Overall Construction Layout

A New Pickle Tank Heating Method
by David Jaye
Presented at the AGA Technical Meeting
Chicago – October 5/7, 2005


Tank heating history and methodology

Hot dip galvanizing has been in existence for over 150 years, and during that time there has been a multitude of methods used to pickle and prepare the steel for the zinc bath. These methods have varied from simple shot blasting to more automated systems utilizing high pressure washing and mechanical brushes. The type of cleaning used in general hot dip galvanizing is, for the most part, done in open top tanks. This discussion is about some of the various methods of heating the acid and preflux solutions used in these open top tanks. They vary from the grossly inefficient to the cost effective, state-of-the-art technology.

Boilers and hot water heaters –

Boilers and hot water heaters are like a separate operation located in close proximity to the tanks to be heated. They require that water be heated in a furnace and then transferred through pipes to the process tanks. Energy losses begin at the boiler, continue
through the piping to the tanks, and end with the heated liquid or steam either injected directly into the tank or run through coils submerged in the process liquid. Even though many boilers manufactured in recent years may have efficiencies up to 80%, by the time the heated water gets to the tank and the heat is transferred into the liquid, efficiencies will have dropped to as little as 60%. Older systems can be even less efficient.

Though some systems still inject live steam or hot water directly into the process tanks, this method is seldom used because of the diluting properties of water and the creation of excess waste. Most boilers and hot water systems use heat exchangers in the process tanks. There are several types available, but all seem to have limited life expectancy due to the corrosiveness found in the process. Elevated levels of chlorides and other impurities can cause rapid failure in some systems.

Special additives have to be injected into the feed water to help prevent buildup of scale inside the boiler tubes and piping, further complicating the operation and cost of boilers. In addition, many localities require that boilers over a certain psi or horsepower have 24 hour monitoring which requires that personnel or recording equipment always be present. Annual inspections are usually required for permitting purposes, requiring scheduled shutdown of the system.

Immersion tube burners –

This method of heating involves installing a steel pipe under the surface of the liquid, usually lengthways down one side of the tank in the lower half. A burner is installed on one end of the tube and fired directly into the pipe. The heat is transferred through the pipe walls and into the liquid bath. Done correctly, this method of heating can be over 80% efficient. Some of the factors that can change the efficiency are burner style, tube length, and condition of tube.

Although there are many types and styles of burners available, the two basic styles are atmospheric and powered. The atmospheric burner is one that draws combustion air into the tube by a venturi as gas is passed into the burner. This method is very simple and reliable, but is limited for efficiency because the mixture of air/gas is subject to change as dirt particles drawn into the burner have a tendency to restrict the air flow over time and change the mixture. These dirt particles also build up on the inside of the tube, creating a thin film that lowers the rate of heat transfer through the tube walls, thus lowering the efficiency. This type of heating is very popular for caustic tank heating, and the simplistic design and operation make it a favorite among its users. To avoid the dirt problems, it is recommended that the burner and tube be inspected and cleaned at least once every 6 months.

The powered burner is one that has combustion air provided by a blower. They typically have pressure ports where air and gas pressure differentials can be measured to achieve proper mixtures. These styles of burners are more efficient than the atmospheric type, and since the air is drawn through a blower, a filter can be installed to reduce the amount of dirt in the air, which will leave the tube walls cleaner.

The immersion tube is seldom used for heating of acid or preflux tanks, due to corrosive nature. Stainless steel, titanium, and other alloys have very limited success, and usually don’t last more that 2-3 months, making the immersion tube a poor choice for all tanks but the caustic.

Waste heat recuperators -

This is typically done by utilizing the waste heat that is exhausted from the galvanizing furnace. The waste heat is passed over a heat exchanger coil which transfers the heat to a liquid which is run through the coil. The liquid is then run through another coil which transfers the heat to the process tank. Although they can only capture a percentage of the available heat, the energy would otherwise be completely exhausted through the stack. There are several designs on the market, and if built and installed properly, they can provide some relief on the energy demand in the pickle tanks.

Hot sticks -

The Hot Stick has been in the industry for about 40 years, and has proven itself over time to be a fairly efficient means of heating. The Hot Stick applies heat to the liquid in the tank by utilizing submerged combustion, which is the practice of heating liquids by bubbling a burner's hot combustion products through them. Although the Hot Stick does use a very efficient means of heat transfer, some of the drawbacks are the lack of precision gas/air mixture, flame supervision, and a sometimes difficult, manual ignition. The design also allows for a portion of the flame to come into contact with the liquid, which quenches some of the flame, causing incomplete combustion of a portion of the gas. The combination of poor mixture and unburned fuel take away from the efficiency of submerged combustion.