Abstract:
An air conditioning system comprising an air mover for circulating air to a space; a vapor compression circuit including a compressor, a condenser, and an expansion device; an evaporator; an air-reheat heat exchanger; and a control system. In one embodiment, the evaporator receives refrigerant from the vapor compression circuit and provides a cooled stream of air to the space. The air-reheat heat exchanger is positioned to receive the cooled stream of air. In one embodiment, the vapor compression circuit, the evaporator, and the air-reheat heat exchanger are operable in combination to provide a plurality of modes of operation. In a preferred embodiment, the control system is configured to compute a Sensible cooling-to-Total cooling (S/T) process ratio and to control an operation of at least one of the vapor compression circuit, the evaporator, and the air-reheat heat exchanger. A method of manufacturing the air conditioning system is also provided.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to air conditioning and, more particularly, to a control system for air conditioning systems employing dehumidification and re-heat. 
     BACKGROUND OF THE INVENTION 
     Air conditioning systems with a re-heating system for active humidity control can overcool the air provided to the conditioned space while performing active dehumidification. This occurs because the re-heat coil is not sized to provide neutral supply air temperature. For example, condenser re-heat systems, like the Lennox Humiditrol® EDA, still have a Sensible cooling-to-Total cooling ratio (S/T) of about 0.25 so that some sensible cooling is occurring while dehumidification is being required by the humidistat setting. 
     A typical control scheme uses “cooling priority” first to satisfy the cooling requirement from the thermostat, and then, if there is excess humidity detected by the humidity sensor, the vapor compression circuit and evaporator continue to cool the air so that the excess humidity can be removed. Since, under most conditions, there is a positive S/T ratio, the space continues to be cooled during this continued dehumidification mode. This results in overcooling of the air and conditioned space. Some thermostats even employ an “overcooling limit” to stop the dehumidification mode from lowering the conditioned air too far below the temperature setpoint even if the desired relative humidity has not been met. The fact that enhanced dehumidification is only enabled after the temperature setpoint has been achieved means that more than the minimum amount of energy is being used to provide the space with conditioned air. 
     Accordingly, what is needed in the art is an air conditioning system that avoids the wasted energy of overcooling the air in order to achieve the desired relative humidity. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, the present invention provides, in one aspect, an air conditioning system comprising an air mover for circulating air to a space; a vapor compression circuit including a compressor, a condenser, and an expansion device; an evaporator; an air-reheat heat exchanger; and a control system. In a preferred embodiment, the evaporator receives refrigerant from the vapor compression circuit and is adapted to provide a cooled stream of air to the space. In a further aspect, the air-reheat heat exchanger is positioned to receive the cooled stream of air. In one embodiment, the vapor compression circuit, the evaporator, and the air-reheat heat exchanger are operable in combination to provide a plurality of modes of operation. In a preferred embodiment, the control system is configured to compute a Sensible cooling-to-Total cooling (S/T) process ratio and to control an operation of at least one of the vapor compression circuit, the evaporator, and the air-reheat heat exchanger. A method of manufacturing the air conditioning system is also provided. 
     The foregoing has outlined preferred and alternative features of the present invention so that those skilled in the pertinent art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the pertinent art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the pertinent art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
         FIG. 1  illustrates a schematic diagram of an air conditioning system constructed according to the principles of the present invention; 
         FIG. 2  illustrates a flow diagram for an enhanced dehumidification control algorithm assembled in accordance with the principles of the present invention as implemented in the controller of  FIG. 1 ; 
         FIG. 3  illustrates a performance chart of three modes of operation of the air conditioning system of  FIG. 1  with a 75° F. indoor dry bulb temperature and a 66% indoor relative humidity; and 
         FIG. 4  illustrates a psychrometric chart covering normal indoor temperature and humidity ranges. 
     
    
    
     DETAILED DESCRIPTION 
     Referring initially to  FIG. 1 , illustrated is a schematic diagram of an air conditioning system  100  constructed according to the principles of the present invention. The air conditioning system  100  comprises a conventional electric motor-driven compressor  110  connected via a conduit  114  to a refrigerant fluid, primary condenser heat exchanger  116  disposed typically outdoors. The heat exchange between fluid flowing through the condenser heat exchanger  116  and ambient outside air is controlled by a fan  118  having a plurality of fixed pitch blades  118   a  and which is driven by a variable-speed electric motor  120 . The variable-speed electric motor  120  may be an electrically-commutated type operating on variable frequency and voltage AC electric power as supplied to the motor  120  via a suitable controller  122 . Fan  118  propels a heat exchange medium, such as ambient “outdoor” air through condenser heat exchanger  116  in a known manner. Condenser heat exchanger  116  may also operate with other forms of heat exchange medium at controlled flow rates thereof. Control of heat exchange medium flowing over condenser heat exchanger  116  may take other forms such as a constant-speed variable pitch fan, air flow control louvers, or control of a variable flow of a liquid heat exchange medium. Condenser heat exchanger  116  is also operably connected to a conventional refrigerant fluid filter and dryer  124  disposed in a conduit  126  for conducting condensed refrigerant fluid to a power-operated or so called motor-controlled valve  128 . Valve  128  may be controlled by a solenoid, for example, and may be of a type commercially available. The solenoid for the valve  128  is also adapted to be controlled by a suitable humidity sensor  130  through controller  122  disposed in a space  132  to be conditioned by the system  100 . In a preferred embodiment, the humidity sensor  130  may be a conventional humidistat with capability to receive input of a desired relative humidity called a relative humidity setpoint RH sp . The humidity sensor  130  is also operably connected to controller  122 . A temperature sensor  134 , disposed within the conditioned space  132 , is also operably connected to the controller  122 . In a preferred embodiment, the temperature sensor  134  may be a conventional thermostat with capability to receive input of a desired temperature called a temperature setpoint T sp . Controlled and conditioned space  132  is represented only schematically in the drawing figures and a return air path from space  132  or another source of air to be conditioned is omitted in the interest of conciseness. 
     Conduit  126  is connected by way of valve  128  to further refrigerant conducting conduits  136  and  138  to a conventional refrigerant fluid expansion device  140  and to an air-reheat heat exchanger  142 , respectively. The compressor  110 , condenser  116 , and expansion device  140  together may be properly termed a vapor compression circuit. The vapor compression circuit is sized so as to enable conditioning of the air returned to space  132  at the desired relative humidity RH sp . Conduit  136  is operable to deliver refrigerant fluid to a heat exchanger or so called evaporator  144  by way of the expansion device  140 . Expansion device  140  is coupled to a remote temperature sensor  140   a  which is adapted to sense the temperature of refrigerant fluid leaving the heat exchanger  144  by way of a conduit  146 . Conduit  146  is commonly known as the suction line leading to compressor  110  whereby refrigerant fluid in vapor form is compressed and recirculated through the system  100  by way of condenser heat exchanger  116 . A suitable valve operator vent conduit  147  is connected between valve  128  and conduit  146 . Heat exchangers  116 ,  142  and  144  may be conventional multiple fin and tube type devices, for example. 
     Air-reheat heat exchanger, also known as an air-reheat condenser  142  is adapted to receive refrigerant fluid from condenser heat exchanger  116  through conduit  138  and discharge such fluid through a conduit  143  and a check valve  145  to conduit  136  upstream of expansion device  140 . The air-reheat condenser  142  is capable of and may be used to raise the temperature of air returned to space  132  to the desired temperature T sp . Under certain operating conditions refrigerant fluid may also be advantageously permitted to bypass the condenser heat exchanger  116  through a conduit  149  and a pressure relief valve  150 . Pressure relief valve  150  includes a closure member  150   a  which is biased into a valve-closed position by resilient means, such as a coil spring  150   b . In response to a predetermined pressure, or range of pressures, acting on the closure member  150   a , the pressure relief valve  150  operates to bypass fluid flowing through conduit  114  around the condenser heat exchanger  116  directly to conduit  126  downstream of the filter/dryer  124 , as shown, and to the air reheat heat exchanger  142 . 
     In the operation of the air conditioning system  100 , controller  122  operates to control a drive motor  152  for a supply air blower or fan  154  of a conventional type. Ambient outdoor air, or air being circulated as return air from space  132 , is propelled by motor driven blower  154  through a suitable duct  156  wherein the heat exchangers  142  and  144  are disposed. Specifically, air-reheat heat exchanger  142  is downstream of heat exchanger  144 . One who is skilled in the art will recognize that the system  100  includes elements of a conventional vapor compression air conditioning system wherein compressor  110  compresses a suitable refrigerant fluid which is condensed in condenser heat exchanger  116  and is conducted to heat exchanger or evaporator  144  through expansion device  140  wherein the condensed refrigerant fluid is expanded and absorbs heat from the air flowing through the duct  156  to provide cooled air to space  132 . This operation is controlled by controller  122  using data demanded by temperature sensor  134  and humidity sensor  130 . Controller  122  operates to control fan motor  152  as well as motor driven compressor  110  and the variable speed fan motor  120  which controls the amount of cooling air flowing over condenser heat exchanger  116 . Controller  122  comprises a microprocessor  125  for management of an algorithm to be described below. 
     If the relative humidity requirements of the space  132  are not being met by operation of the system  100  wherein all refrigerant fluid is being directed from conduit  126  directly to conduit  136 , control valve  128  will be actuated to force refrigerant fluid to and through air-reheat heat exchanger  142  giving up heat to air flowing through the duct  156  into the space  132  thereby raising the temperature of such air and reducing the rate of sensible cooling occurring. Since refrigerant fluid condensed and highly subcooled in the air-reheat heat exchanger/condenser  142  then flows via conduit  143  to expansion device  140  and evaporator  144 , substantial cooling effect is imparted to air being discharged by blower  154  and flowing through evaporator  144  to thereby condense moisture in the air flowing through duct  156 . Blower  154  may also be termed an air mover. Accordingly, air propelled by blower  154  is first cooled by heat exchanger  144  to condense moisture therein and is then reheated by air-reheat heat exchanger  142  to meet the temperature and humidity requirements of the space  132 . If the humidity requirements of space  132  are not being met by the aforementioned operation of system  100 , the controller  122  reduces the speed of the fan motor  120  and fan  118 , thereby reducing the heat exchange taking place by air flow through the condenser heat exchanger  116 . Fan motor  120  may be controlled to continuously vary the speed of fan  118  or motor output speed may be varied in discrete steps. In this way a greater heat rejection load is placed on air-reheat heat exchanger  142 , progressively, thus raising the temperature of the air flowing into space  132  to further reduce the relative humidity. Commonly, the blower  154  is also reduced in speed during enhanced dehumidification operation. 
     In those circumstances where the reduced exchange of heat at the condenser heat exchanger  116  occurs, the configuration of the condenser heat exchanger  116  may be such as to impose a relatively large fluid pressure drop thereacross for refrigerant fluid flowing therethrough, particularly if a substantial amount of such fluid is remaining in gaseous form. However, since a greater amount of condensation is occurring in air-reheat heat exchanger  142 , as the fluid condensing load is shifted from heat exchanger  116  to air-reheat heat exchanger  142 , refrigerant fluid in gaseous form may bypass heat exchanger  116  by way of pressure relief valve  150  and conduit  149  without degrading the performance of the system  100 . 
     Another advantage of the system  100  is that only two refrigerant fluid conduits are required to extend between the indoor portion of the system  100 , as indicated by dashed line  160 , wherein the indoor portion is that generally below the line as shown in the figure. The outdoor portion of system  100  typically includes the compressor  110  and the condenser heat exchanger  116 , as well as the condenser fan and motor  118 ,  120 . In other words only conduits  126  and  146  and control wiring for compressor  110  and motor  120  are required to extend between the indoor and outdoor parts of the system as diagrammatically separated by dashed line  160 . This improved arrangement provides for retrofitting of certain air conditioning systems, since the outdoor portion of an existing system may be unaffected by replacing the original indoor portion of the existing system with the indoor portion of system  100 . 
     At this point, it is desirable to define terms to be used later in the description. Relative humidity setpoint RH sp  and temperature setpoint T sp  have been previously described as the commanded relative humidity and temperature for the space  132 . The humidistat  130  senses and reports to the controller  122  the current relative humidity RH id  of the space  132 . In like manner, the thermostat  134  senses and reports to the controller  122  the current indoor temperature T id  of the space  132 . The absolute humidity ratio for the setpoint ω sp  conditions is defined as the pounds of water (H 2 O) per pound of air for the relative humidity setpoint RH sp  and temperature setpoint T sp  conditions. The absolute indoor humidity ratio ω id  is defined as the pounds of water per pound of air. To calculate the absolute humidity ratio for the setpoint ω sp  conditions, empirical equation 1 is used: 
                     ω   sp     =       [         (       RH   sp     0.4     )     *     (     7.875   +     0.00010438   *     T   sp   3         )       +     (     0.0005   *       (       T   sp     -   50     )     2.2       )       ]     *     [     1   7000     ]               Eq   .           ⁢   1               
To calculate the absolute indoor humidity ratio ω id , empirical equation 2 is used:
 
                     ω   id     =       [         (       RH   id     0.4     )     *     (     7.875   +     0.00010438   *     T   id   3         )       +     (     0.0005   *       (     Tid   -   50     )     2.2       )       ]     *     [     1   7000     ]               Eq   .           ⁢   2               
To calculate the process latent load in Btu/lb of air:
 
 L= 1050*(ω id −ω sp )  Eq. 3
 
To calculate the process sensible load in Btu/lb of air:
 
 S= 0.24*( T   id   −T   sp )  Eq. 4
 
To calculate the Process Sensible to Total cooling ratio (S/T):
 
     
       
         
           
             
               
                 
                   
                     S 
                     T 
                   
                   = 
                   
                     ( 
                     
                       S 
                       
                         S 
                         + 
                         L 
                       
                     
                     ) 
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   5 
                 
               
             
           
         
       
     
     Referring now to  FIG. 2 , illustrated is a flow diagram for an enhanced dehumidification control algorithm  200  assembled in accordance with the principles of the present invention as implemented in the controller  122  of  FIG. 1 . Commencing at Step  205 , the microprocessor  125  within the controller  122  determines if the humidistat  130  is calling for dehumidification. If the answer is NO, then the algorithm proceeds to Step  210  wherein the microprocessor  125  determines if the thermostat  134  is calling for cooling. If the answer is NO, then the algorithm proceeds to Step  215  confirming that both cooling and dehumidification are OFF. The microprocessor  125  then continues to loop  220  and returns to Step  205  wherein the algorithm  200  continues. 
     Returning to Step  210 , if the microprocessor  125  determines that the thermostat  134  is calling for cooling, i.e., the answer is YES, the algorithm  200  proceeds to Step  218 , and normal cooling is commanded by the controller  122 . The microprocessor  125  then continues to loop  220  and returns to Step  205  wherein the algorithm  200  continues. This portion of the algorithm  200  as described constitutes a normal cooling cycle as one who is of skill in the art would expect, except that the algorithm  200  is arranged for dehumidification priority as compared to conventional cooling priority. 
     Continuing from Step  205 , if the microprocessor  125  determines that the humidistat  130  is calling for dehumidification, i.e., the answer is YES, the algorithm  200  proceeds to Step  230 . At Step  230 , the microprocessor  125  determines if the thermostat  134  is calling for cooling. If the answer is NO, the algorithm  200  proceeds to Step  235 , and dehumidification at the minimum S/T is commanded by the controller  122 . If the answer is YES, the algorithm  200  proceeds to Step  240 . 
     At Step  240 , the microprocessor  125  calculates ω sp  from the setpoint temperature T sp  and setpoint relative humidity RH sp  in accordance with Equation 1. The algorithm  200  proceeds to Step  245  where ω id  is calculated from current indoor temperature T id  and current indoor relative humidity RH id  in accordance with Equation 2. The algorithm  200  then proceeds to Step  250  where latent load L is calculated from ω id  and ω sp  in accordance with Equation 3. At Step  255  sensible load S is calculated in accordance with Equation 4 from T id  and T sp . At Step  260 , the Process S/T Ratio is calculated from the Sensible load S and the Total load T=L+S in accordance with Equation 5. 
     Within the microprocessor  125 , there are resident dehumidification modes  1  through n corresponding to n configurations of the various variable elements of the system  100 . In one embodiment, the variable elements may include, but are not limited to, outdoor fan speed, air-reheat condenser  142  active or inactive, indoor fan speed, etc. Associated with each of the dehumidification modes  1  through n is a pre-calculated S/T ratio. The microprocessor uses these pre-calculated S/T ratios, i.e., S/T 1 , S/T 2 , . . . S/T n-1 , S/T n , for comparison with the Process S/T Ratio. At Step  265 , commencing with the first dehumidification mode n=1, i.e., S/T 1 , the Process S/T ratio is compared to the pre-calculated S/T ratios until a condition is found wherein S/T process &lt;S/T m . For example, if S/T 1 &lt;S/T process &lt;S/T 2 , then the microprocessor  125  selects Mode  2  and adjusts settings of the various variable elements to correspond to the corresponding stored configuration for Mode  2 . 
     Referring now to  FIG. 3 , illustrated is a performance chart of three modes of operation of the air conditioning system  100  of  FIG. 1  with a 75° F. indoor dry bulb temperature and a 66% indoor relative humidity. A normal cooling without reheat performance of the system  100  is shown in a first graphical plot  310 . A second graphical plot  320  shows a system configuration of dehumidification (air-reheat condenser  142  active) with the outdoor fan operating at 100 percent and the indoor fan operating at 65 percent. A third graphical plot  330  shows a configuration of dehumidification (air-reheat condenser  142  active) with the outdoor fan operating at 30 percent and the indoor fan operating at 65 percent. With an abscissa scale of outdoor ambient temperature and an ordinate of S/T ratio, the performance of the system  100  will follow the appropriate graph for the selected configuration. As shown in the first graphical plot  310 , the S/T ratio for the normal cooling without reheat stays relatively flat at about 0.61 to about 0.63 over the temperature range from about 75° F. to about 104° F. The S/T ratio for the second configuration is shown to vary substantially linearly from about 0.4 at 75° F. to about 0.25 at about 95° F. The third configuration S/T ratio is shown to vary substantially linearly from about 0.2 at 75° F. to about 0.08 at about 85° F. 
     An example will be helpful in understanding the algorithm  200 . With current indoor temperature at 77° F. and indoor relative humidity at 49%, it is desired to find a process line and system configuration to avoid overcooling and to more directly condition the indoor air to 75° F. and 50% relative humidity. Entering the algorithm  200  of  FIG. 2  at Step  205 , we conclude that both dehumidification and cooling are required, advancing to Step  240  calculate ω sp . From Equation 1, with T sp  at 75° F. and RH sp  at 50% (0.50), ω sp  evaluates to 0.0094. At Step  245  with T id  at 77° F. and RH id  at 49% (0.49), ω id  evaluates to 0.0098 from Equation 2. At Step  250  with ω sp  and ω id  as just calculated, latent load L evaluates to 0.487 BTU/lb of air from Equation 3. At Step  255  with T id  and T SP  as above, sensible load S evaluates to 0.48 BTU/lb of air from Equation 4. At Step  260 , the S/T process  ratio evaluates to 0.497 from Equation 5. At Step  265 , the S/T process  ratio of 0.497 is compared to the known S/T ratios for the three configurations of the system  100 . Within Step  265 , the S/T process  ratio of 0.497 is compared to the S/T Mode 1  ratio of about 0.18. The S/T process  ratio is greater than the S/T Mode 1  ratio, however, it is not yet known if the S/T process  ratio is greater than the S/T Mode 2  ratio. Upon comparing, the S/T process  ratio of 0.497 is found to be greater than the S/T Mode 2  ratio of about 0.38. Again however, it is not yet known if the S/T process  ratio is greater than the S/T Mode 3  ratio. Upon comparing, the S/T process  ratio of 0.497 is found to be less than the S/T Mode 3  ratio of about 0.60. Having satisfied the condition that S/T Mode n &lt;S/T process &lt;S/T Mode n+1 , the algorithm  200  selects dehumidification mode  2  as requiring the least energy to achieve the desired temperature and relative humidity. Dehumidification mode  2  corresponds to a configuration of air-reheat condenser  142  active, outdoor fan speed at 100%, and indoor fan speed at 65%. 
     Referring now to  FIG. 4 , illustrated is a psychrometric chart  400  covering normal indoor temperature and humidity ranges. Normal indoor temperature ranges from about 68° F. to about 82° F. and is indicated along the abscissa. The ordinate covers an absolute humidity ratio ranging from about 0.0 to about 0.020. Current temperature and absolute humidity ratio is represented as a first point  410  at 77° F. and about 0.011 humidity ratio. The desired temperature, 75° F. and about 0.0093 absolute humidity ratio is represented as a second point  420 . The path from the first point  410  to the second point  420  represents the desired process path. Operating the system  100  in dehumidification mode  2  with air-reheat condenser  142  active, outdoor fan speed at 100%, and indoor fan speed at 65% will approximate the desired process path. 
     Thus, an air conditioning system has been described that employs computation of a Process Sensible to Total Cooling ratio and the selection of the air conditioning system configuration having a reasonable close approximation to the Process Sensible to Total Cooling ratio. Of course, other parameters of the air conditioning system may also be included, thereby possibly more closely approaching the desired Process Sensible to Total Cooling ratio. 
     The control algorithm has a loop  220  arrangement. The status of the Process Sensible to Total Cooling ratio is repeatedly evaluated over time and adjustments in air conditioner operation are made in response to changes in the sensible and latent loads. 
     Although the present invention has been described in detail, those skilled in the pertinent art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.