US3691785A

US3691785A - Small centrifugal heat pump

Info

Publication number: US3691785A
Application number: US37779A
Authority: US
Inventors: John D Ruff; Phillip R Wheeler
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-05-15
Filing date: 1970-05-15
Publication date: 1972-09-19
Anticipated expiration: 1989-09-19

Abstract

A variable capacity mechanical refrigeration system for heat pump or cooling operation, with a variable speed centrifugal compressor motor drive that uses an electronic frequency conversion apparatus which is sensitive to and controlled by, discharge or suction pressure and which includes means of preventing overloading during start up of the compressors.

Description

United States Patent Ruff et al.

[ SMALL CENTRIFUGAL HEAT PUMP [451 Sept. 19, 1972 3,355,906 12/1967 Newton ..62/228 [72] Inventors: John D. Ruff, 206 Birch St.; Phillip R. Wheeler, 209 Pine St., both of Pr1mwwExaminerMeyerPerlm Alexandria, Va. 22305 22 Filed: May 15,1970 [57] ABSTRACT [21] APPLNQ; 37,779 A variable capacity mechanical refrigeration system for heat pump or cooling operation, with a variable speed centrifugal compressor motor drive that uses an jll ..62/2I;( )2,56b2/l5/l)3 electronic frequency conversion apparatus which is [58] Fieid 230 215 sensitive to and controlled by, discharge or suction 2 6 pressure and which includes means of preventing overloading during start up of the compressors. [56] Rderences c'ted 2 Claims, 3 Drawing Figures UNITED STATES PATENTS 3,324,672 6/1967 Sones ..62/228 l IZEQV. 4-1:.

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FIG. 1

INVENTOR SMALL CENTRIFUGAL HEAT PUMP This invention is similar in many respects to our earlier inventions, US. Pat. Nos. 3,449,922, 3,447,335, and 3,499,297. Similarly to those inventions the object is to produce an electrically powered mechanical refrigeration system using hermetic, kinetic displacement compressors centrifugal or axial flow) and which has very efficient capacity variation capabilities. As with our other inventions this capacity variation is achieved by variable speed driving of the compressor system. This is done by variable frequency conversion of the electric power supplied from an external source such as electrical power mains or any other source of electrical power. The compressor motor used by this invention is of the squirrel cage induction type and its speed is dependent on the alternating frequency of the electric current supplied to it. This type also has no brushes and can be used in hermetic type systems. Hermetic systems of course do not use compressor shaft seals and with small high speed compressors this is very desirable.

This invention is mainly concerned with the variable speed drive to the compressor system and the control means associated therewith. Such variable speed drives can be applied to all sizes of kinetic displacement compressors, and in all kinds of application (heat pump, air conditioning, refrigeration).

However this invention relates primarily to the methods of varying compressor capacity which depend for their control on the system discharge pressure, with the system being used as a heat pump, or on the system suction pressure when the system is being used for cooling purposes.

Methods are also included which prevent compressor overloading at start up. This is done by building up the system discharge pressure prior to applying normal compressor speed control or by restricting vapor flow through the system by a throttling device for a predetermined time or until the discharge pressure has built up.

This invention comprises:

A variable capacity, variable frequency, hermetic, mechanical refrigeration system using kinetic displacement (centrifugal or axial flow) compressor machinery and driven by a squirrel cage motor (or motors), and a variable frequency inverter to supply current to the motor (or motors).

A capacity controller, to control the inverter frequency, which is sensitive and responsive to the system discharge pressure with the system being used as a heat pump.

A time delaying device which (on system start up) over-rides this discharge pressure sensitive capacity controller and maintains a minimum compressor speed for a pre-set time and which can also be used to de-activate the condenser fan for this preset time so that the system discharge pressure is established more readily.

An alternate means of restricting refrigerant vapor flow, comprising a throttling device which restricts the main flow of refrigerant vapor through the system (during start up) and which can be disengaged by a timer.

A current sensing device which is an alternate means of disengaging the throttling device (after start up), when the discharge pressure has built up and the motor current has dropped to a present level.

A capacity controller, to control the inverter frequency, which is sensitive and responsive to the system suction pressure with the system being used for cooling purposes.

A timing device which (on system start up) overrides this suction pressure sensitive capacity controller and maintains a minimum compressor speed for a preset time and which can also be used to de-activate the evaporator fan for this preset time so that the system suction pressure is established more readily.

An alternate means of restricting refrigerant vapor flow, comprising a throttling device which restricts the main flow of refrigerant vapor through the system (during start up) and which can be disengaged by a timer, (when the system is being used in cooling function).

A current sensing device which is an alternate means of disengaging the throttling device (after start up), when the suction pressure has stabilized and the motor current has dropped to a preset level.

An alternate type of system which uses a dry expansion evaporatorand which does not need any device for preventing overloading at start up due to the operating characteristics peculiar to this type of system.

In the drawings:

FIG. 1 shows system layout in heat pump operation.

FIG. 2 shows the system layout when a dry expansion evaporator is used.

FIG. 3 shows system layout in cooling operation.

COMPRESSOR FIG. 1 shows compressor 2 which is a two stage centrifugal compressor with hermetic enclosure 3.

FREQUENCY CONVERTER FIG. 1 illustrates, in simple form, a high frequency converter of typical specifications for supplying power to the compressor motor. The 220 v. A.C., 60 Hertz (cycles per second), single phase supply is first rectified by the bridge rectifier 4 using solid state diodes 5,6,7,8. This pulsating D.C. output is smoothed by filter capacitor 9 and fed to the silicon controlled rectifiers l0,ll,12,l3,l4,15 which switch into the three phase motor windings 16,17,18 of the motor. Some filtering may be necessary at the windings to achieve optimum wave shape and can be provided as needed. The firing of the silicon controlled rectifiers 10,1 1,12,13,14,]5 is controlled by the variable frequency phase sequencer (or trigger circuit) 19. This circuit triggers and turns off the silicon controlled rectifiers in the necessary sequence for a three phase operation. The frequency of the resulting three phase supply is variable by changing the frequency of the sequencer oscillations.

By this method the speed of rotation of the compressor motor is controlled since this motor is of the squirrel cage induction type and its speed is dependent on its supply frequency/Since the capacity of a centrifugal compressor varies basically as the cube of the rotation speed, then the operating capacity of the compressor unit will be approximately reduced to 40 percent of full capacity by a 25 percent drop in the rotation speed. This would be a typical range of capacity variation, with a span of about 40 F. in evaporator temperature (0 F. 40 F.). Capacity in this case refers to the work done by the compressor as expressed in horsepower. Actually this capacity is dependent very largely, on the pressure against which the compressor is working (or the compression ratio between the condenser and evaporator pressures).

There are several methods 'of achieving the frequency conversion to control motor speed but an electronic converter as described above is preferred since the necessary frequency changes can be accomplished simply by changing the frequency in the trigger circuit 19.

HEAT PUMP OPERATION FIG. 1 shows compressor 2 drawing vapor from evaporator coil 20 which is in contact with the outside air and thus provides a source of heat. Condenser coil 21 is contacting the air inside the heated space and is thus heating it. Controller 22 is a pressure sensitive device which is the means of controlling the frequency of sequencer 19 and is sensitive to the condensing pressure in condenser 21 to which it is connected by line 23. That is, when the outside air temperature is lowered the suction pressure in evaporator 20 is also lowered, and since the pumping head of compressor 2 is relatively constant at any given rotation speed, then the condensing pressure will also be lowered. Controller 22 senses this drop in pressure and causes the frequency of sequencer 19 to be increased. This speeds up the compressor and increases its pumping head and the condensing pressure is thus kept at a reasonable level. It is necessary that the condensing pressure be kept at a reasonable level so that the condensing temperature also can be dept high and thus the heating function of the heat pump maintained. Similarly an undesirable raising of condensing pressure (due to warmer outside conditions) is compensated by a slowing down of the compressor. Controller 22 is shown with switches 24,25,26 which are actuated, a step at a time, by movement of bellows 27 as it responds to varying condensing pressures. Switches 24, 25, 26 short out portions of resistor 28, the resistance of which is the basis of control of sequencer 19 as was explained in more detail in our earlier invention (US. Pat. No. 3,449,297). This arrangement of controller 22 gives four steps of capacity which are engaged automatically in response to pressure variations.

Alternative arrangements are the use of water flowing over evaporator coil 20 (as a source of heat) and the use of water flowing over condenser 21 (with a hydronic circulation system). Controller 22 operates in the same manner with these arrangements.

OVERLOAD PREVENTION When the centrifugal (or axial flow) compressor is starting up after being stopped for any length of time there is a tendency to overload because the pressures in evaporator 20 and condenser 21 are nearly equal and there is a high rate of flow through the compressor. However, (with an air to air system) the evaporator soon cools down and the condenser heats up and normal operation is allowed. Our invention uses a timer 29 which over-rides the switches 24, 25, 26 for a preset time when the system is started up thus causing the compressor to run at the minimum speed and thus minimize overloading. At the end of the preset time, timer 29 disengages and normal operation is resumed.

An alternative method of reducing the rate of flow through the compressor is by the use of throttle valve 30 which can be placed in either the suction line or the discharge line of the compressor. It is electrically activated and controlled by timer 31 during a preset time during starting up of the system. A bypass 32 is used so that a reduced flow can be maintained while the throttle is engaged. Means are shown for switching either of time rs 29 or 31 into the control circuits.

During the engagement of the overload prevention methods described a more positive action is obtained if the evaporator fan 33 and the condenser fan 34 are turned off by the timers. This allows the temperature change (and thus pressure change) in coils 20 and 21 to be more rapid. An alternative to the use of timers is the use of slow acting mechanisms that actuate the throttle valve 30 or the capacity controller 22.

An alternate method of controlling throttle valve 30 is by the use of motor current actuated terminator 35. On start up, throttle valve 30 is engaged and its disengagement is controlled by terminator 35. Pick-up coil 36 detects the level of motor current flowing in one of the motor leads, and when this has dropped to a pre-set level a relay in terminator 35 is actuated and throttle valve 30 is disengaged. This method is most suitable for hydronic systems which take varying and sometimes much longer times to settle down on start up. A timer would not be applicable in this case. Means are shown for switching this alternate terminator into the control circuit.

DRY EXPANSION SYSTEM FIG. 2 shows a heat pump system using a dry expansion evaporator 37. Controller 22 is used to control capacity in the manner already described. However no problems are encountered with start up overloading since there is no quantity of liquid refrigerant in the evaporator at start up as with a flooded evaporator. However special precautions should be taken such as mounting liquid receiver 38 at a level lower than evaporator 37 and pitching the tubes of the evaporator so that any liquid in the evaporator will drain back through expansion valve 39 into the receiver. Evaporator 37 can be contacted by a flow of outside air, or water can alternately be used as a source of heat.

An alternate method of keeping the evaporator 37 free of refrigerant liquid is by a pump down system. When the system operation is to be stopped, solenoid 40 is de-energized and the flow of refrigerant liquid to evaporator 37 is stopped. Compressor 2 continues to run and all of the refrigerant in evaporator 37 is drawn out. The suction pressure then drops and low pressure switch 41 is opened and this switch deactivates the compressor motor. Check valve 42 closes and holds the system refrigerant from flowing back through the compressor and into the evaporator 37. Should some vapor leak through check valve 42 or solenoid 40 the pressure at switch 41 causes it to close and the compressor pumps down again. Condenser 21 can be air contacted, or, alternately it can be water contacted (with hydronic circulation).

COOLING OPERATION FIG. 3 shows the invention in use for cooling operation. Operation is similar to heat pump operation.

Compressor 2 pumps from evaporator into condenser 21. However for cooling operation the controller 43 is sensitive to the suction pressure in evaporator 20 to which it is connected by line 44. And also the action of the controller 43 is the reverse of controller 22. That is, it tends to speed up the compressor on rise of pressure rather than on drop of pressure as with controller 22. Switch points 45,46,47 make on fall of pressure. The action of timer 29 in delaying full capacity application by over-riding controller 43, and the action of throttle 30 controlled by timer 31 and terminator 35 are the same as with heat pump operation. Also a hydronic circulation can alternately be used through evaporator 20, and condenser 21 can alternately be water cooled.

When the system is used for cooling with a dry expansion evaporator (FIG. 2) the operation is similar to heat pump operation but controller 43 is then used instead of Controller 22. Controller 43 is connected to suction pressure in the same manner as in FIG. 3 and in similar function it is reverse acting. Means are shown for switching this alternate controller into the control circuit. Evaporator 20 can be contacted by a flow of inside air or, alternately a hydronic system can be used. Condenser 21 can be air cooled or it can alternately be water cooled.

We claim:

1. In combination a variable capacity heat pump used for heating an air filled space and comprising kinetic displacement vapor compression means direct driven by variable frequency, alternating current, electric motor machinery of the squirrel-cage induction type and the said compression means and motor machinery being combined in a common hermetic enclosure, to pump refrigerant vapor from a dry expansion evaporator to a condenser, a variable frequency inverter means using sequential switching equipment to switch electrical current from an external source into the windings of the said motor machinery in such a manner to provide a variable frequency flux through the said windings, and thus achieve a variable speed rotation of the said motor machinery, since the rotation speed of this type of motor is dependent on the frequency of the current supplied to it, a pressure sensing means which is sensitive only to the refrigerant vapor pressure in the said condenser, and the frequency of the said variable frequency inverter is controlled by this said pressure sensing means so that when operating conditions cause the pressure in the said condenser to be undesirably low the compressor is speeded up until a suitable condenser pressure is reached and similarly when the condenser pressure is too high the compressor is slowed down.

2. In combination a variable capacity cooling system used for cooling an air filled space and comprising kinetic displacement vapor compression means direct driven by variable frequency, alternating current, electric motor machinery of the squirrel-cage induction type and the said compression means and motor machinery being combined in a common hermetic enclosure, to pump refrigerant vapor from a dry expansion evaporator to a condenser, a variable frequency inverter means using sequential switching equipment to switch electrical current from an external source into the windings of the said motor machiner in such a manner to provide a variable frequency ux through the said windings, and thus achieve a variable speed rotation of the said motor machinery, since the rotation speed of this type of motor is dependent on the frequency of the current supplied to it, a pressure sensing means which is sensitive only to the refrigerant vapor pressure in the said evaporator, and the frequency of the said variable frequency inverter is controlled by this said pressure sensing means so that when operating conditions cause the pressure in the said evaporator to be undesirably high the compressor is speeded up till a suitable evaporator pressure is reached and similarly when the evaporator pressure is too low the compressor is slowed down.

Claims

1. In combination a variable capacity heat pump used for heating an air filled space and comprising kinetic displacement vapor compression means direct driven by variable frequency, alternating current, electric motor machinery of the squirrelcage induction type and the said compression means and motor machinery being combined in a common hermetic enclosure, to pump refrigerant vapor from a dry expansion evaporator to a condenser, a variable frequency inverter means using sequential switching equipment to switch electrical current from an external source into the windings of the said motor machinery in such a manner to provide a variable frequency flux through the said windings, and thus achieve a variable speed rotation of the said motor machinery, since the rotation speed of this type of motor is dependent on the frequency of the current supplied to it, a pressure sensing means which is sensitive only to the refrigerant vapor pressUre in the said condenser, and the frequency of the said variable frequency inverter is controlled by this said pressure sensing means so that when operating conditions cause the pressure in the said condenser to be undesirably low the compressor is speeded up until a suitable condenser pressure is reached and similarly when the condenser pressure is too high the compressor is slowed down.

2. In combination a variable capacity cooling system used for cooling an air filled space and comprising kinetic displacement vapor compression means direct driven by variable frequency, alternating current, electric motor machinery of the squirrel-cage induction type and the said compression means and motor machinery being combined in a common hermetic enclosure, to pump refrigerant vapor from a dry expansion evaporator to a condenser, a variable frequency inverter means using sequential switching equipment to switch electrical current from an external source into the windings of the said motor machinery in such a manner to provide a variable frequency flux through the said windings, and thus achieve a variable speed rotation of the said motor machinery, since the rotation speed of this type of motor is dependent on the frequency of the current supplied to it, a pressure sensing means which is sensitive only to the refrigerant vapor pressure in the said evaporator, and the frequency of the said variable frequency inverter is controlled by this said pressure sensing means so that when operating conditions cause the pressure in the said evaporator to be undesirably high the compressor is speeded up till a suitable evaporator pressure is reached and similarly when the evaporator pressure is too low the compressor is slowed down.