressure and temperature
• Sensor probes can be made in SS-316Ti, Hastelloy C-276 or any suitable metal
• Can be used for pipe diameters of 0.60-197 inches (15-5,000 mm)
• Designed to withstand up to 752°F (400°C)
• High turndown ratio of better than 100:1
• Helps with leak detection
• Design to work from too low pressure to high pressure up to 16 bar or more
• No pressure drop directly saves energy
• No change in pipe diameter  
• High accuracy and repeatability
• No moving parts
• Can work in dirty, wet and harsh environments
• Easy cleaning of sensors (option with automatic air purging)
• Easy installation against ultrasonic and pitot tube
• No periodic maintenance needed
• Cost-effective compared to other flow sensor technologies

Thermal flow meter from Leomi Instruments

Conclusion

A calorimetric mass flow sensor can work for almost all industrial gas furnace applications with mini-mum energy loss and high resolution. A versatile design for varying pipe dimensions helps fine-tune furnace parameters with higher control compared to currently available flow sensors.

Leomi’s technical team is researching other alternatives for mass flow measurement solutions for dif-ferent process conditions. For more information, contact Leomi at www.leomi.in. Author Manish Patel is Director of Leomi Instruments Pvt. Ltd. He can be reached at leomi.instruments@gmail.com.

All graphics provided by the author, except where noted.