US2939296A

US2939296A - Air cooling system

Info

Publication number: US2939296A
Application number: US758624A
Authority: US
Inventors: Robert C Coblentz
Original assignee: Individual
Current assignee: Individual
Priority date: 1958-09-02
Filing date: 1958-09-02
Publication date: 1960-06-07
Anticipated expiration: 1977-06-07

Description

June 7, 1960 R. c. COBLENTZ 2,

AIR COOLING SYSTEM Filed Sept. 2, 1958 5 Sheets-Sheet 1 IN VENTOR EM c 6% W n 1960 R. c. COBLENTZ 2,939,296

AIR COOLING SYSTEM Filed Sept. 2, 1958 5 Sheets-Sheet 2 IN VENTOR BY W md/l ATTORNEY June 7, 1960 R. c. COBLENTZ 2,939,296

AIR COOLING SYSTEM Filed Sept. 2, 1958 m 5 Sheets-Sheet 3 z FIG. 4

IN VENTOR ATTORNEY June 7, 1960 R. c. COBLENTZ 2,939,296

AIR COOLING SYSTEM Filed Sept. 2, 1958 5 Sheets-Sheet 4 FIG.5

INVENTOR WWO Maw BY H ATTORNEY R. C. COBLENTZ AIR COOLING SYSTEM June 7, 1960 5 Sheets-Sheet 5 Filed Sept. 2, 1958 FIG],

2 3 .7 4 a 6 0% fl/ f rJ N r 5 m mm W 2 I. 3 o 7 7 v 5 a w W a L .F wmm a w smm w w HH- 0 WWV 7 mm 5 .v a 6 u M I N 1 N m M u 5 F m u 7 n 7 a a A u m 7/ /WJ 2 7 V 5 E M w z 0 7 T n w F 222m wwwwm Y M 2 0M R T P U H INVENTOR.

FIGS

nited States ..2-,9s9,296 .AIR {COOLING SYSTEM Rohert QCoblentz, 1226 W. 9th St., Erie, Pa. "Fnease m, 195s, SenNo. 758,624

a Claims. 62-180) for removalof moisture and accordingly a smaller unit will achieve-the same comfort level without asgreat .a reduction in;the room .a r temperature.

In the accompanyingfdrawings, Fig. l is a diagramof an air conditioning installation;JFig. 2 is a diagram of basement installation; Fig. 3 shows the recirculation control applied to ,a vertical packaged air 'conditioningunit atent 2' I where the air isblown down over the cooling coil; Fig. 4

shows the recirculation control applied toa vertical packaged air conditioning unit Where the air is sucked up over the cooling coil; Fig. 5 'shows two fans, one for controlling the air delivery and the/other for controlling the recirculation; Fig.6 is a diagram of an air conditioning system; Fig. 7 is a diagram of aconventional air conditioning system for achieving results comparable to Fig; 5; and Fig. 8 is a performance diagram forthe Figs. 6 and 7 systems. a

The conventional elements of the air conditioning unit are readily .identified, 1 being the motor driving the

compressor

2, 3 being the condenser, 4 the evaporator or cooling coil, 5 the fan or'blower for circulating .air 'over the sub-freezing temperature evaporator, and 6 the control box. The fan Sruns continuously discharging air to the room or air conditioned area 7 through a duct 8. A floating damper 7a opens at'high rates of flow and closes at low rates of flow thereby insuring suflicient velocity to maintain the proper distribution in the room. Air is returned to the unitthrough a return duct 9 and through an outside air intake duct lfl'controlled by a damper 11. A filter 12 is arranged in the return air duct downstream of the fresh air intake. At a suitable point in the room is a thermostat 13 which ordinarily would control the refrigerating unit either by a valve 14 in the line from the condenser 3 to the evaporator 4 or by directly controlling .the motor 1. When the valve 14 is used to control the refrigerating unit, the motor 1 is controlled by a pressure switch 15 which shuts off the motor whenever the valve 14 is closed and causes a rise in pressure in the compressor output. With the ordinary connection'of the thremostat the room air temperature will have'to be set relatively low to obtain the desired comiortlevel on -hot humid days and the relative humidity in the room will be high. In the present system, the thermostat 13 controls a reversible damper motor 16 aving an arm 17 connected by a link 18 to dampers 19 in a bypass duct 20'which connects thecold air discharge duct 8 back to the intake duct 9. The dampers 19 may be supplemented by a hand damper 21. The combined eifect of the dampers 19 and 21 is to recirculate from 10% to 90% of the cooled air to the comfort level.

"ice

back over the evaporator 4 thereby removing additional moisture without droppingthe 'room' temperature. This permits a higherfisettingbfthe'thermostat 13 without lowering the comfort level. V

The damper 19 is 'of"the--m'odulating or proportioning type, the'motor 16' turning in the direction to open the damper 19 'andd'ncrease the airbypassed through duct 20 when theroom'thermostat 13 is satisfied and turning in the reverse direction when the room thermostat calls for cold. The jresult'ofivarying the amount of air bypassed throughthe 'duct 20 is't'o vary'the quantity of cooled air discha 'gedto'the room 'andto thereby maintain the air flow to" the room atthevalue required for the cooling loafd. Since ,the'air bypassedxthrough the duct 20 loses additional moisture 'asit again flows over the evaporator 4,:the a'ir di'scharged'tothe room will contain less moisture an dth'e relative humidity'of'the room will be less. Furthermore the capacity of the cooling unit'will be materially less. In a typical installation utilizing a 10- ton unit with the fan"5 moving 4,000 c.f.m., the heat removal was9l,600 B.t.u./hr..sensibleheat and 31,000 B:t .u;/hr. "latentheat. 'By changing to a 5 -ton unit with thei'fan 5 moving Z000 icf;m.,'800 c.f.m. being discharged to theroom 1,200 .c.f.m. being' recirculated through the duct20,Ztheheat'removal'was'3l,000 B.t.u. hr. sensiblehe'at and 31*,00'0 'B.t'.u./hr. latent heat. 'While the room air temperature was higherwith the S-ton unit, the comfort level was the samej becauseth'e relative humidity was ,lower with'the recirculating system.

With therecirculating'duct20, theair conditioning unit isused'more'effectively for humidity removal. the roomthermostat 13 is satisfiedthe unit keeps running lowering the humidity of'theair. There'are two controlswhich'shut off the unit, a low temperatureshut-ofi thermostat 22 which closes the'solenoid'valve '1'4Whenever the recycled airdrops to 35' 'Ffiand a wet bulb thermostat or humidistat "23 which 'likewis'e"shuts the solenoid valve when'the room wet bulb temperature drops At all other times, the unit runs and is utilized for cooling and humidity removal as determined'by the setting of the damper1'9. 'Thel'owtem perature shut-0d, while primarily "tor preventing .f 'eezing 'or frosting of the evaporator, is to "some extent .a measure of the 'wet bulb temperature. As the'proporti'on 'ofair'recirculatedgoes up, the air discharged to the room becomes drier and the recirculated air temperature becomes lower. The drier air discharged to the room drops the wet'bulb temperature so 'both controls 22 and 23 are actuated to shut down the unit by the same factors. By varying the percentage of air bypassed or recirculated, the low temperature shut-oil 22 can serve the same function as the humidstat or wet bulb thermostat 23. In fact, the low temperature thermostat 22has proven to be a more reliable and more accurate control for humidity than a humidistat and the humidistat 23 has been omitted in actual practice. It will be noted that the air flowing over the evaporator is always at lower than room'air temperature because it is a' mixture of room air and oithe cooler recycled air. Thishas' anunexpected advantage in maintaining constant humidity under fluctuating loads and also in providing ,a modulated control.

Underlight loads, the recycled air is quickly cooled to V essence All of the systems require smaller refrigerating units for the same comfort level. "The units are used more effectively for dehumidificatiou and the same comfort level is achieved with a higher temperature of the room air; The amount of sensible heatremoved is less and the amount of latent heat'removedis greater than in conventional air conditioning =systems.- 'There is no need to reheat the air as in some installations where dehumidification is achieved by cooling'the air to a low temperature and then reheating before discharge to =the room. The dehumidification is particularly improved at light loads where the quantity of air discharged to the room is kept at a low value.

The evaporator or coolingheat exchanger can be physically smaller because of the higher air velocities which increase the heat transfer. This is due to the recirculated air.

No particular refrigerating system jis required. Although the compression type has been illustrated, the absorption type is entirelyequivalent and in many installations is preferable particularly-there is 'available heat which would'otherwise'be'wasted.

This application claims features disclosed but not claimed-in-copending Patent-2,632I3l5, March 24, 1953.

' Fig. ;6 Shows an air conditioningsystem embodying the basic features 'of the systems o'fFigs. 1 to 5 inclusive. In this system the refrigerating unit '50 supplies a cooling heat exchanger 51 mounted within )3. casing 52 between the return air duct 53 from the room 54 and the fan or blower 55 'for'discharging Cooled air through duct 56*to theroom. In some installations the casing 5,2'with the refrigerating 'unit' SQ Lmaybe in the room While in other cases the casing .and refrigerating unit may be a long distance from .the room,both locations being com mon practice in air conditioning. No control is shown for the fan or. blower 55 .because it normally runs all of .thetime and discharges air to the room at a rate sufficient to' provide a complete change of air every six to eight minutes. air conditioning units.

The parts so far described are standard.

In order to maintain constant low relativehumidity in the room, as is desirable for comfort, there is connected between the cool air duct 56 and the return air duct 53 a'bypass duct 57 which, for example, may return half of the cooled air entering the duct 56 to the return air duct 53. In practice, the duct 57 is usually made over size and the flow adjusted by a damper as is common in heating and cooling systems. Once this adjustment is made at the time of installation, no further adjustment is necessary. In packaged units, the manufacturer would make the recycling duct 57 to the correct size and would not need a damper because the length of the recycling duct would not be subject to variations in length encountered in field installations.

The refrigerating unit is controlled primarily by a thermostat 58 which monitors the cooled air temperature. This thermostat may, for example, be adjusted to respond to a temperature of 40 plus or minus 5 F. and when so adjusted will turn the refrigerating unit on when the cooled air temperature rises to F. and will turn the refrigerating unit off when the cooled air temperature drops to 35 F. Other ranges of adjustment are possible, such as, for example, 45 plus or minus 5 F., plus or minus 5 F. Because the temperature of the cooled air determines the moisture content, it provides an accurate control for the relative humidity of the room. Under commercial practice, there is a complete change of air every six to eight minutes which means that the moisture content of the air in the room quickly falls or approaches the moisture content corresponding to the temperature of the cooled air. It has been found that the cooled air has less moisture than would be expected if the cooled air -had 100% relative humidity. This means that the moisture content of the roomairis 6 ordinarily'lessthan the theoretical maximum amount of moisture which could be suspended or retained in the cooled air.

There are many locations for the thermostat 58, all of which are equally acceptable because all monitor the cooled air temperature. The thermostat 58 may be placed above-or on thedownstream side of the cooling heat exchanger or evaporator 51 because the air leaving-the heat exchanger is all ata temperature of 4O" plus or minus 5 The thermostat 58 can also be placed anywhere in the fan housing 55 or in the duct 56 or in the recycling duct'57. In the case of a remote unit installation, the duct 56 might be 100 or more feet long and the thermostat 58 could be at the end of the duct 56 or 100 or more feet-from the refrigerating unit. Although this could be done, it ordinarily would not be done because there is no need for having the extra wiring. The preferred locations for the thermostat 53 are above the cooling heat exchanger 51 or in the recycling duct 57. In these locations there is substantially no stratification of the air and'there is little need for care in selecting the location of-the thermostat. There is some stratific'ation on the upstream side below the cooling heat exchanger 51 but there are locations on the upstream side wherethe thermostat 58 can .be placed and will respond to the cooled air temperature and monitor the cooled air temperature. These locations require a great deal of skill in selection and are more difiicult. These locations perhaps-would be satisfactory for p ackaged units where the manufacturer could accurately control the position of the thermostat andcould insure that no one-would mcvethe thermostat'from the selected position. In other locations on the upstream side of the heat exchanger -51 the temperature will be the return air temperature or somewhere between the return air and cooled air temperatures. Those skilled in the thermostat art are able to make a thermostat respond to or monitor a temperature different than the actual 'airtemper'atureat the thermostat location. Thermostat 53 should be placed to monitor the cooled air temperature, regardless ofwhatthe room air temperature may be.

:In a typical installation where the thermostat 58 monitors the cooled air temperature to 40 plus or minus 5 F., the room temperature might be 78 F. plus or minus a very-much smaller amount dueto the much greater volume of'the room as compared to the volume of the casing '52 and the average temperature of the mixture of recycled air from the duct .57 and return air from the room might be 60 F. plus or minus something less than 5 F. v

The performance of the Fig. 6 system is shown in Fig. 8. Under full load corresponding to a very hot day when the unit has to run all of the time, the room temperature indicated by line 59 might be F. while the roomhumidity indicated by line 60 might be 40%. These values remain constant so long as the air conditioning load remained at its maximum.

.Under'light load positions shown at the right in Fig. 8, the room humidity indicated by line 61 remains at the same value as that indicated by line 60. This is so because the moisture content is determined by the cooled air temperature which is monitored by the cooled air thermostat '53. The setting of the thermostat 58 determines the moisture content ofthe air Whether the air conditioning load is light or heavy. The humidity accordingly remains the same under all load conditions. it should be noted that the thermostat 58 monitors the cooled air'temperatnre and has no way of knowing what is the temperature in the room or the temperature on the upstream side of the evaporator or cooling heat exchanger 51. While it would be expected that under light load conditions the room temperature would fall to a very low value, surprisingly that has proven not to be the case. The room has so much greater volumethan the casing 52 .for the air conditioning unit that-under long time. From one, aspect, under light load condi: tions the air conditioning ;unit.is effect bringing the temperaturewithin the casing SZ-down to the operating pointof the thermostat, 58. I

Another eflect of the thermostat 58 is that it breaks the cycle of operation of the air conditioning unit up into shorter cycles under light load conditions. With a conventional air conditioning unit under conditions which required operation of'the unit for one third of the time, the unit might be on; for 20 minutes and oir' for 40. minutes. .Underthe samecqnditions the thermostat. 58" mightxtuin the conditioning unit on for '5 minutes and'off for 10 minutes. 7 W Under heavy load conditions,it.- takes a longtime to 7 bring thecooled air in the casing-52 down to the operating point of the thermostat 58 Whilethe heavyair con: 7 ditioning'load quickly brings the cooled air to the upper operating point oigthe thermostat 58 it has shut the refrigerating unit ofi. V a a Under light load conditions, the room temperature may follow a'cnrve suchasrindicated at dz where the roomairtemperature will gradually drop 2 or 3 or 4 degrees; This might happen during a cool spell or at 1 night when the air conditionin-g load drops and is not serious. If this sma-ll change in roorn air temperature is objectionable, the user can turnthe' refrigerating unit by s n athamwe c n a d the flo of air to. the room by the usual dampen; This form of self help is in general use in the air conditioning field. If the user wishes to avoid even these simple steps,'a lo'w limit thermostat 63 can be connected in series with the primary control thermostat 58. The function of the low limit thermostat 63 is to disconnect the power supply to the refrigerating unit 50 whenever the room temperature drops to an objectionably low value. At all other times the lowQlimit thermostat is closed and the refi'ig- V erating unit is operating subject to the primary control of the thermostat 58. The low limit thermostat 63 does not control thefan or blower'SS which runs all of the 7 time. 'The. low limit thermostat during the summer months might not operate at all fora month at a time.

or possibly only during the occasional cooler days or nights. ,It would disconnect the air conditioning unit j duringa cool spell when little or no air conditioning was necessary. ,It would not control the room temperature 7 or humidity.

J In order-to; appreciate the advantages of the air con-' 1 the cooled airhas warmed up, which takes a relatively a any change. a

, refrigeratingunitis ofi,, the air the roomis circu lated and the temperature is not too high because of the light'air conditioning load. As soon as the air conditioning unit is turned on, the air is cooled but the moisture content is not reduced very much 'due to the 60 cooled air temperature and the increase in relative humidity makes the occupant lesscomfortable. This, of course, is highly objectionable to air conditioning manufacturers who dislike complaints that the user is more comfortable when the unit is ofi than when the unit 18 on. 7 I a In order to maintain a relative humidity of which is desirable forrcomfort, the duct 69 is connected to a dehumidifier 71consist ing of a compressor 72. controlled 7 by a room humidistat-73, a-condenser 74 and an evaporator 75. The cooled airfrom the duct 69 is first cooled by the evaporator 75 and-is then heated bythe condenser 74 to' a temperature higher than-the temperature in the duct 69. In atypical dehumidifier'the air fiom the duct 69 might have a temperature of 6Q and be cooled by the evaporator 75 to a temperature 40 F; and then reheated by thecondenser 74 to a temperature 65 F. The reason for the higher-temperatur'eiof the air leaving the dehumidifier is that 100%1efliciency is notobtainable.

By the combinationof units shown in Fig. 7 under full load conditions, the room .temperaturecan be held at the value-indicated by the curve 59 inFig. 8 andthe roomhumidity can be held at the value indicated by the curve 64 or 'in other words,'.the same operation can be obtained as'with the systems of Figs. 1 to 6 inclusive. If the dehumidifier is eliminated, then the relative humidity will be thatindicated by the curve 76. As would be ex pctedjunder load conditions both thetemperature and humidity curves are constant. at maximum capacity allof 'the'time'andthei air condi tioning load is at a maximum sothere is no reason for Under light load conditions, the combination illustrated in Fig. 7 will maintain temperature inthe'room as in:

ditioning systems of this application, there is shown in Fig. 7 "a conventional air conditioningsystem set up to thesaine low relative humidity. This conventional airconditionin'g system oomprises a refrigerating unit 64'suppl'ying'aicooling heat; exchanger 65 Within a correspond to .a moisture content representing a relative humidity ofabout The refrigerating unit would be -t 1rned;,o n and ofi by aroom thermostat 70 which would hold a room temperature of 78 within the usual of; m he mo a e-g 1-2 degrees ;,.,il3ecause. qfithe relatively high humidity, t m

too high, but it fluctuates. V

dicated by the curve 77 and humidity in the room as indicated by the curve 61. Note that the room humidity V will be held constant and the room temperature will also the room reevaporates moisture condensed on the heat exchanger 65. This tends tobring the humidity to a higher than normal value. During the falling part ofthe curve the refrigerating unit is on and the cooling ofthe ,air decreases the moisture but not to a low enough value for comfort. Under light loads not'only is the humidity Very few users of air conditioning are willing to pay for the additional dehumidifier which may costas much as the conditioning unit. With the dehumidifier omit- .ted, reevaporation of the moisture from the cooling heat exchanger is a'serious problem because the air en tering the casing 66is relatively high in temperature. In applicants system as shown in Fig. 6, while the air in the return air'duct 53'is at the same temperature'as the air inthe return air'duct 67, the air which flows over the cooling heat exchanger S1'is much colder due to the mixture of cooled air. This c'older has a much smaller tendency to pick up moisture and the reevaporation is accordingly greatlyireduced. r

With applicants systems, neither a'room thermostat nor a room 'humidistat 'isnecessary. The thermostat monitoring the cooled air temperaturedoes the work of both. For those who do not know when air'conditioning is not necessary, a low limit'thermostat wilkdisconne'ct the power supply to the air conditioning unit and'w1l1 turn'it back on when the need;for air :conditiomngreturns. The low limit thermostat is not'a temperature control.

The primary control of'temperature and also of humidity is in the-cooledairthermostat.

By primarily controlling the unit by "the cooled air temperature, a single air conditioning-nnit does the work of an air conditioning unit plus a dehumidifier, ane'w result not heretofore achieved.

. What'is claimed as new is:

.l. A room'air conditioning systemcompris-ingatduct having an intake from the room and=a.discharge.to:the room, a refrigerating unit supplying a cooling heat exchanger in the duct intermediate the intake and discharge, a bypass passageway leading from the downstream side back to the upstream side of the cooling heat exchanger, a variable speed fan drawing warm air from the room into the intake and over the cooling heat exchanger and discharging cooled air to the room through the discharge, another fan for forcing cooled air back through the bypass passageway to the upstream side of the cooling heat exchanger, a room air thermostat, and means controlled by the room air thermostat for cutting down the speed of the variable speed fan as the room air temperature drops.

2. A room air conditioning system comprising a duct having an intake from the room and a discharge to the room, a refrigerating unit supplying a cooling heat exchanger in the duct intermediate the intake and discharge, a bypass passageway leading from the downstream side back to the upstream side of the cooling heat exchanger, a variable speed fan drawing warm air from the room into the intake and over the cooling heat exchanger and discharging cooled air to the room through the discharge, another fan for forcing cooled air back through the bypass passageway to the upstream side of the cooling heat exchanger, a room air thermostat, means controlled by the room air thermostat for cutting down the speed of the variable speed fan as the room air temperature drops, and means responsive to the room humidity for shutting down said other fan.

3. A room air conditioning system comprising a duct having an intake from the room and a discharge to the room, a refrigerating unit supplying a cooling heat exchanger in the duct intermediate the intake and discharge, means dividing the duct into two passageways through the cooling heat exchanger, fan means drawing warm air from the room into the intake and through one i of the passageways over the cooling heat exchanger and discharging cooled air to the room through the discharge and including means for forcing cooled air back through the other passageway to the upstream side of the cooling heat exchanger, a room air thermostat, and means controlled by the room air thermostat for varying the proportion of the air discharged to the room relative to the air flow through the other passageway as the room air thermostat is satisfied.

4. A room air conditioning system comprising a duct having an intake from the room and a discharge to the room, a refrigerating unit supplying a cooling heat exchanger in the duct immediate the intake and discharge, means dividing the duct into two passageways through the heat exchanger, a fan drawing warm air from the room into the intake and through one of the passageways over the cooling heat exchanger and discharging cooled air to the room through the discharge, another fan for forcing cooled air from the downstream side of the cooling heat exchanger back through the other passageway to the upstream side of the cooling heat exchanger, a room air thermostat, means controlled by the room air thermostat for cutting down the proportion of ai discharged to the room relative to the air flow through 10 theother'passageway as the room air thermostat :is satisfied, "and other means for controlling the otherffan inresponse"tothe room humidity. 1 p

5. 'A' room air conditioning system comprising a duct having an intake fromtheroom'and a=discharge to'the room, a refrigeratingunit supplying a cooling heat exchanger in the duct intermediate theintakfe'and discharge, means dividin'g'the duct into'twopassageways through the heat "exchanger," a fan drawing warm air =from=the room intothe'intake andthrough one ofthe passageways over the cooling heat exchanger and discharging cooled air to the room through :the discharge, another fan for forcingmooled tairafrom .the downstream side ofrlthe coolingrheat aexchanger kback :throughzthe otheripassageway to the upstream side of the cooling heat exchanger, a room air thermostat, means controlled by the room air thermostat for cutting down the speed of the first fan as the room air thermostat is satisfied, and other means for controlling the other fan in response to the room humidity.

6. A room air conditioning system comprising a casing within which air is cooled below room temperature, said casing having an intake from the room for the air to be cooled and a discharge to the room for the cooled air, a refrigerating unit supplying with coolant at sub-freezing temperature a cooling heat exchanger in the casing intermediate the intake and discharge, a bypass passageway leading from the downstream side back to the upstream side of the cooling heat exchanger, fan means drawing warm air from the room into the intake of the casing and over the cooling heat exchanger and discharging cooled air to the room through the discharge and for forcing part of the cooled air from the downstream side of the cooling heat exchanger back through the bypass passageway to the casing on the upstream side of the cooling heat exchanger whereby a portion of the air is recycled over the heat exchanger and is furthercooled, the flow of air over the cooling heat exchanger being unrestricted,

and a primary control for the refrigerating unit monitoring the cooled air temperature andturning the refrigerating unit off when the cooled air reaches a low limit and on when the cooled air reaches an upper limit, the low limit being well below room temperature but above freezing and the upper limit being of the order of 10 F. above the lower limit, said control operating independent of room air conditions.

7. A room air conditioning system comprising a casing within which air is cooled below room temperature, said casing having an intake from the room for the air to be cooled and a discharge to the room for the cooled air, a refrigerating unit supplying with coolant a cooling heat exchanger in the casing intermediate the intake and discharge, a bypass passageway leading from the downstream side back to the upstream side of the cooling heat exchanger, fan means drawing warm air from the room into the intake of the casing and over the cooling heat exchanger and discharging cooled air to the room through the discharge and for forcing part of the cooled air from the downstream side of the cooling heat exchanger back through the bypass passageway to the casing on the upstream side of the cooling heat exchanger whereby a portion of the air is recycled over the heat exchanger and is further cooled, the flow of air over the cooling heat exchanger being unrestricted, a primary control for the refrigerating unit monitoring the cooled air temperature and turning the refrigerating unit off when the cooled air reaches a low limit and on when the cooled air reaches an upper limit, the low limit being well below room temperature but above freezing and the upper limit being of the order of 10 F. above the lower limit, said control operating independent of room air conditions, and a low limit thermostat responsive to room temperature for cutting the system off at a low limit of room temperature.

8. A room air conditioning system comprising a casing within which air is cooled below room temperature, said side back to the nps treani iide ofl the eoo casing shaving anjntakefrotfi the roon 1 :o r the aii eto be cooled and a discharge. to the room for the cooledail a refrig'eratin g unit suii alying with c o.f ola1 1t a .heat

exchanger. in t heicasing intermediate the. intake and dis- 7 ehai ge, a hygass passageway leadingiirom thedqwnstxeanl han minis aw wa nfir p mti the v intake ofthe vcasing ove; the e oling heat x; cha e a n c q ed airto theh nq m hrou h the dischargeand for forcing part fjhe pooled; air from theidownstream side of the cooling heat exchanger; back through the bypass passage'wayto:thecasing-on the';up-

stream side of thecooling heatexeiianger whereby a'por tion of lhe 'air is recycled over theheat exchangernhdis i -jts r 1,988,319" King Jan. 15, 1935 2,022,469 Keighley Nov.' 26, 1935 2,198,449 Atkins Apr. 23, 1940 2236 190 1941