Propeller

PROPELLER

-Hand- Propellers are either “Left Hand” or “Right Hand”. When the propeller is viewed from the blade edge or its side, the blade is angled upward to the left (Left Hand) or upward to the right (Right Hand).
See illustration.

-In 95% of industrial mixing applications, a left handed propeller is used in conjunction with the mixer shaft turning clockwise (viewed from the motor end). This creates a downward flow to produce optimal tank turnover or mixing. If the motor is wired to run counter clockwise the pumping direction is upward. In a limited number of applications, such as draft tubes, this is desirable. A right hand propeller performs just the opposite upward for clockwise rotation and downward for counter-clockwise rotation.

When selecting a propeller for mixing applications, there are several factors to consider as mention below Tank Configuration

-Propeller Diameter

-The Mixing Process Specific Gravity

-Power Requirements

-Viscosity

-Degree of Agitation

-Shaft Speed

-Viscosity

MARINE TYPE PROPELLER

-One of the best shapes for a mixing impeller is that of the 3-blade marine propeller mixing dates back more than 60 years. The propeller is still one of the most efficient mixing devices known in the industry. This impeller is quite sophisticated. It incorporates a complex blade shape that can only be readily manufactured as a casting. There lies its downfall. Cast marine propellers are heavy. They create a mechanically limited design, because of the large shafts and gear reducers necessary to support their weight.

-Fabricated impellers were developed for just this reason. Today the only place we see the marine propeller used on mixing equipment is the smallest portable mixer, where propeller diameters are in the 3″ – 6″ range. It produces high flow at minimal power consumption. Nq =0.5(FLOW NUMBER)

-Marine type propellers are the same as those used on ships. Usually they are driven at speeds above 190 RPM. The flow is primarily axial (discharge flow parallel to the agitator shaft) and are most effective in low viscosity fluids. The leading face can be flat or concave while the back side is convex.

-The pitch is on the basis of its being a section of a helix. Pitch is described as the advance per revolution. Almost all propellers today are square pitch where the pitch is equal to 1.0 times the diameter, or the impeller would advance a distance equal to it’s diameter in one revolution. Propellers having a pitch ratio of 0.7 are becoming more popular, due to the higher efficiency of the low blade angle.

-Propellers are characterized by high discharge capacity with low head. Another characteristic (although seldom mentioned) is the sensitivity to almost any change in viscosity. The propeller is an efficient pumping device when comparing propeller flow to horsepower, however, they are very costly in sizes over 12″ diameter.Propeller mixers are highly effective in blending, dispersion, solids suspension and heat transfer applications. Marine propellers can provide limited application.

-A mixer can be compared to an open centrifugal pump. At a given speed and propeller size, a fluid flow is produced and can be measured in GPM (Gallons per Minute) as a pumping rate or in terms of tank turnovers per minute.

-The pumping rate increases dramatically with the speed and propeller diameter. It is important to keep in mind that changing either of these factors dramatically affects your power or motor horsepower requirements.

 

High Speed VS. Low Speed ed

-Mixers are typically supplied with direct drive motors running at 1750 RPM (high speed) or gear driven versions with output shaft speeds of 170-420 RPM (slow speed). Small propellers (2″ to 8″ dia.) running at high speed are for light liquids in small batches (25-300 gallons) and for dispersions where a higher degree of shear is required to break up particles, crystals or other solids.

-Larger propellers (8″ and up) running at slower speeds are used when higher flow is necessary to the process, or the mixture is thick (viscous). This combination is also effective when there are solids in the solution that need to be suspended. When foaming is undesirable, or colloids are present, a larger prop rotating at slow speed will limit the problem. Effect of Specific Gravity (Density) and Viscosity

-Specific gravity and viscosity do not directly effect the propeller selections, rather the power required. Power requirements increase directly with specific gravity. Viscosity is resistance to flow. It affects power draw in the laminar region only. Generally, the more viscous the material becomes, the application requires larger propellers rotating at lower speeds.

DEGREE OF AGITATION

There are three general categories of mixing or agitation levels.

-Mild – A tank turnover rate of 1.5 per minute,

-Medium – Most common. 2 to 3 tank turnover per minute

-Violent – Over 4tank turnovers per minute.

-Pitch- This defines the angle of the blade and its relationship to the diameter of the propeller. For industrial applications either a square pitch or steep pitch (sometimes referred to as “super pitch”) is used. In terms of pitch ratios, the above are often referred to as 1.0 and 1.5 pitch respectively. Generally a 1.5 or steep pitch prop will produce 50 to 75% higher flow than a square pitch. However, it requires twice the horsepower for a given shaft speed.

APPLICATION

-In Marine Industries.

-In Dairy Technology.

-In Vehicles for cooling of Radiator.

-Laboratory Purpose.

-Liquid-Liquid mixing.

-In cooling tower

-In Manufacturing Process of NaCl

-In aero plane fan

LIMITATION

-Used for limited range of viscosity

-It generates high velocity ( fast motion in fluid) handling is difficult

FOUR BLADE MARINE TYPE PROPELLER

PRINCIPLEE

-This type of propeller works on the principle of axial-flow. It is basically a high speed impeller for liquids of low viscosity.

PURPOSE

-The purpose of this type of propeller is to better working efficiency than three bladed propeller. Also these can be designed for some special cases.

CONSTRUCTION

-Here two of the four blades placed in opposite sides are kept slightly downwards of the other two blades to enhance stirring. The four blades used are curved in nature. In this case the propeller attaches to the shaft and imparts energy to the fluid being pumped. There is no shroud to support the vanes. Pump efficiency is maintained by setting a close clearance between the propeller vanes and the volute or back plate. Since there is no shroud to strengthen the vanes, their use is often limited to small inexpensive pumps. For propeller materials we require a combination of a hard material to resist wear and a corrosion resistant material to insure long life. This is often a conflict in terms because when we heat treat a metal to get the hardness we need, we lose corrosion resistance. The softer metals can have corrosion resistance, but they lack the hardness we need for long wear life. The best materials that combine these features are called the “Duplex Metals”. These duplex materials are now in their second generation. They can be identified by letters and numbers such as Cd4MCu. Generally this marine type propeller uses stainless steel, aluminum, nibrel,etc. as the material of construction.

-A propeller shaft must be properly sized and supported, it is the. Too small of a diameter will cause shaft whip, vibrate and possibly shear, while an excessive diameter is waste of money and weight. The simplest rule is that the shaft should be at least 1/14 the propeller diameter.

WORKING

-The marine type of propeller works on the principle of axial flow.

-The axial flow design is optimised for relatively high volume flow to pressure ratios. Axial flow designs can have an operating life of some 40 years when operating in clean air, but blade life can be reduced to as many months in hostile operating environments despite being protected by hard coatings such as chromium. Where flow control is required, output can be varied while the propeller is running either by changing blade pitch or by altering the angle of a number of radial vanes mounted upstream.

-In this type of propeller they generate currents parallel with the axis of the propeller shaft and so are called axial flow type propellers. It actually gives a flow coming of the propellers of approximately 45o , and therefore have a recirculation pattern coming back towards the centre of the blades.

-Small propellers turn at full motor speed, either 1,150 or 1,750 rpm; larger one turns at 400 to 800 rpm. The direction of rotation is usually chosen to force the liquid downward, and the flow currents leaving the impeller continue until deflected by the floor of the vessel. The highly turbulent swirling column of liquid leaving the impeller entrains stagnant liquid as it moves along, probably considerably more than an equivalent column from a stationary nozzle would. The propeller blades vigorously cut or shear the liquid. Because of the persistence of the flow currents, propeller agitators are effective in very large vessels.

-This marine type of propeller is a revolving propeller which traces out a helix in the fluid, and if there were no slip between liquid and propeller, on full revolution would move the liquid longitudinally a fixed distance depending on the angle of inclination of the propeller blades and in this case the blades are inclined at an angle of 45. The ratio of this distance to the propeller diameter is known as the pitch of the propeller. A propeller with a pitch of 1.0 is said to have a square pitch.

-The four blade marine type propeller is used for special purposes. This type of propeller can exceed 18 in. in diameter as such they are found in 21 in. in diameter for special cases, regardless of the size of the vessel. In a deep tank two or more propellers may be mounted on the same shaft, usually directing the liquid in the same direction. These type of propellers are also available upto 50 in. in diameter.

ADVANTAGES

-In this type of impeller the quality of high speed and high power are combined.

-It also provide maximum speed with minimum cavitation problems. -These types of propellers can be constructed from a variety of materials such as manganese bronze, manganese bronze nickel, etc.

-These type of propellers are excellent in eliminating vibrations and noise ans still give the expected performance.

-We can obtain a high efficiency by using such type of propeller.

-Improved handling and control in rough water conditions and additional bow lift over three blade propellers can be achieved through this type of propeller.

-It gives smoother operation.

LIMITATIONS

-Sometimes, when the propeller blade cavitates, the pressure on part of the blade becomes so low that a near vacuum is formed. This happens more easily than one might think  atmospheric pressure is only 14.7 psi, not a very big number considering the size of a typical propeller and the thrust it is required to produce.

-If the suction on the low-pressure side of the propeller blade dips below ambient pressure atmospheric plus hydrostatic head then a vacuum cavity forms. (To be strictly correct, there is water vapor in the cavity, and the pressure is not a true vacuum, but equal to the vapor pressure of the water.)When these vacuum cavities collapse, water impacts on the blade surface with a local pressure singularity – that is, a point with theoretically infinite velocity and pressure.

-The effect can approximate that of hitting the blade with a hammer on each revolution. Cavitation is a major source of propeller damage, vibration, noise, and loss of performance. And although high- speed propellers are often designed to operate in a fully- cavitating (supercavitating) mode, problems associated with cavitation are frequently a limiting factor in propeller design and selection.

-They cannot be used for highly viscous liquids.

APPLICATIONS

-They are suitable for larger vessels such as: Fast ferries Cruise ships Container ships Cargo vessels Tankers naval vessels offshore vessels

-Such type of propellers are also used in rolls-royce.

-They are used for mixing of low to medium viscous liquids.

-They can handle stringy materials such as paper stock. Such type of propellers were used in Titanic.

-They are also used in jet engines, boats, etc.

-Sometimes such type of propellers are used in household table fans.

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