Electric Heater Power Control by Voltage Variation

Thyristor control is widely used in Oil & Gas as well as power stations for heaters.

You can safely go for it.

Just make sure that the thyristors do not introduce too much harmonics in to your system.

It is generally a combination of some fixed and some variable heaters.

TRIACS should be fine for the size of resistive load you mention. Firing circuits are simpler than Thyristors (SCRs).

The zero-voltage(voltage zero-crossing) firing, integral cycle method is recommended for heaters. Unlike phase angle firing, it does not cause sudden steps in current or DC currents which cause EMI, supply harmonics problems .

Roughly speaking, ON for a variable number of complete cycles out of 100 varies the heating power in fine steps much better than thermostat enabling closer temperature control with RTDs or thermistors as sensors. As mentioned in other posts, one or more heaters can be on continuously to match the minimum heating demand, so not all require Triac control.

Modern microcontrollers can easily handle the necessary selection of cycles, I suggest breaking into groups so average over 100ms is fairly constant, 1 second may thermal cycle the wire elements in the heaters too much. I suggest you consult the heater makers about what cycling will not affect life - most heaters with thermostats stay on for minutes.

One would need a 3-term controller to do that. The equipment can be discovered and selected using an internet search. The next biggest challenge will be tuning the loop.

Simple temperature control driving a solid state contactor. Changed a wash tank over to this type system. Has already paid for itself. The old dish washer heater would wear the tubular heater out in 3 to 4 years. The change over has run for 12 years with no problems. Maintains the water at constant temperature of 140 F.

Something like this?

http://www.circuitstoday.com/diac-applications

Listen, choppers are used when they are needed.

Your system seems slow enough to be controlled with a contactor for the 4th heater. If that works, it should be fine.

You can do it with a silicon switch as well, but in this case, I doubt it will be cost effective. You can make the numbers yourself.

This can best be done with a PID controller and solid-state relay setup.

There are many PID heating controllers available that have SSR drive outputs. Add in a 3-phase SSR with a sufficiently-high current rating and you are mostly there.

Unlike a drive and motor situation where a VFD could be used to save power there is no power saving benefit on heater systems.

It takes a fixed amount of heater wattage to raise you fluid to the desired temperature so the introduction of an SCR will not give you any power savings unless your current system is greatly overshooting temperature and then coming back down.

A good temperature controller with PID function that is properly auto tuned is very accurate as long as heater kW is grossly over sized and will give you a much longer functional life.

If service life of the contactor is a concern I have not notice any better performance with SCR units over a standard contactor as long as they are sized properly to handle the closing arc.

If it is just a fluid that you are heating, and if the fluid is not damaged by higher temperatures, then running the heater at 100% is fine.

Here, I design heating for big (1.5 ton) lumps of butterfat, lard, palm oil and the like, and it is often necessary to back the heating off some to avoid overheating the product. Phase changes of the product introduce a whole new set of problems.

That said, it is not usually necessary to drop the heater power, it usually runs from a PID controller that does a PWM on the heater power.

You can get a PID temperature controller and have it bang the contactors on and off, that's not the problem. The only issue is that when you do that, the heaters are either all the way on or all the way off, so you end up with "overshoot" of the temperature within the cycles. Solid State Relay (SSR) control gives you more ACCURATE control of the heating process (meaning less overshoot) and no wear and tear on contactors, but if accuracy of the control is unimportant and contactors last you a long time anyway, I agree with the others, it's pointless. The down side of SSR control is that the SSRs produce heat; about 1-1/2W per running load amp. That heat must be dissipated properly that that often limits how and where you can mount the controller.

Switching power to control heaters is typically the way it is done. It comes down to having the appropriate digital "word" for the number of heaters available on-line.

Thus, not all the heaters will be "on" at the same time, necessarily, and the milk will not be scorched.

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The proposed change will increase the process time. Go on - justify this change and the effort needed to obtain a result if today's arrangement works satisfactorily, then.