Processing tanks, holding tanks and storage tanks are used throughout the processing industries. Whether containing water, chemicals, oil, asphalt or other fluids, there is often the need to heat tank contents. Heat is required for:
• Preventing freezing or solidification.
• Maintaining minimum viscosity conditions for pumping.
• Initiating a reactor or maintaining chemical feed temperature for reaction effectiveness.
• Providing heat input in a reservoir in a closed circulation system such as a heat transfer system.
One common method to provide tank temperature maintenance and heating is by tapping into existing energy sources at the plant site such as steam or heat transfer system loops. Coils are wrapped around the tank or vessel in order to make up for heat losses, or to impart additional heat into the tank or vessel for processing.
When steam or heat transfer heat sources are either not readily available or are costly to install, electric heating is an option. Electric heating of tanks can be accomplished by external tank warming, which typically comes in the form of heat tracing.
Heat tracing most often is used for freeze prevention in pipes, lines and tanks as well as to make up for heat losses for tank temperature maintenance. On large tanks, however, heat tracing can be costly to install and operate. Because heat tracing is an external heat application, it is used most effectively for makeup losses rather than providing additional heat input to raise the tank temperature contents.
Direct Immersion Heating
Installing a direct immersion electric heater can provide tank temperature maintenance as well as provide tank heatup capabilities. The direct immersion heaters can be placed directly in the tank or installed in an external circulation loop.
Immersion heaters have an electric tubular heater element bundle welded into a threaded NPT fitting or into an ANSI flange (figure 1). The heater then is installed in a mating size nozzle close to the bottom of the tank. This type of heater is virtually 100 percent efficient because heat generated goes directly into the liquid. The heater normally is situated at least 4 to 6 inches off the tank bottom to keep the heater above any sludge or contamination buildup at the bottom of the tank. This also allows the setup of natural convection currents to aid in heat transfer to the liquid. The electric heater element sheath material and power flux (surface watt density) is matched to the fluid to ensure a long service life by preventing tubular element corrosion or buildup of contamination on the outside of the heater sheath.
One potential drawback of direct immersion heating is corrosion of the heater element sheath. Although every attempt is made to mitigate the risk of corrosion in the application, in some cases, direct immersion heating cannot be used due to corrosives involved.
Another potential drawback is contamination buildup on the sheath. This usually comes in the form or mineral deposits when heating water or coking (carbon deposits) of the sheath when heating oils and other organic chemicals. The coating on the heater acts as an insulation barrier and causes the heater element to operate at higher internal temperatures, thus shortening service life.
Another drawback of direct immersion heaters is that removal of the electric immersion heater for servicing or replacement necessitates draining of the tank. In many tank applications, this would be cost prohibitive or a logistical nightmare.
One way to overcome the tank drainage problem is to install a direct immersion heater in an external tank circulation heater system (figure 2). The circulation heater loop consists of a circulation heater (containing the immersion heater), pump, strainer, isolation valving, positive flow sensor, temperature and other sensors and necessary instruments. Often, a control panel is supplied with this type of system on a common skid frame for process control. The tank liquid is circulated through the circulation heater until tank contents reach temperature. Liquid can be circulated continually to maintain tank temperature or can run in on/off mode to maintain temperature within a specified range.
The advantage of this system is that heater removal does not necessitate tank draining. The isolation valves are closed, the heater vessel drained and the heater bundle removed for servicing. The disadvantage is that overall heater system cost is substantially higher when compared to direct immersion heaters or indirect methods of heating. In addition, the circulation vessel must be drained prior to removal of the electric heater bundle. Therefore, storage and disposal of the drained liquids means there are still some costs associated with using this type of heater.
Indirect Immersion Tank Heating
Drywell heaters are an indirect electric immersion heater design that eliminates having to drain tanks for servicing or removal of electric immersion heaters. A drywell heater, also known as pipe-insert heater, comes in various forms. In essence, instead of direct fluid immersion, the electric heater is installed inside a drywell: a closed-end pipe that is installed in the tank. The electric heater element bundle heats the internal portion of the drywell, setting up an oven-like condition where the heater elements radiate, convect and conduct energy to the inner surface of the drywell. The heat is transferred through the wall of the drywell and to the fluid in the tank.
The advantages of drywell heaters are that they facilitate removal of the electric heater without needing to drain the tank, and the heater elements are isolated from tank contents, thus eliminating corrosion and contamination buildup. The disadvantage of drywell heaters is that their size and cost is higher overall than direct immersion heaters.
There are two main styles of drywell heaters: weld-in-place and direct attachment to a tank nozzle (figure 3). The weld-in-place solution often is used when a tank is being fabricated or modified. A hole is cut into the tank. The drywell is inserted through the hole and welded to the tank wall to become a permanent part of the tank.
The direct attachment to a tank nozzle method is more user friendly for retrofit applications (figure 3, bottom). A mating threaded fitting or flange is attached to the drywell for mating to an existing nozzle on the storage tank. In both cases, the drywell heating system should be installed low in the tank with the lower portion of the drywell sitting at least 4 to 6 inches off of the tank bottom.
The higher cost of drywell heaters can be somewhat mitigated by using a less expensive heat source such as an open-coil heater. An open-coil heater element is essentially electrically live resistance wire strung together with ceramic insulators and supports. The main advantage of open-coil heaters is the lower cost vis-à-vis industrial-grade electric tubular heaters.
The main disadvantages of the open-coil heat source products are that they are fragile, electrically live and unsealed. Handling, insertion and removal of open-coil elements may bend supports or break ceramic insulators, requiring repair or replacement. Tanks that experience vibration or are in seismically active areas should not use the open-coil heat source. It is crucial that all insulators are functional because electrically live wires will short circuit if in contact with the drywell. Finally, drywell heaters using tubular electric heaters are sealed units. Should the tank liquid seep into the drywell due to corrosion, the liquid will not short the heater element nor enter the electrical housing.
Application Tips for Drywell Heaters
Before a drywell heater is selected over another type of direct immersion heater, an assessment needs to be made of the cost of failure. In other words, does the investment in a somewhat more expensive drywell heater outweigh the costs and logistical headaches of gaining access to the direct immersion heater for service or replacement? Some cost, time and resource aspects to consider:
• Size of tank and amount of liquid to be drained.
• Heater physical size, cost, expected lifetime and installation costs.
• Temporary storage requirements for the drained liquid.
• Type of liquid and liquid-handling requirements due to toxicity, corrosion and other safety concerns.
• Processing downtime and product losses or spoilage.
• Personnel and equipment availability for liquid transfer and servicing.
In conclusion, direct immersion heating of tank liquids is always preferred, if possible and practical, to indirect methods of heating. Heater size and costs are minimized, and virtually all of the power generated is used to heat the liquid.
However, when the heater needs to be isolated from the liquid due to corrosion or sheath contamination concerns, or a premium is placed on the ability to remove and service the heater without draining the tank, drywell heaters with tubular heater elements are an alternative to tank equipment manufacturers and users in the processing industries.