VFD needed?

[Designer69]

I am reviewing some drawings for comments and one thing I noticed is they went kind of wild on the VFD's.

These following loads have all been specified VFDs:

3HP Boiler Pumps. 480V/3PH

Electric Reheat Coil. 80KW/3Ph

Air handlers as low as 4HP. 480V/3PH


Does this seem normal to you guys based on your experience? Maybe nowadays VFDs are a lot more common even for smaller loads? Also, I cant see why you would put an electric heater on a vfd.


Is there a cutoff where you specify VFDs and where you don't, say 100HP and up?


Thank You

 

[topgone]

I am reviewing some drawings for comments and one thing I noticed is they went kind of wild on the VFD's.

These following loads have all been specified VFDs:

3HP Boiler Pumps. 480V/3PH

Electric Reheat Coil. 80KW/3Ph

Air handlers as low as 4HP. 480V/3PH


Does this seem normal to you guys based on your experience? Maybe nowadays VFDs are a lot more common even for smaller loads? Also, I cant see why you would put an electric heater on a vfd.


Is there a cutoff where you specify VFDs and where you don't, say 100HP and up?


Thank You

If you are dealing with a possible motor load that varies a lot, you will be forced to employ VFDs. Otherwise, size your motor/valves/dampers properly and do away with VFDs.

 

[GoldDigger]

Also, I cant see why you would put an electric heater on a vfd.

It does seem like overkill, since even if you want analog input proportional control instead of on/off switching to set the air temperature a simple controlled Triac circuit would probably do the job just as well. But a VFD certainly gives you a range of control inputs and strategies. You will not be using any of the closed loop feedback capability of the VFD though.

 

[Besoeker]

It does seem like overkill, since even if you want analog input proportional control instead of on/off switching to set the air temperature a simple controlled Triac circuit would probably do the job just as well. But a VFD certainly gives you a range of control inputs and strategies. You will not be using any of the closed loop feedback capability of the VFD though.

I agree. A VFD on a heater makes no sense assuming it is just a heater with electric heating elements. If you want to go solid state a pair of back to back SCRs in two of the phases would do the job. Not sure that you can triacs rated to do the job.

 

[kwired]

Many VFD's won't run a straight resistive load, I think. Even if they will, there has to be a cheaper solution to vary a resistive load, as you only need to reduce effective voltage or shave off portions of current wave to vary the output watts.

 

[Besoeker]

Many VFD's won't run a straight resistive load, I think. Even if they will, there has to be a cheaper solution to vary a resistive load, as you only need to reduce effective voltage or shave off portions of current wave to vary the output watts.

On one project we did it by varying the number of cycles the output was on/off. They were an integral number of full cycles. It avoided non-linear currents.

 

[kda3310]

I am reviewing some drawings for comments and one thing I noticed is they went kind of wild on the VFD's.

These following loads have all been specified VFDs:

3HP Boiler Pumps. 480V/3PH

Electric Reheat Coil. 80KW/3Ph

Air handlers as low as 4HP. 480V/3PH


Does this seem normal to you guys based on your experience? Maybe nowadays VFDs are a lot more common even for smaller loads? Also, I cant see why you would put an electric heater on a vfd.


Is there a cutoff where you specify VFDs and where you don't, say 100HP and up?


Thank You

I don't know about the reheat coil but on our boiler we have to adjust every thing based on the heat and pressure. VFD's are the best way to do that for us. We had a 1.5H 480V/3PH that was geared down to turn 12 RPM. We needed that 1.5H Fuel Feeder Motor to turn at 2 RPM. I wanted to put a VFD on that in case we needed to change that RPM again in the future but do to the price we end up getting a new gearbox that took the motor down to 2 RPM. If they ever want to change the RPM we will have to buy a new gearbox every time.

 

[Besoeker]

Is there a cutoff where you specify VFDs and where you don't, say 100HP and up?
Thank You

Cut off?
No, I don't think there is. You select a variable speed drive if the application requires it, not by size.

 

[petersonra]

VFDs are especially convenient when it is not know with absolute certainty what the rpm of a driven load will be. It is a lot simpler to change the speed of the motor via a VFD than to replace a gear box, or even a pulley system.

 

[Jraef]

In a boiler system, using VFDs would be tied to the boiler control system to vary the feed pump flow based on boiler demand, as opposed to using a proportional control valve on full speed pumps. It reduces the energy consumption of the feed pump system and is likely tied to the overall efficiency rating of the boiler.

Similar for the Air Handlers; you vary the speed of the fan motors as opposed to using a VAV (Variable Air Volume) motorized damper box on the ducts and running the fans at full speed all of the time. Again, it consumes less energy that way.

As to the electric reheat coil, there is nothing inherently WRONG with using a VFD on a resistive load, but in many cases it is more expensive that a simpler SCR controller. In other words there is no BENEFIT to using a VFD for this compared to other more traditional methods of heater control. But; if the overall system is using DDC (Direct Digital Control) from the BMS (Building Management System) and there is a communications protocol involved, it may be that the protocol being used is easier to implement in a VFD than in an SCR controller. Examples might be Siemens Apogee, BACnet, LONWorks or Johnson Controls Metasys. It may be difficult to source an SCR controller with those communication interfaces, but almost all HVAC drives now have them.

 

[Besoeker]

But; if the overall system is using DDC (Direct Digital Control) from the BMS (Building Management System) and there is a communications protocol involved, it may be that the protocol being used is easier to implement in a VFD than in an SCR controller

I don't see this as a reason to go down the VFD route.
We have integrated Soft Starts and building management systems.

 

[Jraef]

I don't see this as a reason to go down the VFD route.
We have integrated Soft Starts and building management systems.

That's why I said "may be"... we are essentially second guessing the EE's thought process here.

 

[Besoeker]

That's why I said "may be"... we are essentially second guessing the EE's thought process here.

I think the OP should question the EE about the reasons for requiring a VFD for a heater.

 

[RumRunner]

It does seem like overkill, since even if you want analog input proportional control instead of on/off switching to set the air temperature a simple controlled Triac circuit would probably do the job just as well. But a VFD certainly gives you a range of control inputs and strategies. You will not be using any of the closed loop feedback capability of the VFD though.

In food processing environment-- precise heat control is crucial.. . . it is not overkill.

Our company (I'm now just a stockholder) had been engaged in servicing the food industry in installing, repair and build of food packaging and pasteurization equipment.
We've had to modernize some pasteurizers that have been built decades ago.

We have had to replace rheostats to maintain the desired temperature inside the pasteurizers and had them replaced with solid state SCRs.

This was during the early days of PWM/VFD controllers and up to now we still do..

PWM/VFDs really come in handy in modernizing this sector of food processing.

Pasteurizers must meet government regulations in terms of eliminating pathogens from growing in every food product that is meant for human consumption.

Heating is one way of killing pathogens- - -irradiation is another way.

The temperature in the pasteurizer is tightly controlled. Temperature cannot run wild or canned goods could burst-- and container could weaken at the seams if pressure builds up due to overheating. It also has to be able to eradicate harmful bacteria by keeping the right temperature.

Wrong setting could have opposite effect. . . it would allow bacteria to grow even more.

There is no overkill—manufacturers are simply playing their game on the side of caution.


And I like the money

 

[Besoeker]

In food processing environment-- precise heat control is crucial.. . . it is not overkill.

But I don't see how a VFD would do than any better than a much cheaper SCR solution.

 

[petersonra]

But I don't see how a VFD would do than any better than a much cheaper SCR solution.

it is not always just about the cost of the hardware. sometimes it is about having to train who knows how many technicains on some new piece of SCR hardware. easier to just use a VFD.

 

[StarCat]

What Kind of Boiler Pumps?

What Kind of Boiler Pumps?

In a boiler system, using VFDs would be tied to the boiler control system to vary the feed pump flow based on boiler demand, as opposed to using a proportional control valve on full speed pumps. It reduces the energy consumption of the feed pump system and is likely tied to the overall efficiency rating of the boiler.

Similar for the Air Handlers; you vary the speed of the fan motors as opposed to using a VAV (Variable Air Volume) motorized damper box on the ducts and running the fans at full speed all of the time. Again, it consumes less energy that way.

As to the electric reheat coil, there is nothing inherently WRONG with using a VFD on a resistive load, but in many cases it is more expensive that a simpler SCR controller. In other words there is no BENEFIT to using a VFD for this compared to other more traditional methods of heater control. But; if the overall system is using DDC (Direct Digital Control) from the BMS (Building Management System) and there is a communications protocol involved, it may be that the protocol being used is easier to implement in a VFD than in an SCR controller. Examples might be Siemens Apogee, BACnet, LONWorks or Johnson Controls Metasys. It may be difficult to source an SCR controller with those communication interfaces, but almost all HVAC drives now have them.

I have to ask what type of boiler pumps are being referred to? If they are return water pumps on a Steam boiler system I would like to understand why, as I have never seen that done. On a pressurized hydronic loop, I can see it. The other thing I want to point out is that I have dealt with Engineering design flaws on very large dollar projects that were not caught until things started going wrong. One in particular led to " repeated " tube failure on a 300 HP Firetube Hot Water boiler and the experts were mystified. While we finally saw through to what was going on, it was neither quick nor easy. The last set of Mechanical and Electrical prints I had to sign off on were loaded with mistakes, and bad choices. Sign of the times.

 

[Besoeker]

it is not always just about the cost of the hardware. sometimes it is about having to train who knows how many technicains on some new piece of SCR hardware. easier to just use a VFD.

If they understand VFDs surely they would no difficulty in understanding a very much simpler SCR piece of kit?

 

[Russs57]

And I was wondering why one needs electric reheat when you have a boiler.

I'd have lots of questions too.

 

[Jraef]

But I don't see how a VFD would do than any better than a much cheaper SCR solution.

I agree. I was involved for a long time as a supplier of temperature controls to Applied Materials, the people who make the machines that make microprocessors (for Intel, Motorola etc.). There is no greater accuracy requirement than theirs in industry. Silicon wafers are divided up into "zones" of concentric rings, heated with parabolic quartz lamp focused onto an area the size of a pencil eraser. A 200mm wafer had 19 zones and heating a zone up to 900 deg. C with 1/10th of a degree different than the adjacent zone meant warping the wafer and wasting $5k worth of IC chips. We did all of that with SCR controls, and it has been done that way since the mid 1970s.