US3508416A

US3508416A - Method of and apparatus for controlling a refrigeration machine

Info

Publication number: US3508416A
Application number: US698552A
Authority: US
Inventors: Charlie D Miller
Original assignee: CHARLIE D MILLER
Current assignee: CHARLIE D MILLER
Priority date: 1968-01-17
Filing date: 1968-01-17
Publication date: 1970-04-28
Anticipated expiration: 1987-04-28
Also published as: JPS4844425B1

Description

an. MILLER REFRIGERATION MACHINE Filed Jan. 17, 1968 April 28, 1970 METHOD OF AND APPARATUS FOR CONTROLLING A INVENTOR. CHARLIE D. MILLER. BY Z ATTORNEY.

United States Patent Office 3,508,416 Patented Apr. 28, 1970 3,508,416 METHOD OF AND APPARATUS FOR CON- TROLLING A REFRIGERATION MACHINE Charlie D. Miller, 141 Mosley Drive, Syracuse, N.Y. 13206 Filed Jan. 17, 1968, Ser. No. 698,552 Int. Cl. F25b 43/02 US. Cl. 62--84 6 Claims ABSTRACT OF THE DISCLOSURE A control for regulating the operation of a capacity control device employed with centrifugal refrigeration machines. The control includes means responsive to the pressure developed by the lubricant pump for regulating the operation of the capacity control system of the machine to either reduce or eliminate foaming of refrigerant in the lubricant sump.

This invention relates generally to refrigeration machinery. More particularly, the invention relates to refrigeration machinery employing a centrifugal refrigerant compressor equipped with guide vanes for varying the capacity of the machine. Still more particularly, the invention pertains to a control system for use with equipment of the kind described in which the control system incorporates means for governing the operation of the machine to avoid excessive foaming of refrigerant in the lubrication sump.

BACKGROUND OF THE INVENTION In the operation of relatively large refrigeration machines having a capacity in range of 100-350 tons and employing electric motor driven centrifugal refrigerant gas compressors, problems are encountered in assuring adequate lubrication of the moving parts and the co rresponding stationary parts that support the moving parts such as bearings. A major cause of these problems is the rapid vaporization of refrigerant, that has migrated from the refrigerant flow circuit into the lubricant sump and mixed in the liquid state with the lubricant, as the pressure varies in the sump due to changes in machine operating conditions. In some instances the vaporization of the admixed refrigerant results in insufficient lubricant passing to the lubrication circuit by the lubricant pump.

The lubricant sump is in communication with the low pressure side of the machine in order that the vaporized refrigerant may return to the refrigerant flow circuit. Accordingly, the vaporization of the refrigerant in the sump responds to the presusre fluctuations in the low pressure side of the system and the temperature conditions in the sump. The usual practice, as far as the temperature is concerned, is to provide a heater in the sump to maintain the temperature at a relatively high level compared to the ambient during the period when the machine is inoperative. When the machine is inactive the pressure equalizes so that an equilibrium condition dependent upon the ambient temperature is obtained. At this equilibrium condition there is refrigerant in the liquid state (admixed with the lubricant) and refrigerant in the vapor state. Elevating the temperature of the body of liquid discourages absorption of the refrigerant by the lubricant at the pressure level encountered at shut down.

Control over the pressure in the low side of the machine is usually accomplished by restricting flow of refrigerant to the compressor by adjustable guide vanes or a suction damper. Thus the capacity of the machine is balanced with respect to the demand on the machine for refrigeration, and at the same time the pressure is prevented from falling to a value such that freezing conditions in the evaporator could obtain. This latter feature is important when the machine is used to chill water for flow in a circuit forming part of an air conditioning installation.

When the machine is started the chilled water is usually at a relatively high temperature so that the control system associated with the machine actuates the guide vanes to an open position in order to increase the pumping capacity of the machine and reduce the pressure in the evaporator. The reduction of pressure in the evaporator or low side of the machine is transmitted to the sump causing violent frothing or boiling of the mixtures due to the vaporization of the liquid refrigerant mixed with the lubricant. The lubricant pump then passes a mixture of lubricant and vaporized refrigerant to the lubrication circuit. Often the quantity of lubricant entering the circuit from the discharge of the pump is insuificient to provide lubrication. This condition is characterized by a drop in the discharge pressure of the lubricant pump. The reduction in pressure is not so severe as to trip the minimum oil pressure safety switch and so operation of the machine continues although insufficient lubricant is being supplied to the lubrication circuit.

While one solution to this problem may be to elevate the set point of the minimum oil pressure safety switch this is not satisfactory because of the necessity to delay restarting after the machine is taken off the line. It will be appreciated that the current draw to the motor at start up is particularly heavy causing the windings to be heated to a substantial degree. Accordingly the motor starting circuit is usually provided with a time delay feature which prevents re-energization of the motor windings until the passage of a period of time sufficient for the windings to cool down.

The chief object of this invention is the provision of a control for regulating the operation of a refrigeration machine wherein adequate lubrication of the parts is assured during periods of operation above the minimum oil pressure safety level.

Another object of the invention is the provision of a control for regulating the operation of a refrigeration machine that maintains the ebullition of refrigerant admixed with lubricant in the sump of the compressor within predetermined limits so as to assure adequate lubrication of the machine particularly at the time operation of the machine is initiated.

As pointed out above, centrifugal refrigeration machines are provided with a control system for regulating the position of the guide vanes throughout the operating range of a machine. This invention involves means for overriding the action of the control system described under those circumstances where it is indicated that delivery of refrigerant-free lubricant or a sufiicient lubricant to the lubrication circuit may not obtain. To this end, the control system serving as the subject of this invention acts to raise the pressure in the low pressure side of the machine so that the pressure in the lubrication sump is also elevated. With the pressure in the sump elevated, flashing of liquid refrigerant in the mixture is prevented. This is accomplished by closing the guide vanes to restrict the flow of gaseous refrigerant to the compressor and is effected in a preferred embodiment by a circuit incorporating a switch responsive to a difference in the pressure of lubricant discharged by the lubricant pump and the suction pressure in the machine.

Previous suggestions for accomplishing the described vane movement are disclosed in United States Letters Patent 3,200,603, assigned to the assignee of this invention. The ararngement proposed here is an improvement over those disclosed in the patent for it eliminates the float members required in the patented control and is responsive to a different machine operating characteristic (lubricant pump pressure). By utilizing the lubricant pump pressure a more reliable and less expensive arrangement is obtained.

BRIEF DESCRIPTION OF THE DRAWING The figure illustrated in the drawing is a schematic representation of a centrifugal refrigeration machine equipped with a control forming the subject of the invention in which certain of the operating parts of the centrifugal compressor are shown in section together with a portion of the control system employed in conjunction with the machine.

DESCRIPTION OF THE PREFERRED EMBODIMENT A centrifugal refrigeration machine of the kind to which this invention applies is shown in part in the drawing. The machine includes an impeller 12 mounted for rotation within a casing including the members 14, 16 and 18. The impeller 12 is driven by an electric motor, not shown, secured to shaft 20, through a gear reduction unit 22. Located within the casing is a sump 24 accommodating a body of lubricant. Lubricant is forwarded from the sump through pump 26 to a network of passages communicating with the portions of the machine such as the seals and bearings requiring lubrication While the pump is illustrated as being remote from the interior of the casing, it will be appreciated that lubrication pumps of the kind under consideration are often secured to the main power shaft of the machine so as to be actuated simultaneously with energization of the motor controlling rotation of the shaft 20.

The impeller receives gaseous refrigerant through the inlet thereof and forwards same under elevated pressure to a discharge line connected with a volute section 28 formed in th casing part 16. The gaseous refrigerant flowing in discharge line 30 is converted to liquid refrigerant in the condenser 32 as it rejects its heat of condensation to a cooling medium flowing in coil 34. Liquid refrigerant formed in condenser 32 is passed via refrigerant expansion device 36 to the evaporator 38 where it is converted to the gaseous phase as it absorbs heat from the source of chilled water flowing through coil 40 disposed within the evaporator 38. The gaseous refrigerant passes from the evaporator 38 through a suction line 42 to a suction inlet member 44 in which a series of circu-mferentially spaced guide vanes 46 are mounted. After flow through the guide vanes, the gaseous refrigerant passes to the inlet of the compressor completing the refrigerant flow cycle.

The lubrication sump 24 is in communication with suction line 42 through conduit 48. Thus, it will be appreciated that the pressure within the lubricant sump is substantially the same as that present in suction line 42 upstream of the guide vanes 46. Water flowing through coil 40, disposed within evaporator 38, forms a part of a closed water flow circuit of an air conditioning installation. The chilled water absorbs heat after leaving evaporator 38 so that its temperature, upon return to the evaporator, is considerably above its temperature upon departure therefrom. The extent to which the temperature of the 64,66 respectively connected across a suitable source of power. Connected in series with secondary coil 60 in one 'leg of thebridge circuit is a resistor 70 forth'e purpose of chilled water rises as it flows in the circuit is an indication of the refrigeration demand being made on the machine. In order that the capacity of the refrigeration machine be adjusted to accommodate a variation in refrigeration demand, mechanism is provided for positioning guide vanes 46 to either restrict or increase the volume of gaseous r refrigerant flow passing through the compressor. Rotation of .the guide vanes 46 to either increase or decrease the amount of gas refrigerant presented to the compressor is accomplished through a linkage 50 shown schematically in connection with the figure presented herewith. The linkbalancing the bridge. The leg of the bridge circuit containing secondary coil 62 includes a variable resistor 72 known as a thermistor. The thermistor is arranged to sense the temperature of the water flowing in chilled watencoil 40 as it leaves the evaporator 38. Its resistance is a function of the temperature of the water and thus changes with a variation in the temperature of water. Connected in parallel with thermistor 72 is a switch 74 for a purpose to be later described. Potentiometer 76 is providedin the bridge circuit for the purpose of adjusting the temperature set point of the chilled water. Potentiometer 78 together with resistor 80 serve as a calibration adjustment of the bridge circuit to compensate for the different tolerances of the bridge componentsflResistor 80 is used as a final trim balance of the bridge. Amplifier 82 is connected across the output of the bridge circuit and is employed to provide an energy source for either relay 84,0r 86 depending upon the characteristic of the signal representing the output of-the bridge circuit. The amplifier 82 is effective to cause selective energization of

relays

84 and 86.'

Considering the operationof the control circuit, it will be appreciated that under normal operating conditions the bridge'circuit is effective to supply a signal to the amplifier 82. The amplifier in accordance with the'electrical char acteristic signal will energize either relay 84 or 86 to complete a circuit through the coils of the shaded pole motor M to provide movement of mechanism 50 to either open or close the vanes depending upon the direction of variation of the temperature of the chilled water, it being understood the vanes will close upon a reduction in refrigeration demand and open upon an increase 'in refrigeration demand.

At the time the machine, illustrated in the preferred embodiment, is started the control system is first operative to energize the chilled water pump, not shown, controlling the flow of water through the chilled water circuit including coil 40 and the lubricant pump in advance of themotor driving the compressor. The flow of relatively high temperature water through the coil 40 represents a large load on the machine so that control element 72 creates on output signal from the bridge circuit urging the motor M to rotate the linkage 50 to open the guide vanes 46 and increasethe capacity of the machine to balance the load upon energization of the impeller.

Pump 26 eventually develops sufiicient pressure to start the motor driving impeller 12. It also develops sufiicient pressure to cause switch 74 to open an permit the bridge circuit to becontrolled from thermistor 72. As pointed out above, the vanes 46 begin to move to an open position. Within a relatively short period of time, the pressure in the low side of the refrigeration machine and the compressor sump is reduced. Some vaporization of the refrigerant mixed with the lubricant in the sump occurs as the mixture achieves equilibrium under the new-temperature-pressure relationship in the sump. The vaporization continues until it affects the discharge pressure in the lubricant pump. Ultimately this pressure is reduced below theset point for actuation of switch 74, causing the switch to close and shunt out sensor 72 in the bridge circuit. At this point,'-the amplifier 82 receives a signal causing energization of the relay' 84 to complete a circuit through

coils

52 and 54 to actuate motor M to cause the guide vanes to close. The pressure in the low side of the machine and in the compressor sump will temporarily rise and terminate the vaporization of the admixed liquid refrigerant present in the lubricant. I

h In response to this condition, lubricant discharge pressure from pump 26 will increase and subsequently cause movement of. the switch 74 to 'an open position. The bridge circuit is againunder control of .sensor 72 and. the vanes move to an open position in themanner described above. The reduction in sump pressure again oceurswith the consequent switch movement as described above causing the vanes to close. The above described cycle is repeated as the refrigerant is alternately vaporized and equilibrium conditions are re-established until vaporization of refrigerant no longer adversely affects the lubricant pump discharge pressure. Thus, an adequate flow of lubricant to the lubrication flow circuit is provided.

While I have described a preferred embodiment of the invention, it is to be understood the invention is not limited thereto since it may be otherwise embodied within the scope of the following claims.

I claim:

1. The method of operating a refrigeration machine including a lubricant flow circuit, a sump for accommodating lubricant, a pump for supplying lubricant to the circuit and a capacity control system for regulating refrigerant flow to a refrigerant compressor which consists in the steps of:

varying the amount of refrigerant passing to the compressor in response to the load imposed on the machine; and

regulating the variation in refrigerant flow to the compressor in response to the pressure developed by said lubricant pump whereby refrigerant flow to the compressor is restricted to elevate the pressure in the compressor sump to inhibit vaporization of refrigerant present in the lubricant.

2. The method set forth in claim 1 wherein the step of regulating the variation of the refrigerant flow is responsive to a predetermined pressure difference between the pressure of lubricant flowing from the lubricant pump and the suction pressure in the refrigeration machine.

3. Refrigeration apparatus comprising a compressor, a condenser, refrigerant expansion means, an evaporator and a suction line for supplying refrigerant from the evaporator to the compressor, connected to form a closed circuit for the flow of refrigerant, a lubrication system including pump means for supplying lubricant to the parts of the compressor requiring lubrication, capacity control means including adjustable refrigerant flow restricting means regulating flow of refrigerant to the compressor to vary the capacity of the apparatus, means for adjusting said flow restricting means in response to a change in demand for refrigeration and means operable during operation of said lubricant pump responsive to predeteranined variation in the pressure of lubricant in the discharge side of said lubricant pump for regulating said adjustable refrigerant flow restricting means independently of the change in refrigeration demand on the machine.

4. Apparatus as set forth in claim 3 wherein said last mentioned means is responsive to a predetermined difference in pressure between the lubricant discharged from said lubricant pump and the suction pressure in said apparatus. 7

5. Apparatus as set forth in claim 3 wherein said adjusting means includes a bridge circuit, the output of which responds to a variation in refrigeration demand.

6. Apparatus as set forth in claim 5 wherein switch means are provided in parallel in said bridge circuit, said switch means being effective upon energization to control the output of said bridge circuit independent of said variation in refrigeration demand.

References Cited UNITED STATES PATENTS 2,199,426 5/1940 Wolfert 62l93 X 2,983,111 5/1961 Miner et al. 62-217 X 3,081,604 3/1963 Namisniak et al. 62192 X 3,200,603 8/1965 Wake et al. 62-84 WILLIAM E. WAYNER, Primary Examiner U.S. Cl. X.R.