Pumps – Processes
Reexamination Certificate
2001-02-15
2003-12-09
Walberg, Teresa (Department: 3742)
Pumps
Processes
C417S282000
Reexamination Certificate
active
06659729
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a screw compressor for accommodating low pressure ratios and pressure variation and the operating method thereof in the case where the screw compressor is applied in a use for compressing gas of relatively high pressure to a constant discharge pressure or for compressing gas of which suction pressure varies from low to near discharge pressure to a constant discharge pressure, that is, in a use in which discharge pressure is constant and suction pressure varies but compression ratio is not large as in the case of a gas fuel compressor of gas turbine booster or a compressor for pressure feeding natural gas; and in the case where the screw compressor is applied in a use for pressure feeding gas to a container of large volume as in the case of pressure feeding gas to a spherical holder of city gas etc., that is, in a use in which discharge pressure varies from near inlet pressure to a predetermined discharge pressure.
2. Description of the Related Art
Variable displacement screw compressors have been used for refrigerators. In the case of a refrigerator, inlet pressure is determined according to the kind of refrigerant and the temperature at which the refrigerant is evaporated at the evaporator. That is, the inlet pressure is kept constant in accordance with the kind of use of the refrigerator but the pressure at the high pressure side of the refrigerating cycle varies according to the temperature and cooling ability of the cooling medium such as cooling water or cooling air which cools the compressed refrigerant gas to condense it at the condenser. Generally, for refrigerator a screw compressor of suitable designed-in internal volume ratio (built-in pressure ratio) is selected from among compressors of low, medium, and high built-in pressure ratio according to the conditions of operation. So, a compressor with a determined internal volume ratio must cope with a certain range of operation conditions, and the polytropic efficiency is maximum at a certain operation condition but decreases at another operation conditions.
There is a type of screw compressor of which the internal volume ratio is controlled automatically from low to high internal volume ratio in accordance with operation conditions. As such a screw compressor is generally provided with a capacity control mechanism, its construction is complicated, the control of the internal volume ratio is difficult, and high polytropic efficiency is difficult to be obtained.
In a screw compressor, the compression pressure P
2
in the groove space enclosed between meshing teeth, i.e. the pressure in the groove space enclosed between meshing teeth just before it is communicated with the discharge port is related with the inlet pressure P
s
and the designed-in internal volume ratio V
i
as shown in the following equation:
P
2
=P
s
×V
i
m
where m is polytropic exponent.
When the difference between said pressure P
2
and the discharge pressure P
d
of the screw compressor, i.e. the pressure at high pressure side of the refrigerating cycle is large, which means excessive or deficient compression, useless work is done, which reduces the polytropic efficiency. Therefore, the designed-in internal volume ratio of the compressor is selected or adjusted or controlled so that said pressure difference is within proper value.
FIG. 8
is a diagrammatic sketch for explaining the compression process of a screw compressor of general use in a refrigerator. In the figure, as a male rotor
12
and a female rotor(not shown) meshing with the male rotor
12
rotate, gas is sucked from an inlet port
15
into the groove space formed by the meshing tooth faces of the both rotors and the inner peripheral wall of a rotor casing
14
. The volume of the groove space increases as the rotors rotate, for the meshing line of the tooth faces moves toward the discharge side. When said volume becomes maximum, the communication of the groove space with the inlet port is shut, the groove space becomes enclosed, and the sucked gas is enclosed in the groove space.
As the rotors further rotate, the inlet suction side meshing line of tooth faces moves toward the discharge side to reduce the volume of the enclosed groove space to compress the gas therein. When the tooth tip
12
b
(in
FIG. 8
, only the tooth tip
12
b
of the male rotor is shown) reaches the beginning edge
17
c
of the cut-off part
17
b
at the discharge side end of a slide valve
17
(actually the beginning edge
17
c
is a beginning edge line parallel to the tooth tip
12
b
), the enclosed groove space communicates with a discharge port
16
, and the gas in the groove space is discharged as the rotors rotate. The internal volume ratio is the ratio of the maximum enclosed groove space volume versus the volume of the enclosed space volume just before the beginning of discharge.
The capacity control for varying the flow rate of gas through the screw compressor is effected by sliding the slide valve
17
which straddles the perimeters of the male rotor
12
and the female rotor(not shown) forming a part of the internal wall surface of the rotor casing
14
and is capable of being moved in the longitudinal direction of the rotors in a way it can not be moved further to the inlet side than the slide valve stopping face
19
. When the slide valve
17
is moved so that its right end
17
a
comes to the location shown by a chain double dashed line
17
a′
, a gap develops between the right end
17
a
and the stopping face
19
. As a result, the groove space is communicated with the inlet port
15
by way of a passage not shown communicating with the inlet port
15
. The beginning of compression which is when the groove space becomes enclosed by the shutoff of communication between the groove space and the inlet port
15
, becomes controlled by the right end
17
a
′ of the slide valve
17
.
Therefore, the farther the slide valve is moved to the left, the smaller the volume of groove space enclosed (hereafter referred to as the suction volume) and the flow rate of gas decreases.
As the beginning edge
17
c
of the cut-off part
17
b
at the discharge side end of a slide valve
17
moves to the left with the slide valve
17
, the timing the enclosed groove space communicates with the discharge port is retarded and the volume of the enclosed groove space just before it communicates with the discharge port (hereafter referred to as the discharge volume) becomes smaller than when full load, i.e. when the right end
17
a
of the slide valve
17
is contacting with the stopping wall
19
. As this decrease of the discharge volume is smaller than the decrease of the suction volume just after the slide valve
17
is moved to the left to depart from the stopping wall
19
, the internal volume ratio is varied. When the slide valve
17
is moved to the left by some extent, the discharge volume which is the volume of enclosed groove space just before it begins to communicate with the axial port formed on the end face of the bearing case
14
a
facing the discharge side end face of rotors before the cutout part of the slide valve
17
begins to communicates with the discharge port varies with about the same rate as the suction volume, and the internal volume ratio does not vary much by controlling capacity.
Recently, as the reliability and durability of a screw compressor is superior than that of a compressor of other type, a screw compressor is required which is able to be used in the field in which a reciprocating compressor or centrifugal blower such as compressor for pressure feeding city gas to a gas turbine, compressor for boosting up the natural gas, etc. has been used.
When a compressor is used for pressure feeding city gas to a gas turbine or for boosting up the natural gas, there may be the case the discharge pressure is constant and the inlet pressure is relatively high or changes largely during operation according to use.
For example, in the case the discharge pressure is 1.8 MpaA and the inlet pressure is 0
Hattori Toshiro
Takahashi Katsuyuki
Tanaka Kiyoshi
Crowell & Moring LLP
Fastovsky L
Mayekawa Mfg. Co., Ltd.
Walberg Teresa
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