Patent Publication Number: US-7219506-B2

Title: Method for estimating inlet and outlet air conditions of an HVAC system

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to a method for estimating the inlet and outlet air conditions of an HVAC system to determine the load requirements of the system. 
   The greenhouse gases emitted to the atmosphere by an HVAC system can be reduced by efficiently utilizing electric power. Electric power can be efficiently utilized by employing capacity control that matches the system capacity to the load requirements of the HVAC system. Capacity control utilizes various refrigerant and air conditions to determine the load requirement of the HVAC system. Sensors are generally utilized in an HVAC system to detect the pressure and the temperature of the refrigerant entering and exiting the compressor, the temperature of the refrigerant entering and exiting the evaporator, and the temperature of the air entering the evaporator. Once the load requirements are known, the compressor can be control so that the system capacity matches the load requirements. 
   The temperature of the air exiting the evaporator and the relative humidity of the air entering and exiting the evaporator also need to be detected to employ capacity control. However, a drawback is that additional sensors must be installed to monitor the temperature of the air exiting the evaporator and the relative humidity of the air entering and exiting the evaporator. In the prior art, humidity sensors, dry bulb sensors, and wet bulb temperature sensors were added to the vapor compression system to monitor these conditions. 
   There are several drawbacks to installing additional sensors in the HVAC system. For one, employing additional sensors is expensive. Additionally, the measurements provided by some sensors may not be reliable due to the complex dynamics of a thermodynamic system. For example, if a sensor is employed to measure the air temperature of the air exiting the evaporator, the turbulence in the outlet air created by a fan can affect the temperature reading. It would be beneficial to determine the temperature of the air exiting the evaporator and the relative humidity of the air entering and exiting the evaporator without using additional sensors. 
   Therefore, the present invention provides a method that utilizes existing sensors to provide an accurate estimation of the inlet and outlet air conditions of the evaporator that are needed for capacity control without additional cost to the system and also provides the information needed for the diagnostic/prognostics of the HVAC system as well as overcoming the other drawbacks and shortcomings of the prior art. 
   SUMMARY OF THE INVENTION 
   A vapor compression system provides cool air to an area when operating in a cooling mode. Refrigerant is compressed to a high pressure in a compressor and is cooled in a condenser. The cooled refrigerant is expanded to a low pressure in an expansion device. After expansion, the refrigerant flows through the evaporator and accepts heat from the air, cooling the air. The refrigerant then returns to the compressor, completing the cycle. 
   Several refrigeration and air properties of the vapor compression system are detected to calculate the load demand of the vapor compression system. The vapor compression system includes sensors that detect the compressor suction temperature, the compressor discharge temperature, the compressor suction pressure, the compressor discharge pressure, the inlet temperature of the refrigerant entering the evaporator, the outlet temperature of the refrigerant exiting the evaporator, and the inlet temperature of the air entering the evaporator. The temperature of the air exiting the evaporator, the relative humidity of the air entering the evaporator, and the relative humidity of the air exiting the evaporator are determined using the values detected by the sensors. 
   The outlet temperature of the air exiting the evaporator is calculated by using the detected inlet temperature of the air entering the evaporator, the saturation temperature of the air (which is approximately equal to the refrigerant saturation temperature) and a bypass factor of the evaporator. 
   The relative humidity of the air entering and exiting the evaporator can then calculated. On a psychrometric chart, the dry bulb temperature is on the horizontal axis, and the humidity ratio is on the vertical axis. A first point is plotted at the intersection of a vertical line extending from the saturation temperature of the refrigerant and the saturation line. The air exiting the evaporator is near saturation, and the relative humidity of the air exiting the evaporator is approximately 95% of the saturation line. Therefore, the relative humidity line of the air exiting the evaporator is known. A second point is defined at the intersection of a vertical line extending from the outlet temperature of the air exiting the evaporator and the relative humidity line of the air exiting the evaporator. 
   A line connecting the first point and the second point is extended until it intersects a vertical line extending vertically from the inlet temperature of the air entering the evaporator at a third point. The third point represents the relative humidity of the air entering the evaporator. 
   By using the existing sensors to determine the temperature of the air exiting the evaporator and the relative humidity of the air entering and exiting the evaporator, the load requirement of the vapor compression system can be calculated without employing additional sensors. Once the load requirements are known, the system capacity can be matched to the load requirement, allowing the electric power of the vapor compression system to be used effectively. 
   These and other features of the present invention will be best understood from the following specification and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of the invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
       FIG. 1  illustrates a vapor compression system including sensors used to detect conditions of the air and the refrigerant flowing through the vapor compression system; 
       FIG. 2  illustrates a vapor compression system showing the sensed values needed to determine the load requirements of the vapor compression system; 
       FIG. 3  illustrates a graph showing the temperature of the air flowing over a evaporator as the air travels through the evaporator; 
       FIG. 4  illustrates a graph showing data about the evaporator; and 
       FIG. 5  illustrates a psychrometric chart showing the procedure for estimating the relative humidity of the air entering and exiting the evaporator. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  illustrates a vapor compression system  20  including a compressor  22 , a condenser  24 , an expansion device  26 , and an evaporator  28 . Refrigerant circulates though the closed circuit vapor compression system  20 . 
   When the vapor compression system  20  is operating in a cooling mode, the refrigerant exits the compressor  22  at a high pressure and a high enthalpy and flows through the condenser  24 . In the condenser  24 , the refrigerant rejects heat to a fluid medium, such as water or air, and is condensed into a liquid that exits the condenser  24  at a low enthalpy and a high pressure. If the fluid medium is air, a fan  30  is employed to direct the fluid medium over the condenser  24 . The cooled refrigerant then passes through the expansion device  26 , and the pressure of the refrigerant drops. After expansion, the refrigerant flows through the evaporator  28 . In the evaporator  28 , the refrigerant accepts heat from air, exiting the evaporator  28  at a high enthalpy and a low pressure. A fan  32  blows the air over the evaporator  28 , and the cooled air is then used to cool an area  52 . 
   When the vapor compression system  20  is operating in a heating mode, the flow of the refrigerant is reversed using a four-way valve (not shown). When operating in the heating mode, the condenser  24  operates as an evaporator, and the evaporator  28  operates as a condenser. 
   Capacity control is utilized to match the system capacity of the vapor compression system  20  to the load requirement of the vapor compression system  20  and therefore effectively use electric power. The load requirement is the required heat exchange that occurs at the evaporator  28 . When the load requirement is known, the compressor  22  can be controlled such that the load requirement of the vapor compression system  20  is met. 
   Several variables are needed to calculate the load demand as an integral part of the capacity control task. As shown in  FIG. 2 , the variables are 1) the compressor suction temperature T suc , 2) the compressor discharge temperature T dis , 3) the compressor suction pressure P suc , 4) the compressor discharge pressure P dis , 5) the inlet temperature of the refrigerant entering the evaporator T 2in , 6) the outlet temperature of the refrigerant exiting the evaporator T 2out , 7) the inlet temperature of the air entering the evaporator T 1in , 8) the outlet temperature of the air exiting the evaporator T 1out , 9) the relative humidity of the air entering the evaporator RH 1 , and 10) the relative humidity of the air exiting the evaporator RH 2 . 
   It is difficult to accurately measure the outlet temperature of the air exiting the evaporator T 1out  due to the non-homogeneous nature of the turbulent airflow produced by the fan  32 . Measuring the relative humidities RH 1  and RH 2  of the air entering or exiting the evaporator  28 , respectively (the wet bulb temperature) is expensive and possibly inaccurate. Therefore, only the sensors that measure the compressor suction temperature T suc , the compressor discharge temperature T dis , the compressor suction pressure P suc , the compressor discharge pressure P dis , the inlet temperature of the refrigerant entering the evaporator T 2in , the outlet temperature of the refrigerant exiting the evaporator T 2out , and the inlet temperature of the air entering the evaporator T 1in  are installed in the vapor compression system  20 . In the present invention, the outlet temperature of the air exiting the evaporator T 1out , the relative humidity of the air entering the evaporator RH 1 , and the relative humidity of the air exiting the evaporator RH 2  are calculated using the values detected by the installed sensors. 
   Returning to  FIG. 1 , the vapor compression system  20  includes a sensor  34  that detects the compressor suction temperature T suc , a sensor  36  that detects the compressor discharge temperature T dis , a sensor  38  that detects the compressor suction pressure P suc , a sensor  40  that detects the compressor discharge pressure P dis , a sensor  42  that detects the inlet temperature of the refrigerant entering the evaporator T 2in , a sensor  44  that detects the outlet temperature of the refrigerant exiting the evaporator T 2out , and a sensor  46  that detects the inlet temperature of the air flowing into the evaporator T 1in . The sensors  34 ,  36 ,  38 ,  40 ,  42 ,  44  and  46  all communicate with a control  48 . 
   By employing the sensors  34 ,  36 ,  38 ,  40 ,  42 ,  44  and  46  that are usually installed in the vapor compression system  20 , the outlet temperature of the air exiting the evaporator T 1out , the relative humidity of the air entering the evaporator RH 1 , and the relative humidity of the air exiting the evaporator RH 2  can be calculated without employing the additional sensors. 
   A bypass factor BPF of the evaporator  28  represents the amount of air that is bypassed without direct contact with the coil of the evaporator  28 . The bypass factor BPF depends upon the number of fins in a unit length of the coil (the pitch of the coil fins), the number of rows in the coil in the direction of airflow, and the velocity of the air. The bypass factor BPF of the coil decreases as the fin spacing decreases and the number of rows increases. The bypass factor BPF is defined as: 
                 BPF   =         T     1   ⁢   out       -     T   s           T     1   ⁢   in       -     T   s                 (     Equation   ⁢           ⁢   1     )               when   ⁢           ⁢   the   ⁢           ⁢   evaporator   ⁢           ⁢   28   ⁢           ⁢   is   ⁢           ⁢   a   ⁢           ⁢   cooling   ⁢           ⁢   coil                           BPF   =         T   s     -     T     1   ⁢   out             T   s     -     T     1   ⁢   in                   (     Equation   ⁢           ⁢   2     )               when   ⁢           ⁢   the   ⁢           ⁢   evaporator   ⁢           ⁢   28   ⁢           ⁢   is   ⁢           ⁢   a   ⁢           ⁢   heating   ⁢           ⁢   coil                           
The saturation temperature of the air is represented by T s . The saturation temperature of the air T s  is approximately equal to the saturation temperature of the refrigerant. The saturation temperature of the refrigerant is calculated using the compressor suction pressure P suc  and the refrigerant property. The refrigerant property is a known value that depends on the type of refrigerant used. Typically, the bypass factor BPF is below 0.2.
 
     FIG. 3  illustrates a graph showing the temperature of the air as it passes over the coil of the evaporator  28 . As shown, as the air travels over and along the length of the coil of the evaporator  28 , the outlet temperature of the air exiting the evaporator T 1out  decreases almost to the saturation temperature of the air T s . 
   The heat transfer rate of the evaporator  28  is defined as:
 
 Q=UA×LMTD   (Equation 3)
 
The heat transfer rate is represented by the variable Q (W). The variable U represents the overall heat transfer coefficient (W/m 2 K), the variable A represents the surface area of the coil of the evaporator  28 , and the variable LMTD represents the logarithmic mean temperature difference.
 
   The variable logarithmic mean temperature difference is defined as: 
   
     
       
         
           
             
               
                 LMTD 
                 = 
                 
                   
                     
                       T 
                       
                         1 
                         ⁢ 
                         in 
                       
                     
                     - 
                     
                       T 
                       
                         1 
                         ⁢ 
                         out 
                       
                     
                   
                   
                     
                       log 
                       e 
                     
                     ⁡ 
                     
                       ( 
                       
                         
                           
                             T 
                             
                               1 
                               ⁢ 
                               in 
                             
                           
                           - 
                           
                             T 
                             s 
                           
                         
                         
                           
                             T 
                             
                               1 
                               ⁢ 
                               out 
                             
                           
                           - 
                           
                             T 
                             s 
                           
                         
                       
                       ) 
                     
                   
                 
               
             
             
               
                 ( 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
                 ) 
               
             
           
         
       
     
   
   Equation 1 can be inserted into Equation 4, and the variable logarithmic mean temperature difference is defined as: 
   
     
       
         
           
             
               
                 LMTD 
                 = 
                 
                   
                     
                       T 
                       
                         1 
                         ⁢ 
                         in 
                       
                     
                     - 
                     
                       T 
                       
                         1 
                         ⁢ 
                         out 
                       
                     
                   
                   
                     
                       log 
                       e 
                     
                     ( 
                     
                       1 
                       BPF 
                     
                     ) 
                   
                 
               
             
             
               
                 ( 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   5 
                 
                 ) 
               
             
           
         
       
     
   
   The heat transfer rate Q can also be calculated from the airside (the load demand) using the following equation: 
                   Q   .     =           m   .     1     ⁢       c   P1     ⁡     (       T     1   ⁢   in       -     T     1   ⁢   out         )         SHR             (     Equation   ⁢           ⁢   6     )               
In this equation, m 1  represents the mass flow rate of air (kg/s), c p1  represents the specific heat of dry air (J/kgK), and SHR represents the sensible heat ratio. The inlet temperature of the air flowing into the evaporator T 1in  and the outlet temperature of the air flowing out of the evaporator T 1out  are in Celsius (° C.).
 
   Combining Equation 3 and Equation 6 results in the following equation: 
   
     
       
         
           
             
               
                 BPF 
                 = 
                 
                   ⅇ 
                   
                     
                       UA 
                       · 
                       SHR 
                     
                     
                       
                         c 
                         P1 
                       
                       ⁢ 
                       
                         
                           m 
                           . 
                         
                         1 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   7 
                 
                 ) 
               
             
           
         
       
     
   
   As shown in  FIG. 4 , for a coil of an evaporator  28  with a two-phase refrigerant flow, the value UA is a function of the sensible heat ratio SHR and the mass flow rate of air m 1 . The evaporator  28  is used in a 30 HP heat pump system. The value UA is inversely proportional to the sensible heat ratio SHR and linearly related to the flow rate change of air. Consequently, the value UA can be approximated using the following equation: 
   
     
       
         
           
             
               
                 UA 
                 = 
                 
                   
                     
                       a 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           m 
                           . 
                         
                         1 
                       
                     
                     + 
                     b 
                   
                   SHR 
                 
               
             
             
               
                 ( 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   8 
                 
                 ) 
               
             
           
         
       
     
   
   In Equation 8, the variables a and b are both constants, and b has a relatively small value. Substituting Equation 8 into Equation 7 demonstrates that the bypass factor BPF is a constant: 
   
     
       
         
           
             
               
                 BPF 
                 = 
                 
                   
                     
                       a 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           m 
                           . 
                         
                         1 
                       
                     
                     + 
                     b 
                   
                   
                     ⅇ 
                     
                       
                         c 
                         
                           p 
                           1 
                         
                       
                       ⁢ 
                       
                         
                           m 
                           1 
                         
                         . 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   9 
                 
                 ) 
               
             
           
         
       
     
   
   Because the bypass factor BPF is a constant for a given coil of the evaporator  28 , its value can be determined either by experiment or by the design model. Using the known bypass factor BPF value and Equation 1, the outlet temperature of the air exiting the evaporator T 1out  can be calculated using the following equations:
 
 T   1out   =BPF ( T   1in   −T   s )+ T   s  when the evaporator  28  is a cooling coil  (Equation 10)
 
 T   1out   =T   s   −BPF ( T   s   −T   1in ) when the evaporator  28  is a heating coil  (Equation 11)
 
   After calculating the outlet temperature of the air exiting the evaporator T 1out , the relative humidity of the air entering the evaporator RH 1  and the relative humidity of the air exiting the evaporator RH 2  can be estimated. 
     FIG. 5  illustrates a psychrometric chart showing the procedure for estimating the relative humidity of the air entering the evaporator RH 1  and the relative humidity of the air exiting the evaporator RH 2 . The dry bulb temperature is on the horizontal axis, and the humidity ratio is on the vertical axis. Points representing the saturation temperature of the air T s , the inlet temperature of the air exiting the evaporator T 1in  and the outlet temperature of the air exiting the evaporator T 1out  are plotted along the horizontal axis. The saturation line RHs is also shown. 
   A vertical line extending from the saturation temperature of the air T s  intersects the saturation line RHs at a point  3 . The coil of the evaporator  28  is designed such that the air exiting the evaporator  28  is near saturation, and the relative humidity of the air exiting the evaporator RH 2  is approximately 95% of the saturation line RHs. Therefore, the relative humidity line RH 2  is known, assuming it to be 95% of the saturation line RHs. The outlet temperature of the air exiting the evaporator T 1out  was previously calculated using the bypass factor BPF and the inlet temperature of the air entering the evaporator T 1in . Therefore, point  2  can be found on the chart at the intersection of a vertical line extending from the outlet temperature of the air exiting the evaporator T 1out  and the relative humidity line RH 2 . 
   A line connecting point  2  and point  3  is extended until it intersects a vertical line extending vertically from the inlet temperature of the air entering the evaporator T 1in  at point  1 . Point  1  represents the relative humidity of the air entering the evaporator RH 1 . The relative humidity line RH 1  can then be determined as it passes through point  1 . 
   If the vapor compression system  20  is operating in a heating mode, the relative humidity RH 1  and the relative humidity RH 2  do not change and can be calculated using the above-described method. Therefore, only the outlet temperature of the air exiting the evaporator T 1out  needs to be calculated to determine the load requirement of the vapor compression system  20 . 
   By using the existing sensors  34 ,  36 ,  38 ,  40 ,  42 ,  44  and  46  in the vapor compression system  20  to determine the outlet temperature of the air exiting the evaporator T 1out , the relative humidity of the air entering the evaporator RH 1 , and the relative humidity of the air exiting the evaporator RH 2 , additional sensors do not need to be added to the vapor compression system  20  to determine these values, reducing the cost and increasing accuracy. By determining these values using the existing sensors  34 ,  36 ,  38 ,  40 ,  42 ,  44  and  46 , the load requirement of the vapor compression system  20  can be calculated. Therefore, system capacity of the vapor compression system  20  can be matched to the load requirement by controlling the compressor  22 , allowing for effective use of electric power without the use of additional sensors. 
   The foregoing description is only exemplary of the principles of the invention. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, so that one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.