Control of process solution temperature is essential in finishing. Therefore, the heat exchanger that adds or removes heat from the finishing tank is a critical piece of equipment.

Solution temperature affects many finishing factors. These include the deposition rates, activation and decomposition of delicate chemistries, the corrosion resistance of equipment, and the ability of deposits to adhere to surfaces. Yet even when other finish variables (solution chemical content and concentration, basis-metal preparation, electrical current and voltage and rack design) are under control, consistency in solution temperature must still be maintained for best results.

Choosing the Right Exchanger
The details of exchanger style, size and choice of materials should be designed in from the start. Correct decisions at the beginning will ensure more profitable, effective plating process operations. Your heating or cooling application may be best served by using one of these heat exchanger designs.

Immersion exchangers. Serpentine coils, “U” coils, pipe coils, grid or plate-style coils and electric heaters offer the simplest answers to heat exchange requirements. Such units are economical, easily installed and maintained, and popular in both large and small systems. They are also available in a variety of sizes and shapes; however, they do take up space in the process tank.

Good design up front can alleviate this problem and make your process run more smoothly. In today’s computer-driven world, heat exchangers can be sized to your specific needs. Using online design technology, you get exactly what you need—no more, no less. This represents an enormous savings from every point of view. What was once considered custom sizing should now be standard (standard delivery and standard pricing as well) in immersion heat exchangers.

External exchangers. These are typically chosen only by finishers with large systems, where in-tank space is a primary concern. Such units require pumps to circulate corrosive liquids from the process tank through the exchanger and back to the tank. External exchangers may either be shell-and-tube style (which can be mounted as a space-saving vertical unit) or plate-and-frame, which also minimizes the exterior mounting space required.

Materials of Construction
Once the exchanger type is determined, the construction materials can be chosen. Some of today’s options are listed in Table I.

Material selection hinges on finding the best mix of economy and performance. Original equipment cost must be balanced against the equipment’s life expectancy and its maintenance costs. Because corrosion is an everyday fact in the finishing industry, mild steel exchangers should be avoided, except for short-run processes. Instead, in most systems where corrosiveness of the atmosphere and solutions is low, type 316 stainless steel provides additional protection at a modest price.

In more corrosive applications, material selection becomes even more important. Some selections are easy:

• Stainless steel for caustics
• Titanium for nickel plating
• Columbian/niobium for fluoride-bearing chromium plating solutions.

Other choices are less obvious and are based on your own experience, as well as that of the technical experts at your heat exchanger manufacturer.

Corrosion charts (Table II) can provide the starting point for more sophisticated material selection. Since actual conditions often vary within the same facility, in-tank testing is often the only way to guarantee corrosion resistance for equipment exposed to new chemical formulations. Chemical suppliers will often provide guidelines for heat exchanger material selection. Be sure to follow such guidelines carefully.

Plastic heat exchangers can solve some of the most difficult corrosion problems. Though they are quite corrosion resistant, they are also among the most expensive, due to their relative inefficiency when compared to metal exchangers. Plastic units may have to be two to four times as large as metal ones in order to provide comparable and sufficient heat-transfer area. This means more mounting space, inside or outside the tank.

Exchanger Rating and Design
After deciding on style and material, the final equipment rating (in Btu/hr) and design can be determined. Equipment engineers usually find that three factors control the amount of heat required to maintain a hot tank at desired temperature: original tank heat-up, loss due to evaporation and loss due to work as it passes through the process.

Heat up. The heat required to bring a tank up to its original operating temperature is related to the size of the tank, tank contents and the final temperature required. This relationship is described in Equation 1:

Q₁ = MCp∆T

Where Q₁ is the total heat input required to heat up the tank; M is the weight of the solution to be heated; Cp is the “specific heat” or a factor that describes the relative amount of heat required by different materials to be heated to the same temperature (Table III lists the specific heat of several common materials) and ∆T is the difference in solution temperatures before and after heating.

In order to see how Q₁ is determined, consider the following situation:

A finishing tank measures 3×10 ft. The solution inside is four ft deep. The temperature is 70°F. You want to elevate the water temperature to 140°F. The tank will be agitated.

You have 120 cu ft (3×10×4) of solution in the tank. Water weighs 62.4 lb/cu ft. Therefore, M = 120 cu ft × 62.4 lb/cu ft or 7,488 lbs. Referring to Table III, you see that Cp = 1.000 (the specific heat of water). Through simple subtraction you can deduce that ∆T = 140 – 70 = 70°F. Completing the calculation, Q₁ = 7,488 × 1.000 × 70 = 524,160 Btu.

This heat requirement measurement gives no indication of how quickly the tank will be heated. In theory, by putting one Btu/hr into a perfectly insulated tank, it would take 60 years to heat the tank to 140°F. Since two to four hours is the typical heat up time for most finishing shops, a four-hour heat up requires 131,040 Btu/hr (524,160 divided by
four).

Temperature Maintenance. The initial heat up calculation is the most common factor in determining heat exchanger size. In very hot tanks or lines where a high volume of work is processed, you must also examine the heat losses experienced. Equation 2 describes the heat input required to balance evaporation.

Q₂ = L × A

Where Q₂ is the heat input required to make up for evaporation; L is loss per square foot (from Table IV) and A is the surface area of liquid in the tank.

Top losses can be reduced by careful choice of ventilation systems and by covering tanks whenever possible.

Losses to Work
Heat is also lost to work passing through the process. You can use Equation 1 to calculate the heat required to make up for losses to the work. For most finishing systems, this factor is insignificant. But, in some where output is measured not in pieces but in tons, it can account for up to one-third of the required heat input.

When you have calculated the above three factors, you can then determine the heat exchanger rating. Usually, the larger of Q₁ or the sum of Q₂ and Q₃ is chosen as the rating number. With the heat exchanger rating now known, the unit size is finally determined.

Size Does Count
For electric exchangers, the rating in Btu/hr is divided by 3.412 Btu/hr/kw to convert it into an electric heater rating.

For immersion and external exchangers, equation 3 is used to relate the rating (Q_Total ) to the size of the heat exchanger measured in square feet:

A = Q_Total /U∆T₂

Where A is the size of the exchanger in square feet; Q_Total is the rating of the exchanger in Btu/hour; U is the heat transfer coefficient in Btu/hr/sq ft*; and ∆T₂ is the difference between the tank temperature and the heating medium.

Table V lists some common “U” values for “watery” solutions used in many types of finishing applications.

You should keep these equations and tables handy. However, an up-to-date heat exchanger manufacturer can provide you with the web tools to make such calculations on line. Once the size of the exchanger has been determined, the manufacturer can provide dimensions to perfectly fit your line and pricing needs.

Cooling Applications
The most common cooling applications in finishing occur when electrical energy is supplied by the process and, thus, must be removed (as heat) from the tank, as described by Equation 4:

Q = volts × amps × 3.412 Btu/kW

Some of the electrical energy is converted into metal deposition, though in practice this is ignored. More important is the “duty cycle” that considers the portion of an hour, day or shift during which the rectifier supplies power to the tank. With the Q from Equation 4, area can be calculated using the appropriate U value from Table V.

The best Web sites can assist you in choosing the style, material and size of the heat exchanger that will best suit your operation. These online tools are the perfect complement to hands-on experience. Exchanger type, size and construction are all critical factors in making your plating line run more efficiently. Design them in from the beginning to ensure smoother processes and a more profitable outcome. PFD

*U factors are based on actual operation and vary depending on the type of equipment specified.