Minimum airflow control

In a central air conditioning system where airflow is regulated by bellows inflation which is controlled by a bleed-type thermostat, a second bleed which is controlled by a regulator is provided to insure a minimum flow for ventilation independent of cooling demand.

BACKGROUND OF THE INVENTION 
The present invention relates to a control for an air distribution unit for 
delivering conditioned air into an area or space. More particularly, it 
relates to a control for maintaining the flow of conditioned air into the 
area at a minimum predetermined rate irrespective of the temperature 
requirements of the space and is an improvement over commonly assigned 
U.S. Pat. No. 4,120,453. 
In the arrangement disclosed in U.S. Pat. No. 4,120,453, a first pressure 
signal is supplied to a 3-way valve from a first pressure regulator which 
is controlled by a bleed-type thermostat so that the first pressure signal 
represents the cooling demand. A second pressure signal is supplied to the 
3-way valve from a second pressure regulator and represents the minimum 
quantity of air for adequate ventilation. The output of the 3-way valve is 
supplied to a bellows which coacts with cutoff plates to regulate air flow 
from the plenum. The output of the 3-way valve is controlled by a 
differential pressure valve which receives the first pressure signal from 
the first pressure regulator and plenum pressure to cause the 3-way valve 
to communicate either the first or second pressure signal to the bellows, 
as required. 
SUMMARY OF THE INVENTION 
The present invention eliminates the 3-way valve and differential pressure 
valve of the U.S. Pat. No. 4,120,453 device while still providing for a 
minimum flow for ventilation purposes. Accordingly, a second regulator is 
connected to the fluid line between the first regulator and the bleed-type 
thermostat. This arrangement produces a regulated bleed flow even when the 
thermostat has shut off the bleed flow in the thermostat and would 
ordinarily cause the bladder to inflate to thereby cut off air flow from 
the plenum. The regulated bleed flow from the second regulator limits the 
amount which the bladder can inflate and thereby establishes a minimum air 
flow from the plenum into the room. 
It is, therefore, an object of the present invention to have an air 
distribution unit with suitable control means whereby a minimum 
predetermined flow of air into the space being conditioned is maintained 
irrespective of the temperature requirements in said space. 
It is another object of this invention to maintain a minimum flow of 
conditioned air for ventilating purposes from an air distribution unit 
without introducing maintenance problems. 
It is a further object of this invention to maintain a minimum flow of 
conditioned air from an air distribution unit without adversely affecting 
the air delivery characteristics thereof. 
These and other objects of the present invention are attained in an air 
distribution unit for discharging conditioned air into an area to be 
conditioned and including means defining a plenum chamber for receiving 
conditioned air, means defining an outlet from the plenum chamber into the 
area to be conditioned, and a damper arrangement to regulate the flow of 
conditioned air from the plenum chamber to the outlet means. The unit 
includes control means including thermostatic means to vary the operation 
of the damper means in accordance with the sensed temperature of the air 
to be conditioned whereby a greater quantity of air is introduced into the 
area when the temperature thereof is substantially greater than a 
predetermined set point, with a lesser quantity of air being introduced 
into the area as the sensed temperature approaches the set point. The 
control means further includes override means operable when the damper 
means substantially terminates the flow of air into the area being 
conditioned in response to said thermostatic means to maintain a minimum 
flow of conditioned air from the plenum chamber to the outlet means 
irrespective of the sensed temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is illustrated a preferred embodiment of an air 
distribution unit 10 in accordance with the present invention. Conditioned 
air is delivered from a central source thereof (not illustrated) to a 
plenum chamber 12 of the air distribution unit. A damper arrangement 
illustrated as inflatable bellows 14 and 15 regulates the flow of 
conditioned air from the plenum 12 to the area or space being conditioned. 
A portion of the conditioned air furnished to plenum chamber 12 flows to 
filter 20 where any foreign bodies entrained therein will be removed. The 
conditioned air passing through filter 20 is used for controlling the 
operation of unit 10. The control air passes from filter 20 to first 
pressure regulator 30 via line 22. The control air passes from regulator 
30 into distributor 40 via line 38 and from distributor 40 via lines 41 
and 42 to bellows 14 and 15, respectively. Additionally, control air 
passes via line 39 to bleed-type thermostat 50 which senses the 
temperature of the area to be conditioned and in response thereto controls 
the magnitude of the control signal supplied to bellows 14 and 15 by 
bleeding control air through bleed port 52. 
A second pressure regulator, the minimum airflow regulator, 60 is connected 
upstream of bleed port 52 via line 58. Except for ordinarily being set to 
open at a different pressure and having different fluid connections, 
regulators 30 and 60 are otherwise preferably identical. 
The damper arrangement employed to control the flow of conditioned air from 
the plenum 12 further includes aligned cutoff plates 16 and 17 which are 
provided with a curved surface for coacting with inflatable bellows 14 and 
15. By varying the inflation of the bellows, the area between each of the 
bellows and the cutoff plates may be varied to regulate the quantity of 
conditioned air discharged into the area or space being conditioned. The 
manner in which inflation of the bellows is controlled shall be explained 
in detail hereinafter. 
OPERATION OF THE PREFERRED EMBODIMENT 
The operation of the air distribution unit and the control system related 
thereto shall now be more fully explained. 
The adjustment of the tension of springs 32 and 62 of regulators 30 and 60 
respectively, will ordinarily be factory set and will be determinative of 
the seating bias of ball valves 31 and 61, respectively, with spring 62 
providing a greater biasing force than spring 32. Alternatively, the 
regulators 30 and 60 can be set upon installation since they will not be 
readily accessible. Thermostat 50 will be set at an adjustable set point. 
Assuming that the area to be cooled is at a temperature substantially above 
the set point, pressurized control air will serially pass through the 
filter 20 and line 22 to pressure regulator 30 where it will cause ball 
valve 31 to open against the bias of spring 32. Pressure inside regulator 
30 is communicated via line 38 to distributor 40 thence via line 41 to 
bellows 14 and via line 42 to bellows 15. The bellows 14 and 15 will be 
inflated to a degree dictated by the pressure in regulator 30 and the 
degree of inflation of the bellows 14 and 15 will dictate the amount of 
conditioned air that will be able to pass from the plenum 12 between the 
bellows 14 and 15 and their respective cutoff plates 16 and 17 into the 
space to be cooled. The pressure regulator 30 is in fluid communication 
with chamber 53 of thermostat 50. The pressure in chamber 53 and hence the 
pressure in regulator 30 and bellows 14 and 15 is controlled by apertured 
sliding plate 54 which controls the amount of air bled from chamber 53 via 
bleed port 52. As the temperature in the area to be cooled approaches the 
set point, flow from chamber 53 via bleed port 52 will be throttled which 
raises the pressure in chamber 53, regulator 30 and hence bellows 14 and 
15 to reduce the flow of conditioned air into the space to be cooled. The 
pressure in chamber 53 will also be communicated via line 58 to regulator 
60 but will be insufficient to unseat ball 61 against the bias of spring 
62. As the pressure in chamber 53 continued to rise, bellows 14 and 15 
continue to inflate until the pressure is sufficient to overcome the bias 
of spring 62 whereby the pressure in regulator 30, chamber 53 and bellows 
14 and 15 is controlled by the bleed flow through regulator 60 to the 
atmosphere via line 68. 
The point at which regulator 60 opens defines the minimum air flow through 
the unit 10 and corresponds to the horizontal portion of the curve in FIG. 
2. If thermostat 50 were the only means of regulating the bleed flow, then 
air flow would follow the dashed portion of the curve. 
Although a preferred embodiment of the present invention has been 
illustrated and described, other changes will occur to those skilled in 
the art. It is therefore intended that the present invention is to be 
limited only by the scope of the appended claims.