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Frequently Asked Questions - Aeration
  • I have a Roots AF-series blower that is obsolete. Will the current URAI-series Roots blowers interchange directly with the AF’s?
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    • Yes, but with some qualifications. If the AF and URAI have the same frame number, ie. 36AF and 36URAI, the URAI will duplicate the performance of the AF turning at the same speed. AF frame sizes 44 & 55 are the only sizes that require a V-belt drive change on the new replacement to duplicate AF performance. For these models, you MUST provide us with the following data in order for us to correctly size a V-drive for the replacement blower:
      1. Motor horsepower and rpm.

      2. Diameter of motor sheave and blower sheave.

      3. The part number of the v-belt currently being used. Regardless of whether a new drive is required, you will have to change the blower sheave bushing because the shafts on the AF series were all non-standard sizes.
      The feet on the URAI units match the same pattern used on the AF’s with the same frame number. If the pipe connection size is different, the new blower has the larger size and a bushing can be used to connect to existing piping.
  • I am going to purchase a blower package for installation in an area where blower noise must be minimized. Which package design is best for this situation?
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    • Here are several suggestions for reducing noise on any new or existing blower package:
      1. A larger blower running slow will produce the same amount of air as a smaller blower running fast but will generally operate at a lower decibel level. Our computer program can provide us with a quick comparison of operating sound levels for different size blowers. Just tell us the CFM and PSI you require.

      2. Install a discharge silencer plus and inlet silencer in addition to the air filter/silencer.

      3. Blower packages can be supplied with enclosures lined with sound-proofing material. This will help reduce noise emitted from the gearbox on a positive displacement blower in addition to the inlet and discharge ports.

      4. Positive displacement blowers are efficient but can be noisier than centrifugal blowers. Ask us about the availability of centrifugal blower models with the CFM and PSI output you require.
  • What kind of oil and how much do I put in the gear case on my positive displacement blower?
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    • The instruction manuals for Sutorbilt Legend, M-D Pneumatics Competitor Plus and Roots URAI blowers (the three brands offered by USABlueBook) all contain information regarding recommended lubricants. If you use regular straight weight, non-detergent petroleum oil, different viscosities are called out for different ambient operating temperatures. The simplest approach is to use synthetic oil that has been specifically selected for use with these series of blowers. One grade of oil covers every operating temperature range for which these blowers have been designed. It flows well at very cold temperatures and does not break down at very high temperatures. The extra cost of using synthetic is minimal considering that most of the gear cases use less than a quart.
      The amount of oil held in a gear case will vary with the mounting position of a blower. If blower shafts are in a horizontal plane, more oil will be required than if the shafts are in a vertical plane. Each gear case has multiple drain plugs for satisfying every mounting position and one breather cap is always included. In any position one drain plug will always be on the bottom and the breather plug should always be installed in the top. The drain plug on the side of the gear case closest to the bottom drain plug is generally the fill level plug, but always read your instruction manual to confirm its precise location. Remember, only the lower portion of the gears (lower gear in the case of a vertical mount) are submersed in oil, which is splashed throughout the gear case as the gears turn. Overfilling will result in excessive heat buildup due to the resistance of the oil to gear rotation.
  • My plant needs more air. Can I speed up my Sutorbilt 4M blower? I have a 7.5 hp motor.
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    • Possibly, but we need to know where the blower is operating within its design limits at this point in time. All of the Sutorbilt, Roots and M-D blowers commonly used in wastewater treatment operate within specific RPM and pressure ranges. With each revolution, a positive displacement blower takes in a given volume of air at a given altitude. The faster the blower turns the more air is moved. We can operate a 4M blower at 2500 RPM with a 3 hp motor or a 15 hp motor, and it will produce the same amount of air at atmospheric pressure. The actual amount of horsepower we need depends on the amount of backpressure at the blower discharge. Several conditions create backpressure in a blower system, some of which include:
      1. Each 2.31" of water over your diffusers creates 1 psi of backpressure.

      2. A rule of thumb is to add 0.5 psi backpressure if you have inlet and outlet silencers.

      3. The size of your air piping and the number of tees and elbows can make a difference if your pipe is small in relation to the volume of air being moved.

      4. Diffuser membranes may require 0.3 psi to open and function properly.

      5. Not having enough diffusers to utilize excess air can cause backpressure to rise.

      6. Dirty air filters can add load to a blower.
      Total backpressure will determine how big a motor is required for operating a blower at a given RPM (CFM output). There is no substitute for having a good pressure gauge in your blower discharge line, which takes much of the guesswork out of trying to calculate total pressure losses. Before you call us, we strongly recommend that you install a gauge in your discharge line to measure actual system pressure. Additionally, using an ammeter to measure actual motor amp draw vs. maximum motor nameplate rating will let you know how much reserve capacity you have with your existing motor.
      We also need to know your existing sheave sizes so that we can determine exactly how fast the blower is turning in relation to its specified operating range. As a general rule we like to limit maximum blower rpm to 80% of maximum rated capacity if they are running long hours on a daily basis.
      Weighing all factors we can provide a reasonable estimate of how much extra capacity we can realize by increasing blower rpm with a sheave change or upgrading motor horsepower.
  • Why do I need diffusers when I can simply drill holes in a length of pipe?
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    • There are several reasons why you should use diffusers, especially in light of their relatively low cost.
      1. Diffusers are designed to help keep the contents of your tank from entering your air supply lines and clogging them up when the blower is not operating. The membranes on coarse and fine bubble diffusers contract and serve as a check valve when air pressure is not applied internally.

      2. Diffusers help balance air flow because the airflow from an engineered diffuser is both predictable and consistent from one to the next. Calculating actual airflow from a length of pipe with drilled holes would be a wild guess at best, and equalizing air discharge over the full length cannot be assured.

      3. Diffusers generate smaller bubbles than those coming from holes drilled in pipe. A large number of small bubbles will transfer far more oxygen than a small number of large bubbles due to the increase in total bubble surface area. Therefore, when using diffusers you can achieve a desired amount of oxygen transfer with less air, and this equates to lower blower operating speeds and lower energy costs. This is especially true if you incorporate fine bubble diffusers into your system.
      If you need help laying out a diffuser system for your plant, contact USABlueBook Tech Support. We will provide a single page questionnaire for you to fill out and return for a complete analysis at no cost to you.
  • I need 150 CFM at 6 psi at my tank, but my tank is 300 ft away from my blower. What size blower do I need?
    View answer
    • A lot depends on your pipe sizes. Airflow through pipe is subject to friction losses just like water. For example, friction loss of 150 CFM flowing through 300’ of 3" pipe is around .054" of H20 per foot. This amounts to a loss of 16.2" H2O over 300'. This is equivalent to 0.60 psi of backpressure, which isn't much. However, if you use 2" pipe, the friction loss is around .34" of H2O per foot. This amounts to 102" H2O over 300' or 3.7 psi of backpressure, which is significant.
      The extra cost of using larger diameter pipe will be far less than the cost of operating a blower at 9.7 psi (6 + 3.7) versus 6.60 psi (6 + 0.60). To put this in perspective, if you were using an M-D 4005 or Roots 45 blower, operating at the higher pressure would require an additional 4 brake horsepower. USABlueBook can provide charts that show friction loss of air in tubing for different pipe sizes for different airflow rates.
  • What kind of blower should I consider – centrifugal or positive displacement?
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    • Positive displacement and centrifugal blowers are found in wastewater treatment plants throughout the country. Both types have advantages and disadvantages, several of which are listed below:
      1. PD blowers operate very efficiently because there is minimal slippage around the lobes. Whatever air goes in comes out. Centrifugal blowers are subject to more internal slippage around the rotor just like a centrifugal water pump, which makes them somewhat less efficient. Therefore, a larger motor may be required than with a PD blower to achieve the same performance.

      2. PD blowers are capable of high pressures up to 15 psi at both low and high flow rates. There are very few centrifugal blowers available that are designed to operate at flow rates below 400 CFM at pressures up to 8+ psi, while this performance can be achieved easily with various models of PD blowers. USABlueBook does offer several models of Siemens 2-stage blowers that cover much of this range.

      3. PD blowers are readily available in emergency situations. The M-D Competitor Plus, Roots URAI and Sutorbilt Legend series blowers, all of which are available from USABlueBook, are functional and dimensional drop-in replacements for one another.

      4. PD blowers can be noisier than centrifugals, even when equipped with silencers, because of gear noise and pulsating airflow. Most centrifugal blowers do not utilize a gear case and airflow is smooth, which translates to lower noise levels. This is a major reason for considering a centrifugal blower.

      5. PD blowers are generally belt driven. This is a plus in that blower speed can be varied to adjust blower output by changing the ratio of the V-belt sheaves. In addition, standard, off-the-shelf motors can be utilized. Centrifugal blowers with outputs less than 400 CFM are generally direct drive. Direct drive centrifugal blowers are very compact, quiet and competitively priced, especially in low-pressure (<4 psi) applications. However, direct drive centrifugal blowers have special motors that are often not economical to replace or service in the field. USABlueBook has two models of belt driven centrifugal blowers for applications up to 250 CFM at 9 psi.
  • My belts are banded together, how do I find them in the book?
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    • There are several different sizes of belts; USABB only carries a small portion of the popular sizes. We have access to almost every size made. Call us with your information and we will provide a quote. In cases that the belts are banded together the part number begins with the number of belts banded together. For instances you have 3 belts banded together the part number would have 3/ in front of the belt number. 3/3VX530.
  • What information do I need to order a blower package?
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    • You will need to provide us with the CFM you are looking for, the systems PSI (how deep are the diffusers), and the voltage requirements. We will run the numbers and suggest a package that will fit your needs.