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I looked the unit up, max flow: 10cfm max pressure: 10psi




Gast brand air pump for sale.3/4 hp motor.new product, Pump can be used for multiple applications.
pump used in lots of bodyshops as a fresh air system pump for painters. Includes two new hoses that attach to pump.

Last edited by LarryHale; 01/27/15 04:11 PM.





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How deep is your pond?


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Is it 10 cfm AT 10 psi, or 10 psi maximum? If it is 10 psi maximum, then an important question is what cfm does it put out at the 10 psi. A performance curve chart would be great if you could find it.

10 psi will cover depths up to 20' or so. Thing of it is, 10 cfm at 10 psi is a bunch. Enough for several diffusers at a depth of 20'. But if cfm drops off quite a bit at depth (back pressure), it might not have 10 cfm at depth.

What kind of pump is it? Rotary vane? Kind of what it looks like by the picture.

Ideally you want to know the max depth in the pond you will use the pump, then look at a performance curve and see what the cfm is at that depth (pond depth in feet times .5 = approx psi the pump will be looking at). Once you know that you can look at diffuser specs and see what type and how many diffusers it will support.

3/4 hp run continuously will definitely cause a rise in your electric bill.

Last edited by snrub; 01/27/15 05:38 PM.

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A lot depends on the pressure drop across the diffuser as well. Pressure drop will increase with flow rate.

Example, the normal operating pressure of my system this time of year is 14 psig. I have one 4 stick membrane diffuser operating in 4 feet of water at about 2 cfm. The line to the compressor is 3/8 for about 60 feet. Line size and length are also factors to consider for pressure drop. So for me, a 10 psig max pressure compressor would not work with my current setup and air flow.

Last edited by Bill D.; 01/27/15 05:53 PM.

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Date code is July of 1998 when it came off the line. Don't think they attached plumbing to new units, but give it a whirl and see what's up. Pretty easy to rebuild unless it's pretty worn out inside. Sounds about right on the stats if it were a fresh pump. They do last a while tho. Gast has been doing this for a very long time.

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This is a used unit. Advertised for $500. I figure if I need to rebuild it, it's still a deal. What do you think? Surface area of the pond is .29 acres with a max depth of 10'. I figure I can get away with a double diffuser. Plan to run 1" pvc 150' to the pond then 5/8" weighted tubing to the diffuser. Checked the specs, free air flow is 10cfm @ 10psig.

Last edited by LarryHale; 01/27/15 08:17 PM.





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I only have limited experience with aeration, having only been doing it a year.

I would have two concerns. Well really just one because the first one is easily rectified.

10 cfm is a bunch of air. The 9" Matala diffusers I am using call for a range of 1.4-4.2 cfm per disk or in other words 2.8-8.4 for a dual station. Lots of different types of diffusers with different specs out there. Using my diffusers as an example, you would need three of them if you were to use all the air the pump produced. The other option would be to "bleed off" part of the air and not use all of it. Of course this means you are wasting electricity. If you over expand a diffuser (with excess air flow) it will burst it or severely shorten its life span.

In past threads there have been formulas to figure electric cost. I don't have that info right off hand, but I would not be surprised of that 3/4 hp motor did not cost 30-40 dollars a month to operate continuously.

So the first problem, too much air, could be rectified easily by bleeding some off. For me, the electric cost would be my concern. You might only need a quarter or third horsepower pump for the size pond you have, and the electric cost for the larger pump might quickly eat up any savings in your lower pump cost price. That would be my concern. Something running 24/7 for 6-8 months out of a year can get expensive in electric cost. Maybe that is not a consideration in your case.

Some helpful links:
aeration links

Edit: It is my understanding that some diffusers use restrictions on the input of the diffuser. In my case the Matala diffusers do not. But it is another consideration when balancing pump output to diffuser.

Last edited by snrub; 01/27/15 08:55 PM.

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snrub,

Thanks for the quick reply and the link. Cost of electric is something I hadn't considered.






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You can figure that pretty easily. There are on line calculators and they probably have estimates for what a 3/4 hp motor draws or you could get the info from the specs. Then all you need to know is your cost per kilowatt hour, which should be on your electric bill.


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For that size pond (area and depth) you can get by with 1/3 to 1/4 hp depending on the compressor performance and diffuser performance.

It's not just about putting air into the pond, it's about how many GPM the diffusers will move water wise.

The more effecient the diffusers are, the less CFM your compressor needs to be, which shows up on your electric bill.


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Looked up a Gast 1023. They do 10 cfm at low psi and 9 cfm at 10 psi. Rotary vane. And you can figure 14 amps for full 3/4 hp on 110 VAC. The current draw will drop if you run it low pressure, but still will cost. I agree it's oversized for such a small pond.

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OK, should I go with rotary vane or rocking piston given the depth of my pond (10') and it's kidney shape? My calculation on the pond are as follows:

New figures based on survey today

Surface area: 14,880 sq ft
Surface acreage: 0.35
Total acre feet: 3.42
Avg Depth: 10 ft
Max Depth: 15 ft
Total Gallons: 1,117,500






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I can't give you a legitimate answer to that because have had very little experience with such pumps, and what experience I have had is in other applications than supplying air.

But some generalities:

A piston type pump by its design has a design that lends itself to higher pressures, thereby can operate at deeper depths. If you have a shallow pond, this requirement might not be needed thereby perhaps not being the most efficient at delivering relative high volumes at low pressure.

A rotary vane pump from my experience on other applications tends to be noisy and they are not particularly good as pressures increase. They like higher volumes at lower pressures.

Now having said all that on compressor design, it may be more splitting hairs rather than useful information. I would look at what is available, what is commonly used for pond aeration use, figure your volume requirements of cfm flow needed at the head pressure (water depth based on the water flow you need and the diffuser(s) you are going to use. Start with the diffusers, then see what pump best fits the needs to supply those diffusers based on initial cost, longevity, dependability, and operating cost.

The diffusers are the key to aeration. Then find a pump that operates the diffusers correctly.

There may be trade offs. IE, a very dependable pump that will run for years with little maintenance may cost more in electric cost. Are you willing to change pumps out more often or do maintenance in trade for a lower monthly operating cost? Or would you prefer to set it and forget it, even if it costs a little more monthly to operate it?

These are questions I do not have the answers for. But questions I came up when considering my own system. But there are guys on this forum that have operated systems for many years that have good opinions and experiences to help answer those questions. All the good I can do you is help you ask the right questions. The answers need to come from those with more experience.


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Originally Posted By: DNickolaus
Looked up a Gast 1023. They do 10 cfm at low psi and 9 cfm at 10 psi. Rotary vane. And you can figure 14 amps for full 3/4 hp on 110 VAC. The current draw will drop if you run it low pressure, but still will cost. I agree it's oversized for such a small pond.


Unfortunately, these types of canned motors are not required by law to meet energy efficiency "mandates". 14 amps for a 3/4hp motor at 110V is really, really bad. Most of the decent 3/4hp canned motors will have a FLA (Full Load Amp) rating in the 10 amp range at 115V. A really good one will be in the low 6A range, but don't count on finding one in canned motor flavor.

I was looking at a 3/4hp shallow well pump. 12.2 Amps at 115V, Yikes!!! Then I saw one that was listed 5 Amps at 120V, and about fell off my chair laughing laugh When did the Chinese come up with single phase PMAC motor technology and able to offer a pump/motor combo thru a nation wide distributor for about 75 bucks wink

Electric motor voltage schemes for the USA are going to be 115/230/460VAC. That's because most places in the USA have power schemes of 120/240/480VAC.

Quite a bit to this electric motor stuff, but the vast majority of people are stuck with the offerings at hand and gravitate toward what is the least cost to initially purchase. Could be really expensive to operate tho.

Another thing with these canned motors (a motor made specifically for a product that won't fit anything else), is that they list the HP rating as what it consumes, not what it produces at the shaft, as it should be.

I got a close-up of a 3/4HP rig as stated with the stickers on it. Thought the calculator in my cell phone was on the fritz. Long story short, the HP rating is what it ate, not what it produced. Compressor motor was actually 1/3HP, not 3/4HP wink

Yep, kinda stuck with the offerings, but people make the best of it.


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