Blower CFM and restriction on outflow

[Dolly Llama]

someone help me understand this;

it seems logical to me that a straightest path with the largest dia opening out of a bower offers the least resistance...i understand that part.

But now put bends and other restrictions on it..where does the CFM "go"??
here's what I don't understand ......Wouldn't the same amount/volume are air still be expelled??
I understand that it would have to push harder and take more HP to do so.
i also understand a certain amount of volume would be compressed before exhausting .
But it would still be the same amount/volume exiting , no???

the only way "my" little brain could see otherwise would be for the air to "slip" over the lobes and not make it to the down stream side ..other wise, the same volume HAS to come out..regardless of the restriction ..it just has to work harder to overcome it...yes??


..L.T.A.

 

[sam miller]

I dont think backflow restriction is the way to look at it I would guess that an open free exhaust 4.5 blower running the same rpm as a 4.7 blower with exhaust heat exchangers at the same rpm's would crate the same cfm because of the restriction!

But remove the exhaust heaters and You would increase your cfm because the limit is the exhaust until You put the wand to the carpet then its the carpet.

Its all about what load the manufacturer puts on the blower in the first place rpm flow restriction they could get 500 cfm out of a blower or 550 cfm wide open drop blower exhaust heater and lower rpm same blower may only produce 400 cfm.

I have a 405 and I have thought about dropping the exhaust heaters and adding a propane heater more heat and cfm its a thought.

 

[bob vawter]

yes...same amount Larr...unless the restriction causes the blower to bog down some.....
my 90 degree elbows make SQUAT difference......
thats my opinion

 

[Hoody]

Larry,

Let me see if I can explain this. When you put a bend on plumbing you're increase the length of it. There is a way to calculate how much each type(sharp 90, sharp 45, sweeping 90, sweeping 45, ect ect) of bend increases length based on diameter of hose , but I don't feel like trying to look it up, and deciphering how[has to do with pressure inside the tube vs atmospheric pressure, and pressure change inside the tube bladdy blah blah] - the formulas are quite extensive.

You know that at longer hose runs the CFM decreases as the length of hose increases. Same concept here, more bends = longer length of hose the air has to travel through.

House power isn't going to lessen the length of the plumbing. Only way to "make up" for the loss in CFM is spin the blower faster to increase the CFM at the blower. You would need a sensor to constantly monitor the CFM, and a module that would increase the RPMs of the motor when CFM fell below a set point to make up for it. You'd be running through a lot of blowers doing that, and its not practical for longevity of the engine and blower.

By the way.... most people think 45 degree angles are best if you have to have one, when in fact 30 degree angles provide the least amount of restriction. Back pressure has little to do with the whole scenario, it has more to do with damage to the blower and in catalytic HXs cases damage to the heads on the engine.

 

[bob vawter]

Steven...next time you build a machine and have to decide on the design....
whether it be a purty mandrel bent 3" auto exhaust cold roll steel that WILL rust..........with NO muffler....
OR a stainless system with a 3" muffler with a 3" 90 degree bend that has negligible effect on CFM.....

get back to me.........

 

[Hoody]

Yes Bob I know the steam genie has nice smooth 90's coming out of the blower, and to the exhaust pipe to the muffler.

 

[Dolly Llama]

'Dink, (et al) I understand the length and size of vac hose restricting CFM that can only be compensated for by increasing the HG (pressure) ...no problem there.

but leave the vac side open for this discussion and restrict only the exhaust end.
the pressure would automatically increase to expel the volume of air being pushed .

as long as the blower is spinning at same RPM, it "has" to expel the same amount.
OR......slip past the blower lobes

It can't be any other way, can it???


..L.T.A.

 

[Lee Stockwell]

The blower moves a constant volume of air per revolution. Slippage loss is usually minimal.

All the blower "sees" is "load" whether it is measurable vacuum on the inlet side or measurable pressure ("backpressure") on the outlet side.

Any plumbing element on the inlet side will measurably INCREASE the amount shown on the vacuum gauge before any hoses are connected. The system should be designed to keep this gauge reading as low as possible. A high vacuum reading with no hoses connected is just a constant waste of power.

Sadly most systems don't have a corresponding fitting and gauge on the exhaust side of the blower to measure "backpressure". This number should be as low as possible because it does nothing to help cleaning performance. A gauge here would indicate clogged exchangers or mufflers etc. A high pressure gauge reading here would ALSO be a constant waste of power.

All unneeded restrictions bog down the blower rpms and directly related CFMs.

 

[Ross Buettner]

The term is called "friction loss"

It's a term used in calculating gallons per minute, overall pressure, vacuum lift, and so on.

Every bend causes it. The ribs in your hose add to it. It's small per item that causes it however. But, if you add more hose footage that smaller figure increases. Add more "head feet" or lift, such as stairs and the numbers increase again. The more pathway the air needs to travel, plus the amount of water it's trying to move adds as well.

I had to use those figures in sizing pumps for years. Even Cat pumps But it works relatively with vacuum as well.

 

[bob vawter]

if the inlet and exhaust PORTS are of significant enough in size......
again the answer is negligible............

 

[Hoody]

The blower moves a constant volume of air per revolution. Slippage loss is usually minimal.

All the blower "sees" is "load" whether it is measurable vacuum on the inlet side or measurable pressure ("backpressure") on the outlet side.

Any plumbing element on the inlet side will measurably INCREASE the amount shown on the vacuum gauge before any hoses are connected. The system should be designed to keep this gauge reading as low as possible. A high vacuum reading with no hoses connected is just a constant waste of power.

Sadly most systems don't have a corresponding fitting and gauge on the exhaust side of the blower to measure "backpressure". This number should be as low as possible because it does nothing to help cleaning performance. A gauge here would indicate clogged exchangers or mufflers etc. A high pressure gauge reading here would ALSO be a constant waste of power.

All unneeded restrictions bog down the blower rpms and directly related CFMs.

Right on Lee! The loss in CFMs is very minimal.... the amount of available lift before hoses are connected however is greatly effected as Lee explained. CFM is more greatly affected by how much water is extracted. Before water extraction CFM is a moot point.

Take extracting a flood vs extracting carpet.... more water coming through the hose means less space the air available which creates a loss in CFM, and a loss of available lift. Once the hose clears CFMs increase, and available lift is increased as well.

 

[Larry Cobb]

Meat;

You need to really think about the Vac System as a whole.

The real restrictions (friction loss) to airflow are the:

1. Silencer for blower (needs to be low restriction model)

2. Heat Exchanger (preferably design with straight tubes)

3. Waste Tank (preferably with no baffles)

4. Vac Hose ( 2.5" or 2" depending on cleaner preference)

5. Wand Lip-Carpet Interface (14" wand & Glides help)

6. Vac Inlet Filter (check the actual mesh size & open area)

All of these increase the blower lift losses (and slippage thru the blower).

Eventually the vac relief opens . . .

and CFM thru the wand is minimized.

Larry Cobb

 

[truckmount girl]

Think of it like the exhaust in a car. If you restrict it a lot you get too much back pressure which is harmful to the system and causes performance loss.

Take care,
Lisa

 

[joe harper]

Can this TheOrY...be confirmed with a "golf ball & a garden hose"... :?:

 

[Leslie Judson Jones]

'Dink, (et al) I understand the length and size of vac hose restricting CFM that can only be compensated for by increasing the HG (pressure) ...no problem there.

but leave the vac side open for this discussion and restrict only the exhaust end.
the pressure would automatically increase to expel the volume of air being pushed .

as long as the blower is spinning at same RPM, it "has" to expel the same amount.
OR......slip past the blower lobes

It can't be any other way, can it???


..L.T.A.

one part of this video is not right can you tell me what part ?

click on photo

 

[Walt]

Great video Les!

Thanks for making it.

 

[Hoody]

Think of it like the exhaust in a car. If you restrict it a lot you get too much back pressure which is harmful to the system and causes performance loss.

Take care,
Lisa

Exactly, think what would happen if you took a vehicle with a lot of bends and turns in the exhaust and straight piped it with a flared exhaust tip. You'd increase the HP, and there is less restriction on the engine.

 

[bob vawter]

wrong Dink...SOME of those bends in a header system are there 'specialy to create a "scavageing" situation....where one pulse of the exhaust is close enough to hep pull the next pulse out with it.....

 

[truckmount girl]

wrong Dink...SOME of those bends in a header system are there 'specialy to create a "scavageing" situation....where one pulse of the exhaust is close enough to hep pull the next pulse out with it.....

Yes, but we're not dealing with multiple cylinders. So what Steve said applies.

take care,
Lisa

 

[Duane Oxley]

someone help me understand this;

it seems logical to me that a straightest path with the largest dia opening out of a bower offers the least resistance...i understand that part.

But now put bends and other restrictions on it..where does the CFM "go"??

the only way "my" little brain could see otherwise would be for the air to "slip" over the lobes and not make it to the down stream side ..other wise, the same volume HAS to come out..regardless of the restriction ..it just has to work harder to overcome it...yes??


..L.T.A.

Here's my take on it:

A given blower has a capacity to generate a specific amount of pressure (PSI) or vacuum (Hg. / H20").

So, that specific amount of force it can generate is the amount of force it can push or pull airflow with.

As airflow encounters resistance, some of the available force is used to overcome that resistance. And as a result, the amount of force remaining to move airflow is reduced accordingly. A reduce in "available" ("unconsumed", or "unspent") force, reduces the amount of airflow.

The slippage in a blower does factor in, as said above and in Les' video. And how it factors in is that, the more resistance the airflow encounters, the more resistance the airstream through the blower offers. And that resistance means that the airflow tends to move more slowly, even when the blower is trying to get it to move faster. And that difference between the two affects the amount of slippage present in the blower. The more the resistance, the more the slippage.

Bends in plumbing offer resistance. And while Bawb is right in that over- sized plumbing has a much reduced effect than "textbook" plumbing, even there, some efficiency is present, because every inch of the way through any turn, the airflow is trying to go straight, but is being pushed into a circle, which creates resistance.


And although it is different for pressure vs. vacuum, in terms of how much difference it makes (pressure is more efficient for a blower to generate, if I remember correctly here...), there is still a pretty close correlation between the two. In other words, if resistance is generated on one "side" of the blower, it affects the airflow through the other side...

 

[Bob Foster]

one part of this video is not right can you tell me what part ?

Almost the last statement you made on the video. You said 275 to 300 CFM and you meant to say 275 - 300 degrees temperature.

 

[Hoody]

'Dink, (et al) I understand the length and size of vac hose restricting CFM that can only be compensated for by increasing the HG (pressure) ...no problem there.

but leave the vac side open for this discussion and restrict only the exhaust end.
the pressure would automatically increase to expel the volume of air being pushed .

as long as the blower is spinning at same RPM, it "has" to expel the same amount.
OR......slip past the blower lobes

It can't be any other way, can it???


..L.T.A.

one part of this video is not right can you tell me what part ?

click on photo

Toward the end you said the maximum temperature for a blower is 275-300 CFM instead of degrees.

Edit... I need to learn to refresh....

 

[bob vawter]

very nicley put Duane......however one has to measure the advantages over disadvantages
in deciding whether or not a mandrel bent smooth exhaust is WORTH the noise it encompasses by the lack of room to put a primary muffler off the blower.......

 

[sam miller]

Great video Les and fast response nice. So maybe having a killer inline filter 2.5 in ports and dropping the filter basket and vacuum filters would be better for cfm???

 

[Dolly Llama]

excellent vid, Les

at appx 3:40 or so in the vid is what explained it and made it clear to me

thanks

..L.T.A.

 

[Mikey P]

bump

 

[Joel D]

Wow, excellent explanation and video. Thanks!