Steam heating is rapidly yielding ground to electric heat because electricity lends itself to infinite control with little environmental impact. Modern electric heaters coupled with advanced control schemes can deliver precise amounts of energy efficiently.

Process heating requirements have traditionally been approached via the “stove-top” method of heater sizing, that is, sizing based on heat-up time only. This approach completely disregards other significant sizing factors and overlooks some basic engineering decisions regarding the choice of a heater.

All process heating requirements follow the same thermal laws, namely, the calculation of energy requirements to elevate temperature and provide additional energy to compensate for operation losses.

There are four facets to selecting any heater for process heating requirements: heater sizing, thermally and physically, heater construction and installation.

Heater Sizing—Thermal
There are usually six considerations in thermally sizing heaters for surface finishing applications:

1. Initial heat-up
2. Surface losses
3. Parts to be finished
4. Solution additions
5. Side wall losses
6. Fouled heater losses

The basic unit of measure used in heating is the British thermal unit (Btu). One Btu is the amount of heat required to raise one lb of water 1°F. This unit of measure is then converted into watts or pounds of steam or gallons of heating fluid to obtain a specific heating requirement.

Basic Equations

1. Initial Heat-up = Weight of solution (lbs) × specific heat of the solution (Cp) × the temperature change (°F) divided by 3,412 (BtuH/kW). Divide the result by heat-up time (hr).
2. Surface Losses = Surface area of the tank (sq ft) × surface loss factor (BtuH/sq ft/F) divided by 3,412 (BtuH/kW). Double this loss for air-agitated solutions.
3. Parts Being Finished = Work load per hour (lb/hr) × specific heat of the parts (Cp) × the temperature difference between the parts and the solution (F) divided by 3,412 (BtuH/kW).
4. Solution Additions = Weight of solution added each hour (lb) × specific heat of the added solution (Cp) × the temperature difference of the added solution to bath temperature (F) divided by 3,412 (BtuH/kW).
5. Side Wall Losses = The outside area of the tank (length × width + 2 × length × depth +2 × width × depth, sq ft × side wall loss factor [BtuH/sq ft/F] divided by 3,412 [BtuH/kW]).

Figure 1. Electric immersion heaters are available in side-mount or bottom-mount configurations.

Fouling from a buildup of solids can partially or completely insulate an immersion heater, thus reducing the thermal efficiency. If fouling is anticipated, you can prolong heater life and increase intervals between cleaning by increasing the surface area.

Unlike steam heaters, the electric heater’s surface temperature will climb as a result of fouling. At some point, this temperature increase will cause heater failure. Reducing the heater’s watt density reduces the heater’s “clean” surface temperature. Table I gives some recommended watt densities for various solutions. Once the various losses have been determined, the electric heater can be thermally sized. Heat-up requirement is the initial heat-up loss plus surface losses (equation 1 + equation 2). Operating requirement is the surface losses plus parts being finished losses plus solution make-up losses plus side wall losses (equation 3 + equation 4 + equation 5).

The larger of the two requirements, heat-up vs. operating, will determine the electric heater size. (You may wish to increase the initial heat-up time to reduce heater size.)

Figure 2. Cross section through a process tank shows relationship of heater to solution and sludge levels.

Heater Sizing—Physical
After determining required wattage and watt density, the next task is to determine the size and configuration of the actual heater. Using the manufacturer’s catalog and your knowledge of operating conditions within the tank, sketch out tank size, parts package size, anodes, filters or filter pipe allowances, internal pipes, air agitation spargers and sludge depths. Figures 1 and 2 show typical side- and bottom-mount heater configurations and a cross section through a process tank.

Since sludge or solids buildup will cause overheating and premature failure, it's necessary to locate and configure the heater to keep it out of the sludge. In most electroplating applications, where parts are placed in the tank with an immersion heater, it's wise to install a side-wall style heater. A recirculating tank or off-line tank where precipitates or sludge are not usually formed can use a bottom heater.

Heater Construction
Electric immersion heaters are all basically metallic in construction. Some typical metals used include: carbon steel, 18-8 stainless steel, Incoloy and titanium.

The use of fluoropolymer-sheathed metal heaters has gained acceptance for the following reasons:

• Fluoropolymers exhibit almost universal corrosion resistance to electroplating chemistries.
• By design, the watt density of fluoropolymer heaters is almost half of their equivalent metal heaters, reducing solids buildup potential.
• The fluoropolymer surface provides a release surface for easy removal of any solids that might build up.

Unlike metal heaters, some fluoropolymer heaters are inconvenient or impossible to electrically ground. Be sure to select a fluoropolymer-sheathed heater that can be grounded so that a heater failure does not expose the operator to potentially lethal voltages.

Quartz (pure silicon glass) heaters share some of the corrosion resistance characteristics of fluoropolymer-sheathed heaters but are generally unsuitable in highly alkaline chemistry. Again, be sure to select a quartz heater that can be electrically grounded.

When selecting the proper material of construction (most cost effective), it is best to consult with your process chemistry supplier. Corrosion tables are extremely general by necessity and often provide only partial information that can result in improper material selection. Use these tables to verify your process chemistry supplier’s recommendation.

You must also consider materials of construction for those parts of the heater that are above solution level. Electrochemical gassing in your plating tank creates an aerosol of the plating chemistry. This vapor coats everything near the solution surface, including the heater junction box. Check for a corrosion-resistant junction box, sealed junction box, corrosion-resistant electrical conduit and convenient mounting to permit disassembly for cleaning.

Figure 3. An air agitation sparger is designed to minimize thermal stratification effects.

Finally, to ensure consistent and approved heater construction, look for third-party inspection listing (UL, CSA, FM, etc.) that certifies that a minimum design and construction criteria has been and is being met.

Heater Installation
Crude but effective, non-indicating bulbs and capillary control were traditionally used to directly control the on/off electrical cycling of immersion heaters. While this approach works and is economically attractive, it totally disregards important safety considerations. Capillary controls can (and most often do) fail in the “power-on” condition in the process tank.
They fail due to life cycling, mechanical damage or corrosion.

Modern digital circuits allow the economical design and manufacture of quality process temperature controllers that include “open” and “shorted” thermal sensor protection systems, minimizing the unsafe effect of a damaged sensor. Redundant safety controls sense liquid level relative to the heater’s “hot zone,” while heater surface temperature cut-off devices provide additional protectionin the event of loss of liquid. All safety elements should be electrically wired using relay contacts that must be energized by the elements to allow heater operation. This means that control element failure will result in a heater shutdown. It is also important that this control circuit be provided with a manual reset or start scheme so that the operator has an opportunity to evaluate any shutdown condition prior to returning power to the heater.

Modern GFCIs (ground fault circuit interrupters) will sense a power leak to the tank and shut down the heater. These devices sense power imbalances as low as five milliamps and can reduce the incidence of shock hazard in the event of heater sheath failure. Figure 2 also illustrates some required relationships between liquid levels, heated zones and sludge levels. Always take these into account when installing a heater.

A final note: thermal stratification occurs in any unagitated heated tank. To minimize this effect, install a single-lane air-agitation sparger similar to that shown in Figure 3. In lab tests, temperature differences of more than 120°F were measured between top and bottom of a heated tank. An air sparger brought this difference down to 8°F. PFD