U.S. patent number 3,811,291 [Application Number 05/314,993] was granted by the patent office on 1974-05-21 for method of operating a refrigeration plant and a plant for performing the method.
This patent grant is currently assigned to Svenska Rotor Maskiner Aktiebolag. Invention is credited to Laurtiz Benedictus Schibbye.
United States Patent |
3,811,291 |
Schibbye |
May 21, 1974 |
METHOD OF OPERATING A REFRIGERATION PLANT AND A PLANT FOR
PERFORMING THE METHOD
Abstract
A refrigeration plant including an oil-injected compressor, an
oil separator, a condenser and an evaporator. The plant comprises
means to introduce liquid refrigerant to the compression phase of
the cycle ahead of the oil separator. The plant further comprises
means to adjust the amount of liquid refrigerant introduced in
response to the temperature difference between the condenser and
the oil separator, in order to keep the difference on a constant
level of 5.degree.C to 15.degree.C.
Inventors: |
Schibbye; Laurtiz Benedictus
(Algvagen, SW) |
Assignee: |
Svenska Rotor Maskiner
Aktiebolag (Nacka, SW)
|
Family
ID: |
10485338 |
Appl.
No.: |
05/314,993 |
Filed: |
December 14, 1972 |
Foreign Application Priority Data
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|
|
|
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Dec 28, 1971 [GB] |
|
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60274/71 |
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Current U.S.
Class: |
62/84; 62/468;
62/473; 62/192; 62/470 |
Current CPC
Class: |
F25B
1/047 (20130101); F04C 29/042 (20130101); F25B
31/008 (20130101) |
Current International
Class: |
F25B
1/04 (20060101); F25B 1/047 (20060101); F25B
31/00 (20060101); F04C 29/04 (20060101); F25b
043/02 () |
Field of
Search: |
;62/84,192,193,468,470,473 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wye; William J.
Attorney, Agent or Firm: Flynn & Frishauf
Claims
I claim:
1. A method of operating a refrigeration plant comprising: a
refrigerant flow circuit including a compressor of the screw rotor
type, a condenser and an evaporator; means for circulating oil and
for injecting said oil into the compression chambers of said
compressor; an oil separator provided in said circuit between the
outlet of said compressor and the inlet of said condenser; and
means for introducing liquid refrigerant into a circuit portion
between the inlet of the compressor and the inlet of said oil
separator;
comprising controlling the introduction of the liquid refrigerant
into said circuit portion such that the temperature in the oil
separator is kept at a level only slightly above the liquefaction
temperature of the refrigerant at the pressure prevailing in the
oil separator but is prevented from falling to said liquefaction
temperature.
2. A method as defined in claim 1, comprising keeping the
temperature in the oil separator at a temperature level which is
5.degree.C to 15.degree.C higher than said liquefaction
temperature.
3. A refrigeration plant comprising:
a refrigerant flow circuit including a compressor of the screw
rotor type, a condenser and an evaporator;
means for circulating oil and for injecting said oil into the
compression chambers of said compressor;
an oil separator provided in said circuit between the outlet of
said compressor and the inlet of said condenser;
means for introducing liquid refrigerant into a circuit portion
between the inlet of the compressor and the inlet of said oil
seperator;
means responsive to at least one parameter indicative of the
difference between the temperature in the oil separator and the
temperature in the condenser;
adjustable means for varying the quantity of liquid refrigerant
introduced into said circuit portion; and
means connecting said responsive means with said adjustable means
to control said adjustable means such that said temperature
difference is kept small but is prevented from dropping down to
zero.
4. A plant as defined in claim 3, comprising means for controlling
the capacity of the compressor, said capacity controlling means
being operatively connected to said adjustable means to reduce the
quantity of liquid refrigerant introduced during operation at
partial capacity.
5. A plant as defined in claim 3, wherein said responsive means
includes a temperature sensing device in each of the condenser and
the oil separator, said sensing devices being combined to produce a
signal representing said temperature difference, said connecting
means controlling said adjustable means as a function of said
signal to keep said difference substantially constant.
6. A plant as defined in claim 3 wherein said responsive means
includes a temperature sensing device and a pressure sensing device
coupled to said oil separator, said devices being connected to a
computer for producing a signal representing the difference between
the temperature in the oil separator and the liquefaction
temperature of the refrigerant at the pressure prevailing in the
oil separator.
7. A plant according to claim 3 wherein said connecting means
controls said adjustable means such that said temperature in the
oil separator is about 5.degree.C to 15.degree.C higher than the
temperature in said condenser.
8. A plant according to claim 3 wherein said liquid refrigerant is
introduced into the compression chambers of said compressor and
said adjustable means comprises a valve means coupled in the path
of said liquid refrigerant to said compressor, said valve means
being opened to increase the quantity of said liquid refrigerant
introduced into said compressor.
9. A plant according to claim 3 wherein said liquid refrigerant is
introduced into a discharge conduit of said compressor and
including return conduit means returning liquid refrigerant to said
condenser, said adjustable means comprising a valve means in said
return conduit, said valve means being closed to increase the
quantity of liquid refrigerant introduced into said discharge
conduit of said compressor.
10. A plant according to claim 4 wherein said connecting means
comprises a motor coupled to said adjustable means via a gear
mechanism, said capacity controlling means also being coupled to
said adjustable means via said gear mechanism.
11. A plant according to claim 4 wherein said controlling means
includes a cam arrangement operatively coupled to said adjustable
means.
12. A plant according to claim 5 wherein said connecting means
comprises a motor coupled to said adjustable means via a gear
mechanism.
13. A plant according to claim 6 wherein said temperature sensing
and pressure sensing devices are in said oil separator.
14. A method as defined in claim 1 comprising controlling the
introduction of said liquid refrigerant into the compression
chambers of said compressor.
15. A method according to claim 1 comprising controlling the
introduction of said liquid refrigerant into a discharge conduit of
said compressor.
Description
The present invention relates to refrigeration plants comprising a
refrigerant flow circuit including a compressor of the screw rotor
type, a condenser and an evaporator and more praticularly to such
plants which also include means for circulating oil and for passing
said oil through the compressor and having an oil separator
provided in said circuit between the outlet of the compressor and
the inlet of the condenser.
In refrigeration plants of this kind the oil is injected into the
compression chambers of the compressor in order to cool the gaseous
refrigerant during the compression and to seal the clearances at
the intermesh between the rotors and at the periphery and end
planes of the rotors. Further, in compressors having
non-synchronized rotors the oil also serves to lubricate the
rotors.
After having passed the compressor the oil has a rather high
temperature and therefore it must be cooled before it is again
injected into the compressor. Hitherto this cooling has usually
been effected in an oil cooler using for instance water as a
cooling agent. Due to the fact that there are large heat quantities
to be removed from the oil these oil coolers become rather bulky.
Further, when the refrigerant consists of ammonia the cooler must
be made from steel which renders it expensive to manufacture.
It has previously been suggested to use liquid refrigerant as a
cooling agent by introducing it into the compressor of a
refrigeration plant. Some of these known proposals represent
attempts to attain the desired cooling of the compressed gas
without using oil while other are intended to solve specific
problems in the operation of certain types of refrigerant
compressors. However, liquid refrigerant has very poor sealing and
lubricating properties and therefore the replacement of oil by
liquid refrigerant in screw rotor compressors has not been
successful.
The invention has for its object to provide a refrigeration plant
of the type described in the introductory paragraph of this
specification and further comprising means for introducing liquid
refrigerant into a circuit portion between the inlet of the
compressor and the inlet of the oil cooler, and a method of
operating such a plant. According to this method the introduction
of the liquid refrigerant is controlled such that the temperature
in the oil separator is kept at a level only slightly above the
liquefaction temperature of the refrigerant at the pressure
prevailing in the oil separator but is prevented from falling to
said liquefaction temperature.
SUMMARY OF THE INVENTION
The plant according to the invention is characterized by means
responsive to at least one parameter indicative of the difference
between the temperature in the oil separator and the temperature in
the condenser, adjustable means for varying the quantity of liquid
refrigerant introduced into said circuit portion, and means
connecting said responsive means with said adjustable means to
control said adjustable means such that the temperature difference
is kept small but is prevented from dropping down to zero.
The oil must be removed from the gas before the gas enters the
condenser because oil entrained by the gaseous refrigerant will
eventually accumulate in the evaporator. On the other hand, if
liquid refrigerant is separated from the gaseous refrigerant in the
oil separator together with the oil this will cause severe
disturbances and even failure of the whole plant due to evaporation
of the liquid refrigerant within the oil pump and in the bearings
of the screw rotor compressor which are pressure lubricated by part
of the oil passing through the pump. Due to the high operating
pressures of refrigerant compressors and the relatively small
distance between the rotor shafts the radial bearings must be of
the plain bearing type and therefore require perfect
lubrication.
For the above reasons the gas must be prevented from condensing in
the oil separator. On the other hand it is desirable to decrease
the temperature of the oil as much as possible in order to increase
the cooling effect of the oil when injected into the compressor.
The invention makes it possible to attain a desired low oil
temperature without the use of a separate oil cooler while
obviating the risk of condensing of the gaseous refrigerant within
the oil separator.
The invention will now be described more in detail with reference
to the accompanying drawing in which
FIG. 1 diagrammatically illustrates an embodiment of a
refrigeration plant according to the invention,
FIG. 2 is a detailed view of operating means included in the plant
according to FIG. 1,
FIG. 3 is a modification of FIG. 1 and
FIG. 4 is a further modification of the invention.
DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS
The refrigeration plant shown comprises a refrigerant flow circuit
which includes a compressor 10 of the screw rotor type, a condenser
12 and an evaporator 14. In the embodiment shown the condenser 12
is of the type in which the refrigerant passes through straight
tubes extending between headers at the ends of a preferably
cylindrical jacket which surrounds the tubes and through which
cooling water is circulated in a known manner. The evaporator 14 is
provided in a refrigerating chamber 16 and liquid refrigerant is
supplied to the evaporator through a conduit 18 and via an
expansion valve 20 which is controlled in a known manner in
response to the temperature in the refrigerating chamber 16.
The compressor 10 may be of substantially the same type as shown in
U.S. Pat. No. 3,432,089. Thus, oil from a supply conduit 22 is
injected into the compression chambers of the compressor for
sealing and cooling purposes and leaves the compressor together
with the vapourous refrigerant through a conduit 24 leading to an
oil separator 26 of suitable type. The oil-free high pressure
refrigerant vapour flows through a conduit 28 to the condenser 12
where it condenses while the oil is removed from the oil separator
26 by a pump 30 and delivered to the conduit 22 which forms the
discharge conduit of the pump.
According to the invention there is provided a conduit 32 between
the outlet of the condenser 12 and the compressor 10. In the
compressor 10 there is provided at least one nozzle which is
connected to said conduit 32 and located such as to inject liquid
refrigerant into the compression chambers of the compressor before
the pressure in said chambers has reached the discharge pressure of
the compressor. In this manner the pressure of the liquid
refrigerant will be sufficient for injecting the refrigerant into
the compression chambers against the pressure prevailing in said
chambers during this stage of the compression.
In the conduit 32 there is provided a metering valve 33 adjustable
by means of an operating device 34. This device 34 is controlled by
a thermocouple having one joint 35 located at the inlet of the oil
separator 26 and exposed to the temperature of the compressed fluid
entering the separator and a second joint 36 located within the
condenser 12. The operating device 34 contains means for
transforming the current produced by the thermocouple into
adjustment forces for actuating the valve 33 such that at
increasing temperature difference between the joints 35 and 36 the
valve 33 is actuated in an opening direction and at decreasing
temperature difference in a closing direction. In this manner it is
possible to maintain the temperature difference substantially
constant and at a value of about for instance 5.degree. to
15.degree.C. Thus, irrespective of the condensing temperature of
the gaseous refrigerant the temperature in the oil separator 26 is
slightly higher so that no liquefaction can take place in the
separator. Further, the temperature of the oil leaving the
separator through the conduit 22 is also always only slightly
higher than the condensing temperature resulting in the best
possible cooling effect when the oil is injected into the
compressor.
The capacity of the compressor 10 is variable by means of a slide
valve as described in U.S. Pat. No. 3,432,089, so that the
temperature in the refrigerating chamber 16 may automatically be
kept substantially constant in a known manner. In the embodiment
shown the slide valve is adjustable by means of a hydraulic
servomotor 38.
If the temperature in the refrigeration chamber 16 tends to sink
below the desired predetermined value the slide valve is moved to
decrease the capacity of the compressor. This reduction of the
capacity involves a reduction of the compression work and
consequently also of the demand for cooling. However, the speed of
the oil pump 30 is not changed and therefore the injected oil
quantity remains unchanged. Further, at partial capacity the
counter pressure in the compressor at the condensate injection
point is lower than the pressure at full capacity and therefore the
injected condensate quantity is at first increased. For these
reasons the discharge temperature of the compressor will at first
decrease before the pressure in the high pressure portion of the
cirquit has assumed the somewhat lower value which will be the
result of the reduced capacity of the compressor. It is evident
that in this way there is a risk that the temperature in the oil
separator 26 may temporarily sink down to the condensing
temperature.
In order to eliminate this risk the piston rod 40 of the
servo-motor 38 extends out through the end of the servo-motor
remote from the compressor. The projecting outer portion of the
piston rod carries a cam 42 cooperating with a cam follower 44
operatively connected to the operating device 34. When the cam 42
moves rightwards from the position shown in FIG. 1 and
corresponding to the full capacity position of the slide valve as
the slide valve is adjusted towards a partial capacity position the
cam follower 44 actuates the operating device 34 to bring this
device to adjust the metering valve 33 in a closing direction so
that the injected condensate quantity is decreased.
The cam 42 may be shaped such that for each position of the slide
valve the cam determines a preliminary specific setting of the
metering valve 33 in which the supply of condensate to the
compressor is sufficient to keep the temperature of the compressed
gas at a level higher than the liquefaction or condensing
temperature of the gaseous refrigerant at the pressure prevailing
in the high pressure portion of the circuit when the plant operates
under steady state condition at the actual position of the slide
valve. It is to be noted that the drawbacks associated with an
unnecessarily high temperature in the oil separator are smaller
than those resulting from condensing and therefore the metering
valve 33 is governed such that at all changes of the capacity of
the compressor the temperature in the oil separator is well above
the condensing temperature during the transition periods between
different compressor capacities. Once a steady state condition is
reached the thermocouple 35, 36 and the operating device 34
ascertain that the temperature in the oil separator 26 is only
5.degree. to 15.degree.C higher than the condensing
temperature.
The operating device 34 is shown in detail but diagramatically in
FIG. 2. According to this Figure the two conduits 50 from the
thermocouple 35, 36 are connected to a control device 52 which is
adapted to supply current to an electric motor 54 to drive this
motor in either direction in response to signals received from the
thermocouple. The shaft 56 of the motor carries a worm 58 engaging
a worm wheel 60 mounted on a shaft 62 perpendicular to the motor
shaft 56. Also mounted on the shaft 62 is a spur gear 64 which is
drivingly connected to the worm wheel 60 via a slip coupling (not
shown) and meshes with a rack 66. This rack 66 is adapted to
actuate the metering valve 33.
At its upper end the cam follower 44 is provided with a head member
68 having a longitudinal slot 70 slidingly receiving a pin 72
mounted at the lower end of the rack 66.
In the Figures the cam 42 is shown in a position corresponding to
full capacity of the compressor 10. In this position the pin 72 is
located at a small distance from the lower end of the slot so that
the rack 66 is free to move in the slot 70 in both directions in
response to signals from the thermocouple 35, 36 for fine
adjustment of the metering valve 33 during the steady state
condition.
When the slide valve of the compressor is adjusted rightward
towards a partial capacity position the cam follower 44 is raised
and pushes the rack 66 upwardly. Such movement of the rack 66 is
possible due to the action of the slip coupling between the spur
gear 64 and the worm wheel 60.
When the rack 66 moves upwardly the metering valve 33 is
successively throttled so that the quantity of injected liquid
refrigerant is decreased substantially in proportion to the
decreasing compression work of the compressor. In this manner the
temperature of the compressed fluid is prevented from falling down
to the condensation temperature during the adjustment movement of
the slide valve of the compressor. When the temperature in the
refrigerating chamber 16 approaches the desired predetermined value
the slide valve is slowed down and stopped and the thermocouple 35,
36 takes the control of metering valve 33 for fine adjustment of
the temperature in the oil separator in relation to the temperature
in the condenser. During steady state conditions at the partial
load thus attained, the pin 72 is located near the lower end of the
slot 70.
When later on the temperature in the refrigerating chamber 16 tends
to rise the slide valve is caused to move towards the full capacity
position and the cam follower 44 is pulled downwardly. This
downward movement takes place without the rack 66 being actuated
because the slot 70 can move downwardly relatively to the pin 72.
Therefore, the metering valve 33 remains in its partly throttled
position until the thermocouple 35, 36 responds to the increasing
temperature and causes the motor 54 to actuate the valve 33 in an
opening direction.
In FIG. 3 is illustrated a modification of the control device just
described. According to this modification the liquid refrigerant is
injected into the discharge conduit 24 of the compressor where the
pressure is somewhat higher than the pressure in the condenser 12.
For this reason there is provided a pump 74 to which the liquid
refrigerant from the condenser is supplied through a conduit 32A.
The pump 74 is of the type having a non-return valve in its outlet
and means for by-passing a variable liquid quantity which is
returned to the condenser 12 through a conduit 76. In this conduit
76 is provided a valve 33A adapted to be operated by a device
similar to the device 34 described above. However, in order to
decrease the injected refrigerant quantity the valve 33A is
actuated in an opening direction, the maximum quantity being
attained when the valve 33A is fully closed.
As shown in FIG. 4, since the liquefaction or condensing
temperature of a gas varies with its pressure the thermocouple 35,
36 may be replaced by a temperature sensing device 80 and a
pressure sensing device 81 provided in the oil separator 26. These
devices may be connected to a computer 82 adapted to produce a
signal representing the difference between the temperature in the
oil separator and the liquefaction temperature of the refrigerant
at the pressure prevailing in the oil separator, this signal being
used to control the supply of cooling liquid refrigerant.
* * * * *