A bath medium is heated by a submerged firetube and the hot fluid either heats a separate submerged coil bundle indirectly or heats the fluid directly to the desired temperature. Let's look at the types of baths commonly used for both applications.
TYPES OF BATHS: INDIRECT
Low Pressure Steam
Water vapor is an exceptional heat transfer medium. A steam bath heater is configured so that the firetube is submerged but the process coils are placed above the water level to allow the steam to condense on the cooler coils. The bath operating temperature is typically around 250 °F with allows for slightly higher process outlet temperatures. The heat transfer coefficient on the process coil for steam is significantly higher than other methods of heating.
The vessel must be designed per ASME IV code to allow the vessel to operate at 15 PSIG. The process coils should be manufactured from 304 stainless steel for corrosion protection. Water chemistry should be monitored and should be free of impurities to avoid dissolved solid formation.
Molten Salt
Heat transfer Molten Salt is generally a blend of sodium nitrate and potassium nitrate which have better corrosion performance than the chloride in sodium chloride (table salt). This mixture reduces the melting point and aides freeze up potential. It has favorable heat transfer properties and is used in heaters with high temperature requirements (400 °F to 900 °F).
Material selection is important and depends on the salt mixture and operating temperatures. Due to a melting point significantly above ambient temperature, heater design must include heat tracing and additional uniform heating equipment to maintain elevated temperatures when the unit is not operating. The risk of freezing is significant, and the salts expand when they re-melt.
Hot Oil
Also called thermal or heat transfer oils and fluids. Comprised of a wide variety of engineered synthetic hydrocarbon formulas specifically designed for heating applications. They have excellent thermal transfer properties and are available in a range of temperatures (See Figure 1).
Inert blanketing systems should be installed to keep oxygen from contacting hot oil. Additionally, all water must be boiled out of the system as some synthetic oils can create organic acids and cause corrosion issues at elevated temperatures. If the only circulation in the heater will be from natural convection then a circulation pump is recommended.
Figure 1
Source: https://www.therminol.com/heat-transfer-fluids
Water
Is often used as a heat transfer fluid as it is the cheapest fluid, has excellent thermal properties and is safe if kept above freezing. The bulk temperature of a heater is typically controlled at 190 °F to 200 °F depending on elevation to stay below the boiling point and reduce vaporization and water losses. One must consider adding alternate fluids or heat tracing in climates where the water will freeze.
Water/Glycol
These mixtures are used when heaters are located in climates where the water bath was the potential to freeze. Fluid analysis should be performed periodically to confirm pH balance which will determine if any degradation has occurred and if inhibitors should be added. Some fluid manufacturers include inhibitors and additives in their glycol.
TWO FORMS OF GLYCOL
Ethylene Glycol (EG)
Odorless and colorless organic compound commonly used as a heat transfer medium and found in many household items such as laundry/dishwasher detergent, brake-fluid, and antifreeze. A small amount is toxic and can be fatal. Dissolves in water and adds the benefit of lowering the freezing point but reduces the overall heat transfer properties and decreases the overall heat transfer to the process by up to 20%.
Propylene Glycol (PG)
Non-toxic polyether compound is widely used in the chemical production, food, medical, and industrial industries. Frequently selected in antifreeze applications in lieu of EG as it is non-toxic and considered more environmentally friendly than EG. The tradeoff is that the heat transfer characteristics are not as favorable as EG or water. Industrial grade PG contains inhibitors which makes the concentration toxic. Although both EG and PG are not harmful to aquatic life in the event of a spill, both absorb oxygen in water. PG needs twice the oxygen to degrade than EG which leads one to question whether PG is in fact the environmentally friendly option.
TYPES OF BATHS: DIRECT
Triethylene Glycol (TEG)
An alternate glycol liquid with superior water absorption properties. It is typically favored over diethylene glycol to remove water from natural gas in gas dehydration systems for several reasons. TEG regenerates easily and permits higher dew point depressions. Since glycol in the presence of oxygen creates corrosion, inhibitors are used to maintain proper a pH level of 7-8. Bulk heater temperatures should be maintained at 390 °F to avoid damaging the fluid because TEG with a concentration of 99% by weight decomposes at 404 °F. Even operating under these conditions, degradation of the fluid over time will occur in TEG reboiler applications. Continual circulation of the TEG and water absorbed in the process prevents rapid fluid degradation.
Amines (MEA, MDEA, DEA, etc.)
Alkanolamines bond chemically with acid gases and create unstable salts which can be broken down with controlled temperature and pressure. Commonly used in process equipment to remove H2S, CO2 and mercaptans from sour gases to meet desired sales gas specifications. A variety of amines including proprietary solvents have been present in gas treating processes for many decades. MDEA is most popular due to its low regeneration costs, low corrosion properties and flexibility in selectively treating CO2 and H2S. Bath temperatures in a direct firetube amine reboilers are typically maintained at 260°F or less as fluid degradation increases with temperature and film temperatures may exceed recommendations above this temperature.
Figure 2 – Recommended Bath Temperatures for Various Fluids
THE HEATER THAT COVERS IT ALL
HydroFlux is THM’s product line that covers both indirect and direct systems. Give us a call today to discuss your unique application.