Stainless Steel Steam Heating Tubes
Magnesium oxide (MgO) powder is familiar to many of us and plays a decisive role in electric heating tubes. But how much do you really know about its functions? Your understanding might be partial. To help everyone fully understand the role of MgO powder in electric heating tubes and gain more knowledge, Messi Biology has compiled the following information:
1. What is Magnesium Oxide Powder?
Magnesium oxide (MgO), commonly known as caustic calcined magnesia, light-burned powder, or magnesia, is a pale yellow powder with a density of 2.94. It has a melting point of 2852°C and a boiling point of 3600°C. It is soluble in acids and ammonium salts but difficult to dissolve in water and ethanol. Its specific surface area is 180 m²/g when calcined at 400°C, but drops to only 3 m²/g at 1300°C. It deliquesces easily in the air and is a gas-hardened cementitious material.
MgO powder is classified into several grades: High-grade lubricant grade, food grade, medical grade, silicon steel grade, high-grade electromagnetic grade, and high-purity magnesium oxide.
Part of MgO powder is used to manufacture refractory materials. Why? In the United States and most countries that consume magnesium products, the steel industry is the largest consumer. Dead-burned magnesia, obtained from magnesite ore, seawater, salt lakes, or well salt, is primarily used to manufacture refractory products for metallurgical furnaces. Light-burned magnesium powder is widely used in various fields:
- Water Treatment: Used to remove silica and heavy metals from industrial wastewater.
- Environmental Protection: Used as a neutralizing agent for industrial exhaust gas. It reacts with
SO2SO_2SO2in flue gas to produce magnesium sulfate, which is more effective for desulfurization than calcium compounds.
Different manufacturing methods result in different physical states of MgO:
- (1) Burned Magnesium: This is a product of pure metallic magnesium heated and oxidized in air. It has good electrical insulation but poor heat transfer properties, making it unsuitable for heating tubes.
- (2) Magnesite (
MgCO3MgCO_3MgCO3): It has a high heat transfer coefficient but decomposes intoMgOMgOMgOandCO2CO_2CO2at 500–800°C. Therefore, it is not used in electric heating tubes. - (3) Caustic Magnesium: It is produced by sintering magnesite at 800–1000°C (
MgCO3=MgO+CO2MgCO_3 = MgO + CO_2MgCO3=MgO+CO2). In a compressed state, it has good thermal conductivity and satisfactory electrical insulation (though it produces magnesium hydroxide when combined with water). Thus, caustic magnesium is only suitable for dry environments or electric heating tubes with excellent sealing. - (4) Fused Crystalline Magnesium Oxide: This has superior characteristics compared to caustic magnesium. It is produced by melting magnesite in an electric arc at 2800°C to obtain large crystalline blocks. After crushing, grinding, sieving, and magnetic separation, fused crystalline MgO powder is produced, with purity reaching over 96%.
- Once compressed in a heating tube, its specific gravity is twice that of caustic magnesium (approx. 2.85–3.2 g/cm³).
- Thermal Conductivity: At a specific gravity of 2.85 g/cm³, the thermal conductivity is 0.0215 kcal/cm·°C·h at 250°C and 0.015 kcal/cm·°C·h at 950°C. The average value between 250–950°C is approximately 0.018 kcal/cm·°C·h.
- Well-crystallized MgO does not react with water or carbonic acid gas. It is an excellent filler for heating tubes operating at temperatures above 400°C.
Addressing Insulation Failure:
In heating tubes using fused crystalline MgO as filler, if the surface temperature exceeds 500°C, the hot insulation resistance will drop significantly as the temperature rises. Failure often occurs due to insulation breakdown caused by leakage between the spiral heating wire and the metal sheath. This leakage involves both electronic and ionic conduction. To reduce electronic conduction, 2% Boron Nitride is often added; to reduce ionic conduction, 0.01 mole% Lithium Oxide is added.
2. Structure of the Electric Heating Tube
An electric heating tube is composed of a heating wire (resistance wire), a metal sheath, and insulating magnesium oxide powder. The material of each component is vital, but if the quality of the MgO powder is substandard or if it is installed incorrectly, the service life of the heating tube will be directly affected.
Rules for Magnesium Oxide in Heating Tubes:
- Insulation and Conduction: MgO powder is located between the heating wire and the tube wall for insulation. Simultaneously, it possesses high thermal conductivity. However, MgO powder is highly hygroscopic (absorbs moisture), so it must undergo moisture-proof treatment (via modification of the powder or sealing the heating tube).
- Temperature Range: MgO powder is categorized into low-temperature powder and high-temperature powder. Low-temperature powder is typically modified and is only used below 400°C.
- Particle Size Distribution: MgO powder is granular. The powder used in heating tubes is configured by mixing different particle sizes according to a specific mesh ratio (proportion of coarse to fine grains).