Heater Efficiency affected by kW and time on / time off 4

[PaulKraemer]

Hi,

I have been working on a few different glue melting tanks. I have two tanks of what seems to be an identical design. Each tank is of the same dimensions and has four seemingly identical resistance heating elements (physically and electrically) that are in the same locations in each tank. The heater resistance is the same in both tanks. One tank has all four heaters connected in parallel. One tank has has the heaters connected in a parallel-series combination, with two pairs of two heaters connected in series, with the two pairs connected in parallel. If my calculations are correct, the tank with all four heaters in parallel will be able to produce twice the power (kW) of the tank with the parallel-series combination.

In both tanks, I have closed-loop temperature control in which an SCR is used for time-proportioned control, cycling power to the heaters on and off as necessary to achieve and maintain setpoint (which is adjustable by the operator). For both tanks, we are able to achieve and maintain setpoint with no problem. I assume (although have not verified), that the tank wired for lower power likely ends up with the heaters powered a higher percentage of the time than the tank wired for higher power.

My question is, considering that temperature transfer from the heaters to the material in the tank is through the surface of the heating elements (which is the same in both tanks), is there any reason I should expect heat transfer to be more or less efficient in the tank with lower power that is turned on a higher percentage of the time, or in the tank with higher power that is turned on for a lower percentage of the time.

Any feedback will be greatly appreciated.

Thanks and best regards,
Paul

 

[Gr8blu]

PaulKraemer: The standard voltage/current relationships in an electric circuit hold true.
Let's take a set of four 500 W heater elements rated for 120 V and connect them all in series to a 120 V supply. That means each heater sees 120/4 = 30 V. This in turn means each heater is putting out (30/120)^2 * 500 W ... or about 31 watts. Since there are four heaters, the total wattage applied to the process is 4*31 = 125 W.
Now let's connect them as 2 parallel circuits of 2 elements each to that same 120 V supply. Each heater now sees 120/2 volts, which means it puts out (60/120)^2 * 500 W ... or about 125 W. The total heat into the process then becomes 4*125 = 500 W, which is significantly more than in the first "series" case.
The only difference in the "efficiency" of the heat transfer between the two approaches is whether the higher wattage version brings the material in the tank (that is in close contact with the heating elements) to a temperature where its properties change to something less desirable. If this is the case, the lower wattage approach will be a better method. If it makes no difference to the material chemistry or physical properties, there is no real benefit for either approach - other than that the number of switching events will be much higher for the higher-wattage connection, resulting in more wear-and-tear on the controller.

Converting energy to motion for more than half a century

 

[waross]

Compare the temperature charts of both tanks. Particularly compare the overshoots and the undershoots.
This may give you a clue.
Note that higher overshoots may imply more losses through the tank insulation.
Heat in equals heat out.
Losses are part of heat out.
Higher temperature overshoots mean higher losses.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LionelHutz]

Two heaters in series see 1/2 the voltage which causes half the current. 1/2 both V and I means quarter power.

If you have 2 setups then you can test both with a power meter. That beats theory every time.

Temperature stability is likely the better thing to use when determining which method is better.

 

[Compositepro]

"Two heaters in series see 1/2 the voltage which causes half the current. 1/2 both V and I means half power."

Shouldn't that be quarter power?

The power required by both tanks should be identical, and would equal the sum heat losses plus heat required to melt any added resin.

One of the most important requirements of a glue melter is to not overheat the glue so that it degrades and chars. When using time-proportional power control, the most uniform temperature is achieved using a fast cycle time and the duty cycle is 50%. So the heater power should be selected to get close to a 50% duty cycle from your temperature controller. With contactors you generally select a 5 to 10 second cycle time to maximize contact life. With SCR's a one second cycle time is appropriate.

One other consideration is that during warm-up of the equipment far more power is desired to reduce the time it takes. In that case you can operate normally at 10% duty cycle, if that does not cause any problems with temperature uniformity.

None of this has any significant effect on heater efficiency. I would only have concerns with your breaker panel. Operating many pieces of equipment at only 10% typical power requires a larger supply panel, to handle all of the equiqment warming-up at once.

 

[LionelHutz]

Yes, I meant that. Was in a hurry and didn't write it though.

I think in your last 2 sentences you missed a zero - 100%?

 

[PaulKraemer]

Hi Gr8blu, Waross, Lionel, and CompositePro,

Thank you for your responses, and for pointing out my calculation error. It is now clear to me that the series-parallel would result in 1/4 power.

I am going to have an opportunity to play with these two tanks sometime this week. You have put me in a good position to make the most of my time working on them.

I appreciate your help!

Best regards,
Paul

 

[Compositepro]

Waross, I meant that during "normal" production that the duty cycle may be 10%, or 10% of full power so that 100% power is available during warm-up. Equipment like resin melters can take hours to warm-up due to their large mass. Then you have to choose between keeping them on over-night or waiting a long time for warm-up in the morning.

Another option is a switch to give low, medium, or high power. Some ovens have this for improved temperature control when operating at different temperature ranges, or for the warm-up issue.