Abstract:
A strategy for determining the critical supply duct pressure in variable-air-volume heating, ventilating, and air-conditioning systems that compensates for duct leakage and variable loads, enabling its use during normal system operation. The strategy consists of a static pressure sensor, an airflow sensor, a supply fan, a fan modulating device, a controller coupled to the static pressure sensor and the airflow sensor, and a data processing algorithm for analyzing results from a function test using these components. The functional test involves changing the supply duct pressure setpoint, waiting for equilibrium, recording pressure, flow, and time, then changing the supply duct pressure setpoint to the next setting in the sequence.

Description:
[0001]     “This invention was made with State of California support under California Energy Commission Grant number 02-03. The Energy Commission has certain rights to this invention.” 
     
    
     BACKGROUND  
       [0002]     1. Field of the Invention  
         [0003]     The following invention relates to controls for variable-air-volume heating, ventilating, and air-conditioning (HVAC) systems, specifically to supply duct static pressure control.  
         [0004]     2. Description of Prior Art  
         [0005]     Modern buildings typically have complex heating, ventilating, and air-conditioning systems to control indoor temperature, pressure, ventilation rate, and other variables in a way that makes efficient use of energy. One way to conserve energy in these systems is to use a so-called variable-air-volume design. Key components of a variable-air-volume system are a supply fan and terminal units. The supply fan is a prime mover that causes air to move. A terminal unit contains a throttling damper that regulates an amount of air supplied to a space in a building that it controls in order to regulate temperature and ventilation in that space.  
         [0006]     In a variable-air-volume system, a flow rate of conditioned air supplied to a building is adjusted so that no more air than necessary is used. Variable flow is achieved using controls on or near the supply fan and by the use of controls on the terminals. The supply fan controls adjust the speed of the fan, an angle of the fan blades, an angle of guide vane at an inlet or outlet of the fan, or by adjusting a damper upstream or downstream of the fan that throttles the flow. The controls on the terminals determine how much air flows through each terminal.  
         [0007]     The most common control strategy for the supply fan of variable-air-volume systems is to regulate a static pressure in a supply duct at a point downstream of the supply fan. This strategy seeks to keep the static pressure at a measurement point constant at all times. Control strategies based on a constant static pressure in the supply duct have been proposed in U.S. Pat. No. 4,437,608 to Smith (1984) and U.S. Pat. No. 6,227,961 to Moore et al. (2001). U.S. Pat. No. 4,836,095 to Wright (1989) describes a variant of this strategy for systems that have multi-speed fans rather than fans in which the speed is continuously variable. A rule of thumb for this strategy is to locate the pressure sensor two-thirds of the distance from the supply fan to the end of the supply duct. A problem with this strategy is that it is inefficient at part-load conditions, when the supply flow rate is significantly lower than a design flow rate, which is the flow rate at which the system should operate when the fan is running at full speed.  
         [0008]     A control strategy that overcomes the problem of constant static pressure control is one in which a static pressure setpoint is reset based on a position of a terminal damper that is most open. Control strategies that reset the static pressure based on the position of the terminal damper that is most open have been proposed in U.S. Pat. No. 4,630,670 to Wellman and Clark (1986) and U.S. Pat. No. 5,863,246 to Bujak (1999). An objective is to keep this damper nearly open or completely open. Doing so keeps the supply duct pressure near the critical pressure, reducing throttling losses at part-load conditions. The critical pressure is the lowest supply duct pressure at which all of the terminal dampers are still controlling. When the supply duct pressure is below the critical pressure one or more terminal dampers will be fully open yet unable to get enough air.  
         [0009]     A report published by the California Energy Commission (CEC publication number P500-03-052F, 2003) showed that the critical supply duct pressure is correlated with the supply airflow rate. This fact is exploited in U.S. Pat. No. 6,719,625 to Federspiel (2004), which describes a static pressure reset strategy that adjusts the static pressure setpoint based on the supply airflow rate. This strategy overcomes many of the problems of static pressure reset strategies that rely on terminal damper position measurements. However, it requires some knowledge of how the critical supply duct pressure is related to the supply airflow rate.  
         [0010]     Accordingly, a need exists for a strategy that will allow the relationship between critical supply duct pressure and a supply flow rate to be determined so that the static pressure reset strategy based on supply airflow rate can be optimized.  
       SUMMARY OF THE INVENTION  
       [0011]     In accordance with the present invention, a strategy for determining the critical supply duct pressure of a variable-air-volume heating, ventilating, and air-conditioning system comprises the supply fan, a fan modulating device, a static pressure sensor, an airflow sensor, and a controller coupled to the static pressure sensor. The controller is commanded to a sequence of static pressures. Supply airflow at each static pressure setpoint is recorded, and the data are processed using a model-based analysis technique that determines the critical supply duct pressure, the leakage coefficient, and the rate of change of the load at the test condition.  
       OBJECTS OF THE INVENTION  
       [0012]     Accordingly, a primary object of the present invention is to provide a strategy for determining the critical supply duct pressure of variable-air-volume heating, ventilating, and air-conditioning systems so that a static pressure reset strategy can be configured and optimized.  
         [0013]     Another object of the present invention is to provide a strategy for determining the critical supply duct pressure of variable-air-volume heating, ventilating, and air-conditioning systems that can be implemented during normal system operation.  
         [0014]     Another object of the present invention is determine the leakage rate of the supply air duct in variable-air-volume heating, ventilating, and air-conditioning systems.  
         [0015]     Another object of the present invention is to provide a strategy for determining the critical supply duct pressure of variable-air-volume heating, ventilating, and air-conditioning systems that can compensate for load changes that occur while the strategy is implemented.  
         [0016]     Other further objects of the present invention will become apparent from a careful reading of the included drawing figures, the claims, and detailed description of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a schematic diagram of a portion of a variable-air-volume (VAV) heating, ventilating, and air-conditioning (HVAC) system.  
         [0018]      FIG. 2  is a graph of supply duct pressure versus supply airflow. The points in the graph show the measured pressures and flows from a laboratory test. The curves in the graph show the starved and controlling models. The pressure at the intersection of the curves is the critical pressure. 
     
    
     REFERENCE NUMERALS IN DRAWINGS  
       [0019]    
       
         
               
               
               
               
             
           
               
                   
               
               
                   
               
             
             
               
                 11 
                 supply fan 
                 12 
                 fan modulating device 
               
               
                 13 
                 supply duct 
                 14 
                 terminal duct 
               
               
                 15 
                 terminal unit 
                 16 
                 terminal unit controller 
               
               
                 17 
                 static pressure sensor 
                 18 
                 airflow sensor 
               
               
                 19 
                 supply fan controller 
                 20 
                 terminal damper 
               
               
                 21 
                 starved-mode model 
                 22 
                 controlling-mode model 
               
               
                 23 
                 supply duct critical pressure 
               
               
                   
               
             
          
         
       
     
       DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]      FIG. 1  shows the components of a variable-air-volume heating, ventilating, and air-conditioning system that are relevant to the critical pressure determination strategy. These components include a supply fan  11 , a fan modulating device  12 , a supply duct  13 , two or more terminal ducts  14 , two or more terminal units  15 , two or more terminal unit controllers  16 , a static pressure sensor  17 , an airflow sensor  18 , and a supply fan controller  19 . The system also contains other components such as heat exchangers and filters not shown in  FIG. 1 , which are used for other functions such as heating, cooling, and cleaning air. Supply fan  11  could be a centrifugal fan or an axial fan. Fan modulating device  12  could be a variable-speed drive, variable inlet guide vanes, a throttling device such as a damper, or a device to adjust the pitch of the fan blades. Supply duct  13  is an elongate sheet metal structure with rectangular cross-section used to transport air. Each terminal duct  14  is also an elongate sheet metal structure used to transport air. Each terminal duct  14  contains a terminal unit  15 , which contains at least one terminal damper  20  used to regulate a flow rate of air in the terminal duct  14  in response to commands from the terminal unit controller  16 . Static pressure sensor  17  is located downstream of supply fan  11 . Static pressure sensor  17  indicates the static pressure in supply duct  13 . Airflow sensor  18  indicates a flow rate of air in supply duct  13 . Airflow sensor  18  may be located either upstream or downstream of supply fan  11 . Alternatively, the airflow readings from terminal units  15  may be added together to measure the supply airflow rate. Supply fan controller  19  may be an electronic device with a microprocessor and memory, an analog electrical circuit, or a pneumatic device.  
         [0021]     Determination of the critical supply duct pressure involves implementing a functional test on the air-handling unit, then processing the data from the functional test using a model-based procedure. The data processing uses a dual-mode model of a variable-air-volume air-handling system. The two modes are “controlling” and “starved”. The supply fan in most variable-air-volume air-handling systems is used to regulate the static pressure at a point in the supply duct. The static pressure should be sufficiently high that all terminals served by the air-handling unit get enough air to meet their load. If it is too high, then even the most-open variable-air-volume terminal will be throttling considerably, and energy will be wasted. The critical supply duct pressure occurs when the most-open variable-air-volume terminal is 100% open and just meeting the load because this condition minimizes throttling losses while keeping the system in control. When the supply duct pressure is high enough that all of the terminals are meeting the load, the system is operating in the controlling mode. When one or more terminal dampers are 100% open and not meeting the load, the system is in the starved mode. The lowest supply duct pressure that keeps all the terminals in control is called the critical pressure.  
         [0022]     The controlling-mode model contains three terms. The first is a constant term that represents the cumulative flow rate through the dampers at the beginning of the functional test used to determine the critical pressure. The second is a term to account for duct leakage, which can be very significant in some systems. The third is a time-dependent term that accounts for the fact that the loads, and therefore the supply flow, may change over the course of the functional test if it is conducted during normal operation. Mathematically, the controlling model is as follows: 
 
 Q   c =Q 0   +C   p   P   n   +C   t   T    (1) 
 
 where Q c  is the total supply airflow rate when the system is in control, C p  is the leakage coefficient, and C t  is the rate of change of the supply airflow rate due to changing load conditions. The first term on the right-hand side of Equation 1 is the controlled cumulative terminal flow (cumulative flow through the terminal dampers) at the start of the functional test. The second term is leakage flow, and the third is the time-varying component of the controlled cumulative terminal flow. 
 
         [0023]     When C t =0, Q 0  is held constant as long as the terminal dampers can change the system flow coefficient according to the following relation:  
               C   Q     =       Q   0       P   n               (   2   )             
 
 where C Q  is the system flow coefficient. 
 
         [0024]     When the supply duct pressure drops below the critical pressure (starved mode), the relationship between flow coefficient and pressure in Equation 2 no longer holds. The flow coefficient becomes less that that of Equation 2, and Q 0  becomes a function of the pressure. In the starved mode, the flow coefficient is modeled a quadratic function of pressure as follows: 
 
 C   Q   =c   0   +c   1   P+c   2   P   2    (3) 
 
 where the polynomial coefficients c 0 , c 1 , and c 2  must be determined empirically. The starved-mode model is as follows:  
               Q   s     =         (         c   0     ⁢     P   n       +       c   1     ⁢     P     1   +   n         +       c   2     ⁢     P     2   +   n           )     ⁢     (     1   +         C   t     ⁢   T       Q   0         )       +       C   p     ⁢     P   n                 (   4   )             
 
         [0025]     The starved-mode model has three additional parameters besides the three parameters of the controlling-mode model (Equation 1). The term C t T/Q 0  compensates for the fact that only a fraction of the terminal flows (those of unstarved terminals) may be changing with time in response to changing loads.  
         [0026]     The preferred functional test procedure for determining the critical supply duct pressure involves the following sequence of operations: 
        1. start at a sufficiently high supply duct pressure setpoint     2. wait for the terminals to reach equilibrium (e.g., 2 minutes for laboratory experiments, 15 minutes for field experiments)     3. take a reading of supply flow, static pressure, and time     4. reduce the supply duct static pressure setpoint by a small amount (e.g., 0.1 in. w.c.)     5. wait for the terminals to reach equilibrium again     6. take a reading of supply flow, static pressure, and time     7. repeat steps 4-6 until the supply flow is less than a pre-determined limit (e.g., 70% of the starting flow)     8. increase the pressure by a small amount (e.g., 0.1 in. w.c.)     9. wait for terminals to reach equilibrium again     10. take a reading of supply flow, static pressure, and time     11. repeat steps 8-10 until the pressure equals the starting pressure        
 
         [0038]     The preferred analysis procedure for determining the critical pressure from the functional test data is as follows: 
        A. Assign the first N high-pressure points at the beginning of the test and the M low-pressure points at the end of the test to the controlling-mode model. Estimate the coefficients of the controlling-mode model with least squares. Determine if the time-dependent term can be dropped from the model using a t-test with a decision probability of 0.02.     B. Assign the remaining data points to the starved-mode model. Estimate the three coefficients of the starved-mode model using the coefficients determined from the controlling model.     C. Compute the variance of the combined residuals.     D. Repeat steps 1-3 for all allowable values of N and M.     E. Choose the values of N and M that produce the lowest variance     F. Determine the pressure at which the flow predicted by the starved model equals the flow predicted by the controlling model. 
 
  FIG. 2  is a graph showing the functional test data and results of the analysis procedure for a test conducted on a laboratory air-handling unit. The gradual slope of controlling-mode model  22  is due to duct leakage. Starved-mode model  21  shows a rapid decrease in pressure below the airflow rate corresponding to critical supply duct pressure  23 , which is caused by terminals becoming starved. Experiments conducted over a wide range of conditions demonstrate that the standard error of values from the preferred embodiment is 6% of the true critical supply duct pressure. 
 
 Conclusion, Ramifications, and Scope 
       
 
         [0045]     Accordingly, the reader will see that the critical pressure determination strategy of this invention has a number of advantages including the following: 
        (a) It can be implemented during normal system operation,     (b) It can determine duct leakage,     (c) It can compensate for time-varying loads.        
 
         [0049]     This disclosure is provided to reveal a preferred embodiment of the invention and a best mode for practicing the invention. Having thus described the invention in this way, it should be apparent that various different modifications can be made to the preferred embodiment without departing from the scope and spirit of this disclosure. Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.