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Author Topic: Centrifugal tube blower question.  (Read 3992 times)
2E0ILY
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« on: October 27, 2012, 03:32:28 PM »

Henry 2000D RF Generator being converted to single band linear. The original 3CX3000A7 blower was converted here in the UK to an RS Components 230 volt motor. http://uk.rs-online.com/web/p/ac-motors/2483637/?searchTerm=248-3637&relevancy-data=636F3D3126696E3D4931384E525353746F636B4E756D6265724D504E266C753D656E266D6D3D6D61746368616C6C26706D3D5E5C647B337D5B5C732D2F255C2E5D5C647B332C347D2426706F3D313426736E3D592673743D52535F53544F434B5F4E554D424552267573743D3234382D333633372677633D4E4F4E4526Generator has been idle for years. Fired up blower on its own, to check. Found air flow vane switch seized. Cleaned and freed but thought blower air flow was a bit pitiful. I have never seen any other valve blowers to compare it with, but the RS sites shows the replacement motor (on the original fan and case) is 2000RPM. The original Dayton 4C006 115 volt motor, which I don't have, was 2200 RPM.

I was expecting this thing to be hellish noisy, more than a loud vacuum cleaner, with a mega air flow you couldn't hold in front of your face. it's actually very quiet, and v=creates a good breeze. Is that to be expected? I can't see an extra 200 RPM making a HUGE difference, it may be fine as is. Should these things be hurricane force, noisy as hell, or quiet enough to share the shack with and just a good waft from it? Thanks.
« Last Edit: October 27, 2012, 03:48:59 PM by BASIL » Logged

Best regards, Chris Wilson.
WX7G
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« Reply #1 on: October 27, 2012, 04:03:53 PM »

With the UK RF power limit being 400 watts it shouldn't need much of a blower.
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W8JI
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« Reply #2 on: October 27, 2012, 06:08:02 PM »

If a blower is not being choked by inlet restriction and you increase wheel speed 10%, output flow volume increases 10%.

When driving backpressure, if wheel speed is increased 10%, the pressure increases 21%. Pressure is the square of the wheel speed change. If you are running the wheel at .9 rated speed, you lost about 19% of pressure and about 10% of maximum open air flow-rate. That is considerable pressure loss.
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WB2WIK
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« Reply #3 on: October 27, 2012, 06:09:56 PM »

The air flow through a 3CX3000A7 isn't much, compared to the air flow out of the blower by itself without blowing through the tube.

The blower, ducted into "free space" would be very powerful.  With the back pressure of the tube, not so.  You should barely feel the air coming through the tube, and might have to wet your hand to actually feel it.
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W8JI
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« Reply #4 on: October 28, 2012, 02:58:38 AM »

The air flow through a 3CX3000A7 isn't much, compared to the air flow out of the blower by itself without blowing through the tube.

The blower, ducted into "free space" would be very powerful.  With the back pressure of the tube, not so.  You should barely feel the air coming through the tube, and might have to wet your hand to actually feel it.

Not so.

A 3XC3000A7 has a fairly open anode. It requires at least 40 CFM of air, preferably 80 or more in hard use. Forty cubic feet of air in a minute is a very strong airflow when it comes out of an seven square inch area.

If he measures water column, he should have around 1" of column across the anode cooler.
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2E0ILY
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« Reply #5 on: October 28, 2012, 06:20:47 AM »

I have managed to source a 2300 RPM motor that I can machine an adapter to fit. That puts the RPM a little OVER the original spec 115 volt motor, and I retain the OE fan and housing, so should be adequate. I was just surprised by how little puff these things have, I was expecting something far more powerful that could be felt six feet away. I had read a lot of stories about folks wanting a linear with a centrifugal blower outside of their shack due to the noise of the fan, but even the 2300 RPM motor would not give me an issue should  I have to sit on the amp! Being a bit deaf probably helps. Thanks for the info, I don't feel it necessary to build a manometer given I am now back at the Henry spec where the thing was running full tilt, 100% duty cycle. Our usage will be far less than that. Cheers.
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Best regards, Chris Wilson.
G3RZP
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« Reply #6 on: October 28, 2012, 08:37:18 AM »

The replacement for the 4C006 is rated at 2450 rpm, and 0.74 amps at 115 volts, or around 85 watts input. That RS motor is rated at 24 watts, so my suspicion is that the back pressure is enough that the fan is windmilling and you aren't getting even 2000 rev/min, because the power just isn't there from the motor. That would explain why it's a gentle wafting. The Dayton motor is probably about 1/10 HP: the RS one sounds like about 1/30 HP.

It's not just the rev/min you need to have, but the motor power to develop those rev/min when under load. The manometer will tell you....or measure the time to fill a 50 litre rubbish sack with air, and convert to f.p.m.
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2E0ILY
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« Reply #7 on: October 28, 2012, 09:24:32 AM »

The replacement for the 4C006 is rated at 2450 rpm, and 0.74 amps at 115 volts, or around 85 watts input. That RS motor is rated at 24 watts, so my suspicion is that the back pressure is enough that the fan is windmilling and you aren't getting even 2000 rev/min, because the power just isn't there from the motor. That would explain why it's a gentle wafting. The Dayton motor is probably about 1/10 HP: the RS one sounds like about 1/30 HP.

It's not just the rev/min you need to have, but the motor power to develop those rev/min when under load. The manometer will tell you....or measure the time to fill a 50 litre rubbish sack with air, and convert to f.p.m.


Hmmm, you may well have hit it here. I can't believe this is enough air FLOW, I don't know why the machine was de-commissioned, maybe they had tube issues because of a lack of air flow? Maybe it coped, but with the price of a new tube being a lot higher than a well specced blower I guess I ought to go for something I am CERTAIN is correct. Thanks for bringing flow through a restriction to attention, rather than just free, unloaded RPM. You wouldn't know of a UK source for something suitable at a fair price, would you Peter? This one is about 1.7 times the free air flow of the OE one, with the correct motor fitted. Air flow V pressure seems to hold up as well as most devices of similar free air flow and current draw.

 http://uk.rs-online.com/web/p/centrifugal-fans-blowers/3324078/


Space wise, shape wise and mounting wise, no issues. It just blows into a sealed chassis section through the base plate. No problem milling the hole bigger..

Thanks
« Last Edit: October 28, 2012, 09:58:59 AM by BASIL » Logged

Best regards, Chris Wilson.
G3RZP
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« Reply #8 on: October 28, 2012, 10:35:28 AM »

It looks about the right power, anyway. I'd suggest that you look up the airflow requirements for the valve to start with: that will give you a minimum requirement. (Personally, I'd add something like 33% or more, but I am somewhat conservative on these matters). There are various UK blower manufacturers: I seem to remember Airflow Developments as one of them. Google will be your friend........

You want the lowest speed blower you can get that gives adequate air flow at the back pressure, because a low speed blower is quieter. Likewise, you want plain bearings as being quieter than ball or roller bearings - not that you are likely to get roller bearings in these sizes.

As I say, be generous in providing volume against back pressure. It is said the only damage from over cooling is blowing the tube out of the socket......I have seen it said you can cool it so far that the filament emission is affected, but I find that unlikely.
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2E0ILY
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« Reply #9 on: October 28, 2012, 10:53:47 AM »

Great, I did have a slight concern re over cooling. I'll aim somewhat for overkill after looking at the Eimac specs. Noise won't be an issue, the thing is in its own room, and it's going to the US when my pal goes back, so it's his problem Smiley I'll try not to blow the tube from the socket or balloon the chassis compartment! Wink I need a water pump for the dummy load, but I am OK with electric water pumps, water flow rates,  and heat rejection via water to air rads, as it's more race car orientated than air blower stuff. Thanks again Peter, everyone.
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Best regards, Chris Wilson.
GM3SEK
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« Reply #10 on: October 29, 2012, 01:32:59 AM »



A 3XC3000A7 has a fairly open anode. It requires at least 40 CFM of air, preferably 80 or more in hard use. Forty cubic feet of air in a minute is a very strong airflow when it comes out of an seven square inch area.

If he measures water column, he should have around 1" of column across the anode cooler.

Agreed with Tom, and also with Peter RZP that the motor is seriously under-powered.

Two ways forward:

1. Remember that the important figure for cooling the tube is the volume flow rate. You can measure this by timing how long it takes to inflate a lightweight plastic bag attached to the air outlet. In Europe (where you are) a standard kitchen swing-bin liner holds close to 50 litres when fully inflated.

Obviously the bag needs to be fully flattened at the start, and the measurement can only be done after the blower is up to full speed. You will need to develop a method to attach the bag quickly and securely, using cardboard gaskets and duct tape. It's usually OK to hold the gasket in place with both hands for the few seconds it takes to inflate the bag.

With care and a little practice you can make a measurement to about 10%, which is probably more accurate than the pressure drop method (because the measured pressure depends greatly on the positioning of the probe, and you don't know where the Eimac engineers positioned their pressure probe when they did the measurement for the data sheet).

2. Check whether that blower motor has any chance of working correctly. A major problem with US-made amplifiers exported to Europe is that AC induction motors run more slowly on our 50Hz mains. The volume flow delivered by the blower depends on the rpm squared so changing from 60Hz to 50Hz will cut the volume flow rate down to 70% - which can make all the difference between "adequately cooled" and "under-cooled".

But changing to a smaller, under-powered motor can reduce the rpm by far more, causing the flow rate to collapse completely... which we all suspect has happened here.

To estimate the blower performance, take a look at this spreadsheet: http://www.ifwtech.co.uk/g3sek/in-prac/best-of.htm#blower. It cannot account for all the little tricks of blower design, but the method developed by G8WRB is firmly based on the underlying physics so it is several steps ahead of the usual guesswork. In particular it shows how the back pressure capability of a centrifugal blower depends on three key factors: rpm, wheel diameter and width of the wheel.

The chart shows two lines: the red line shows the demands of the tube (back-pressure created by various levels of flow rate) and the blue line shows the capability of the blower (volume flow delivered at various levels of back-pressure). The 'operating point' of the whole system is where those two lines cross.

The square marker on the red line shows the minimum pressure and flow rate requirements from the tube data sheet. The blue and red lines need to cross above this point for the tube to be adequately cooled.

Blower performance depends on three main factors: rpm and wheel diameter, both of which strongly which affect the pressure capability; and the width of the wheel which affects only the volume delivered.

Imagine that the default 2700 rpm in the spreadsheet is what you'd get with 50Hz mains and a powerful induction motor. In cell C14, change this to 2400 which is what your little motor claims - and see how the blower performance collapses! Regardless of the specific design and performance of your particular blower, it will collapse in a very similar way with inadequate rpm.

And because your motor is almost certainly not achieving 2400rpm, the collapse will be even worse... which is pretty much what you're seeing.


73 from Ian GM3SEK





« Last Edit: October 29, 2012, 02:14:08 AM by GM3SEK » Logged
KD0REQ
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« Reply #11 on: October 29, 2012, 02:32:45 PM »

only way you can have overcooling with room temp forced air is if you blow the front panel off and the tube out of the socket.  we can get to 40 below in my neck of the woods, so I'd not use outside air supply Wink
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G3RZP
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« Reply #12 on: October 30, 2012, 04:06:00 AM »

I remember reading about a station at a base in the arctic where they did use outside air, suitably arranged to keep snow out. Worked very well apparently, but I read that even after running a load of RTTY, the tube anode was still cold enough that fingers would stick to it! (HV off, of course!)
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2E0ILY
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« Reply #13 on: October 30, 2012, 04:19:36 AM »

GM3SEK Ian and all who have assisted. Many thanks indeed for the detailed replies That gives me plenty to work with, for this and future projects (a VHF valved amp with one of your highly regarded "Triode Boards" fitted is awaiting collection, in an incomplete state, requiring, amongst other things, a blower sourcing). I have decided to just go for a new blower with more than enough known air flow to meet the Eimac flow rates. The alternative, finding another motor that's mechanically compliant with the existing original case and fan rotor, won't be much cheaper, plus I am still using a lot of used parts. I can mill out the hole in the chassis plate to accommodate a bigger exit on the new blower, and have more than enough room in the cabinet. Good thinking re the bin bag inflation, I will try that as a matter of interest on the blower that's currently on it, but I just know it's inadequate, it would struggle to blow out a one year old's birthday cake decoration Smiley I would rather go for overkill than wonder if the valve's life is being compromised, given their cost.  Thanks again.
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W8JI
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« Reply #14 on: October 30, 2012, 01:23:51 PM »

If you borrow a gauge from someone in the HVAC business, all you need do is measure the pressure in the anode exit area and compare it to pressure in the air inlet plenum.

That is how tubes are actually rated, or by measuring temperature of various areas of the tube.

My amplifiers' inlet air system ducts in from below the floor in the summer, and out to the attic.
The air is reversed in the winter, and sucks down from the attic and exhausts under the floor in the winter. I do not use room air.
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