The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Zinc barrel plating solution is overheating

I need conformation on the formula or experience in the cooling requirements for an acid zinc barrel plating plant. We have installed a fully automatic barrel plating plant for the purpose of plating fasteners. The plant capacity is 900 Kg of fasteners per hour utilising an Ammonium Chloride Zinc solution. The chemical supplier claimed an anode efficiency of 95%. WE calculated the total rectifier input in the bath to be 50 Kw. We further have a filter pump of 4 Kw on the tank and barrel rotation motors of 4 of at 0.37 Kw. We calculated the heat to be 2,5 Kw in chemical efficiency 1Kw on the pump and 0,5 Kw on the barrel rotation giving 4 Kw of heat. We doubled this as a safety and installed a chiller plant of 8 Kw with the circulating medium water of 5 °C. The problem however is the chiller plant is not coping. Any input regarding this will be appreciated.

John Kleyn
Plating Plant design and manufacture - Cape Town, Western Cape, South Africa
2003

---

Sorry to be the bearer of bad news, John, but your chiller is drastically undersize. The problem develops from a misunderstanding of chemical efficiency vs. heat input. Efficiency describes where the current goes, but not where the voltage goes. Let's look at the voltage...

A very small portion of the voltage is lost across the bus work (maybe half a volt). Only about a volt is spent in the reduction of Zinc ions to metallic zinc. The balance (5 volts? 7 volts?) is consumed as resistance across the plating solution. The plating solution is a resistance heater with a wattage equal to Amps times Volts. The Amps is 100 percent of the actual current measured by the rectifier; and the voltage lost to plating solution resistance is probably about 80 percent of your actual voltage as measured by the rectifier. If the actual OUTPUT KW of the rectifiers is 50 KW, you need roughly a 40 KW chiller. If that was the INPUT KW, and the rectifier efficiency is 75 percent, then you need 30 KW of cooling.

If the bath runs significantly over room temperature, you can some cooling from that, but your 8KW chiller is unfortunately not even in the ballpark. Sorry.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003