pamungkas
First you need to describe the heating devices, power topography and the precision level at which you want to provide control. In other words, is this a precision continuous control situation where you are using analog outputs from a PID controller to continually maintain a temperature? Or are you thinking of just turning a heater element on for a few minutes, then off for a few minutes once it reaches temperature, then back on when it drops too low again (also called Hysteresis control)? Then as far as power topography, are you supplying the same voltage as what your heater device is rated for, or are you using a transformer to power them?
In a nutshell: [ul]
[li]if you are doing simple Hysteresis control (On - off), then all you need is a solid state contactor, which will be Zero Cross. By its very nature, Zero Cross means it will NOT present significant harmonics or noise to the line.[/li]
[li]If you want continuous analog control, i.e. from a PID controller, the Zero cross becomes a special version called Variable Time Base Zero Cross Control. This method is essentially On-off control, but is done at the sine wave level. For instance, if you want 50% output, the controller turns on the SCRs for 10 cycles, then off for 10, then on for 10, then off for 10 etc. etc. But because the On parts still always take place at the Zero cross, again, no harmonics or noise. This is the least expensive analog control method and is the one most widely used. The only drawback is that it does not work well with inductive loads, i.e. transformers. That's why I asked that question earlier on the power topography. If you are feeding it with 208V for instance, but want to run the heaters at 480V to keep the current down, then Zero Cross is not a good idea, you need to go with phase angle control.[/li]
[li]Phase Angle Control is a more complicated method of analog SCR control involving gating the SCRs at whatever part of each and every sine wave cycle is necessary to accomplish the variable voltage output. It works great, can be highly accurate, allows for ramping (which is good for heater elements that have high coefficient of temperature ratios) and works with transformers. The drawbacks are comparably higher cost over zero cross, plus it creates harmonics when operating at anything less than 100% output. It's impossible to quantify the THD because it varies with firing angle, but it can be quite substantial at lower output levels (albeit at a lower demand level). This is one of the reasons many people choose not to use it.[/li][/ul]
Keith, they often do it with heat tape on the piping or heating elements in a receiver vessel, so yes, it's electric heating.
