Perfect 10 answer..

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

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 . . .

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

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 1^st interlock is activated, 2^nd and 3^rd 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.

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?

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.