Understanding The Concept Of Chamber Functions
By Dean Smith
With regards to chambers (or autoclaves as they are also called), the basis and function has to be understood to realise their potential for operation and failure.
Without going into OTR or raw steam type, there are essentially 2 types of chambers, electrically heated and indirect steam heated. There are operational advantages and disadvantages with each. They both basically operate on the same procedure; it’s just that the heat source medium is different. When using an electric chamber, the only supplies needed are electricity (around 150A @ 480V), and an air supply (of around 100-120psi) which will be regulated for the chamber and manifold.
When using steam, there will be electricity for the control circuits (around 20-30A) air and steam (90-125 psi) this will require a boiler, nominally around 25hp per chamber on start up, and around 10hp during operation.
When using an electric chamber, the heat is obtained using electric heating elements (nominally 6 x 12K elements) located in the top 2 quadrants of the chamber.
When using steam (and depending on the design of the chamber), will obtain heat from radiators in the top 2 quadrants, or radiators in all 4 quadrants of the vessel. The heat will be the same; it will only be the time to reach the heat that is a factor.
Different chamber manufacturers have different ways of obtaining information displays on the chamber heat and air pressure readings. Pressure will be a constant throughout the chamber, but based on where temperature thermocouples and probes are placed, readings could vary by a few degrees (these readings being on the pen chart, the thermometer and/or the digital thermometer) so do not be alarmed if there are slightly different readings).
A chamber basically consists of two separate air circuits (three for bias tyres cured on rims) which must not interconnect with each other. If chamber pressure (15 psi above envelope manifold pressure after initial vacuum start-up) leaks through an envelope seal or hose or fitting, it will pressurize the envelope manifold to that of the chamber resulting in probable failed tyres, therefore care must be taken to ensure a good seal between the envelope and the tyre bead and all hoses and fittings must be leak free. The 15psi differential is there to promote even tread pressure and stabilization of location of the tread on the casing. However some companies have adopted the vent to atmosphere procedure, which places the entire chamber pressure onto the envelope and tread area.
It is important that the operator has a good knowledge of the workings of a chamber and that it is constantly monitored. This is the part of the retread process where the most tyres can be lost or fail.
These chambers are heated by the aid of electricity; they consist of heating elements that are resistance orientated. They are switched on and off by means of a thermostat strategically located in the area of the chamber for optimum effect, with a pre-set differential setting (at a temperature that they activate, and de-activate). It is difficult to ensure they are working correctly unless a visual indicator of heater on /off is used or by using a hand held meter (or current clamp) placed around the main supply cable to each element to show the current draw when active. If no boiler is available these are excellent for the purpose of retreading, and are more trouble free than steam boilers. If you have a boiler, you may decide on steam chambers. As I said earlier they both perform the same, but ifs and buts may sway you.
These chambers are heated using a boiler, to send steam through a network of pipes and a series of radiators. The temperature settings for the chamber will be he same as electric, but controlled through steam traps and pneumatic or solenoid valves instead of contactors. The one difference is that extra pipe work (steam) and pipe insulation is necessary. Steam will come from the boiler (hopefully treated with a softener and an oxygen scavenger (to reduce pipe degradation)), and heat up the radiators. These radiators will require filters/strainers and steam traps to control the dissolved solids and steam/condensate flow. It may sound complicated but is pretty straight forward. This however means there are more preventative maintenance items to consider than on electric chambers.
A filter/strainer stops any impurities etc due to rust in the pipeline; the traps block steam and allow condensate (cooled steam) from the system to flow thus being heat efficient. Efficiency of the radiators can be monitored using a thermometer. The inlet to the filter/trap should be around the chamber temperature.
The outlet from the steam trap should be substantially lower. Should the inlet temp be low, the chances are the filter or trap is blocked and water/condensate is building up in the radiators lowering the curing temperature. Should the outlet temp be around the same as the inlet temp, the chances are the trap is stuck open and wasting boiler time and pressure!
Whichever way you prefer, chambers are the final stage of retreading. Whether electric or steam, they are needed. No system overrides the other; it’s just a matter of preference; or a matter of having a boiler or not. Remember: Monitor your chamber, understand its needs and requirements, and you won’t go wrong.