US2179700A - Refrigeration system - Google Patents

Description

Nov. 14, 1939. w R. MILLER 2,179,700

REFRIGERATION SYSTEM Filed Oct. 15, 1936 l6 5 8 WayZavzdR.MiZZe7 Patented Nov. 14, 1939 UNITED STATES PATENT OFFICE REFRIGERATION SYSTEM ware Application October 15, 1936, Serial No. 105,841

6 Claims.

This invention relates to refrigeration control devices and more particularly to an arrangement for automatically and proportionately varying the capacity of a refrigeration compressor in accordance with the prevailing load.

In refrigeration systems of large capacity, such for instance as those employed in air conditioning systems, or cold storage plants, the refrigeration load varies considerably from period to period, due to changes in weather conditions, and also due to changes in heat emission within the enclosure, such as caused by changes in the number of persons present or changes in operation of machinery within the enclosure. Where mechanical refrigerating systems are employed, it has been customary to meet these changes in load by changing the speed of the compressor, by varying the number of compressors in operation, or by throttling. These various control 0 schemes while being satisfactory in some respects are unsatisfactory in others. The changing of the compressor speed involves complicated and expensive apparatus and precludes the use of synchronous motors which are highly desirable from a power rate viewpoint. The changing of the number of compressors in operation is not feasible where large units are employed, as the control secured is not flexible, and involves very complex control apparatus in order to secure automatic operation. The throttling method while being simpler than the other methods men: tioned, however has the serious disadvantage of being uneconomical, due to the large loss of energy occurring when the refrigeration load is light.

It is an object of my invention, therefore, to provide an automatic control device for varying the capacity of a refrigeration compressor without changing its speed, and which does not materially affect the efficiency thereof.

A further object of my invention is to provide a flexible control arrangement which is rugged in construction and simple in operation and Hal which automatically balances the capacity of a 3 compressor against the load it, must carry.

Another object is to maintain a constant pressure in the suction line of a compressor by automatically and proportionately varying the capacity of the compressor to meet the changes in suction pressure.

A further object is to control the operation of an air conditioning system by varying the capacity of the refrigeration system compressor proportionately with the load upon the system.

Another object is to maintain a constant pres- For a full understanding of my invention, ref-' erence is made to the accompanying drawing, in which the single figure diagrammatically illustrates an air conditioning system to which one form of my invention is applied.

Reference character I indicates an air conditioning chamber in which is located a cooling coil 2. Chamber I communicates at one end with'the space 3 to be conditioned. At its other end the chamber' I communicates with a fan 4 which discharges through a duct 5 into the space 3. While for the sake of clearness I have not illustrated other air conditioning devices such as air washers, filters, reheaters, etc., it is to be understood that such apparatus may be employed. In order to supply liquid refrigerant to coil 2 and to withdraw gaseous refrigerant therefrom, I

have shown a compressor generally indicated at 6. While I have illustrated this compressor as bein of the double acting reciprocating type it is to e understood that any type of positive action compressor may be employed. 4 Compressor 6 is provided with a branched discharge pipe I which discharges into a condenser 8.

A pipe 9 leads from the outlet of condenser 8 to the expansion valve l0 which is located at the inlet of coil 2.. The outlet of the evaporating coil 2 is connected by a branched pipe II to eachend of the compressor cylinder.

Connected to each end of the compressor cylinder are a pair of clearance pockets generally designated at I3 and 14. Clearance pocket I3 is connected to the upper end of the compressor cylinder by means of a conduit I5, while clearance pocket I4 is connected to the lower end of x the cylinder by conduit I6. Each clearance pocket is formed of a cylinder block I8 having bored therein a cylindrical chamber I9. Fitted into each chamber I9 is a piston 20 which is secured to a piston rod 2| having at its end a gear rack, 22. The clearance pockets I3 a'nd I4 are'displaced from each other a distances'ufiicient to accommo date a pinion gear 25 which'cooperates with each rack. Movement of pinion 25 in a clockwise direction causes the pistons to be moved away from each other, thereby reducing the volume of the chambers l9 which are in communication with the compressor. Movement of pinion 25 in the opposite or counter-clockwise direction moves the pistons towards each other, thereby increasing the volume of the chambers which are in communication with the compressor. Each cylinder block I8 is fitted with a cylinder head 26. Each cylinder head is formedwith an opening for the piston rod, this opening being fitted with a packing gland 21 for preventing the escape of refrigerant from the clearance pocket.

By varying the volume of the clearance pockets the output of the compressor may be controlled. As is well known, the volumetric efficiency of a compressor is dependent, among othof gas sufiicient to fill the clearance volume must below the suction'pressure before gas will be' drawn from the suction line into the compressor cylinder. The effect of clearance, therefore, is to reduce the quantity of refrigerant which will be handled by the compressor per stroke. The greater the clearance volume, the more .gas will be trapped in the compressor at the end of each stroke and hence the further the piston must move on its suction stroke before gas will be drawn from the suction line. Thus by increasing the clearance volume the capacity of the compressor may be reducedwhile by decreasing the clearance volume the capacity may be increased. Rotation of the pinion 25 in a clockwise direction, therefore, acts to increase the capacity of the compressor while rotation of the pinion in the opposite direction will decrease the compressor capacity.

By varying the clearance volume in the manner described the compressor capacity may be varied even though it is operating at constant speed, and this variation in capacity is secured without a substantial loss in eificiency. The gas which is trapped in the clearance pockets at the end of each stroke expands when the suction stroke begins and in this manner returns to the pistonthe energy imparted to it during the compression stroke. The only loss in energy is due to a small amount of throttling in the connecting conduits l5 and I6 and due to friction betweenthe clearance pockets of somewhat exaggerated size and as being conne ted to the compressor by relatively long conduit it is to be understood that in practice the clearance pockets would be placed as close to the compressor cylinder as possible in order to avoid excessive uncontrolled clearance volume.

.Pinion 25 is secured to a shaft 30. This shaft in turn is secured to a gear 3| of a gear train generally indicated at 32. The end of this gear train comprises a gear 33 cooperating with a worm gear 34. Gear 34 is carried by shaft 35 which also carries armatures 36 and 31 of induction type electric motors. Field coil 39 cooperates with the armature 36 and field coil 39 cooperates with the armature 31. These coils are arranged to cause their respective armatures to rotate in opposite directions. Thus-by energizing field coil 38 worm gear 34 will be rotated in one direction while by energizing the coil 39 the worm gear will be rotated in the opposite direction. Specifically, energization of coil 38 causes rotation of the gear 34 in a counter-clockwise direction as viewed from its outer end, while energization of coil 39 causes the gear 34 to be rotated in a clockwise direction.

Energization of field coils 38 or 39 is controlled by a relay generally indicated as 49. This relay comprises two actuating coils 4| and 42 connected together. Their opposite ends are connected across the secondary 44 of a step-down transformer 45 by means-of wires 46 'and 41. The

primary 48 of transformer 45 is connected across line wires 49 and 50. In the magnetic field of coils 4| and 42 is located an armature or plunger 5| which is connected by means of a rod 52 with a switch arm 53. Switch arm 53 is pivoted at 54 and cooperates with two contact points 55 and 56. When plunger 5| is moved to the right under the action of coils 4| and 42, the switch arm 53 is movedto engage contact 56. When the plunger moves in the opposite direction the switch arm 53 engages with the contact 55. When plunger 5| is in its midposition the switch arm 53 is likewise in a midposltion and is not engaged with either contact. Switch arm 53 is connected'to the line wire 49 by means of a wire 59. Contact 56 is connected by means of a wire 60 with one terminal of the limit switch 6|. The other terminal of limit switch 6| is connected by wire 62 with one end of the motor field coil 38. Contact 55 is connected by means of a wire 63 to one terminal of a limit switch 64, the other terminal of this switch being connected to one end of the motor field 39 by means of a wire 65. The other ends of the motor fields 38 and 39 are connected together and by means of a wire 66 are connected to the line wire 50. It will be seen, therefore, that when contact arm 53 engages contact 56 the motor field 38 is energized by a circuit as follows: line wire 49, wire 59, contact arm 53, contact 56, wire 60, limit switch 6|; wire 62, field coil 38 and wire 66 to line wire 59; Similarly, when contact arm' 53 engages contact 55 the field coil 39 is energized by the following circuit: line wire 49, wire 59, contact arm 53, contact 55, wire 63, limit switch 64, wire 65, field coil 39, and wire 66 to line wire 50.

The limit switches 6| and 64 are illustrated as rection, this resulting in movement of the pistons 20 to decrease the volume of the chambers l9,

'and so long as contact 53 remains engaged with contact 56, the pistons will be moved until they reach the end of their travel and at this point the tripping member 12 will engage limit switch 6|, thus breaking the motor operating circuit. In a similar manner, when contact arm 53 engages contact 55 the pistons will be moved in a direction to increase the volume of chambers l8. and the energizing circuit formotor 38 will be broken by limit switch 64 when the pistons 20 reach the end of their travel if switch arm 53 has not disengaged from contact 55 meanwhile.

Located in the space 3 is a humidostat generally indicated as 19. This instrument comprises a bell crank lever having a contact arm 8|, an actuating arm 83, and being pivoted at 82.

Below the actuating arm 83 and connected thereto by'a suitable link is a humidity responsive device consisting of a plurality of strands 84a of hair or other moisture responsive material, these strands being secured together at each end by means of clamping members 84 and 85. lower clamping member 84 is secured to a suitable fixed element as 88 while the upper clamping member is secured to the bell crank lever arm 83. A tension spring 81 is connected at one end to the contact arm 8|, and at its other end is secured to a suitable fixed element, this spring serving to bias the bell crank lever 80 against the action of the humidity responsive elements 84a, thereby maintaining such elements under tension. The contact arm 8| of the bell crank lever is arranged to contact with a resistance element 88. Upon an increase in humidity, the strands of hair 840. will expand, thereby permitting the spring 81 to cause rotation of contact arm 8| across the resistance element in a clockwise direction.. Conversely, a decrease in humidity will cause shrinking of the strands of hair 84a causing rotation of the contact arm 8| in a counter-clockwise direction.

Connected to the suction line II is a pressure responsive devicegenerally indicated at 90. This device comprises an expansible and contractible bellows 9| having one end thereof connected to the suction line II by means of a tube 92. Fixed to the other end of the bellows 9| is an actuating arm 93 which is pivotally connected at 94 to a contact arm 95. Contact arm 95 is pivoted at 95 and is arranged to wipe across a potentiometer.

resistance 91. Upon an increase in pressure in the suction line H the bellows 9| will expand, thus causing the contact arm to be rotated in a clockwise direction across the resistance element 91. Conversely, a decrease in pressure will cause the bellows 9| to contract, this resulting in the contact arm 95 being moved in a counterclockwise direction across the resistance element 91.

The left-hand end 'of resistance element 91 is connected to the right-hand end of the relay coil 42 by means of wires 99, I00, and protective resistance MI. The right-hand end of resistance element 91 is connected by a wire I02 and protective resistance I03 with the left-hand end of the relay coil 4|. The left-hand end of the humidostat resistance 88 is connected by means of wires I04, I05, I00 and protective resistance |0I with relay coil 42, while the right-hand end of said resistance is connected by means of wires I06, I01, and protective resistance I03 to the left-hand'end of relay coil 4|. It will be seen, therefore, that the potentiometer resistance element 88 of the humidostat 80 and the corresponding resistance element 91 of the pressure responsive device 9| are connected in parallel across the relay coils 4| and. 42. Contact arm 8| is connected by a wire M0 to a rheostat III which is in turn connected to wire I I2 which leads to the junction of coils 4| and 42. The contact arm 95 of the pressure responsive device 9| is similarly connected to the junction on relay coils The III! of a balancing potentiometer generally indicated at-II4. Contact arm 5 is connected by means of a wire I I8, rheostat 'I I1 and wire 8 to the wire 2 and is therefore connected to the junction of relay coils 4| and 42 in parallel with the contact arms 8| and .95 of the humidostat and pressure responsive device respectively. The. upper end of the resistance coil I20 of the balancing potentiometer H4 is connected by wire I2I to wire I01 and hence to the left end of relaycoil 4|, while the lower end of the resistance I20 is connected by wire I22 to the wire I05 and hence to theright end of relay coil 42. The balancing potentiometer H4 is therefore connected in parallel to the relay coils 4| and 42 along with the potentiometers'of. the humidostat 80 and pres-. sure responsive device 90.

Operation i that no current is fiowing to or from the junction of coils 4| and 42 owing to the intermediate positions of the three contactfingers. If thepressure in the suction line II should increase, the expansion of the bellows 9| will cause movement of the contact finger 95 in a clockwise direction across the resistance 91. As this contact finger 95 is connected to the junction of coils 4| and 42, the movement thereof in a clockwise direction will decrease the portion of the potentiometer resistance which is in parallel with the coil 4| and increase the portion of the potentiometer resistance which is in parallel with the relay coil 42. Therefore, a part of the current which had been flowing through coil 4I- will flow through the right-hand portion of the resistance element 91. Similarly, a part of the current which had been flowing through the left-hand portion of 53 will engage contact 56. This will cause energization of the motor field 38 and will cause rotation of the worm gear 34 in a direction to reduce the volume of chambers I 9. As the pistons are being thus moved, the contact finger 5 will be rotated in a clockwise direction across the resistanceelement I20. This action will act to increase the portion of resistance I20 which is in parallel with the relay coil 4| and to decrease the portion of the resistance I20 which is in parallel with the relay coil 42. This will act to decrease the fiow of current through coil 42 and to increase the flow through the relaycoil 4| When the contact finger H5 is moved sufliciently across its cooperating resistance the energization of coils 4i and 42 will again become equalized, thus causing the armature 5| to move the contact finger 53 to its mid-position thus breaking the energizing circuit for field 38 and causing the motor to stop.

Due to the worm gear drive, the pistons2ii will be locked in their new positions. As they have been moved toreduce the volume of the chambers IS, the clearance volume of the compressor will have been decreased thus increasing the volumetric efliciency of the compressor and causing it to assume a greater load. An increase in pressure, therefore, acts to adjust thei clearance volume so as to cause the compressor to carry the increased load as indicated by the increase in pressure.

Upon a decrease in pressure in the suction line H, the bellows 9| will contract thus causing the contact arm 95 to be moved in a counter-clockwise direction across the resistance element 911. This will have the effect of increasing the energization of relay coil 4| and decreasing the energization of relay coil 42. This efiect will cause the armature M to be moved towards the left thus moving the contact arm 53 towards the contact 55. When the decrease in pressure has become sufi cient, armature M will be further moved to the left and bring contact arm 53 into engagement with contact 55. This action will energize the motor field 39, thus causing the pinion 25 to be rotated in a clockwise direction, this action moving the pistons 20 towards each other and increasing the clearance volume. In the manner stated previously, this increase in clearance volume will reduce the capacity of the compressor. While the pistons 2t arebeing moved towards each other, the contact arm M5 is rotated in a counter-clockwise direction. This results in decreasing the portion of resistance 0% which is in parallel with the relay coil M, and in increasing the portion of the resistance which is in parallel with the coil 42. When the movement or contact finger H5 in the clockwise direction becomes suflicient to cause balancing of the current flows in coils 4| and 42, the armature M will be moved towards the right thus moving contact arm 53 from engagement with contact 55, thereby opening the operating circuit for motor It will be seen, therefore, that an increase in pressure in the suction line ill. will cause the pistons 2G to be moved in a direction which reduces the clearance volume of the compressor and. that the amount of such movement will be proportional to the increase inpressure. Similarly, upon a drop in pressure in the suction line H the pistons wil be moved in a direction to increase the clearance volume and the amount of movement will be proportional to the fall in pressure.

The action of the rheostat Bill which is interposed between the junction of coils 4| and 42 and the contract finger H5 of the balancing potentiometer H4 is to reduce the sensitivity of said potentiometer. This rheostat in effect chokes the flow of current through the contact finger H5 thus necessitating a relatively large movement of said contact finger across the resistance I20 to effect a relatively small variation in the energization of coils 4| and 42. Thus when contact finger 95 of the pressure responsive device moves across this resistance 91 it effects an unbalancing of the energization of coils 4| and 42. This, in the manner previously described, causes energization of either motor field 38 or 39, this resulting in rotation of contact finger H5 in a direction to rebalance the energization of relay coils 4| and .42. Due to the choking action of rheostat H, contact finger |5 must move across its resistance -|2|l in order to rebalance the energization of coils 4| and 42,-a greater distance than the movement of the contact arm 94 moved to cause the initial unbalancing. Thus by properly adjusting the rheostat I H a slight movement of contact arm 95 may cause the balancing potentiometer 4 to traverse its complete range in order to rebalance the energization of coils 4| and 42. In this manner the operating range 01 the pressurestat potentiometer may be made less than'its total range. That is, a movement of contact arm 95 through but part of its range may cause the pistons 20 to be moved from one extreme position to the other.

Upon an increase in humidity the contact finger 8i will be caused to move across the resistance 8 in a clockwise direction. This will have the eiTect of decreasing the energization of relay coil 4| and increasing the flow of current through relay coil 42. When the change'in humidity becomes sufiicient, relay coil 42 will be energized sufficiently greater than relay coil 4| as to cause the armature 5i to move to the right and cause contact arm 53 to engage the contact 56. This will cause energization of the motor field coil 38 thus causing rotation of the worm gear 36 in a direction which acts to move the pistons away from each other, thereby decreasing the clearance volume of the compressor. At the same time, the contact finger N5 of the balancing potentiometer M4 will be moved in a clockwise direction across the resistance 02%. This action will act to decrease the energization of relay coil d2 andincrease the current flow in relay coil 4|. When the movement of contact finger H5 becomes suflicient to rebalance the current flow through the relay coils, armature 5!] will swin back towards its mid-position, thus breaking the motor actuating circuit. In this manner an increase in humidity causes the clearance volume of the compressor to be decreased, thereby causing the capacity of the compresor to be increased. This results in a decrease in pressure in the evaporating coil 2 which causes its temperature to be lowered. Due to this lower temperature an increased amount of moisture will be condensed on the cooling coil 2, thereby removing moisture from the air and lowering its relative humidity. An increase in humidity therefore acts to lower the coil temperature and pressure and the amount that the temperature and pressure is lowered is proportional to the humidity increase. the greater the increase in humidity the lower the coil pressure that will be maintained.

Upon a decrease in humidity, contraction of the humidity responsive strands 84 wii cause rotation of the contact finger 8| in a counter-clockwise direction. This action results in decreasing the current flow through the relay coil 42 and in increasing the current fiow through relay coil 4|. This will result in the armature 5| being moved to the left, thereby bringing contact. arm 53 into engagement with the contact point 55. This will cause energization of the motor field coil 39, thereby moving the pistons 20 towards each other to increase the compressor clearance volume. Simultaneously the contact finger H5 will be That is,

moved in a counter-clockwise direction across resistance I20, this action tending tobalance the current flows in relay coils 4| and 42. When the movement of contact finger I I5 has become sufficient to cause balancing of the current flows in said relay coils, the contact arm 53 will be disenance the effect thereof.

gaged from the contact point 55 thereby stopping the motor at this point. A decrease in relative humidity, therefore, acts to decrease the capacity of the compressor and this reduction in capacity will be proportional to the decrease in humidity. The reduction in the compressor capacity results in the pressure rising in the suction line H and evaporating coil 2 thereby causing the temperature of the coil 2 to increase, which causes it to remove less moisture from the air passing thereover.

It will be observed that rheostat III is interposed between the contact finger 8| and the junction of relay coils ll and 42. This rheostat acts to decrease the sensitivity of the humidostat 80. Due to this decrease in sensitivity the effect of the humidostat upon the system may be controlled.

As noted herebefore, the effect of the rheostat H1 is to reduce the sensitivity of the balancing potentiometer I20 thereby causing the operating range of the pressurestat potentiometer to be less than its total range. In other words, a movement of the contact finger 95 across but a portion' of resistance 91 will cause movement of the pistons 20 from one extreme to the other. The center of the operating range of the pressurestat potentiometer is the position of the contact finger 95 which causes the balancing potentiometer contact finger H5 to assume a mid-position. Should the contact finger 95 move clockwise from this operating range center it will cause movement of contact finger H5 in a similar direction to bal- Similarly, should the contact finger 95 move to the left of this operating range center, corresponding movement of the balancing contact finger H5 will take place.

The efi'ect of the humidostat 80 is to shift the location of the pressurestate operating range within its total range. It will be observed that the effect of the humidostat on the system under an increase in humidity is similar to the effect of the pressurestate under an increase in pressure. In other words, an increase in humidity will cause the relay coil 42 to be energized to a greater extent than the relay coil II and an increase in pressure will have this same effect. When the humidostat 80 is in its mid-position the center of the operating range of the pressurestat potentiometer will coincide with the center of its total range. Should the humidity increase, the contact finger 88 will be moved clockwise, thereby decreasing the current how in relay coil ll and increasing the cu rrent flow in relay coil 82. In order for the pressurestat potentiometer .to rebalance the current flows in relay coils ll and 82, the contact finger must move in a counter-clockwise direction across resistance 91, which requires a decrease in pressure in the suction line H. The effect of a rise in humidity, therefore, is to shift the center of the operating range of the pressurestate potentiometer to the left, thereby causing it to maintain a lower pressure in the evaporator, Similarly, a decrease in humidity will cause contact finger 8| to move in a counter-clockwise direction across resistance 88, thereby increasing the current flow in relay coil H and decreasing the current fiow through relay coil 82. In order for the pressures at potentiometer to rebalance the relay the contact finger 85 must be moved in a clockwise direction across the resistance 81, this requiring an increase in pressure'in the suction line II. The effect of a decrease in humidity, therefore, is to shift the tentiometer within its total range to cause a Y higher pressure to be maintained in the evaporating coil' 2 when the humidity is low and to cause progressively lower pressures to be maintained in the evaporating coil as the humidity increases.

In view of the foregoing it will be apparent that I have provided an arrangement for modulating the capacity of a constant speed compressor in order to cause it to operate to carry the desired load, this arrangement being extremely flexible and causing variation of the compressor opera- .tion without sacrificing efficiency forany load condition from zero to full load. While I have illustrated my invention in connection with a compressor for an air conditioning system and in which the compressor is controlled in accordance with the pressure in the suction line, and in accordance with humidity in the space to be conditioned, it is to be understood that my invention may be applied to any positive displacement type of compressor and may be responsive to other factors related to the compressor load, such, for instance, as the temperature in the conditioned space. It is also to be understood that the control device may be positioned in accordance with suction line pressure alone, humidity alone, or in accordance with any other condition affecting or aifected by the load.

As many modifications of my invention will suggest themselves to those skilled in the art, I wish to be limited only by the scope of the appended claims and the prior art.

I claim as my invention:

1. In an air conditioning system, in combination, an evaporator in heat exchange relationship with a space to be conditioned, means for supplying liquid refrigerant to the evaporator, a positive displacement compressor connected to said evaporator, movable means associated with the interior of the compressor for varying the volume of refrigerant drawn into the compressor per cycle thereby varying its capacity, positioning means for graduatingly positioning said movable means, said positioning means including a device influenced by the temperature at which the liquid refrigerant in the evaporator evaporates and acting to position said movable means in a manner to increase the volume of refrigerant drawn into the compressor upon increase in said evaporating refrigerant temperature and to decrease the volume of refrigerant drawn into the compressor upon decrease in said evaporating refrigerant temperature, and a controller respon sive to a condition of the air in said space which is affected by said evaporator, said controller adjusting said positioning means for varying the evaporating temperature of the refrigerant in accordance with variations in said spacecondition. j

2. In an air conditioning system, in combination, an evaporator in heat exchange relationship with a space to be conditioned, means for supplying liquid refrigerant to the evaporator, a positive displacement compressor connectedto said evaporator, means for varying the clearance volume of the compressor, positioning means for graduatingly positioning said clearance volume a device influenced by the temperature at which liquid refrigerant in the evaporator evaporates and acting to position said clearance volume varying means for decreasing the clearance volume upon increase in said evaporating refrigerant temperature and to increase the clearance volume upon decrease in said evaporating refrigerant temperatura-and a controller responsive to a condition of the air in said space which is affected by said evaporator, said controller adjusting said positioning means for varying the evaporating temperature of the refrigerant in accordance with variations in said space condition."

3. In an air conditioning system, in combination, an evaporator in heat exchange relationship with a space to be conditioned, means for supplying liquid refrigerant to the evaporator, a positive displacement compressor connected to said evaporator, movable means associated with the interior of the compressor for varying the volume of refrigerant drawn into the compressor per cycle thereby varying its capacity, positioning means for graduatingly positioning said movable means, said positioning means including a device influenced by the temperature at which the liquid refrigerant in the evaporator evaporates and acting to position said movable means in a manner to increase the volume of refrigerant drawn into the compressor upon increase in said evaporating refrigerant temperature and to decrease the volume of refrigerant drawn into the compressor upon, decrease in said evaporating refrigerant temperature, and a controller responsive to the moisture in the air in said space, said controller adjusting said positioning means in a manner to lower the evaporating temperature of the refrigerant as the moisture "in the air increases and to raise the evaporating temperature as the moisture in the air decreases.

4. In an air conditioning system, in combination, an evaporator in heat exchange relationship with a space to be conditioned, means for supplying liquid refrigerant to the evaporator, a positive displacement compressor connected to said evaporator, movable means associated with the interior of the compressor for varying the volume of refrigerant drawn into the compressor per cycle thereby varying its capacity, positioning means for graduatingly positioning said movable means, said positioning means including a device influenced by the temperature at which the liquid refrigerant in the evaporator evaporates and acting to position said movable means in a manner to increase the volume of refrigerant drawn into the compressor upon increase in said evaporating refrigerant temperature and to decrease the volume of refrigerant drawn into the compressor upon decrease in said evaporating refrigerant temperature to thereby maintain the evaporating temperature of the refrigerant substantially constant, the evaporating temperature influenced device having a total range of operation greater than necessary for maintaining the evaporating temperature constant, and a controller responsive to the condition of the air in said space, said controller being arranged to shift the operating range of said device within its total range to cause different evaporating temperatures to be maintained for different values of said condition.

5. In an air conditioning system, in combinacontrol means also including a device influenced by the temperature at which the liquid refrigerant in the evaporator evaporates for controlling said switching means, said control means acting to position saidmovable means in a manner to increase the volume of refrigerant drawn into the compressor upon increase in said evaporating refrigerant temperature and to decrease the volume of refrigerant drawn into the compressor upon decrease in said evaporating refrigerant temperature, and a controller responsive to a condition of the air in said space which is affected by said evaporator, said controller acting to adjust said control means for varying the evaporating temperature of the refrigerant in accordance with variations in said space condition.

6. In an air conditioning system, in combination, an evaporator in heat exchange relationship with a space to be conditioned, means for supplyng liquid refrigerant to the evaporator, a positive displacement compressor connected to said evaporator, an adjustable clearance pocket for said compressor for varying the clearance volume of said compressor, a reversible electric motor for adjusting said clearance pocket, control means for saith reversible electric motor including a switching means for selectively'causing said motor to travel in either direction or to remain stationary, said control means also including a device influenced by the temperature at which the liquid refrigerant in the evaporator evaporates for controlling said switching means, said control means acting to cause decrease in the clearance volume upon increase in said temperature and to cause increase in clearance volume upon decrease in said temperature and a controller responsive to a condition of the air in said space which is affected by said evaporator, said controller acting to adjust said control means for varying the evaporating temperature of the refrigerant in accordance with variations in said space condition.

WAYLAND R. MILLER.

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