Cooling of Plating Solutions

 

by Peter Gennaro

Camac Industries

 


With tighter controls by the EPA, water shortages, higher water costs and disposal costs for water, maintaining plating solutions at optimum operating temperatures is becoming more of a problem every day. With the use of modern refrigeration systems, however, these problems can be eliminated and temperature maintenance can become easier and more economical. With a basic understanding of the capabilities of the refrigeration systems now available, the metal finisher can expect to realize greater efficiency at lower costs.

 

Cooling

When a metal finishing process develops sufficient heat to raise the solution temperature, cooling must be provided to maintain the most desirable temperature. Some solutions that usually require cooling are:

 

Solution Best Operating Temp. (F)

Cyanide zinc 70

Acid zinc 110

Acid tin 65

Copper sulfate 110

Nickel sulfamate 120

Nickel fluoborate 120

Chromium 130

Carbonate removal 25

Anodizing, sulfuric acid 70

Anodizing, organic acid 50-80

Anodizing, hard coat 28

 

Since heat is generated at the rate of 3.412 Btu/watt of electric input, use of the following simple formula will convert input to Btu/hr that must be removed to maintain the desired temperature:

 

Volts x Amperes x 3.412 = Cooling load in Btu/hr

 

Other factors to consider when determining heat load are tank insulation (or lack of it), tank size, summer temperature of plating room, and temperature and weight of parts being plated. Wet bulb temperature for the area of the country where the equipment will be installed also affects proper cooling tower design.

A solution may be cooled by using well water, city water, cooling tower water or a refrigeration chiller, depending upon what temperature is to be maintained. To maintain 28 to 80F, a refrigeration chiller is recommended. To maintain 60 to 100F, well or city water can be used if the temperature of the water is low enough and a sufficient amount of water is available to properly design the heat exchangers or coils.

When using city water or well water, it is advisable to check the water for hardness and mineral content. Use of filters and water softeners may be required. The cost of this additional equipment, price of the water to be used and cost of water disposal should determine whether city or well water would be economical for your cooling requirements.

To determine proper chiller size required, divide the applicable Btu/hr figure by 12,000 to convert to tons required. Once you have determined the tons required for cooling, the proper chiller selection can be made. When tonnage has been established, it is advisable to correlate tons to horsepower (hp).

Example: A 25hp low temperature water-cooled chiller, rated at 173,000 Btu/hr (to maintain tank at 28F) provides 14.4 tons of refrigeration while a 25hp high temperature water-cooled chiller, rated at 253,000 Btu/hr (to maintain tank at 70F) provides 21 tons of refrigeration. The hp vs. tons figure will vary, depending on the chiller manufacturer.

Todays modern chiller systems are designed to operate with either air- or water-cooled condensers. Air-cooled units, designed and constructed for outdoor installations, are equipped with fans for cooling the condenser and should be equipped with low ambient controls for year-round operation in cooler climates where freezing is possible.

Water-cooled units designed for indoor operation require approximately 3 GPM of 85F water per ton to cool the condenser. This water is usually supplied by a cooling tower (see Fig. 1).

Because packaged chillers are designed by the manufacturer as completely integrated systems, savings on installation and assembly costs can be considerable. Packaged chiller systems are available with a built-in control panel. This control unit can augment and simplify single or multiple installation requirements for refrigeration by coordinating entire systems of heat exchangers, acid pumps, water pumps, cooling towers, recirculation and storage tanks.

Figure 2 illustrates a chiller system with master control panel for multiple tank cooling. As shown in the illustration, the use of three-way actuated ball valves is recommended on multiple tank systems to ensure constant flow to the chiller regardless of whether or not cooling is required on all tanks. An indicating temperature controller is used to sense tank temperature and actuate the chiller compressor and ball valves, sending chilled water to the heat exchangers or diverting the water back to the chiller head tank. A flow control switch should also be installed to prevent damage to the chiller by stopping the compressor if water is not flowing or if the chilled water flow is insufficient. With this type of system, one chiller could maintain any number of tanks with individual, on-command cooling.

 

Carbonate Removal by Refrigeration

An additional application for chillers in plating shops is carbonate removal. Carbonates form as a result of cyanide hydrolysis and anodic oxidation as well as the reaction between caustic and carbon dioxide.

Excessively high carbonate concentrations reduce the bright plating current density range and may cause grainy or otherwise unsatisfactory deposits. To remove the carbonates, the bath must be cooled to 32F and the excess carbonates will then crystallize.

The solution is then filtered back to the plating tank. The exact temperature used is determined by the nature of the solution and its concentration.

Where tanks are shut down at night or over a weekend, another method is recommended. A panel coil is placed in the tank and chilled water at 12F (40% ethylene glycol) is circulated through the coil. The carbonates will then crystallize on the coil and, when carefully removed, they can be flushed off with warm water. To be effective, this method of carbonate removal must be done regularly.

 

Cooling Towers For Condensers and Tank Cooling

Cooling towers are designed to provide a constant source of 85F cooling water (at 78F wet bulb) which can be used either for refrigeration condensers or cooling of solutions operating above 100F. The temperature of the water obtained from a tower will depend on wet-bulb temperature in the area of the country where the tower will be installed. Towers will cut down both fresh water costs and wastewater disposal, for an approximate 95% saving over city water use.

When towers are used for cooling the condenser of a chiller, the standard heat removal is 12,000 Btu/hr per ton, but when used for direct heat removal on a plating tank, approximately 25% more heat than the actual load imposed by the process can be dissipated. When designing for this type system, therefore, a figure of 15,000 Btu/hr per ton can be used.

In areas where temperatures drop to 32F, a cooling tower with an indoor storage tank is needed to prevent freeze-up. Without the tank, the tower must be provided with heaters, which will prevent freeze-up but are expensive to operate. A 3/8 line (see Fig. 1) should be installed between the tower inlet piping and the gravity drain from the tower to ensure complete draining of lines and prevent freezing.

An additional bleed line should be installed to bleed off water (GPM bleed required is determined by the size of tower) to prevent mineral build-up in the tower storage tank due to evaporation. A liquid-level control installed in the head tank will replenish water lost from evaporation and bleed off.

 

Indirect Free Cooling With Cooling Towers

With sufficient reduction in the wet-bulb temperature (as will occur seasonally and, in some locations, daily), there will obviously come a time when the cold water temperature produced by the tower is low enough to satisfy the requirements of the process without assistance by the chiller. At those times, with a properly equipped and arranged piping system, the cooling tower water could serve the load and the expense of added heat or compressor operation could be avoided.

 

Heat Exchangers Cooling and Heating

The next consideration is whether to use internal coils or external heat exchangers.

Coils are popular, but have shortcomings. They consume valuable room in the tank and are subject to physical damage from parts being inserted and removed, and to electrolytic attack from stray currents. Coils also do not provide uniform temperatures in the tanks. Their advantage is that they do not require an external solution pump, as does the external exchanger.

External exchangers provide more even temperatures by taking the solution from one end of the tank and returning it to the other end. They are not subject to mechanical or electrolytic attack, and they free the tank of all but the work being plated. An external exchanger is also more efficient, requiring less transfer area than coils.