US2244551A

US2244551A - Air conditioning system

Info

Publication number: US2244551A
Application number: US198850A
Authority: US
Inventors: Robert B P Crawford
Original assignee: Honeywell Inc
Current assignee: Honeywell Inc
Priority date: 1938-03-30
Filing date: 1938-03-30
Publication date: 1941-06-03
Anticipated expiration: 1958-06-03

Description

J1me 1941- I R. B. P. CRAWFORD 2,244,551

AIR CONDITIONING SYSTEM Filed March 30 r 1938 3m2entor Robert 113.11 Crawford Ittomeg Patented June 3, 1941 AIR. CONDITIONING SYSTEM Robert B. P. Crawford, Miami, Fla., assignor to Minneapolis-Honeywell Regulator Company,

Minneapolis, Minn, a corporation of Delaware Application March 30, 1938, Serial No. 198,850

3 Claims.

' This invention relates to an air conditioning system, and is more particularly concerned with an air conditioning system for cooling and controlling the humidity of a space. Y

It is customary in such systems to. pass outdoor air over cooling coils whereby the temperaature as well as the humidity of the air may be reduced. 'Wheredehumidifi'cation of the air is required,itis necessary to cool the air below its dewpoint, with the result that reheating of the air may be required before the air is passed into the space. One method that has been used to reheat theconditioned air is to recirculate some of the air in the space and mix this air with the air that has been cooled. Another system has been to by-pass some of the fresh air around the cooling coils and mix it with the cooled air, thereby. reheating this air. In systems of this type, it iscustomary to control the temperature of the cooling coils in accbrdance with the amount of dehumidification that is required, and to control the proportion of air that is passed over the cooling coils, and the proportion of recirculated or by-passed air, in accordance with the temperature in the space being conditioned.

In a system of this type, as the proportion of fresh airand of return or by-passed air is varied, the load on the cooling unit varies, thus requiring a varying tonnage of the refrigerating unit. to cool the cooling unit. The variation inthe refrigeration tonnage may be so marked in such a system that when maximum tonnage is required to effect the desired amount of dehumidification, that due to the temperature within the space, there may be a very small amount of air passing over the cooling unit, so that a very small temperature rise in the cooling unit results, and the tonnage of the refrigeration system maybe reduced far below the maximum value obtainable when the maximumamount of fresh or outdoor air is going through the cooling unit. v

I have overcome this defect which is common to such systems by providing a fixed by-pass around the cooling unit so that the sameamount of air is at all times passed over the cooling unit and by-passed around the cooling unit. The cooling unit is controlled by, the outdoor wet bulb tempera'ture and'variations ,in the requirements for sensible heating are taken care'of ,in the first placebyproviding a refrigerant air heater andv liquid-coolen which: may conipris a coil in the h l ng chamber .dowhfstrea th ou h-which sionvaive of jthe from, the refrigerant passes. 'w

of the refrigeration system at a minimum. cooling unit may also be controlled from the outdoor 'dewpoint temperatures refrigeration system, whereby the air is reheated and the liquid in the cooling coil is further cooled to give a lower dcwpoint without affecting the delivery'air temperature. In this way, the delivery air temperature may be efiectively controlled without affecting the tonnage on the refrigeration system. As the outdoor wet bulb temperature varies, the demands on the cooling unit vary, and the tonnage may be controlled by controlling the expansion'valve and/or by employing a sluicing control for the cooling unit as taught in my Patent 1,673,552, issued June 12, 1928.

I have also devised various means for economically providing for further reheating of the conditioned air if necessary. The various means may be operated in sequence, the air heater liquid cooler above mentioned being the first one. The cooling water which has been used for precooling the air entering the air conditioning chamber may next be used for reheat purposes, this water having been warmed by removing heat from the air entering the chamber. This same water may also be used for cooling the condenser of the refrigeration system. If more reheating is required, some of the air may be caused to circulate through a jacket surrounding the motor which drives the compressor of the refrigeration system. In this way, the motor is cooled making for more efiicient operation thereof, and this heat, which would otherwise be wasted, is utilized for reheat purposes. Under severely abnormal conditions, it may be required to provide for further reheating of the air, and a steam coil may be used to provide this additional reheating. Under normal conditions, however, the other reheating means will be sufficient to reheat the air, so that the expense of reheating is reduced to a minimum.

It is accordingly an object of my invention to provide an air conditioning system wherein an air conditioning chamber having a cooling unit therein is provided with a fixed by-pass around the cooling unit, the cooling unit being controlled in accordance with outdoorwet bulb temperature, the indoor dry bulb temperature being controlled by suitable reheating means, whereby proper condititons of temperature and humidity are maintained within the space being conditioned, and whereby the load on the refrigeration system is unaffected by the requirements for sensible cooling in the space for a given outdoor Wet bulb temperature, thus keeping the tonnage The Another object of the invention is the provision of an air conditioning system wherein new and economical means for reheating the cooled and dehumidified air are provided.

Another object of the invention is the provision of an improved control arrangement for an air conditioning system.

Other objects will become apparent upon an examination of the specification, claims and appended drawing, wherein is illustrated a preferred form of the invention.

Referring more particularly to the drawing, an air conditioning chamber is represented generaliy by the-reference character ID. A fan H driven by a motor I2 is provided for drawing fresh air through the inlet I4 of the air conditioning chamber l0 and discharging it from the outlet l3 into a space to be conditioned. Located at the inlet I4 of the chamber In is a counter flow heat transfer coil l6 for precooling the air entering the chamber. This coil is connected by means of a pipell to a counter flow heat transfer coil l8 which serves to reheat the air which passes over the cooling elements to be described.

The means for cooling and dehumidifying the air passing through the chamber I0 may comprise a coil which forms the evaporator of a refrigeration system. This system may include a compressor 2| driven by a motor 22, compressor 2| being connected to a condenser 23 by means of a pipe 24, the outlet of this condenser being connected 'by means of valve 25 and pipe 26 to the expansion valve 21 from which expanded refrigerant flows to the evaporator 20. The evaporated refrigerant flows through a pipe 28 from the outlet of the evaporator to the inlet of the compressor 2|. The compressor motor 22 may be controlled in any well known manner, and for purposes of illustration I' have shown a suction pres-sure controller 30 which may comprise a bellows 3| connected by a tube 32 to the inlet of the compressor, this bellows operating a mercury switch 33 mounted on a lever 34 biased against the bellows by means of a spring 35. As the suction pressure of the compressor drops to a predetermined low value, the bellows 3| contracts and tilts the mercury switch to the circuit breaking position, thus stopping the operation of motor 22. will be understood that any other conventional controlling means for the motor 22 may be used.

The condenser 23 is cooled by water which flows through the precooling coil I5 and pipe I1. This water may flow either through the reheat coil 18 or through a by-pass 38 around the coil I 8 and through the pipe 39 to the condenser 23, the water leaving the condenser through a pipe 4.0.

Thecooling effect of the evaporator 20 is controlled by the expansion valve 2'! in a manner to be described and also by means of the spray 42 mounted in front or up stream of the evaporator 20. Water is sprayed from the spray 42 onto the evaporator 20, the spray 'being controlled by means of a valve 43 controlled in a manner to be described. The amount of water sprayed onto the evaporator 20 will control the cooling and dehumidifying effect of the evaporator as more fully set out in my Patent 1,673,552,

issued January 12, 1928. The-cooling effect of the evaporator will decrease as the formation of frost on the evaporator increases. By spraying water onto the evaporator, the formation of frost This form of control is well known and it U on the evaporator may be reduced with the result that the cooling effect thereof is increased as the Mounted in the path of air leaving the evaporator 20 and flowing through the by-pa'ss 45 is a set of eliminator plates 46 for removing particles of water from the air. A suitable sump may be provided below the eliminator plates, this sump being provided with a drain 48. These eliminator plates may be-of any suitable form such as the form illustrated in the drawing, or they may be shaped as illustrated in my aforementioned patent. Mounted just down stream of these eliminator plates is a coil 50 for reheating the air leaving the evaporator 20. The inlet of this coil is connected by means of the valve 25 with the outlet of the condenser, and when the valve 25 is properly set the refrigerant leaving the condenser will first flow through the coil 50 before it reaches the expansion valve 21. Valve 25 is a three-way valve and in one extreme position all of the refrigerant will flow through the coil 50,

in the other extreme position'none of the refrigerant will flow through this coil, and between these extreme positions the amount of refrigerant flowing through the coil will .be proportional to the setting of the valve. It will be understood that when the refrigerant is caused to flow through the coil 50, the refrigerant will be further cooled below the temperature at the condenser, in this manner reheating the air passing thereover and causing therefrigerant entering the evaporator 20 to be further cooled, thus increasing the cooling and dehumidifying effect of the evaporator.

The coil 18 is mounted just downstream of the coil 50 and forms a second reheat means for the cold and dehumidified air. The flow of water through this coil is controlled by a three-way valve 52 similar to the valve 25. In one position of this valve, all the water flowing to the condenser 23 flows through the pipe 38 which is mounted outside of the chamber l0, and in the other extreme position the water all flows through the coil I8. used for precooling the air by means of the coil I6, it will be understood that this water is relatively warm when it reaches the coil l8 and will have a heating effect on the air passing thereover and this water will alsobe cooled so as to increase the efficiency of the condenser 23 when the water flows through the-coil Hi.

The heat generated by the driving motor 22 for thecompressor 2| may also be utilized for reheating the air. A casing may be provided about the motor through which air is caused to circulate by means of conduits 52. and 53. The

flow of airthrough these conduits may be con- .dampers are moved towaids closed position some of the air will tendto flow through the conduit It will, however, be of a size which,

Since this water has been 52 to the motor casing and thence through the conduit 53 back to the chamber II). In this manner, the heat generated by the motor is effective- 1y utilized and at the same time the motor is cooled.

Under normal conditions the reheating devices described will be sufiicient to maintain the temperature of the air leaving the chamber I sufiiciently high for proper comfort conditions in the space I5. Under extreme conditions, however, it may be necessary to provide for further reheating of the air and for this purpose a steam coil 55 may be provided, the flow of steam through this coil being controlled by a valve 56.

Mounted directly up stream of the eliminator plates 46 is a dewpoint controller which may be composed of a bulb 58 connected by means of a capillary tube 59 to a bellows 60, this tube, bulb, and bellows being filled with a suitable volatile fluid whereby variations in the temperature in the bulb 58 will cause the bellows 60 to expand or contract. Since the bulb 58 is located just in front of the eliminator plates, it will be subjected to a spray caused by the separation of the water particles from the air and this bulb will, accordingly, respond to the dewpoint temperature of the air flowing thereover. A bellcrank lever pivoted at H has an arm 62 biased against a pin or projection 63 connected to the top of the bellows by means of a spring 64. The other arm 65 of the lever is arranged to sweep over a resistance element 66, the portion of the arm which engages the resistance being suitably insulated from the rest of the lever. As the dewpoint temperature in the bulb 58 decreases, the bellows 60 will contract and the arm 65 will move to the left under the influence of the spring 64, and conversely as the dewpoint temperature increases arm 65 will be caused to move to the right. This controller is arranged to control the operation of a step controller generally indicated by the reference character I6. This controller may include a proportioning motor II which may be of the type illustrated in Patent No. 2,028,110 issued to D. G. Taylor on January 14, 1936. Briefly, this motor may comprise a pair of armatures operated by selectively energized field windings, the direction of the rotation of the motor depending upon which of the field windings is energized. A pair of opposed balanced relay coils is connected together at one end, the junction of these coils being connected to the motor terminal I2 and the opposite ends of the coils being connected to the motor terminals I3 and I4. These relay coils control the energization of the two field windings and when one coil is energized to a greater extent than the other, the motor is caused to rotate in one direction or the other depending upon which of the coils is the more highly energized. When the coils are equally energized, neither field winding is energized and the motor remains at rest. Leads and I6 are connected to a suitable source of power, not shown, and'serve for energizing the field windings as well as the relay coils, the ends of the relay coils being connected to the source of power. The resistance 66 is connected in parallel with these relay coils, the left end of the resistance being connected to the terminal I3 by means of conductors II and I8, and the right end of the resistance being connected It will now be apparent that when the arm 65 is in the mid position illustrated, the relay coils connected to the motor terminals will be equally energized and the motor will remain at rest. If the arm 65 is caused to move to the right or left by reason of a change in the dewpoint temperature of the air passing over the bulb 58, the relay coils will become unbalanced and the motor will be caused to rotate. The motor II also includes a balancing potentiometer which is connected in parallel with the relay coils, the arm of which is caused to move by the motor II in a direction to counteract the effect of the potentiometer 66 so that after the motor has operated a sufficient length of time, the balancing potentiometer will counteract the effect of potentiometer 66 and the relay coils controlling the operation of the motor will again be equally energized. In this manner, the motor II is caused to operate an amount proportional to the movement of arm 65 over the resistance 66 and in a direction depending upon this movement.

A second potentiometer 85 is connected in parallel with the potentiometer 66. The control arm 86 of this potentiometer is pivoted at 81 and has a portion 88 biased by a spring 89 against a pin 90 projecting upwardly from the bellows 9 I. This bellows is connected by means of a capillary tube 92 to a bulb 93 which is mounted outdoors, the tube, bulb, and bellows being filled with a suitable volatile fluid. The bulb 93 responds to the wet bulb outdoor temperature and may be provided with a suitable cloth 94 wrapped around the outside thereof, this cloth being kept moist by means of a tank 95 or any other suitable and 1 well known means may be employed for causing to terminal I I by means of conductors I9 and the bulb 93 to respond to the wet bulb outdoor temperature. An increase in outdoor ,wet bulb temperature causes expansion of bellows 9| and movement of arm 86 to the right over resistance 85. 'Upon a decrease in outdoor wet bulb temperature, arm 86 moves to the left over resistance 85. If desired, bulb 93 may be replaced by the bulb of a dewpoint temperature controller or other device for indicating dewpoint, such as described in Labus and Crawford Patent No. 2,106,101 issued January 18, 1938. The arm 86 is connected by means of the conductor 98, variable resistance 99 to the junction of conductors 8i and 82. The right end of the resistance is connected by conductor II]! to the junction of conductors I1 and I8 and the left end of the resistance 85 is connected by means of conductor I02 to the junction of conductors I9 and 88. Since the potentiometers 85 and 66 are connected in parallel to the motor II, it will be apparent that each will exercise a controlling function on the motor. By reason of the resistance 99 connected in series with the arm 86 of the wet bulb responsive potentiometer, this potentiometer will have less effect for a given movement thereof than will the potentiometer 66. In other words, a small movement of the arm 65 will cause the motor II to operate from one extreme position to the other whereas it will take a relatively great movement of the arm 86 to cause the same movement of the motor. Potentiometer 85 is therefore a compensating potentiometer and has the effect of shifting the control range of the potentiometer 66 in accordance withvariations in outdoor wet bulb temperature. In other words, as the wet bulb temperature rises causing movement of arm 86 to the right, the controlrange of the potentiometer 66 will be shifted to the right so that a higher dewpoint temperature will be maintained at the bulb 58. The'resistance 83 insures that the operating diflerential of the arm 65 will be substantially the same regardless of whether the control point is at the center of the resistance 66 or at either end thereof.

Operated by the motor H are switch arms I05 and I06 cooperating with resistance elements I01 and I08, respectively. These resistance elements are so arranged as to be sequentially engaged by the arms. In other words, the resistance element I08 is not engaged by the arm I06 until the arm I05 has moved from the extreme left end of the resistance I01 to the extreme right end, as illustrated. Arm I05 and resistance element I01 from a control potentiometer for a proportioning motor IIO similar to the motor H. An arm III is operated by the motor H and is connected by means of a link II2 with the stem II3 of valve 21. III and consequently of the valve 21 will depend upon the relative position of the arm I05 with respect to the resistance I01. When the arm I05 is in the extreme right end of resistance I01, as illustrated, the valve .21 will be in wide open position and as the arm I05 is caused to move to the left the valve 21 will close an amount proportional to this movement.

The arm I06 and resistance I08 control a proportioning motor II5 which is connected by means of arm H6 and link II1 to stem IIB of valve 43. When the arm I I6 is in the position illustrated, the valve 43 is in its closed position but as the arm I06 begins to move over the resistance I08 the arm H6 is rotated counterclockwise an amount proportional to this movement and the valve 43 is opened an amount proportional to the extent of the movement of arm The operation of the evaporator 20 and spray 42 may now be briefly summarized as follows: assuming that the outdoor wet bulb temperature is at 70, the dewpoint controller 58 will operate to maintain a dewpoint temperature of approximately 54. Should the dewpoint temperature rise above this value, arm 65 will move toward the right and cause operation of motor H in a direction to move the arms I05 and I06 in a clockwise direction. Valve 21 is already wide open and further movement of arm I 05 in a clockwise direction will have no effect on the motor IIO. As the arm I06 moves in a clockwise direction, the motor II5 operates to open the valve 43 an amount proportional to the extent of this movement whereupon water is sprayed onto the evaporator 20 ata rate depending upon the movement of the arm I06, thus increasing the dehumidification eifected by the evaporator 20 in the manner set forth in my aforementioned patent. Should the dewpoint temperature at 58 decrease below the desired temperature, motor IIO will operate to move arms I05 and I06 in a counter-clockwise direction. The water issuing from spray 42 will gradually decrease and if the dewpoint temperature has not risen to the desired value when this water is completely shut off, the controller I05 will start to operate the motor III) to shut down on the expansion valve 21, thus decreasing the amount of refrigerant supplied to and the dehumidifying efiect of the evaporator 20 still further until the dewpoint temperature at 58 has reached the desired value. As the outdoor wet bulb temperature rises, the control point of the potentiometer 66 will rise to maintain a higher dewpoint at 58, and conversely, if

. trol point of the potentiometer 66 is lowered.

The position of arm the outdoor wet bulb temperature falls the con- 75 The amount of dehumidification and cooling required by the evaporator 20 will depend primarily upon the outdoor wet bulb temperature as will be apparent since this temperature is a measure of the total heat within the air and it will therefore be seen that for a given outdoor wet bulb temperature the load on the refrigeration system will remain substantially constant since the amount of air passing over the evaporator is the same at all times.

Mounted within the space being cooled is a thermostat indicated generally by the reference character I 20. Thisthermostat may comprise a bimetallic element I2I having an arm I22 connected thereto and operated thereby in accordance with variations in the dry bulb temperature of the air within the space I5. Operated by the thermostat I 20 is a proportioning motor I25 of a step controller generally designated by the reference character I26. The terminal I28 of this motor is connected by means of conductors I30 and I3I to the center tap resistance I32 over which the arm I22 of the thermostat slides. Motor terminal I21 is connected by means of conductors I33 and I34 to the right end of a resistance element I35 which forms the control potentiometer resistance for the motor I25. The opposite end of this resistance is connected by means of conductors I36 and I31 to the terminal I29 of the motor. If the temperature within the space I5 varies, the motor I25 operates in the same manner as the motor H as will be apparent. The effect of the thermostat I 20 on the motor I21 may be compensated by the outdoor dry bulb temperature responsive controller I40. This controller may comprise a bellows I4I connected by means of capillary tube I42 to the bulb I43, this tube, bulb, and bellows being provided with a suitable volatile fill. The control arm I45 is biased by means of spring I46 into engagement with the pin I41 carried by the bellows and this arm is caused to move over the resistance I50 in response to changes in the outdoor dry bulb temperature. The arm I45 of this controller is connected by means of conductor I5I, variable resistance I52 and conductor I53 to the junction of conductors I 3| and I30. The right end of this resistance is connected by means of conductor I54 to the junction of conductors I33 and I34, and the opposite end of this resistance is connected by means of conductor I55 to the junction ofconductors I36 and I31. It will, accordingly, be seen that the resistance I50 and arm I45 form a potentiometer connected in parallel with the potentiometer I35 to the motor I25 and this controller serves to shift the control range of the controller I20 in accordance with variations in outdoor dry bulb temperature. Thus as the outdoor dry bulb temperature falls, the control range of the thermostat I20 will be lowered 0r shifted towards the right. It will therefore be seen that the motor I25 willoperate in accordance with the outdoor dry bulb temperature to maintain the indoor dry bulb temperature at a value which depends upon the outdoor dry bulb temperature.

Operated by the motor I 25 are the arms I 60, NH, I62, and I63 which are arranged to move sequentially over the resistance elements I64. I65, I66, and I61, respectively. The arm I controls the operation of a motor I 10 which may be a proportioning motor. An arm I1I operated by the motor I 10 is connected by a link I12 to the stem I13 of the three-way valve 25. When the arm I60 is in the position illustrated, the arm III is in one extreme position and the three-way valve is likewise in an extreme position in which no refrigerant is permitted to flow through the reheat coil 50. Should the indoor dry bulb temperature start to fall below the desired value, the arm I60 is moved in a clockwise direction by the motor I25 which in turn causes the motor I'I0 to operate to move the valve 25 in a direction to permit the flow of refrigerant through the coil 50, the amount of refrigerant flowing through the coil depending upon the extent of movement of the arm I60.

The arm I6I controls the operation of the proportioning motor I15, the arm I16 which is operated thereby being connected by a link IT! to the valve stem I18 of the three-way valve 52. In the position illustrated, the three-way valve is in one extreme position in which no water is permitted to flow through the coil I8 but all the water flows through the by-pass 38. Should the temperature in the space I drop sufiiciently so that arm I6l begins to move over resistance I65, this valve is operated to permit water to flow through the coil I8 at a rate proportional to the amount of movement of arm I6I over the resistance I65.

The arm I62 controls a proportioning motor I80 having an arm I8I operated thereby and connected by means of a link I82 to the dampers 54. When the arm I62 is in the position illustrated, the dampers are positioned to permit maximum flow of air therethrough. These dampers will begin to move towards closed position as the arm I62 begins to move over the resistance I66 thus-causing some of the air to flow through conduit 52, motor casing 22, and

conduit 53. The arm I63 controls the operation of the proportioning motor I85, the arm I 86 of which is connected by means of the link I81 to the stem I88 of the valve 56 which controls the supply of steam or other heating medium to the coil 55. This valve will remain in closed position until such time as the arm I63 begins to move over the resistance I61 whereupon it will begin to open and permit the flow of steam or other heating fluid through the coil 55, the amount of steam which flows through this coil being dependent upon the position of the arm I63 with respect to this resistance I61.

It will be noted that the resistances I64 and I61. are so arranged that each successive resistance will not be engaged by its control arm until the control arm of the preceding resistance has moved through the full range of the resistance. In other words, resistance I65 will not be engaged by the arm I6I until arm I60 has moved to the extreme right end of resistance I64 and so on. It will therefore be seen that. the various reheating devices will be operated sequentially. Thus if the space temperature drops below the desired value, the coil 50 will be utilized for reheating purposes and if after the valve 25 is positioned to permit all of the refrigerant flowing to the evaporator 20 to flow through the coil 50 the space temperature has not risen to the desired value, then the Water flowing through pipe I! will begin to flow through the coil I8 and additionally reheat the air passing thereover. If the temperature in the space is still too low after all the water flowing through the pipe I1 has passed through the coil I8, the dampers 50 will begin to move towards closed position and cause some of the air passing through the valve 56 starts to open so that the desired degree.

of reheating will be accomplished without the necessity of utilizing any extra heat supply,-thus increasing the efliciency of the system appreciably. The use of the various reheating devices with the e'..ception of the coil 55 all tend to increase the efliciency of the system. The flow of refrigerant through the coil 50 will tend to lower the temperature of the coil 20 thus increasing its capacity and the flow of water through the coil I8 will increase the cooling efiected by the condenser 23 which will increase the efliciency of the refrigeration system. Also by causing a circulation of air through the easing of motor 22, this motor will be cooled and the heat generated therefrom will serve a useful purpose which also increases the eiflciency of the system.

With the various control devices in the positions illustrated, the outdoor wet bulb temperature is approximately 70 and the outdoor dry bulb temperature is approximately 100", the indoor dry bulb temperature is approximately and the dewpoint at 58 is about 53 The amount of refrigerant flowing through the evaporator 20 is at a maximum, the spray 42 is shut off and none of the reheating devices are in operation, the by-pass 45 providing the necessary amount of reheat to maintain the space temperature at the desired value which may be assumed to be 85 for an outdoor dry bulb' temperatureof As the outdoor dry bulb temperature drops, the control range of thermostat I20 will shift to the right and when the outdoor dry bulb temperature drops to 70, the indoor dry bulb temperature will be lowered from 85 to 72. Likewise, as the outdoor wet bulb temperature drops to'60, the control point of the dewpoint controller 58 will drop to 50 and as the outdoor wet bulb temperature rises to 80, the control point of the dewpoint controller will rise to 57.

Since there is always a uniform amount of air flowing past the evaporator 20 and since the dehumidifying effects of the evaporator is contro'lled in accordance with the outdoor wet bulb temperature, the available tonnage of the refrigeration system available for dehumidification may be kept at a maximum for maximum requirements. Should the sensible load within the space I5 decrease due to a decrease in the sun eliect or a decrease in the number of persons within the space without any change in the outdoor dry bulb temperature the temperature of the air entering the chamber I5 will have to be increased in order to compensate for the decreased sensible cooling load. The valve 25 will be operated to increase the amount of refrigerant flowing through the coil 50 which will tend to raise the temperatlre of the air passing thereby and at the same time it will lowerthe cooling efiect of the evaporator 20, which will tend to slightly lower the dewpoint temperature at 58. Since the air leaving the chamber I0 has a somewhat higher temperature but a somewhat lower dewpoint, the relative humidity of the air will be maintained at a comfortable value and it is therefore seen that the relative humidity of the space is effectively controlled without the provision of any humidity responsive devices.

The provision of the variable resistances 99 and I 52' in the circuits to the control arms 86 and I45 make possible the adjustment of the effect of these controllers with respect to the controllers 65 and I20. Other variable resistances may be also provided in the circuits to the controllers 66 and I 20 to shift the control range of these controllers for a given position of control arms 86 and I45, if desired.

It should be understood that the temperature values maintained by the various controllers are illustrative only and any desired temperature schedule may be set up in actual practice.

While I have illustrated a preferred form of my invention, many modifications may become apparent to those skilled in the art and I wish it to be understood that my invention is limited only by the. scope of the appended claims.

I claim as my invention:

1. In an air conditioning system, an air conditioning chamber, means including an evaporater of a refrigeration system for cooling and dehumidifying air passing through said chamber, said refrigeration system also including a. condenser, a precooling coil in the path of fresh air entering said chamber, means utilizing the fluid leaving said coil for cooling said condenser, means responsive to the conditions in the space being cooled, and means under the control of said last named means for utilizing the fluid leaving the precooling coil for reheating the air leaving the cooling means before the fluid reaches the condenser.

2. In an air conditioning system, the combination of, an air conditioning chamber, cooling and dehumidifying means for cooling and dehumidifying air passing through said chamber including a refrigerating apparatus having evaporator means for cooling and dehumidifying the air, a

condenser for condensing refrigerant, a compressor for circulating refrigerant through the evapoator means and condenser and an electric motor for operating the compressor, a precooling coil in the path of fresh air entering said chamber, means utilizing the fluid leaving said coil for cooling said condenser, reheating means including means utilizing heat generated by the electric motor and the fluid leaving the precooling coil before the fluid reaches the condenser for reheating the cooled and dehumidified air, a controller responsive to the temperature of the cooled and dehumidified air for regulating the cooling and dehumidifying action of the cooling and dehumidifying means, and a controller responsive to the dry bulb temperature of the air in the space being cooled for controlling the reheating means to regulate the dry bulb temperature of the space, both of said controllers coacting to maintain desired dry bulb temperature and relative humidity conditions and hence effective temperature conditions in the space.

3. In an air conditioning system, the combination of, an air conditioning chamber, cooling and dehumidifying means for cooling and dehumidifying air passing through said chamber including refrigerating apparatus having an evaporator and a condenser, a precooling coil in the path of fresh air entering said chamber, means utilizing the fluid leaving said coil for cooling said condenser, reheating means including means utilizing the fluid leaving the precooling coil before the fluid reaches the condenser for reheating the cooled and dehumidified air, a controller responsive to the temperature of the cooled and dehumidified air for regulating the cooling and dehumidifying action of the cooling and dehumidifying means, and a controller responsive to the dry bulb temperature of the air in the space being cooled for controlling the reheatmg means to regulate the dry bulb temperature of the space, both of said controllers coacting to maintain desired dry bulb temperature and relative humidity conditions and hence effective temperature conditions in the space.

ROBERT B. P. CRAWFORD.