Belt slipping on AHU waisting KW

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Microwatt

Microwatt

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North Dakota
We have a company trying to get our HVAC guys to covert all of the AHUs to cogged belts and shives because they claim V-belts can slip and waste energy. They are saying the heat generated from the belt slipping is the waste. Currently they have a data recorder on a constant velocity AHU with V-belts of ours to record KW. In a couple of days they are coming to change the belts and shives to the cogged type. They will then record the KW again for the same amount of time and they think the numbers will show proof. Where we are a little sketchy is if the shives are not exactly the same size as the existing ones they could make the data show whatever they want it to.

The way I see it is if the v-belts are slipping the fan isn't spinning as fast so it is moving less air which in turn puts less load on the motor which would be less KW. Am I thinking of this incorrectly?
 
130405-1108 EDT

Microwatt:

You are correct that they could prove anything they wanted.

If there was a lot of wasted power in the V-belt, then its life would be short and it would smell.

An instantaneous power reading, kW, is sufficient for the demonstration. kWh measurements, energy, would require precisely the same time intervals. Also any tests need to be performed with exactly the same voltage at the motor terminals, and the motor at a stable operating temperature.

See http://www1.eere.energy.gov/manufacturing/tech_deployment/pdfs/replace_vbelts_motor_systemts5.pdf

When efficiency is as high as indicated in the above discussion, then to prove anything useful requires a very accurately controlled experiment. However, there may be reason to change from a standard V-belt to a notched V-belt, but probably not to a synchronous belt.

You probably have other ways to save a lot more energy than going to synchronous belts.

.
 
You can you use a tach on the fan . If the fan turns the same speed then the KW results will be proof.
 
Exactly as Gar stated, if there was any belt slip you would hear the squeal and smell the burning rubber. Belts slip a little bit at start up, which reduces the shock torque load to the blower.

One of the sheaves is an adjustable, variable geometry sheave and the V belt rides higher or lower in the sheave, changing transmission rpm, and changing motor current draw. Changing sheaves for sheaves, the test would have to also measure and confirm resultant blower rpm, which determines motor current draw. Matching blower rpm exactly, there could be some marginal savings going to a different sheave and belt type, but the saving calculation would have to include maintenance. The new sheave system would have to be proven lower maintenance, longer lived, to be a winner.

I've seen similar claims for the linked type V belts, but have no experience with them. There may be reasons for looking into this further, but belt slip is not one of them.
 
Thanks guys. We were thinking the same thing. The unit does currently have the notched v-belts and there are three of them and they are PM'd regularly. The slip now is very minimal as the sheaves (thanks for the correct spelling) are cool to the touch right after the unit is shut down. The unit is on a VFD, that is set at a specific Hz, so there is no squealing at start up.
 
I would want to see airflow measurments before and after the change to make sure the airflow doesn't drop.
 
steve66 said:
I would want to see airflow measurments before and after the change to make sure the airflow doesn't drop.
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I think the suggestion by Sierrasparky and a tach measurement probably would be more precise and take care of that.

I would take an instantaneous kw reading, use motor rpm to make sure your comparing apples to apples, and proceed from there. I would venture a guess and say you will probably not be buying any new belts and sheaves.
 
Standard V-belts are 90-95% efficient, depending on how well the tension is maintained. The compression of the rubber as it goes around the pulley causes heating. You gain another 2% efficiency by using notched belts to reduce this rubber compression.

The HTD belts don't slip but are noisier because of the air that is forced suddenly out of the notches. The no-slip can be an issue in applications where slip might help reduce equipment damage like during a jam on a conveyor belt. The cogged belts are about 98% efficient and they have the added advantage of no slip even during high-torque, unlike the V-belts.

If the tension is maintained on notched V-belts and high torque is not an issue, I don't think the efficiency savings will pay for the expensive cogged belt setup (but you would have to consider the difference in maintenance costs).
 
While there is some loss with any type of transmission of power, belt drives can be noisy and the fact you have the maintenance of replacing the belt and maintaining the correct tension, this is one reason why most home HVAC company's have gone to direct drive systems, I have tried the cogged belts in my own furnace and over the years on my older cars that used V belts, one thing is for sure is they do not last as long when compared quality to quality Goodyear or Gates, with todays vehicles using the serpentine belts that have a much greater surface area of contact, so even though it eliminated belt slippage this also creates more friction that again can cause a transmission loss, some car manufactures have also gone the way the HVAC company's did by eliminating as many belt driven loads as possible by directly driving off the cam shaft or other methods, I think I remember someone telling me about one that didn't have a single belt, but I cant remember which cars it was?

I can't see a big enough change to warrant the change to a cogged belt system, maybe having a way to change to a direct drive system would result in more savings as it eliminates the friction losses but other then that I just don't see it, and I think they will realize that any slight savings they might see will be eaten up with having to replace the belt more often.
 
130405-2335 EDT

hurk:

I have electric power steering and it seems to work well and probably contributes to less fuel consumption. This is not because of no belt drive to a pump, but rather there is very little power load from the power steering system when not much assist is needed.

Also the fan is electrically driven.

.
 
With the advent of readily available IR thermal guns now, I would think one could easily prove a difference in heat losses in 2 belt drive systems just by looking at the comparitive heat rejection while running the EXACT same load. One would think that if they are legit, that youd be a good picture to have on their literature eh?

And to re-re-emphasize the previous points, any change in transmission geometry will result in a change in load, oldest trick in the book. Remember that load will vary at the CUBE of the change in speed. So even if they drop the speed by what appears to be an insignificant "measurement error" of just 2%, the load will drop to .983 or .94, so a 6% change! That can be made to look like a significant "energy savings" to the unwary. So measure fan speed with a tach or air flow with an anemometer, either one, to make sure they are not scamming you.
 
Microwatt said:
We have a company trying to get our HVAC guys to covert all of the AHUs to cogged belts and shives because they claim V-belts can slip and waste energy. They are saying the heat generated from the belt slipping is the waste. Currently they have a data recorder on a constant velocity AHU with V-belts of ours to record KW. In a couple of days they are coming to change the belts and shives to the cogged type. They will then record the KW again for the same amount of time and they think the numbers will show proof. Where we are a little sketchy is if the shives are not exactly the same size as the existing ones they could make the data show whatever they want it to.

The way I see it is if the v-belts are slipping the fan isn't spinning as fast so it is moving less air which in turn puts less load on the motor which would be less KW. Am I thinking of this incorrectly?
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Their claim is legitimate. The standard belts over time stretch and the slip increases and so are the losses. Cogged, or synchronous belts are like timing belts and maintain a constant rate of speed increase/reduction. It is creases.questionable if their initial friction losses are less, because their actual surface contact is greater than simple V-belts, but over time their efficiency remains relatively constant, while V-belts' increases.
 
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gar said:
130405-2335 EDT

hurk:

I have electric power steering and it seems to work well and probably contributes to less fuel consumption. This is not because of no belt drive to a pump, but rather there is very little power load from the power steering system when not much assist is needed.

Also the fan is electrically driven.

Gar what your saying is like that there is no frictional losses with using a V type drive belt? unless I not understanding what you stated, which I find hard to believe from the industry I know you have worked in? if you have ever tried to turn a shaft with a pulley that is not connected to anything but has a belt to another shaft with just a pulley you will see that there is quit a bit of frictional resistance from just the friction of the belt sliding in and out of the V groove, this is of course with the tension on the belt to the required ?" deflection, I was designing a couple of drive shafts and had the shafts mounted on pillow blocks with the pulleys not even yet greased, before I put the belt on you could turn the shafts very easily and they would coast for just a bit, after installing the belt it was much harder to turn the shafts.

My 2010 Toyota Corolla also has electric assist steering as well as electric assist brakes, I find the steering has a strange feel but is very responsive, I found an article on why they were going to all electric systems like this and was floored that it wasn't in response to saving gas as it was to the development of the hybrids and electric cars, so that these parts could be manufactured for both types eliminating the need to manufacture two different types of systems making parts supply's simpler and more cost efficient.

.
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weressl said:
Their claim is legitimate. The standard belts over time stretch and the slip increases and so are the losses. Cogged, or synchronous belts are like timing belts and maintain a constant rate of speed increase/reduction. It is creases.questionable if their initial friction losses are less, because their actual surface contact is greater than simple V-belts, but over time their efficiency remains relatively constant, while V-belts' increases.
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I guess I didn't understand what the OP was really describing, I was thinking of the cogged belts that were developed back in the "70's" to reduce the friction cause by a V belt running in a V groove pulley by having less belt material in contact with the V pulley.

I would guess that a belt with grooves like a timing belt running in a gear grooved pulley could lower the friction because they would not have to tensioned as heavily and the positive gear drive effect that would prevent and slippage at a less tension, so yes I could see some savings if it is not eat up in the cost of the change or replacement duration. even though I would still say direct drive is by far the best choice if such choice can be given.

I could also see a cost savings on bearing wear as again the less tension these belts can run at and still drive without slippage would put less ware and tare on the accosted bearings, we have encoders and tachs that use a small belt just like this design, these belts are thinner but have cords much like a tire that don't allow it to stretch and they can be run with almost no tension at all just adjusted to a point where they cant jump teeth sort of speak, this alone would reduce almost all the friction that a V belt has, but these little belts were quit pricey.

Another common use that came about back in the late "70s" was these types of belts was used to replace the drive chains on motorcycles, as they would not stretch like a chain and were much quieter, as well as didn't place as heavy of the load on the bearings so your axle and final drive shaft bearings lasted much longer, but the only draw back was it didn't last as long as a chain.
 
mivey said:
Standard V-belts are 90-95% efficient, depending on how well the tension is maintained. The compression of the rubber as it goes around the pulley causes heating. You gain another 2% efficiency by using notched belts to reduce this rubber compression.

The HTD belts don't slip but are noisier because of the air that is forced suddenly out of the notches. The no-slip can be an issue in applications where slip might help reduce equipment damage like during a jam on a conveyor belt. The cogged belts are about 98% efficient and they have the added advantage of no slip even during high-torque, unlike the V-belts.
Click to expand...
The no slip comment reminds be of a problem I was called out to look at a good many rears ago.

It was a test set up with two winches, each driven by a DC motor. One winch was winching in and puling rope off the other while it winched out (rendered).
This was how the winch manufacturers tested the system at rated load.

The symptoms were that one of the drives would give a loud thump at fairly regular intervals. The DC motors had four quadrant SCR variable speed drives. I'd come across cases where SCRs would fire at the wrong time due to pick-up or some other cause. What I heard sounded exactly like that. I looked at the current feedback with a 'scope and it looked perfectly fine. And the speed was constant. But still the motor would bump now and again.

It's mechanical problem I thought and said so. The mechanical engineer, the winch designer, was having none of that.
However we looked at the drive train together and found the problem. The drive from the motor to the winch input gearbox was a toothed bely. Every now and again it would slip a notch hence the bump. Correctly tensioning the belt fixed the problem.

It was an interesting project. The winches were for a tin mining dredge up the jungle in Malaysia. The winches, two on the port side, two on the starboard, pulled the dredger back and forth across a fairly shallow pond (paddock) while a big bucket elevator scooped big bucket-loads of tin bearing soil.
 
Here is an update on this. Remember they were only claiming the change would save on KW and did not factor in bearing wear or anything else. They were here to change to the syncronous belt today. We did have a recording of the RPM of the fan before the change and we told them that would have to stay consistant. Before the change the fan RPM was 1925, after the change it was 2200. Good thing it was on a VFD. We slowed the fan down by reducing the output frequency of the VFD. Looking at the initial numbers, the KW output of the motor is the same.

I don't think we are going to see much of a savings.
 
Microwatt said:
Here is an update on this. Remember they were only claiming the change would save on KW and did not factor in bearing wear or anything else. They were here to change to the syncronous belt today. We did have a recording of the RPM of the fan before the change and we told them that would have to stay consistant. Before the change the fan RPM was 1925, after the change it was 2200. Good thing it was on a VFD. We slowed the fan down by reducing the output frequency of the VFD. Looking at the initial numbers, the KW output of the motor is the same.

I don't think we are going to see much of a savings.
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As it should be (output kW).

Do they guarantee any savings, looks to me like if you did not reduce the speed with the VFD you would have actually seen an increase in kW.:(
 
kwired said:
As it should be (output kW).

Do they guarantee any savings, looks to me like if you did not reduce the speed with the VFD you would have actually seen an increase in kW.:(
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I haven't seen anything in writing that it is guaranteed but this is why they are doing this test and recording the KW with a meter. This unit was a free-bee to change over in hopes they can prove the change will save money.

Yes we would have seen an increase in KW but the funniest part is there is a chance the fan would not have lasted long at 2200 RMP. She was a screamin when it was first started.
 
130409-2153 EDT

Microwatt:

Whenever you run an experiment to try to prove a point, and the % change expected is small, then it is necessary to run a very tightly controlled experiment with the correct instruments.

As example in the old days if you wanted to measure the output impedance (resistance) of a regulated power supply and all you had available was a Simpson 260 type instrument, then you might not even see the needle move if the meter was reading the full output voltage of the power supply. Such as a 24 VDC supply.

But if you can create a separate stable, at least for the time of the measurement, voltage supply that is approximately adjusted to the no-load output voltage of the supply under test, then you could use the 250 mV range and get a 200 times increase in the sensitivity of the measurement.

.
 
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