Screw rotor type wet vacuum pump

Details Screw rotor type wet vacuum pump Screw rotor type wet vacuum pump

2000-09-22

2002-04-23

Vrablik, John J. (Department: 3748)

Rotary expansible chamber devices

Heat exchange or non-working fluid lubricating or sealing

With condition responsive control of non-working fluid

C418S087000, C418S097000, C418S100000, C418S201100, C418S201200

Reexamination Certificate

active

06375443

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a positive displacement vacuum pump, more particularly to a screw rotor type wet vacuum pump which can draw in by itself a sealing water supplied from a suction side. The wet vacuum pump can prevent direct contact of a pump casing with rotors due to thermal expansion caused by heat generated during an adiabatic compression step of the pump. The adiabatic compression step reduces an energy for driving the rotors.
BACKGROUND OF THE INVENTION
A screw rotor type vacuum pump has been used for many applications in various fields such as gas vacuuming, gas suction, cleaning, and pneumatic conveying of powder, particles, and viscous materials.
FIG. 8
is a general illustration showing a sludge stripping unit which is an application example of the vacuum pump. In the sludge stripping unit, a sludge collection hopper tank
1
receives an end of a sludge suction pipe
2
. The suction pipe
2
has a flange
3
positioned outside the hopper tank
1
. The flange
3
is connected to a hose
4
for drawing in the sludge. The hopper tank
1
has a top wall provided with a conduit
7
communicating with an inner pipe
6
of a separator
5
. The separator
5
has an air duct
8
positioned at an upper portion thereof. The air duct
8
is connected to a suction inlet of a vacuum pump A. A discharge side portion of the vacuum pump A is connected to an exhaust pipe
10
via a silencer
9
.
Operating the vacuum pump A reduces the inner pressure of the separator
5
and the hopper tank
1
. A worker puts the leading end of the sludge suction pipe
2
on the sludge, so that an air is drawn in together with the sludge through the sludge suction pipe
2
into the hopper tank
1
. The sludge hits a top wall of the hopper tank
1
to be splashed backward, which allows a primary separation of the sludge and the drawn-in air.
The sludge having a comparatively large specific gravity falls to accumulate on a bottom wall of the hopper tank
1
, while the air passes through the conduit
7
to flow downward in the inner pipe
6
of the separator
5
. Then, the air passes through a liquid filled in the separator
5
, allowing a secondary separation of the sludge and the air.
That is, the sludge included in the air is captured by the liquid, and only the air flows upward through a space outside of the inner pipe
6
into the air duct
8
.
The air that has flown into the air duct
8
is drawn into the vacuum pump A, and then the air is discharged from a discharge port of the vacuum pump A into the silencer
9
. Finally, the air is discharged in the atmosphere from the silencer
9
through the exhaust pipe
10
.
When the sludge is stripped by means of the vacuum pump as described above, small amounts of entrained matters such as dust and pebbles still remain in the air even after the secondary separation. There is a fear of a damage of a sealing portion of the vacuum pump which draws in the air.
The screw rotor type vacuum pump A has a construction as illustrated in a longitudinal sectional view of FIG.
6
. The pump has a housing
11
consisting of a main housing
12
having an inner cylinder
12
a
, a gear housing
13
closing a right end portion of the inner cylinder
12
a
, and a side cap
14
closing a left end potion of the inner cylinder
12
a.
The main housing
12
is provided with a suction port
15
communicating with the inner cylinder
12
a
, and the side cap
14
is provided with a discharge port
16
communicating with the inner cylinder
12
a.
The housing
11
accommodates a pair of screw rotors
17
(one of which is illustrated in
FIG. 6
) each consisting of a screw portion
17
a
and a shaft portion
17
b
provided at each end of the screw portion
17
a
. The screw portions
17
a
are of a Quimby type. The screw portion
17
a
has a normal section envelop consisting of a circular arc and a quasi-Alchimedean spiral curve.
The shaft portion
17
b
is rotatively supported by a fixed bearing
18
provided in the side cap
14
and by an expansion side bearing
19
provided in the main housing
12
.
By sealing lines provided by the engagement of the pair of the screw portions
17
a
of the screw rotors
17
and the inner cylinder
12
a
of the main housing
12
, there is defined an enclosed chamber
20
.
By means of a pair of gears
21
each secured on each shaft portion
17
b
, the pair of screw rotors
17
rotate in opposite directions at the same speed as each other. Thereby, a fluid is drawn in from the suction port
15
of the main housing
12
into the enclosed chamber
20
, and then the fluid is discharged from the discharge port
16
when the enclosed chamber
20
has moved to communicate with the discharge port
16
.
For reducing the driving force of the vacuum pump, there is provided a discharge outlet
24
(see
FIG. 2
) for adjusting the open degree of the discharge port
16
to compress the drawn-in fluid at a compression rate of about 1/1.6 before discharging it.
FIG. 7
is a graph showing a relationship between a pressure (P) and a volume (V). The pressure is indicated by a vertical coordinate, and the volume is indicated by a horizontal coordinate. A one-step Roots vacuum pump and a screw rotor type vacuum pump with no adiabatic compression step each have an energy shown by a square area defined by points A, B, C, and D. Meanwhile, a screw rotor type vacuum pump with an adiabatic compression step has an energy shown by a semi-square area defined by points A, B, E, and D, which saves an energy shown by a diagonally shaded area of &Dgr;E.
The vacuum pump with an adiabatic compression step has to be a wet type pump to prevent direct contact of the screw rotor
17
and the housing
11
due to thermal expansion by heat generated during the adiabatic compression. The wet type pump generally draws in a sealing water by a vacuum generated in a suction port of the pump.
However, the water drawn in from a suction port
15
flows along a screw channel in an axial direction of the pump, and the water under pressure hits a discharge side shaft sealing portion
22
(see
FIG. 6
) like a water jet.
As, illustrated in
FIG. 9
, the shaft sealing portion
22
receives an increased force by the water pressure exerted thereon. The increased force produces no adverse effect for a service life of the shaft sealing portion
22
, when the water is clean and includes no entrained matter such as dust or pebbles.
Meanwhile, when the water includes an entrained matter such as dust or pebbles, the shaft sealing portion
22
will have a reduced service life. If the shaft sealing portion
22
suffers a damage, the sealing water leaks into the fixed bearing
18
adjacent to the shaft sealing portion
22
, which causes a breakage of lubrication of a grease fed in the fixed bearing
18
. In addition, the deposition of the entrained matter such as dust or pebbles on the fixed bearing
18
tends to cause a damage of the fixed bearing
18
.
To prevent the damage of the fixed bearing
18
, a method is proposed, in which a slinger (or flinger)
23
is mounted on the shaft portion
17
b
as illustrated in FIG.
10
. The slinger
23
rotates together with the screw rotor
17
to throw away a water leaked from the shaft sealing portion
22
to discharge it outside of the housing
11
. Nevertheless, the method has the disadvantages that the discharged water makes an area surrounding the pump dirty and that a shortage of the sealing water occurs when a circulation system is applied for the water to save it.
In the present invention, when the suction pressure of a fluid is between a normal atmospheric pressure and −380 Hg, the fluid pressure of the discharge side becomes higher than the normal atmospheric pressure because, the fluid is compressed at a compression rate of about ½ in the discharge side. Meanwhile, when the suction pressure of the fluid is lower than −380 mmHg, the fluid pressure of the discharge side becomes lower than the normal atmospheric pressure. This lower discharge side pressure provides no leak of the sealing water from the shaft se

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