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### Electric Heater Operation by Thyristor

09/24/2011 6:53 AM

Dear Members,

Consider the case of a electric heater to be used in instrument air dryer unit in oil/petrochemical refinery:

Application: Heating the wet instrument air from 40 degree C to 185 degree C.For every 8 hours, it is switched ON for 5.5 hours for heating cycle and switched OFF for 2.5 hours at the end of 5.5 hours.

Design factors: Required power as per process calculations (for heating ther gas from 40 degree C to 185 degree C)= 81 kW, Installed power= 96 kW (including spare heating elements).Heater has 3 bundles each of 27 kW (3* 27 =81 kW) + 15 kW spare elements. Each element rating is 3 kW.

Control:2 bundles are contactor based and last bundle is thyristor based as per client specifications.

Question: Please explain the operation for heating the gas from 40 degree C to 185 C. Whether all bundles should run at full load or any bundle will run at lesser load than required? what is the purpose of providing thyristor control only for 1 bundle? It cud have been provided to control all bundles for fine control of process temperature.

Pls advice if any further data is required to complete the question.

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#1

### Re: Electric Heater operation by Thyristor

09/24/2011 12:37 PM

Consider the case of an unemployed control systems engineer that has seen his job outsourced to India. He goes to one of his favorite engineering chat sites to relax and finds a process control problem posted from India. From the poor, ambiguous structure of the proposed problem he concludes that the Indian author either has a poor grasp of English or the author has a poor grasp of process control. Does the unemployed engineer either try to correct the English ambiguities of the baffling question or does he point out the similarities and differences of a mechanical contactor versus the family of devices known as thyristors?

It's a good thing I'm actually employed. If I wasn't I might ask for payment for my services here. But this still begs the question, how can I communicate through such uncertainty?

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#6

### Re: Electric Heater operation by Thyristor

09/26/2011 9:46 AM

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#2

### Re: Electric Heater Operation by Thyristor

09/24/2011 2:51 PM

Two contactors plus one smaller thyristor are less costly than one larger thyristor.

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#3

### Re: Electric Heater Operation by Thyristor

09/25/2011 12:45 AM

being from India, i understand the communication and the question slightly better . . . or at least i presume i do . . .

1. the "contactor" goes ON and OFF as commanded.

2. the "thyristor" is capable of giving a variable output depending on the way it is controlled.

3. your question does not tell us whether the 5.5 hours heating is FULL HEAT after the temperature is already brought to 185 deg (and heat is supplied to maintain the temperature) or whether the heating is done to raise the temperature from 40 deg to 185 deg over 5.5 hours. the OFF period of 2.5 hours is understood.

4. your question does not tell us whether there is any thermostatic control that selects the heater bank as and when required. if that is so then is it s straight CUT-IN and CUT-OUT control or is it a multi layer control where the thyristor controlled heater acts as fine control and the contactor operated heaters act as coarse control points.

5. since you already have the equipment with you, it may be possible to search for a manual of operation and maintenance. you may even correspond with the equipment supplier and obtain such a manual. when you have access to a control wiring and power wiring diagram you may share it with us. that will help us to serve you better with an answer.

6. are you the electrical engineer at the plant? or are you the process engineer? maybe you should request a senior process person to write out the process so that any electrical engineer, control engineer or instrument engineer can understand enough to help you.

just in case you get the answer you are looking for, here or elsewhere, please post here and inform everyone so all can move on the next big question . . .

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#4

### Re: Electric Heater Operation by Thyristor

09/25/2011 11:32 AM

Well I'm glad to see that somebody from India is also uncertain about the conditions and point of this question. I hope my little story did not embarrass or scare away our Original Poster from filling in the details. I enjoy answering people's questions here. Just look at my running total of posts and good answers. So Mayank.Technip, please fill in what we don't know so that we may help you.

Let me return to what I think is a part of the original question itself. There appears to be three heating bundles that heat some kind of moving petrochemical vapour or fluid. Two of these heater bundles are controlled by a contactor while one of them is controlled by a thyristor (presumably a silicon controlled rectifier and not a thyratron). Now an implied question in this set up is why one set of heaters would be controlled by a contactor and another would be controlled by a thyristor. The variability of the RMS voltage a thyristor can apply to the heater is the key here and is precisely what you mention soebfatehi. This heater stage can thus apply a range of heat energy to the stream and not just be ON or OFF like the other two contactor controlled heaters. Presumably all of these heaters are nichrome elements, so there will be some non-linearities in the instantaneous heat produced based on the actual nichrome temperature. [I do not wish to "steal your thunder" soebfatehi]

Since this system is being used in a petrochemical refinery, there are other significant power sources that affect the temperature of this gas (gasoline or vapour) that cannot be ignored from a controls perspective. There maybe catalysts in the gas stream that are performing a "cracking" of the long hydrocarbon chains present in the gas. Thus more of a particular useful or valuable grade of volatile hydrocarbons (gasoline, diesel, etc.) and less of the less valuable very long chain hydrocarbons (asphalt). Depending on the average temperature of the gas, the length of time this gas is exposed to the catalyst, and the volatile mix concentrations the amount of energy produced in the gas itself by this process will change. Lastly there are also ambient temperature variations that will change the amount of heat required to be applied or withdrawn from the system to maintain the temperature of the gas for proper refinement. Ambient temperature changes do tend to vary slowly over eight hour periods. This might be the 5.5/2.5 hour contribution to the original question.

My point here is that control systems in a petrochemical refinery are very complicated systems. To properly explain what is happening and why a system has been designed the way it is requires precise knowledge of the operation itself. What I suspect has actually happened here is the OP in an effort to not reveal perceived corporate secrets has posted such a nebulous question that nobody will ever be able to know what the OP is actually asking.

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#5

### Re: Electric Heater Operation by Thyristor

09/25/2011 12:00 PM

the thunder belongs to the Lord . . . ours for sharing!

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#7

### Re: Electric Heater Operation by Thyristor

09/26/2011 2:38 PM

Dear All,

Being from mechanical background, I am not able to highlight the electrical aspects & parameters of this package and I also apologize for the pathetic English being used by me. But I try once again to explain the question and operating principle in more detail as follows. As I wanted to keep the length of the question short, hence I didn't explain it clearly. And sincerely there are no corporate secrets or proprietary information involved in this.

Operating principle of Instrument air dryer unit:

Aim- To remove the moisture content from inlet instrument air from to some desired value (say from fully saturated to 78 ppm)

Consider 2 vessels filled in with desiccant material for removing moisture from inlet wet air (inlet temp=40 degree C and flow rate= 2500 Nm3/hr). One vessel(A) is removing the moisture continuously for 8 hours and another vessel (B) is simultaneously going through heating (5.5 hrs)/cooling cycle(2.5 hrs) (assuming this vessel has already removed moisture in previous cycle) with the help of electric heater . Heating cycle removes the moisture from desiccant material, and cooling cycle cools the desiccant down to inlet air temperature .At the end of 8 hours, cycle changeover takes place & vessel A undergoes heating/cooling cycle and vessel B starts removing moisture.

And all this process involves automatic changeover vales and a local PLC based control system.

A part of inlet air is sent to a Electric Heater which is a part of this package. The heated air from electric heater is used to heat and dry out the desiccant material (to remove moisture adsorbed) inside the vessel. 5.5 hours is the total time during which we heat the entire desiccant material inside the vessel till it reaches a temp. of 185 degree C. Hence it's a continuous heating process. Now this 185 degree C temp. is called as regeneration temperature at which desiccant releases the moisture from it and it is now ready for another adsorber cycle and must be allowed to cool before it can be reused again.

During heating cycle the air is heated from 40 degree C to 185 degree C and we calculate the required heater wattage as around Q = 59 kW (as per process calculations Q= m *c*delta T-where m is mass flow rate of air, c is sp. heat of gas and delta T is 185-40=145 degree C).Hence this wattage is the min. power required to heat the air from 40 to 185 degree C continuously for 5.5 hours. And after 2.5 hours we arrive from 185 Degree C to around 40 degree C.

The above 2 paragraphs answers Point no-3 of soebfatehi's query.

Heater construction as per client requirement (their standard specification document mandatory calls for it):

Provide 3 bundles of equal wattage. 2 bundles contactor based and last one thyristor controlled. Thyristor panel must be provided to have fine control of this regeneration temperature.

Earlier client spec. used to call for contactor based heater only and in that case sometimes regeneration Temp. used to reach around 200 or up to 250 degree C (because bundle was just On or OFF and there was no precise control) and that affects the life of desiccant material badly due to high temperature above 200 degree C.

Hence later client changed their spec. and instead of proving complete heater as thyristor controlled they decided to provide a thyristor based bundle along with contactor based bundle.

We are under progress of detail engineering and supplier of this heater has selected 3 bundles each of 22 kW rating (hence total 66 kW for 3 bundles).Each element rating is 3 kW. Final Heater rating selected is 81 kW considering spare heating elements (15 kW electrically unconnected spare elements, total 5 spare elements each of 3 kW rating and hence 66 + 15= 81kW)

Point no-4 of soebfatehi's response is the question, which I have in my mind. Whether during heater start up, contactor based bundles operate first or all operate simultaneously and when does the role of thyristor controlled bundle starts? How does thyristor based bundle adjusts it power to suit the process requirement.

As in our present case: 66 kW is the required wattage.

As per my understanding both contactor based bundles should generate 22 kW each (total 44kW) & operate at full load for 5.5 hours and thyristor control panel should operate at 15 kW (total 44 +15= 59 kW as per process requirement). How does thyristor adjust itself to control the regeneration temperature to 185 degree C. process Parameter?

Process interlocks involves in this are: 1- Trip heater bank 1 if regen. Temp. exceeds 185 degree C, 2- heater bank 2 if regen. Temp exceeds 190 degree C and 3- if it reaches 200 degree C, trip off the complete heater.

This interlock is also confusing because once 1st interlock is activated, 2nd and 3rd wud never reach.

Point no-5 of soebfatehi's response: Equipment under fabrication, no manual available

Point no-6 of soebfatehi's response: this package involves interface with Process engr, who has little idea about heater operation and electrical engr has little idea about process details and im just left to act as a interface between the two guys.

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#8

### Re: Electric Heater Operation by Thyristor

09/26/2011 5:10 PM

I just finished an unscheduled crazy day here at work. Tomorrow will be a scheduled crazy day at work. So I won't be able to give you a good response until tomorrow evening EDT but it looks like you've provided enough information here that I can guide you what process engineering and electrical engineering needs to know to do their jobs properly. Until then....

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Off Topic (Score 5)
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#9

### Re: Electric Heater Operation by Thyristor

09/28/2011 11:46 AM

Ok, I've had some time to look over your information and I have a few suggestions. First, you will need to add to the PLC system a method to read the temperature of the gas being moved into and out of the desiccant at all times. Ideally this will be a sensor that can also handle relative humidity. These one or two parameters will provide the critical system feedback required to know how much of the thyristor controlled heat will be needed to maintain the desired temperatures and humidities. It might be a good idea to include some gas flow rate sensor. An added system benefit to measuring the flow and humidities before and after the desiccant will be that you can now test the effectiveness of the whole desiccant system. This will permit timely predictions of desiccant fatigue and any unanticipated flow restrictions for repairs and maintenance. This will guarantee that your product was fabricated under the precise conditions required.

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Associate

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#10

### Re: Electric Heater Operation by Thyristor

09/28/2011 1:08 PM

The temperature indication from Desiccant vessel is already going to local PLC based control panel. But Whether PLC has any interface with thyristor? Unless it is there, how can thyristor control the required power to the heater?

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#11

### Re: Electric Heater Operation by Thyristor

09/28/2011 1:19 PM

This depends on the specifics of the thyristor drive circuitry and the capabilities of your PLC. Likely your PLC system reads the analog temperature level (not just a Pass/Fail temperature status bit) with an analog channel or module. Most modules that can handle an analog input also have an analog output. This analog output can be easily used as the control voltage to the thyristor drive circuitry. These are all simple capabilities for your electrical engineer that is handling the programming of your PLC.

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