Supply airflow control for dual-duct system

Two variable position supply air valves, one for modulating warm airflow to a comfort zone and the other for modulating cool airflow, each have a constant intermediate open position to provide at least a desired minimum airflow, and both can be controlled so that only one valve is open at any one time to minimize any mixing of the airstreams. Potentiometers are used to adjust the intermediate open position of each valve and to adjust at least one valve's relationship of valve opening to zone temperature.

TECHNICAL FIELD 
This invention generally pertains to HVAC (heating, ventilating, and air 
conditioning) equipment and more specifically to the control of a VAV 
(variable air volume) heating valve and a VAV cooling valve to provide at 
least a minimum supply airflow rate while minimizing the mixing of warm 
and cool supply air. 
BACKGROUND OF THE INVENTION 
The temperature of a comfort zone within a building can be controlled by 
modulating the airflow rate of warm or cool air supplied to the zone. This 
is typically accomplished with the use of at least two VAV valves, one for 
a warm air supply duct and another for a cool air supply duct. Both supply 
air ducts serve the same zone and may share a common blower. Downstream of 
the blower(s), however, the ducts convey separate airflows corresponding 
to separate heat exchangers, one for heating and the other for cooling. 
The rate of airflow through each valve is modulated to meet the comfort 
zone's temperature conditioning demand as determined by a thermostate. At 
low demand, both valves may be substantially closed, and the speed of the 
supply air blowers may be reduced to save energy. However, it is usually 
desirable to maintain at least a minimum airflow rate for ventilation 
purposes, even though the temperature conditioning demand has been 
satisfied. When the temperature of the zone has reached its set point, 
many controls provide minimum airflow by slightly opening both the heating 
and the cooling valves an equal amount. When a demand for heat or cooling 
arises, the valves begin to open further or close accordingly. As the 
demand further increases, the valves continue to move, and eventually one 
fully closes and only the other is controlled to meet the demand. 
Accurate modulation of airflow can be difficult when operating in a 
relatively narrow minimum airflow region, where both valves are only 
partially open a slight amount around the set point temperature. This is 
because accurate measurement of low airflow rates requires a relatively 
sensitive flow sensor. In addition, a slight change in valve position can 
cause a dramatic change in airflow at low airflow rates. A further 
complication arises when the heated air supplies more heat than an equal 
amount of cooled air can remove, or vice versa. This imbalance should be 
compensated to avoid a net heating or cooling effect when the temperature 
conditioning demand has been satisfied, i.e., the zone temperature equals 
its set point. In addition, the mixing of heated and cooled air should be 
avoided whenever possible to minimize energy consumption. 
SUMMARY OF THE INVENTION 
An object of the invention is to minimize the mixing of warm and cool 
supply air whose flow rate is modulated by two VAV valves for satisfying 
the temperature conditioning demand of a comfort zone. 
Another object is to maintain a constant minimum airflow to a comfort zone 
when its temperature conditioning demand is substantially satisfied. 
Another object is to provide means for adjusting the minimum airflow 
setting of each VAV valve. 
Yet another object of the invention is to provide means for adjusting the 
width of a dead band where either one of the two valves is at a relatively 
constant intermediate open position. 
A feature of the invention is a relatively narrow region of hysteresis 
within the dead band to minimize alternate valve cycling near the comfort 
zone's set point temperature. 
These and other objects and features of the invention are provided by a 
novel HVAC apparatus for conditioning the temperature of a comfort zone. 
The HVAC apparatus includes two VAV valves, one for modulating warm 
airflow to a comfort zone and the other for modulating cool airflow. The 
opening of the valves are controlled such that only one valve is open at a 
time to minimize any mixing of warm and cool supply air. In addition, each 
valve has a intermediate open position for providing at least a 
predetermined minimum airflow when the zone temperature is near its set 
point.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A dual-duct VAV system, shown in FIG. 1, is connected to condition the 
temperature of a comfort zone 10, such as a room within a building. Two 
blowers 12 circulate temperature conditioned air through zone 10 by way of 
two separate supply air ducts 14 and 16 and return air duct 18. Supply air 
in duct 16 is heated by a heater 20 before it is delivered to zone 10. 
Heater 20 represents any device for heating air such as an electrical 
resistance heating element, a combustion gas heat exchanger, a steam coil, 
or a refrigeration condenser. Likewise, the supply air in duct 14 is 
cooled by heat exchanger 22 before being supplied to zone 10. Heat 
exchanger 22 represents any device for cooling air such as a water coil or 
a refrigeration evaporator. After passing through zone 10, the air returns 
to the suction side of blowers 12 by way of return air duct 18. A heating 
valve 24 and a cooling valve 26 are VAV valves that modulate the airflow 
through duct 16 and 14 respectively. Exemplary VAV valves are disclosed in 
U.S. Pat. Nos. 4,749,000 and 4,749,001, specifically incorporated by 
reference herein. The degree of opening of valves 24 and 26 is controlled 
by a controller 28. 
The actual control scheme of controller 28 will be best understood by first 
referring to the hypothetical control schemes shown in FIGS. 2 through 4. 
If no minimum airflow was ever required, a desirable valve opening to zone 
temperature relationship may look as shown in FIG. 2. Both the heating and 
cooling valves would be closed when the zone temperature is at its set 
point 30. As the zone temperature deviates from set point 30, the 
appropriate heating or cooling valve would open to meet the demand. 
If a minimum airflow was desired for ventilation purposes, the curves of 
FIG. 2 can be shifted to cross at set point 30, as shown in FIG. 3. Dashed 
line 32 represents the valve opening that provides the desired minimum 
airflow when one of the two valves is completely closed. Coordinating the 
proper actuation of both valves requires close tolerance flow sensors when 
operating below line 32, because the slope of the curves are relatively 
steep. Moreover, due to the non-linearity of the curves, the amount of 
ventilating airflow at set point 30 is greater than the desired minimum at 
line 32. To compensate for these problems, the relationship between valve 
opening and temperature can be modified to provide a linear relationship 
in the control region below dashed line 32, as shown in FIG. 4. 
Unfortunately, providing such a control scheme is not only relatively 
complicated but also results in excessive mixing of both warm and cool 
supply air near the set point. 
Control 28 avoids the problems of the hypothetical controls by providing a 
relationship of valve opening to temperature as shown in FIG. 5. Control 
28 includes a dead band region 34, i.e., the temperature range over which 
the position of the furthest open valve (valve 24 or 26) does not vary as 
a function of temperature. However, the dead band region does not include 
the temperature range over which either valve, 24 or 26, is fully open. 
Within dead band region 34, one of the two valves is at a a predetermined 
minimum position 40 or 42 (FIG. 1) while the other is held at a generally 
constant intermediate open position 36 to provide the minimum desired 
airflow rate for adequate ventilation. The term, "minimum position" is any 
position less than fully open, and the term, "intermediate open position" 
is any position between fully open and the minimum position. In one 
embodiment of the invention, the minimum position is substantially closed. 
When the zone temperature is below set point 30 but within dead band 34, 
heating valve 24 is at its generally constant intermediate open position 
36 while cooling valve 26 is at its minimum position 40, e.g., nearly 
closed. When the zone temperature is above set point 30 but within dead 
band 34, cooling valve 26 is at its intermediate open position 36 whle 
heating valve 24 is at its minimum position 42, e.g., fully closed. 
In one embodiment of the invention, set point 30 is at an unstable point of 
transition where one valve opens while the other closes. The valves may be 
actuated sequentially or simultaneously, depending on the specific control 
details. Regardless of the actuation sequence, set point 30 does not 
represent a stable condition where both valves are maintained in a 
partially open position according to one embodiment of the invention. 
A narrow range of hysteresis may be present at set point 30 to minimize 
alternate cycling of the valves. In other words, once the valves have 
switched, the zone temperature must deviate a predetermined amount from 
the point of switching before the valves can switch back. Although a 
certain degree of hysteresis is desirable, it is not a requirement of the 
invention. 
Although FIG. 5 shows a slight separation 43 at set point 30, between lines 
45 and 47, the separation is shown only to illustrate the distinction 
between the heating and cooling curves. In actuality, the two lines may 
completely overlap, cross, or the location of the two lines may be 
interchanged due to the hysteresis just mentioned. 
Beyond dead band 34, control 28 operates in a somewhat similar fashion as 
the controls represented by FIGS. 2 through 4. However, the non-linear 
relationship between valve position and temperature has been modified 
using straight line approximation for simplication. 
The straight line approximation can be avoided by employing an airflow 
sensors 62 and 100 for sensing the actual airflow rate through valves 24 
and 26. Although control 28 can vary valve position as a function of 
temperature without the use of flow sensor 62 as described above, using 
sensor 62 can provide a greater degree of control. When an airflow sensor 
is used, the term "minimum position" used herein represents any airflow 
rate that is less than that provided by the fully open position, and the 
term "intermediate open position" used herein represents any airflow rate 
that is between that which is provided by the fully open and minimum 
position. 
Further details of control 28 are shown in FIG. 6. A thermostat 44 provides 
a signal 46 representing the temperature difference between an actual 
temperature associated with zone 10 and its set point 30. A signal 
conditioner 48 provides a transfer function that transfers the incoming 
signal 46 to an output signal 50 that is adapted to control valves 24 and 
26. Signal conditioner 48 can provide any one of a variety of commonly 
used transfer functions such as proportional, integral, or proportional 
plus integral control. Signal 50 is conveyed to cooling valve 26 by way of 
a resistor 52 and a summing junction 54. Summing junction 54 delivers a 
control signal 56 to valve 26 upon comparing signal 58 to an airflow 
feedback signal 60 generated by an airflow sensor 62. Resistor 52 is of 
sufficient ohmic resistance to enable potentiometer circuit 64 and 66 to 
impose predetermined minimum voltage levels at node 68. 
Potentiometer 70 adjusts the intermediate open position 36 of cooling valve 
26 is indicated by numeral 72 of FIG. 5. And potentiometer 74 determines 
the cooling valve's minimum position 40 as indicated by numeral 76 of FIG. 
5. In response to the level of output signal 50, a switching circuit 78 
automatically determines which predetermined minimum voltage should be 
imposed on node 68. As indicated by FIG. 5, switching occurs at 
approximately set point 30. Diode 80 allows the voltage at node 68 to 
exceed either of the two predetermined minimal voltage levels for 
controlling the valves in the region above line 32 of FIG. 5. 
Referring back to FIG. 6, a second signal conditioning circuit 82 provides 
a heating valve control signal 84 in response to signal 50. Circuit 82 
includes a potentiometer 86 that varies the relationship of temperature to 
heating valve position as indicated by numeral 90 of FIG. 5. It should be 
clear that varying the relationship as indicated by numeral 90 also varies 
the width of dead band 34. Similar to the cooling valve coontrol, a 
potentiometer 92 determines the heating valve's intermediate open position 
36, as indicated by numeral 93 of FIG. 5. And a potentiometer 94 
determines the valve's minimum position 42, as indicated by numeral 95 of 
FIG. 5. Resistor 96, summing junction 98, and airflow sensor 100 are the 
heating valve's counterpart to the cooling valves's resistor 52, summing 
junction 54, and airflow sensor 62 respectively. Switch 102 provides a 
means of bypassing circuit 82, whereby valves 24 and 26 can be operated in 
a second mode where both valves move in unison rather than in opposition. 
This feature is useful in some systems where it is desirable to operate in 
the second mode where both ducts provide heating, or both supply cooling, 
or to simply convey unconditioned air through both valves simultaneously. 
FIG. 5 represents the first mode of operation. 
It should be noted that potentiometers 70, 74, 86, 92, and 94 represent any 
means (e.g., EEPROM) for adjusting the response of control 28 to provide 
the response described above. 
Although the invention is described with respect to a preferred embodiment, 
modifications thereto will be apparent to those skilled in the art. 
Therefore, the scope of the invention is to be determined by reference to 
the claims which follow.