BFF or soft start

[Arkansas74]

We have a 750hp boiler with a 50hp 480v motor for the blower which starts full York. They are wanting to put a soft start or vfd on the fan motor which one would be the best? I was thinking vfd with it being a blower fan. What is the pros or cons?

 

[petersonra]

We have a 750hp boiler with a 50hp 480v motor for the blower which starts full York. They are wanting to put a soft start or vfd on the fan motor which one would be the best? I was thinking vfd with it being a blower fan. What is the pros or cons?

I guess the first thing I would find out is why they want to do this.

A VFD makes sense if they might want to run the fan at a speed other than 60 Hz.

If all the want to do is lower the startup jolt, a soft starter will work fine and cost less.

It is possible the boiler is not designed to operate with the fan not running at design speed. Before putting a VFD on it, I would want the people that supplied the boiler to bless the idea.

personally, I am not convinced a motor as small as 50 HP really benefits all that much from a soft starter.

keep in mind that both soft starters and VFDs introduce additional points of failure into the system as a whole.

 

[Dale001289]

We have a 750hp boiler with a 50hp 480v motor for the blower which starts full York. They are wanting to put a soft start or vfd on the fan motor which one would be the best? I was thinking vfd with it being a blower fan. What is the pros or cons?

If you're only concerned about starting torque soft start may be a better application. If however, you need to vary the frequency after you've started, then VFD is the better choice.

 

[Besoeker]

We have a 750hp boiler with a 50hp 480v motor for the blower which starts full York. They are wanting to put a soft start or vfd on the fan motor which one would be the best? I was thinking vfd with it being a blower fan. What is the pros or cons?

Full York - I like that........
Does the blower have dampers to adjust the airflow? Does it start with cold air? That increases the load.

 

[Russs57]

I would not do this without something in writing from boiler manufacturer, boiler inspector, and insurance agent. Modifying boilers is a big no-no.

 

[Besoeker]

I would not do this without something in writing from boiler manufacturer, boiler inspector, and insurance agent. Modifying boilers is a big no-no.

I agree but I think that should be done by the customer who wants the change, not the contractor being asked to do the work.

 

[Arkansas74]

They are having trouble with the fan bearings and want to be able to loosen the belts to keep some of the pre load off the bearings. With the belt loosened to lower the load on bearings the squalls really loud and burns up the belts. The boiler technician is the one that wants the belts looser to prolong bearings.

 

[Besoeker]

They are having trouble with the fan bearings and want to be able to loosen the belts to keep some of the pre load off the bearings. With the belt loosened to lower the load on bearings the squalls really loud and burns up the belts. The boiler technician is the one that wants the belts looser to prolong bearings.

A soft start might help with that.

 

[Arkansas74]

Yes it has dampers and it does start with cold air

 

[Besoeker]

Yes it has dampers and it does start with cold air

Are the dampers used to control the airflow?

 

[Ingenieur]

if you do not plan on controlling speed a vfd is overkill
a ss should do it
I assume lt runs continuously? or is it cycled?

 

[Arkansas74]

Yes sir they are used to control the air flow to the burner. It runs continually

 

[Besoeker]

Yes sir they are used to control the air flow to the burner. It runs continually

If the dampers are used to control the airflow then a VFD could reduce your customer's energy bill. And be gentler on the belts.

 

[drcampbell]

They are having trouble with the fan bearings and want to be able to loosen the belts to keep some of the pre load off the bearings. With the belt loosened to lower the load on bearings the squalls really loud and burns up the belts. The boiler technician is the one that wants the belts looser to prolong bearings.

This isn't an electrical problem; it's a ID-ten-T problem, and a soft starter isn't likely to solve it. Installing a soft starter might eliminate the squeal during start, but with the belts this loose, they will slip a lot and their life will be short. I would encourage the customer to identify & solve the fundamental problem, not a secondary problem caused by Appalachian Engineerin'.

 

[Ingenieur]

does it start with dampers open or closed?

 

[Russs57]

Tell us more about the boiler and this fan. Most of my experience is with Cleaver Brooks and their fans have no belts. If you have a draft fan with split pillow block bearings there are other solutions to the problem.

Running loose belts can do more harm because sheaves will get hot and the heat will be transferred to bearings. The belt tension load is a radial load and a very small fraction of the bearings rated radial load capacity.

Be sure to let us know if this fan is moving any particulate matter and if temperature of air it moves is close to a constant or changes a lot as boiler fires and ramps up.

Also if this fan is integral part of boiler or sits adjacent to it.

Oh, wasn’t clear if it is fan shaft bearings or motor bearings and how long they last.

 

[Ingenieur]

loose belts are rough on shafts and bearings
vibration, wobble, looseness, not good
fans are typically easy to start, ie, no load, builds up with rpm

did you measure current?
overloaded?
needs resheaved?
check alignment and tension?

 

[Besoeker]

fans are typically easy to start, ie, no load, builds up with rpm

If you ignore the inertia.

 

[Ingenieur]

If you ignore the inertia.

bearings and balance

 

[ptonsparky]

adding a belt or two to the setup may work. Who set up the original design? You would think or hope this isn’t their first time.