US2979265A - Room ventilating method and apparatus - Google Patents

Description

April 1961 c. s. sTocK 2,979,265

ROOM VENTILATING METHOD AND APPARATUS Filed Dec. 51, 1956 v 3 Sheets-Sheet 1 ROOM AIR INLET-3 FIG. I

FIG.3

DEW POINT CONTROLLER 37 e CENTER COWARTMENT"! I I F I 35 I RELAY 3| 22 I I PRESSURE SELECTOR I29 I H I 30 I I 24 I as RIGHT END I COMPARTMENT AIRSTREAM I g I I ROOM THERMOSTAT la n g THERMOSTAT I I DAMPER s'zcnou LEFT END 20 I AIR SUPPLY I COMPARTMENT I I B I I 2| I FIG. 2

I INVENTOR. CHARLES S STOCK April 11, 1961 c. s. STOCK 2,979,265

ROOM VENTILATING METHOD AND APPARATUS Filed D80. 31, 1956 3 Sheets-Sheet 2 FIG. 4

ROOM TEMPERATURE, P '10 7| '12 7s '74 '75 I00 I I ou'roooR I OUTDOOR DAMPER VALVE I DAMPER I Am HEATER VALVE so I I I PERcENT OPEN 0|234567s9|ou|2|3|4|5 FIRST BRANCH PRESSURErPOUMJS PER sQ INCH LOCALIZED RELATIVE HUMIDITY, PERCENT [00 so 9| I00 I '15 OUTDOOR DAMPER INVENTOR. PERCENT CHARLES s STOCK 25 OUTDOOR oAMPER-:

A ril 11, 1961 c. s. STOCK ,979, 65

ROOM VENTILATING METHOD AND APPARATUS Filed Dec. 31, 1956 3 Sheets-Sheet 3' e3 e4 37 .376. I0 i 11 ffi fl 66 J76. A INVENTOR.

CHARLES S. STOCK ATTORNEY ROOM VENTILATING METHOD APPARATUS Filed Dec. 31, 1956, Ser. No. 631,732

4 Claims. Cl. 236-44) This invention relates to a method and apparatus for controlling the operation of unit ventilators.

It is generally accepted practice in rooms with high occupancy rates, such'as school classrooms, to bring a quantity of outdoor air into the room for ventilating purposes during most of the occupied time. This practice is common in extremely cold weather as well as moderate weather. If the room temperature is at the desired level, so that neither heating nor cooling of the room is required, this cold outdoor air is tempered either by mixing it with recirculated room air or by adding heat to it while it passes through the heating element within the unit ventilator, or both. The tempered air is then discharged into the room from the unit ventilator. If additional heat is required to maintain the desired room temperature, that is, where there exists an imbalance between heat gain from students, lights and solar radiation, 7

United States Pam: Q

2,979,265 Patented Apr. 11, 19 1 Although the above solution achieves the desired re-- 'It is the primary object of this invention to provide a control arrangement for unit ventilators which results in substantial fuel economies.

Another object is the provision of a control system for unit ventilators which will provide a more healthful environment in classrooms during cold weather.

A further object is the provision of a control system which will substantially minimie if not avoid the proband heat loss from leakage and conduction, extra heat is added to the air before it is discharged from the unit ventilator.

The amount of heat, required to temper the ventilating air brought into the unit from outdoors, is substantial at the lower outside temperatures, and, in rooms where an established minimum percentage of outdoor air, such as 50%, is introduced into the room, the additional heat required adds greatly to the cost of heating the room. Furthermore, the introduction of a considerable quantity of comparatively cold outdoor air results in the lowering of the relative humidity of the room air. For example, out

door air at 30 F. containing 0.002# of water per pound of dry air and having a relative humidity of 58%, has a relative humidity of only 13% when its temperature is elevated to 70 F.

It has been found that a comparatively low relative humidity in a heavily occupied room has several deleterious effects. One is that the occupants have an increased susceptibility to respiratory infections, due, it is believed, to the lowered effectiveness of the particle trapping mechanism in the nasal passages when they dry out. Another is that the furniture and woodwork release their absorbed moisture to the relatively dry air, and may ultimately split or crack.

A solution,-to the disadvantages inherent in introducing substantial amounts of outdoor air for ventilating purposes during the occupied periods, is to operate the unit ventilator with total recirculation so long as the room temperature and humidity are not too high. Thus, since outdoor air is not added, fuel economy is achieved. Also, since the occupants exude moisture which is added to the room air being recirculated, the relative humidity of the room tends to rise. While no direct ventilation of the room occurs with total recirculation of roomair, leakage through structural cracks provides ventilation which tends to prevent the room from becoming odoriferous or stale. Also, should the room overheat at times, tempered outside air of sufficient quantity to cool the room is then introduced and this freshens the room.

lem of moisture condensation on interior structural surfaces while at the same time achieving the aforesaid objects.

In carrying out my invention I may use a room unit ventilator of the type conventionally comprising? a motorized fan operating continuously to discharge a fan-inducedfiow of air into the room; motorized damper means operative one way to increase the relative amount of fresh I air and another way to increase the relative amount of room air in said fan-induced flow; an air heater for said fan-induced flow; and a conventional control system. The latter conventionally comprises: a high pressure airsupply line; a first air pressure branch line; a thermostatic control valve connecting supply and first branch lines and operating, in're'sponse to rising and falling room temperatures, respectively to increase and decrease the first branch line 'air pressure; a pneumatic heater-control valve operating, in response to rising and falling first branch line pressures, over a predetermined range, respectively to reduce and increase the quantity of heat supplied to said air heater; and a pneumatic damper-control means normally operating, in response to rising and falling first branch line pressures, over a relatively higher air pressure range, respectively to operate said damper means one way and the other. a

My invention resides in the provision of dew point controller means responsive to the humidity of localized room air, along a room surface which reflects the outdoor temperature, and operative, when the humidity of said localized air approaches to within a predetermined degree of its dew point, to override the normal'operation of said damper-control means and to operate said damper said one way to increase the relative amount of fresh air in i said fan-induced flow. One satisfactory dew point controller means, integrated into the aforesaid control system, comprises: a second air pressure branch line; humidity control valve means connecting the supply and secondbranch lines and operating, in response to rising and falling values of the humidity of said localized air, respectively to increase and decrease the second-branch line air pressure; and balance means placingthe pressure of the firstand second-branch lines in opposition and operative to subject the damper control means to the stronger of said opposed pressures. The humidity control valve means may comprise: a second control valve connecting the supply and second branch lines; and humidity sensing means operating, in response to rising and falling values of said localized humidity, respectively to operate said second control valve in the opening and closing directions to increase and decrease the second-branch line air pressure. 7

In the present invention, and in contrast to previous practice, the temperature control components are adjusted to maintain the damper means in a position closed to outdoor air except when room cooling is required,

while the humidity control components are adjusted to cause the damper means to open to outdoor air only when the humidity of the localized room air, along a window pane or other room surface which reflects the outdoor temperature, approaches to within a predetertermined degree of the dew point of that localized air. In other words, this control operates, when moisture is about to condense on the windows of the room, to override the conventional control and open the fresh air damper.

The principle of operation of the invention is best explained in connection with the conditions which must exist to permit the formation of condensation. Condensation forms on a window pane or other cold surface of a room when the localized film of air, which is adjacent to and cooled by that surface, is saturated i.e., at 100% relative humidity. Since the localized film is considerably cooler than the air throughout the remainder of the room, it is necessary to keep the moisture content of the room at a level, which, when considered relative to the temperature of the localized film, is below the saturation level of the localized film. This is effectively accomplished by sensing the relative humidity of the localized film and, when it reaches a predetermined value at or near its dew point or saturation level, then opening the damper means in response to the elevated humidity condition to introduce a quantity of outside air.

By mixing, with the recirculated room air, a quantity of cold outdoor air, which usually contains much less water vapor, the relative humidities of both the localized film and the room, are lowered. If the reduction in the relative humidity of the localized film is sufficient, the humidity control means again relinquishes its control of the damper means, thus permitting the conventional control to take over and close the damper means to outdoor air. If the relative humidity of the localized film again rises to said predetermined level, the humidity control means will again override the conventional control.

An embodiment of the invention is illustrated in the accompanying drawing, wherein:

Figure 1 is a perspective view of an exterior wall with the unit ventilator mounted against the wall, and a humidity sensitive device mounted on a window sill;

Figure 2 is a somewhat diagrammatic view of the control components and a unit ventilator;

Figure 3 is a fragmentary section of a unit ventilator in its mounted position against an exterior wall with the damper means closed to outdoor air and open to room air;

Figure 4 is a partly broken elevation of the humidity sensitive device in operative position adjacent a window pane;

Figure 5 is a section taken along line 5-5 of Figure 4;

Figure 6 is a graph showing the percentage of opening of the air heater valve and the outdoor air damper in relation to the operative range of first branch pressures and a corresponding operative range of room air temperatures;

Figure 7 is a graph showing the percentage of opening of the outdoor air damper in relation to various pressures in the second branch;

Figure 8 is a partly diagrammatic view of a conventional room air thermostat;

Figure 9 is a partly diagrammatic view of a conventional or low limit thermostat.

Figure 10 is a partly diagrammatic view of a conventional pressure selector of the type adapted for use in the invention; and,

Figure 11 is a partly diagrammatic view of a conventional intermediate relay of the type adapted for use in the invention.

Unit ventilator The type of unit ventilator as contemplated herein is exemplified by Patent 2,723,616, issued November 15,

1955, to Arthur F. Hubbard. Such a unit ventilator basically comprises: a cabinet having an outdoor air intake opening, a room air intake opening and discharge air opening; a series of motor driven, centrifugal fans for drawing air into the cabinet through the intake openings and discharging it into the room; an air heating element supplied with suitable heating medium; and damper means regulating the admission of air into the unit through the intake openings.

As seen in Figures 1-3, a unit ventilator cabinet 1 is located with its rear side against an exterior wall 2. The cabinet is provided with room air intake slots 3 in the lower front side, an outdoor air intake opening 4 in its lower rear side, and discharge grilles or openings 5 along its top surface.

As shown in Figures 2 and 3, the interior of the cabinet 1 is divided into three compartments, viz: a right end compartment 6; a center compartment 7 which serves as a mixing chamber for recirculated and fresh air; and an opposite or left end compartment 8. End compartment 6 has mounted therewithin a motor 9 which drives a series of spaced, centrifugal fans 10 within scrolls 11 located in the upper portion of center compartment 7. An air heater 12, having valve means 13 supplied with heating medium through pipe 14, is horizontally disposed below the scrolls 11. The air heater 12 is preferably constructed and operated like that described in Patent 2,532,550, issued December 5, 1950, to Arthur F. Hubbard. An air filter 15 is positioned immediately below the air heater. It operates to filter both recirculated room air and outdoor air before that air passes through the air heater.

The damper means, located in the lower portion of center compartment 7 comprise: an outdoor air damper 16 mounted on a shaft 17 for rotational movement around its longitudinal axis; a room air damper 18 mounted on shaft 19 for rotational movement; connecting linkage 20; and crank arm 21. The damper means are shown in Figure 3 with the outdoor air damper 16 in fully closed position to prevent the admission of outdoor air into the cabinet and with the room air damper 13 in fully open position to permit the maximum recirculation of room air through the cabinet. In left end compartment 8, the connecting linkage 20, which is attached to the projecting ends of shaft 17 and shaft 19, is arranged so that as the outdoor air damper 16 rotates in a clockwise direction toward its open position, room air damper 18 simultaneously rotates in a clockwise direction toward its closed position. The linkage arrangement is described in Patent 2,723,616, issued November l5, 1955, to Arthur F. Hubbard and will not be further described herein. Broadly speaking, my invention may be employed with either the recirculating air damper 18 omitted or with one damper designed to serve as a combination room air and outdoor air damper.

Control system The control system has two air pressure branches, viz: a first air pressure branch having control components operating in response to temperature changes; and a second air pressure branch operating in response to localized humidity changes. Both branches are supplied through conduit 22 with a suitable pneumatic pressure from a remote source and both terminate in a common,

pneumatically operated damper control means responsive to the branch having the highest pressure.

First branch The first air pressure branch comprises: thermostatically operated valve 23, under the control of a room temperature thermostat associated with it, and designated by the same numeral 23; pneumatic operating means 24 controlling air heater valve 13; thermostatically operated exhaust valve 25, under the control of the temperature sensing element or bulb 26 located in the air flow stream of the center compartment, the element 26 being connected to thevalve 25 by air pressure line 27; air pressure line 28 connecting valve 23 to both pneumatic means 24 and valve 25; balance means 29; air pressure line 30 connecting valve 25 to balance means 29; pneumatically operated damper-control means 31; and, air pressure line 32 connecting balance means 29 to damper control means 31.

Thermostatically operated valve 23 which may be a so: called direct acting room thermostat of the type desig-' nated as a T400 and manufactured and sold by the Johnson Service Company, regulates the pressure received by line 28 from supplyair pressure line'22, and operates, in thermostatic response to rising and falling room temperatures, respectively to increase and decrease the pressure in line 28. As somewhat diagrammatically illustrated in Figure 8, the room thermostat valve 23 is conventional and includes an exhaust port 50 adapted to be more or less restricted by pivoted lever 51 actuated by bi-metal strip-52, and a built in restrictor 53 which restricts the rate at which air is received from the supply air pressure line 22. The bi-metal strip 52 causes the lever 51 to move toward and away from the exhaust port in response to respectively increasing and decreasing temperatures, within limits, sensed by the strip 52. Thus the first branch pressure in line is correspondingly increased and decreased according to the temperature sensed by the room thermostat.

Pneumatic means 24 is of any suitable design and operates, within limits and in response to increasing and decreasing pressures in line 28, respectively to modulate air heater valve 13 in a closing and opening direction to regulate the heating medium flow to air heater 12.

The exhaust valve 25, which is commonly referred to as a low limit or air stream thermostat and which is available in one suitable form as. a direct acting T-333 manufactured and sold by the Johnson Service Company, connects line 28 to line 30, as is operative, when the bulb 26 senses an air flow temperature through the unit ventilator below a predetermined value (usually 55 or 60 F.), to exhaust or bleed off pressure from the first branch. This prevents the unit ventilator from discharging air which would be sufficiently cold to cause drafts in the room. As somewhat diagrammatically illustrated in Figure 9, the airstream thermostat 25 in cludes an exhaust port 54 adapted to be more or less restricted by pivoted lever 55 actuated by displacement of diaphragm 56 in accordance with the thermal expansion and contraction of the vapor charge in remote bulb 26 and capillary line 27. A built in restrictor 57 re stricts the rate at which air from the line 28 is received by the airstream thermostat. The exhaust port 54 will normally be closed by lever 55 except when the air sensed by the bulb 26 is of a sufficiently low temperature, such as 55, that the lever 55 is moved away from the exhaust port and the air pressure in line 30 is then correspondingly decreased.

Balance means 29, one suitable form of which is available as the type H, C-120 duplex pressure selector from the Johnson Service Company, receives the pressure from line 30, balances it against a pressure received from a line in the second branch (to be described later), and connects line 32 to the branch having the highest pressure. As somewhat diagrammatically illustrated in Figure 10, the balance means 29 receives air pressure from line 30 through bypass line 58 connected to feed one side of a diaphragm 59 enclosed in diaphragm chamber 60, and from second branch line 38 through bypass line 61 connected to feed the opposite side of diaphragm 59. If line 30 pressure exceeds line 38 pressure (the position shown in Figure 10), the imbalance upon the diaphragm causes the double-ended valve member 62 connected to the diaphragm to be displaced in a direction preventing air from line 38 from entering the pper part of main air chamber 63 and permitting air from line 30 to enter control means 31 and line 32 connecting the balance the lower part of main air chamber63. Oppositepressure conditions (line 38 pressure exceeding line 30 pressure), results in the pressure of line 38 being passed to line 32 while line 30 pressure is prevented from being so passed. Both the upper and lower parts of the main air chamber 63 are connected,-as shown, to line 32 leading to the damper operator 31.

Damper-control means 31 receives the pressure in line 32 and converts the air pressure by suitable means, such as a bellows arrangement, into mechanical movement of pivoted lever 33 which is connected by a vertical rod 34 to crank arm 21 of the damper means. The dampercontrol means 31 include suitable adjustable springs opposing the movement of lever 33 in a direction which would result in the damper means opening to outdoor air. This permits setting the damper-control. means for operation within predetermined limits of air pressure in the first branch. 2 r

The control components and arrangement thereof thus far described as comprising the firstbranch are, with the exception of the balance means 29, commonly used in controlling the operation of unit ventilators.

Second branch i The second air pressure branch comprises: humidity controlled exhaust valve or dew point controller 35; valve 36, operating under the control of dew point controller 35; air pressure line 37 connecting dew point controller 35 .to valve 36;]balance means 29; air pressure line 38 connecting valve 36 to balance means 29; and dampermeans and damper-control means.

It will be noted that balance means 29, damper-control means 31 and line 32 are common to both the first and second air pressure branches, thereby providing an arrangement whereby either branch may override the other and assume control of the damper means.

The dew point controller 35 (Figs. 4 and 5) comprises: a vertically disposed plate-like base 39 mounted near a surface, such as a window pane 40, which is subjest to outside temperatures; a generally L-shapedmemher having one leg 41 horizontally extending through an opening 42 in the base and terminating adjacent the win dow surface 40, and the other leg 43 extending downwardly and terminating beyond a leakport or air exhaust opening 44 so as to form a variable closure therefor; a bracket 45 rigidly mounted on the base; a pivotal support on the bracket for the L-shaped member; a leakport assembly 46 on the lower portion of the bracket; a plurality of vertically extending hair elements 47 positioned in the space between the base 39 and outside surface 40, and secured at their lower ends to the base 39 and at their upper ends to the adjacent end of the horizontally disposed leg 41; and a spring 48 biasing the inner surface of the vertical leg 43 against the leakport 44. One suitable form of this so-described dew point controller is available from the Johnson Service Company as its H-305 Windowstat.

The dew point controller 35 operates in the following manner. The hair elements 47 are mounted to be in tension when relatively-dry, and exert a pull on the horizontal leg 41, thereby tending to rotate L-shaped mem her in a clockwise direction about its pivoted support so as to move the lower end of vertical leg 43 away from the leakport 44. The spring 48 is likewise in ten sion and opposes the rotational pull exerted by the hair elements. As the hair elements absorb moisture, 'due to an increasing localized humidity, they expand and thus lessen the tensile force exerted on the end of horizontal leg 41. Since the force exerted by spring 48 is relatively unaffected by humidity, the L-shaped member rotates slightly in a counterclockwise direction which tends to close the leakport. Then, ifthe localized humidity decreases, the hair elements lose moisture and their tensile force increases, thereby rotating the L-shapecl member and again moving the lower end of vertical leg 43 in a direction tending to open the leakport.

The valve 36, one suitable form of which is available as a direct acting R-35l intermediate relay from the Johnson Service Company, regulates the pressure received by line 38 from air pressure supply line 22 and operates, in response to rising and falling localized humidity at the dew point controller 35, respectively to increase and decrease the .pressure in line 38. To aid in understanding the invention, the dew point controller 35 and valve 36 interaction will be described in connection with the somewhat diagrammatic illustration of the valve or relay 36 in 'Figure 11.

The valve 36 includes a supply air bypass 63 with a built-in restrictor 64 which permits a minor portion of the supply air from line 22 to pass through a pilot chamber 65 in valve 36, into line 37, and thence to the leakport assembly 46. If the localized humidity at the dew point controller is relatively low, the restriction of the leakport 44 by leg 43 is effectively less than the built-in restrictor 64 of valve 36, and the air leaks to atmosphere as quickly as it reaches the leakport. As the localized humidity at the dew point controller increases, the vertical leg tends to increasingly restrict the leakport, and, when the leakport restriction exceeds the built-in restriction of valve 36, the pressure in line 37 and the pilot chamber 65 of valve 36 increase. The resulting increasing pressure in line 37 and pilot chamber in valve 36 exerts an increasing force on a diaphragm 66 forming one wall of the chamber. This diaphragm is mechanically linked to a valve member 67 which regulates the opening between supply line 22 and line 38. In this manner, with an increasing and decreasing localized humidity at the dew point controller, the valve 36 operates respectively to increase and decrease the pressure in line 38.

Operation The operation of the unit ventilator and its associated controls, as arranged in the present invention, will now be described. During the occupied period the unit ventilator fans normally operate continuously to recirculate air from the room successively through the room air intake slots 3, filter 15, air heater 12, fan scroll 11 and outlet which discharges the conditioned air back into the room. Supply air pressure of fifteen p.s.i. is furnished to valve 23 and valve 36 through supply line 22.

Figure 6 is a chart showing how the first branch control components are adjusted .to operate in accordance with the present invention. The upper horizontal scale is the room temperature sensed by the thermostat associated with valve 23, while the lower horizontal scale is the corresponding pressure received by the first branch at these particular room temperatures. The curve marked Valve shows the percentage of opening of air heater valve 13 to the flow of heating medium at the corresponding pressure in the first branch, and the curve marked Outdoor Damper shows the percentage of opening of the outdoor air damper 16 to fresh air at the corresponding pressure in the first branch.

Figure 6 shows that, when .a temperature sensed by thermostatically operated valve 23 is 70 F., there is a pressure of 2 p.s.i. in the first branch. This pressure, or any lower pressure, permits a pressure-opposing spring in pneumatic means 24, to hold air heater valve '13 fully open to the flow of the heating medium. With a slightly higher room temperature, thermostatic valve 23 operates to increase the pressure in the first branch and air heater valve 13 is modulated by pneumatic means 24 toward a closed position. At 71.6 R, which corresponds with'6 p.s.i., the air heater valve 13 is fully closed to the flow of the heatingmedium. With the room temperature between 71.6 F. and 72.4 F., both the air heater valve 13 and outdoor air damper 16 are closed and total recirculation of room-air occurs.

Then, if the room temperature rises above 724 F. (8 p.s.i.), the damper control means 31 operates the damper means in a direction to increase the relative proportion of outdoor air. If, due to this influx of fresh air, the temperature of the air flow through the unit ventilator falls below the predetermined limit to which exhaust valve 25 is adjusted, the exhaust valve 25 will operate to exhaust air from the first branch. The pressure-opposing springs of damper control means 31 will then simultaneously operate to cause the damper means to decrease the relative proportion of outdoor air. As the temperature of air flow rises above the predetermined adjusted limit of exhaust valve 25, the pressure in the first branch will again increase if cooling of the room is still required, until, at 74 F., the damper means will be fully open to outdoor air. It will be understood that during comparatively cold weather, the room rarely overheats to the extent that outdoor air is required, but such an arrangement is desirable to permit maximum cooling during moderate weather.

As the room temperature falls, due to the mixing of outdoor with recirculated room air, thermostatic valve 23 exhausts pressure from the first branch so that the damper means move to decrease the relative proportion of outdoor air admitted to the unit. If the temperature should fall to below 71.6 F., the air heater valve 13 will, of course, be operated by pneumatic means 24 to permit heating medium to flow into the air heater 12. Thus, with the first branch controls adjusted for operation as shown in Figure 6, the air heater valve 13 operates from an open position at 2 p.s.i. or below, to a closed position at 6 p.s.i. or above, and the damper means operate from a position closed to fresh air at 8 p.s.i. or below, to a position fully open to fresh air at 12 psi. or above.

It is to be understood that the first branch operation described constitutes a part of the invention only insotar as it cooperates with the operation of the dew point controller and second branch, and the foregoing is set out simply to aid in the understanding of the manner in which the dew point controller means are integrated into the circuit.

The function of the dew point controller, as integrated into the control arrangement already described, is to respond to humidity of localized room air along a surface which reflects the outside temperature, and operate, when the humidity of the localized air approaches to within a predetermined degree of saturation, to increase the pressure in the second branch to an extent which permits it to assume control of the damper means and move the outdoor damper toward its open position.

Figure 7 shows the operating range of the damper means when the second branch components are adjusted to operate in accordance with the invention. The upper horizontal scale is the relative humidity of the localized air. The lower horizontal scale is the second branch pressure corresponding to the relative humidity of the upper scale. The curve marked Outdoor Damper shows the percentage of opening of the outdoor damper in the range of operation controlled by the second branch. It will be noted that, since the balance means 29, damper control means 31, and connecting line 32 are common to both the first and second branch, the outdoor damper operates between the same air pressure limits, irrespective of the controlling branch.

The dew point controller 35 and valve 36 are preferably adjusted to regulate the pressure received by the second branch so that when the localized relative humidity approaches saturation, the second branch receives sufiicient pressure to begin opening the damper means to fresh air. Figure 7 illustrates this preferred operation by showing that the dew point controller means have been adjusted to pass 8 p.s.i. to the second branch when the localized humidity is 91%.

Upon an increase in relative humidity of the localized air above 91%, the pressure in the second branch increases and the outdoor damper moves in a direction which will increase the relative proportion of fresh air admitted to the unit. If the relative humidity reaches 100%, the outdoor air damper is at its maximum opening in response to second branch pressure, this being 25% in the example illustrated in Figure 7.

A limit, such as 25% fresh air admitted in response to maximum second branch pressure, is desirable since the low limit exhaust valve 25 associated with the first branch is not connected to exhaust second branch pressure. Such a limit thus avoids the possibility of relatively large quantities of very cold fresh air being momentarily introduced into the room and causing drafts. Another reason for limiting fresh air introduced in response to maximum localized humidity is that it is desirable for reasons of health to maintain the humidity throughout the room at as high a practicable level as possible without condensation forming.

As the relatively dry outside air is mixed with recirculated room air, the relative humidity of the localized air and, consequently, the second branch pressure, decrease. When the pressure in the second branch decreases below that of the first branch, the control of the damper means again reverts to the first branch and,

assuming no cooling of the room is required, the damper means close to fresh air.

It is to be understood that the ranges of operation described have been selected to illustrate a preferred manner of operation, and by adjusting the spring pressures against which the air pressures delivered to the control components operate, the ranges of operation may be changed for slightly different operation.

Having described my invention, I claim:

1. A control system for a room unit ventilator of the type having a motorized fan operating continuously to discharge a fan-induced flow of air into the room, motorized damper means operative one way to increase the relative amount of fresh air and another way to increase the relative amount of room air in said fan-induced flow and an air heater for said fan-induced flow, comprising: an air pressure supply; a first air pressure branch; 21 thermostatic control valve connecting the supply to the first branch and operating, in thermostatic response to rising and falling room temperatures, respectively to increase and decrease the first branch pressure; a pneumatic airheater control valve operating, in response to rising and falling first branch pressures, over a predetermined range, respectively to reduce and increase the quantity of heat supplied to said air heater; a pneumatic damper-control means normally operating, in response to rising and falling first branch pressures, over'a range relatively higher than said predetermined range, respectively to operate said damper means one way and the other; and dew point controller means including, a second air pressure branch,

humidity control valve means connecting said supply to said second branch and operative in response to rising and falling values of humidity of localized air along a room surface which reflects the outdoor temperature respectively to increase and decrease said second branch pressure, and balance means placing the pressures of said first and second branches in opposition and operative to subject said damper control means to the stronger of said opposed pressures, whereby said dew point controller means is operative when the humidity of said localized air rises to within a predetermined degree of the dew point to override the normal operation of said damper control means and to operate said damper said one Way to increase the relative amount of fresh air in said fan-induced flow for preventing the formation of condensation upon said room surface.

2. The system of claim 1 wherein said humidity control valve means includes: a second control valve connecting said supply to said second branch, and humidity sensitive means operative in response to said rising and falling values of said localized air humidity respectively to operate said second control valve in an opening and closing direction to increase and decrease said second branch pressure.

3. The system of claim 2 wherein: said damper control means is adjusted for operation over an air pressure range having a lower limit adjacent the upper limit of said predetermined range of operation of said air heater control valve.

4. The system of claim 3 wherein: said dew point controller means is adjusted to limit said second branch air pressure to a predetermined maximum value when said localized air is saturated, said predetermined maximum value corresponding to a position of said damper means restricting said fresh air to a minor proportion of said fan-induced flow.

References Cited in the file of this patent UNITED STATES PATENTS 1,753,157 Otis Apr. 1, 1930 2,171,109 Downs et al. Aug. 29, 1939 2,226,865 Kirchhoif Dec. 31, 1940 2,261,852 Mathis Nov. 4, 1941 2,279,582 Russel Apr. 14, 1942 OTHER REFERENCES Control Manual for Heating Ventilating and Air Conditioning, copyright 1940, by Minneapolis-Honeywell Regulator Company, Minneapolis, Minnesota, pages 148-153 inclusive.

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