Belt Drive or Direct Drive?

[atlas06]

Some manufacturers claim that using Direct Drive is much cheaper in maintenace because no need for belts adjustments and so on. I am talking about fans over 1000 CFM, not 200 to 700 CFM kind.

From an operation, equipment life, maintenance and Performance stand point, should one specify a direct drive fan or a Belt drive for large fans?

 

[AbbyNormal]

Well if your fan wheel is perfect at 1760 RPM it would ramp up great with a VFD.

Belt drive allows you to be much more compact on the big equipment, not going to have a direct drive 15 horse blower motor spider mounted to a fan scroll.

Plug/plenum fans can end up high RPM,the 'plenum cabinet' you install them inside would get a couple/few feet longer with direct drive.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[atlas06]

Good to hear from you AbbyNormal, long time didn't read a post from you.

I was presented with Fan Wall Technology lately, which consists of direct drive vane axial fan and VFD's used in the 100,000 CFM plus kind of units for Arenas, large factories, etc.. I am being told that Direct drive with VFD's in the way to go for this application.

What's you take on it?

Thanks

 

[AbbyNormal]

I have seen the fan wall ads courtesy of penton. Reminds me of trying to replace lennox multizones, some company was slapping 10 residential pulses together to make a replacement unit

All kidding aside I could see some uses for the fan wall, and you have a little redundancy as well.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[willard3]

Used to be that with belt-drive it was easier and cheaper to adjust fan volume, but with VSD, this is no longet true.

 

[AbbyNormal]

that is an apple and an orange though willard

Most of the motors in hvac are nominally 1800 RPM, the belt/drives spin the fan at the speed needed. A VFD can vary the motor and hence fan speed up/down as needed

Could be a big forward curved fan, maybe it runs at 600 RPM to do the job. May have to re-engineer motors rather than run at 20Hz on the top end. Plug fans, BAF fans tend to be high RPMs sometimes exceeding the 1800 RPM.



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[RossABQ]

I've just come off a project where the client's standard practice is belt-drive fans with VFD's. I was not privy to the rationale for this, but it appears to be economics. It allows an off-the-shelf fan to be installed in a compact space, using a very standard motor, and the VFD provides a soft-start effect that is good for the belts (vs. across-the-line starting). No special technical staff capable of aligning the shafts is required. These are in largely unattended service (one of those "do more with less" situations) and we found the belts to be in pretty sorry shape. They optimize the package around the motor HP, i.e., if they need a 20 HP motor they sheave it so that at 60 Hz it is max'd out.

I am also familiar with the fan-walls used in semiconductor plants, where very high volumes with very high reliability is needed. The fan-walls' big sales point is once again economics, not technical superiority. Many commodity grade small motors are cheaper than a single high-quality, higher HP motor. (Note this is not a direct vs belt-drive issue, belts' rubber dust is unacceptable in such plants).

Personally, I think the choice of belt or direct drive depends on the client's maintenance staff, the attention they will give the equipment, and of course, what the client tells you they value more, first-cost or life-cycle cost. On small, in-ceiling FCU's I would go direct if it were up to me.

 

[quark]

The problem with plug fans is that, in case of VFD failure, the fan runs at runout condition(bypass mode), overloads and trips.

Belt driven fans also run for ages, without problems, if proper alignment is done. You can take care of the runout condition by pulley sizing and no problems even in case of VFD failure.

The dust from rubber belts is a latent idea. I have used belt driven fans in class 100 areas. Nevertheless, it can be a good marketing tactic. It is very easy to forget all those filters installed downstream. Who cares weighing the belts during their life period!

 

[atlas06]

Quark, could please elaborate on this?

"The problem with plug fans is that, in case of VFD failure, the fan runs at runout condition(bypass mode), overloads and trips"

why does it do that? can it be prevented from running in the condition you describe?

Thanks

 

[AbbyNormal]

A lot of times a plug fan is a BAF wheel without the housing.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[RossABQ]

VFD's only go to bypass automatically if 1) you have a bypass (it isn't standard), and 2) if you program it to do that. In HVAC service bypasses are rare in my experience. They are expensive.

 

[willard3]

And that, sir, is a problem thet the Engineer can solve by specifying a bypass and not letting someone who is not responsible for the design "value" engineer it away.

W/O a bypass, how do you propose to fix stuff and keep the fan running? Shut down the building service.......

 

[RossABQ]

You wouldn't always want a piece of equipment to run at 60 Hz if the VFD failed. Exs: plastic-piped pumped systems, boiler combustion air blower motors, make-up airhandlers, etc.

In my industry there are either completely redundant units that auto-start on loss of the lead unit, or the unit is non-critical and an outage is tolerated.

 

[marcoh]

We have been looking at direct drive backward curved fans for AHU's in the 10,000 L/s range. Hard to match a direct drive to a normal backward curved centrifugal fan to required fan duty and most systems are variable speed anyway and excessive speed adjustment using the VFD means you loose on operational speed range.

Plug fans seem less efficient due to the discharge chamber so any gains in drive efficiencies are offset by losses in the discharge.

 

[atlas06]

Quark, where the heck are you?,
Get back in here and elaborate on what you got everyone started.
Thanks for all the relplies guys, please keep up the debate, and take it anywhere you deem it necessary.

 

[RossABQ]

This is why engineers "get the big bucks", it's all about the right fan/system for the application. Direct drive isn't "right" for every application, neither is belt drive. They each have their strengths. I would have to have a really good reason to use belts on anything over maybe 20-25 HP, especially if it had a daily start/stop operation. The "soft start" provided by belts on large equipment frequently involves smoke and a lot of noise and wear on the belts, not pretty. I ran across one brand new 40 HP belt-drive unit in back-up service that set off the smoke detectors every time it started, ended up tripping off all the rest of the system!

 

[AbbyNormal]

Motors are made for various rpm and the common one in HVAC is 1800 RPM, (1760 RPM) etc

Wheels work for the job at different RPMs.


forward curve fans are lower RPM than BAF

VFD ramps a motor up to 60 Hz. Need the belt to get the right RPM at 60 Hz until they redesign the motors

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[quark]

Atlas,

You should first check the type of plug fan. Backward curved fans are non overloading type but not all plug fans are backward curved.

When you select a forward curved plug fan(low static application) with any other speed lower than the rated speed (hihger rpm - compact fan -low initial cost), your option is to use a variable frequency drive. Incase of VFD failure, the fan runs at rated RPM, generates more air flow and draws more power. As your motor doesn't have enough capacity, it trips.

The problem is more critical when you have a continuous requirement for ex. pharmaceutical AHUs.

When you have a faulty VFD, you can temporarily control the flowrate by discharge damper but you should watch out unstable portion of the fan curve to avoid hunting.

Having said that, I am not against this but you should know pros and cons before making a decision.

 

[sterl]

The application and the duty requirements will tell you enough of the story to make a fan selection. Motor and drive selection will follow.

We use semi axial direct drives for 6" plus H2O at temps to minus 50 deg. With restart at minus 50 deg. We've even put in filtration for minus 50 deg. Biggest problem, besides lubrication, is the weldment that supports the motor moves around as the system cools...we get a comes and goes vibrational mode.

We've installed equipment for 100,000 cfm of 99% HEPA at 35 deg. DB 0 Deg Dew Point and that sidewinder plug fan runs at 410 RPM Fixed Speed with a Soft Starter... Why? Cause the price of tearing up a fan shaft is this great big wheel walks its way right through the wall of the air handler. Better that than the finned coil.

Its a point of what you need for what you got, and the piece part vendors are not going to follow through on these...