Author: Skip Schaefer
Back in the 1970's, when I was selling electric immersion heaters and liquid level controls, there was not much on safety. The best plating tank available was old rubber battery casings, left over from some type of military requirement. The heater was in the tank, and they would let the level drop below the cold zone bracket, then the heater was hot enough to make the rubber casing smolder and stink. A liquid level control, 2 prong type, was incorporated into the control system to shut the immersion heater off.
Polypro was just coming into popularity in the 80's which made things simple. You design a tank, cut the polypro, weld it up, and put an immersion heater in the coating tank. Except when you have a repeat of the above scenario, the outcome is entirely different.
Thermal cutoff devices were incorporated in heaters and in my opinion, gave the customers a false sense of security. Liquid level safety controls were, and still are a safer application, if applied correctly. Cleaning and checking solution level safeties is part of a maintenance schedule. There are several type of level safety controls and I will present them by relative cost.
A capicitance solution control mounts outside the tank and is set at a specific level. Once mounted, the customer must make sure he orders the correct replacement heater. With the below conductivity level safety systems, it is simple to adjust. You trim the probes from the bottom-up, and adjust it using the set screw on the bracket.
The 2-prong conductivity sensor passes 12 VAC across the probes or prongs in the presence of solution. When the solution level drops below the probes, it opens the holding coil contactor momentarily, usually I design for instant power off of the immersion heater. Some finishing lines use a 3 prong sensor, which can add more solution to the tank when the low probe is exposed. The water or solution valve opens and fills the tank until the top probe is reached.
Thirdly, is the bottom-up float type control. This is an inverted, polypro float in a CPVC tube, with 1 amp rated float. This is for solutions where crystalline build-up doesn’t occur in the tank. Hot seal tanks, hygroscopic, acid base solutions, and Di Water are examples of solutions best for the bottom-up float.
This solution level control introduces air pressure through a CPVC or PTFE tube that is set to the cold zone of the heater. Air is introduced and if liquid is present at the tube tip, back pressure occurs in the tube and the differential pressure switch stays closed. If the solution drops, the air blows without pressure, opening the switch. Air bubbler level safety systems work in all chemistries and temperatures.
Electroless nickel plating is the perfect application for this system. No conductivity probes to electrify the solution causing plate-out. Capacitance staying on from plate out on the tank. Bottom up, getting frozen shut in the on position. Air bubbler level sensors are also ideal in Copper, Gold, Rhodium, Palladium, or Platinum plating tank systems.
Now we have the basic four solution level safeties choices, but what still needs to be discussed what happens when there is air agitation in the tank and a tank failure. These devices are usually connected to the holding coil of the contactor. If the solution is at the critical point of the tip of these devices and a “boil action” occurs and starts the contactor coil to “chatter”– going on and off frequently and repeatedly, this may cause the contact points to “weld shut”. Once they are welded shut the heater is 100% on al the time and not even a thermal cutoff can save it.
It is imperative that a manual reset system be in place on all systems. This consists of a relay and reset button wired so once the probes, float, capacitance sensor, or air bubbler, has one incident of exposed appliance– the electrical system open the contactor. All the basic 4 level safety systems all react before the solution drops below the heater “cold zone” bracket. The thermal cutoff device must have the heat generated by the electric immersion heater which exposes the plastic tank to needless heat and possibility the integrity of the exposed area.
In a tank failure as Mr. Goad pointed out, it is a catastrophe and it happens quick. With the “basic 4”, they will shut the system off instantaneously. With the thermal cut off device the heating element will discolor the metal sheath and begin emitting infrared radiation. Then it is a race for time. Does it shut off in time or not. As a responsible manufacturer of the equipment, it is not a chance I care to take and neither should you.
In this case I used one of our AIR BUBBLER Liquid level controls. This system induces air into a piece of CPVC piping and if there is solution at the tip of the tube, restricting air flow, the system operates correctly. Once liquid is removed and no back pressure- the system shuts off.
Now that is simple, easy, and safe.