Note: Descriptions are shown in the official language in which they were submitted.
109304~
The present invention relates to a process for supply-
ing gas in varying quantities and to a gas compressor train in-
cluding a rotary compre sor which compresses gas to an intermedi-
ate degree followed by a reciprocating compressor compressing the
S gas from the rotary compressor to a final predetermined pressure.
The basic concept of compressing gas comprising a train
including a rotating air compressor for compressing to an inter-
mediate pressure followed by a reciprocating compressor for com-
pressing to a final higher pressure is old and well-known, see
for example, United States Patent No. 132,891, November 12, 1872.
This type of system i8 often advantageous where the quantity of
ga~ uch that a rotary compress~r cannot be used to raise the
gas to a required final pressure but where a reciprocating com-
pressor is necessary. Normally such applications require multi-
stage compressors.
However, this type of device in the prior art has re-
sulted in control methods for applications in which there is a
wide variation in the final discharge pressure. The present case
as opposed to that is for applications where the end use of the
~20 gas is at a substantially constant pressure but where the pres-
sure i9 controlled by the using process or at least controlled
by de~ices external to the compressor train itself. An example
of this is the wet air oxidation process which uses compressed
,' air at high pressure and in which the process itself fixes and
controls the pressure of the air supplied.
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This invention is particularly useful in applications
for gas compression for which a reciprocating compressor is re-
quired due to the combination of small capacity and high pres-
sures required for the end use. In the case of air compres-
sion, it is generally considered that the limit for rotarycompressors of the centrifugal or axial type is a minimum of
200 actual cubic feet per minute at the ~ charge. For smaller
actual discharge volumes reciprocating air compressors are
xequired.
Standard "of~-the-shelf" rotary air compressors, such
as turbine or other centrifugal compressors, are mass produced
and readily available for service in the range of 100 to 150
psig discharge pressure. On the other hand reciprocating com-
pressors for service greater than about 150 p~ig, particularly
in larger sizes, are generally custom made and are rather
expen~ive. In addition the lower pressure cylinders for multi-
stage machines, particularly for large capacity machines, are
very large, have many valves, and are expensive to maintain.
The use of a rotary compressor up to an intermediate pressure
of about 100 pounds eliminates these large, low pressure
cylinders in the reciprocating machine. The rotary compressor
itself is generally less expensive than the reciprocating
machine.
Acc~rding to the present invention there is provided
a gas compressor train in combination with a wet air oxidation
reactor, which comprises a rotary compressor adapted to compress
a gas to an intermediate pressure with means connecting the
outlet from said rotary compressor to the inlet of a recipro-
cating compressor adapted to compress the gas to a final pres-
sure, and means conneCting the outlet from the reciprocating
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compressor to the wet air oxidation reactor, and means operableto vary the discharge pressure of the rotary compressor and
thus to vary the mass flow of gas discharged from the xeci-
procating compressor to the wet air oxidation reactor, the
wet air oxidation reactor being adapted to be maintained at
a predetermined pressure.
According to the present invention the capacity of
a rotary reciprocating compressor train ~s controlled by oper-
ating the rotary compressor in a well known manner so as to
vary the discharge pressure of the rotary compressor, the
reciprocating compressor operating at substantially constant
speed, so that, since the actual inlet volume of the reci-
procating compressor is constant, the quantity of air in
standard cubic feet or weight taken into the reciprocating
compressor varies with the inlet pressure, thus varying
the dellvered capaclty o~ the entlre com-
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pression train. The discharge pressure of the rotary com-
pressor is, as stated, varied according to well known means,
for example, referring to "Compressed Air and Gas Data" by C.W.
Gibbs, Ingersoll-Rand Co., 1969-71, page 9-32, said pressure
canbe regulated by ~1) Qpeed control, (2) gas blowoff of ~3)
throttling the intake.
Reference is made to the accompanying drawing wherein;
Figure 1 is a diagram showing one embodiment of the
invention; and
Figure 2 shows an alternative structure.
In Figure 1 the reference numeral 10 indicates a
motor which drives, through any convenient means such as the
crank 12, a conventional reciprocatory piston and cylinder
compressor 14 having an outlet 16 ~P2) to a machine or process
18 such as ~or instance a wet air oxidation apparatus.
Connector 20 ~Pl) leads compressed gas into the top
of the compressor 14 from the rotary or turbine compressor
22 driven directly by a motor 24.
The centrifugal compressor 22 supplies gas at a vari-
able pressure to the cylinder of the reciprocatory compressorwhich is driven at a con~tant speed. The centrifugal compres-
sor is equipped with adjustable guide vanes which allow the
discharge pressure to be varied. If this discharge pressure
is increased more air will be taken into and consequently
discharged from the cylinder of the reciprocating compressor14.
If the discharge pressure from the centrifugal compressor is
decreas2d, the reverse of course occurs, that is less air will
discharge from the reciprocating compressor 14 to the prccess or apparatus
18. While controlllng the capacity of the entire tram ln the ~x~e manner,
the discharge pressure from the reciprocating compressor 14 ln the pipe 16
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remains constant. It ~ill be noted that the pressure ratio across
the reciprocating compressor will vary and it may be said that
this iB a disadvantage. It is true that the reciprocating com-
pressor must be designed for a certain maximum pressure ratio
which represents the lowest capacity of the compressor train. The
train and other parts of the reciprocating compressor must be
designed for this maximum pressure ratio but it is easier and more
economical to do this than to equip the compressor with other con-
trol methods such as clearance pockets, unloaders, or air blow off
control devices.
The rotary air compressor may be a centrifugal or axial
machine or could be one of the type known as a screw compressor.
The reciprocating compressor can be a single stage machine or a
multiple stage machine as required by the final pressure.
A preferred type of machine for the rotary compressor i~
the so-called "4-poster" packaged air compressor in which two or
more centrifugal wheels (most often four wheels, thus the name
"4-poster") are driven by a common bull gear. Such machines are
manufactured in quantity and are economical and readily available.
Such machines are designed to give a variable discharge pressure
and are thus ideal for practlcing this invention.
An actual installation is shown in diagram form in
Figure 2. A wet air oxidation plant is supplied with air by an
air compressor train. Two four-poster centrifugal air compressors,
A and B, supply air to a 4 stage reciprocating compressor C. The
discharge pressure, P2, of the reciprocating compressor is con-
trolled by the process at 2950 psig. The air intake to the
centrifugal compressors i9 atmospheric at approximately 14.7 psia.
The centri~ugal compressors are driven by electric motors at con-
stant speed. The actual inlet flow rate of the reciprocating
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machine is constant at 2342 actual cubic feet per minute. The
characteristics of the centrifugal machines are such that Pl can
be varied from 50 to 115 psig. The temperature at the discharge
of the centrifugal machine is 120F. Therefore, the capacity of
the compression train can be varied continuously from 705 to 1415
pounds of air per minute. Test operation of these machines has
confirmed that this is possible.
Although the Figures show electric motors driving all
machines it would be possible to use other prime movers such as
steam turbines, gas turbines, etc.
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