Patent Application
20090191083
1. Field of the Invention
The present invention relates generally to the field of mechanical vacuum pumps and blowers used in industrial and municipal applications, and more particularly relates to double rotor multi-lobe type blowers commonly known as positive displacement rotary blowers or simply the Roots blowers, and more specifically relates to an isothermal air jacket for equalizing the casing temperature distribution of a rotary blower.
2. Description of the Prior Art
Rotary blowers are widely used in industrial and municipal applications such as for loading a bulk truck by pneumatic conveying bulk materials, for cleaning municipal sewer lines by vacuum suction or for generating air bubbles in an aeration tank of a modern waste water treatment plant. They are desired because of their unique performance characteristics inherent from the rotary positive displacement nature: they deliver an almost constant air flow at varying pressure levels. The ability of varying pressure at constant flow makes rotary blowers the ideal tools for pneumatic conveying applications where material clogging can be quickly cleared with increasing the discharge pressure or suction vacuum while maintaining the transfer capability.
Rotary blowers are typically operated with a discharge pressure range between 5 to 20 psig, or an inlet vacuum range between 5 to 17″ Hg in a single stage compression. The high limit is equivalent to a compression ratio about 2.5:1, defined as a ratio of the absolute discharge pressure over the absolute inlet pressure. Compression ratio is often preferred because it correlates to the temperature ratio, which stays the same no matter whether it is a pressure or a vacuum application.
Under this compression ratio, the temperature rise from blower inlet to outlet could be as high as 300-400 F depending on efficiency and internal clearances. This temperature difference causes great thermal distortion for the rotary blower main casing, often called the cylinder, resulting in a “banana shape” with the outlet side (hot side) bowing towards the inlet side (cool side), as illustrated in
However, while the cylinder is “banana shaped” in hot condition, the rotors remain its original straight shape and uniform temperature, because they continuously experience the cyclic cool and hot air temperature during each rotation. This condition creates an uneven internal clearance at rotor tips and rotor ends between rotor and casing. Some clearance is increased from the cold state, say near discharge side, causing more internal leakage while other clearance is decreased, posing potential rubbing and seizure failures. The later scenario often forces the design clearances set to be larger than necessary to avoid any potential contact. The result is more leakage flow and the recycled hot leakage gas raises the inlet temperature further more, further increases the discharge temperature, which is already high due to higher compression ratio.
Various approaches have been developed to address the higher temperature and associated thermal distortion problems at high compression ratio. U.S. Pat. No. 6,817,844 disclosed a method for cooling the oil reservoir of a blower, as illustrated in
However, it fails to correct the root cause of the problem: the high temperature difference between inlet port and outlet port of the cylinder, which remains the “banana shaped”. This failure could still lead to more internal leakage, high vibration and noise, shortened bearing life at the highest pressure ratio and create reliability problems.
Accordingly, it is always desirable to provide a new design and construction of rotary blowers that can effectively deliver the desired pressure with lower vibration, noise and less leakage than conventional rotary blowers, so that they can operate more reliably, quietly, efficiently and has a longer life.
The present invention is directed to a rotary blower that utilizes an isothermal air jacket with flow passages arranged on the outer surface of the blower, directing cooling flow in a general direction from the bearing housing to the cylinder so that whole casing temperature tends to be more uniform. Integral fans mounted on rotors own shafts provide the air source as part of the isothermal air jacket.
It is an object of the present invention to provide a new and unique design and construction of a rotary blower with an isothermal air jacket to effectively reduce the temperature difference and thermal distortion between the casing inlet port and outlet port.
It is also an object of the present invention to provide a new and unique design and construction of a rotary blower with an isothermal air jacket that tends to have a more uniform clearance distribution and smaller internal clearances to increase the blower efficiency.
It is also an object of the present invention to provide a new and unique design and construction of a rotary blower with an isothermal air jacket to have less vibration, lower noise and longer blower life.
Although specific embodiments of the present invention will now be described with reference to the drawings, it should be understood that such embodiments are by way of example only and merely illustrative of but a small number of the many possible specific embodiments which can represent applications of the principles of the present invention. Various changes and modifications obvious to one skilled in the art to which the present invention pertains are deemed to be within the spirit, scope and contemplation of the present invention as further defined in the appended claims.
As a brief introduction, the present invention rotary blower with an isothermal air jacket includes an inner enclosed casing having a flow suction port and a flow discharge port and an internal bearing support structure, and an outer isothermal air jacket. The isothermal air jacket is shaped to surround the blower outer body, but is oversized to provide adequate flow passages from a fan inlet opening near the bearing housing to flow over most of the blower outer casing surfaces.
The present invention rotary blower with an isothermal air jacket also includes two parallel multi-lobe rotors mounted on two parallel rotor shafts respectively, where the rotor shafts are supported by an internal bearing support structure of the inner casing and interconnected through a set of timing gears to rotate the rotors in synchronization for propelling flow from the suction port to the discharge port. Cooling fans are mounted on rotor own shafts at locations adjacent to the inlet openings of the air jacket for circulating flow through flow channels between the outer cover and the blower outer skin with a general direction from the oil reservoir to the outlet port and then to the inlet port for the final exit.
It is therefore an object of the present invention to teach the construction of an isothermal air jacket for reducing the temperature difference between the inlet port and outlet port. More specifically, the isothermal air jacket of the present invention is an apparatus constructed to engage in contact with a rotary blower so that it could cool the blower oil reservior and its outlet port casing and heat the inlet port casing at the same time. The present invention rotary blower with an isothermal air jacket is capable of delivering the desired pressure with lower vibration, noise and less leakage than conventional rotary blowers, so that they can operate more reliably, quietly efficiently and has a longer bearing life.
Referring to
As an important novel and unique feature of the present invention, an isothermal air jacket apparatus is surrounding the rotary blower 10 of the present invention, and its cross-section is illustrated in
When the rotary blower 10 is equipped by the isothermal air jacket apparatus 50 of the present invention, there exists both a reduction in temperature differences of the cylinder 20 between the inlet port 36 and the outlet port 38 as well as a reduction in temperature of the mechanical components such as the bearings 24 and the seals 26.
The theory of operation underlying the isothermal air jacket apparatus 50 of the present invention is as follows. With the fan 40 generating an air flow to cool the hot outer surface of the oil reservoir and hot surface of the outlet port 38, the raised temperature of air flow is then directed to heat the cold surface of the inlet port 36, hence tend to equalize temperature differences between the inlet and outlet ports. This will lead to less thermal distortion of the cylinder 20, which in turn will decrease the internal end clearance and tip clearance, and at the same time increase life expectancy of the bearing 24 and the seal 26 and improve the blower 10 reliability.
It is apparent that there has been provided in accordance with the present invention a rotary blower with an isothermal air jacket for effectively reducing the high temperature differences caused by high compression ratio in a rotary blower. While the present invention has been described in context of the specific embodiments thereof, other alternatives, modifications, and variations will become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations as fall within the broad scope of the appended claims.
