Patent Publication Number: US-9890980-B2

Title: System and method of freeze protection of a heat exchanger in an HVAC system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is related to, and claims the priority benefit of, U.S. Provisional Patent Application Ser. No. 61/882,918 filed Sep. 26, 2013, the contents of which are hereby incorporated in their entirety into the present disclosure. 
    
    
     TECHNICAL FIELD OF THE DISCLOSED EMBODIMENTS 
     The presently disclosed embodiments generally relate to heating, ventilation, and air-conditioning (HVAC) systems, and more particularly, to a system and method of freeze protection of a heat exchanger in an HVAC system. 
     BACKGROUND OF THE DISCLOSED EMBODIMENTS 
     Generally, HVAC systems increase their overall efficiency by closely matching airflow to the refrigerant system capacity. Generally, lower airflows, such as approximately 325 cubic feet per minute (CFM) per ton, yield a higher seasonal energy efficiency ratio (SEER) due to lower electrical consumption of the fan. Open-loop airflow control systems, such as a permanent split capacitor to name one non-limiting example, lose airflow performance at high system static pressures. In order to avoid freezing of the heat exchanger, the open-loop control systems generally require higher airflows, such as approximately 400 CFM/ton, when filters clog, registers are obstructed, etc. There is, therefore, a need for a system and method to increase SEER efficiencies in an open-loop airflow control system while avoiding the possibility of a heat exchanger freezing. 
     SUMMARY OF THE DISCLOSED EMBODIMENTS 
     In one aspect, a HVAC system is provided. The HVAC system includes an indoor unit assembly, operably coupled to an outdoor unit assembly. The HVAC system further includes a controller operably coupled to the indoor unit assembly and the outdoor unit assembly. 
     In one embodiment, the indoor unit assembly includes a heat exchanger, including a suction port and a liquid port, generally associated with a refrigerant medium, but may be associated with any medium used to reduce a temperature of the heat exchanger. The indoor unit assembly further includes a fan configured to circulate air across the heat exchanger into an interior space. The indoor unit assembly further includes a temperature sensor operably coupled to the heat exchanger. In one embodiment, the temperature sensor is affixed to the suction port. In one embodiment, the indoor unit assembly may be an air handler. In another embodiment, the indoor unit assembly may be a furnace in combination with an evaporator coil. In one embodiment, the outdoor unit assembly may be an air conditioner. In another embodiment, the outdoor unit assembly may be a heat pump. 
     In one aspect, a method of heat exchanger freeze protection for an HVAC system is provided. In one embodiment, the method includes the step of operating the indoor unit assembly and the outdoor unit assembly in a cooling mode, and operating the fan at an initial airflow. 
     In one embodiment, the method includes the step of operating the temperature sensor to measure a temperature value of the heat exchanger, at a time period. In one embodiment, the time period is adjustable. In one embodiment, the time period is less than or equal to approximately five minutes. 
     In one embodiment, the method includes the step of determining whether the temperature value is less than or equal to a first temperature preset value and whether the temperature value is greater than or equal to a second temperature preset value. In one embodiment, the first temperature preset value and the second temperature preset value are adjustable. In one embodiment, the first temperature preset value is less than or equal to approximately 35 degrees Fahrenheit (F.). In one embodiment, the second temperature preset value is greater than or equal to approximately 37 degrees F. 
     In one embodiment, the method includes the step of determining whether a current airflow multiplier is equal to a maximum airflow multiplier limit and equal to a minimum airflow multiplier limit. In one embodiment, the maximum airflow multiplier limit and the minimum airflow multiplier limit are adjustable. In one embodiment, the maximum airflow multiplier limit is greater than or equal to approximately 1.50. In one embodiment, the minimum airflow multiplier limit is less than or equal to approximately 1.00. 
     In one embodiment, the method includes the step of increasing the current airflow multiplier by an airflow offset if the current airflow multiplier is not equal the maximum airflow multiplier limit and the temperature value is less than or equal to the first temperature preset value. In one embodiment, the method includes, decreasing the current airflow multiplier by the airflow offset if the current airflow multiplier is not equal to the minimum airflow multiplier limit and the temperature value is greater than or equal to the second temperature preset value. In one embodiment, the offset airflow multiplier is adjustable. In one embodiment, the airflow offset is approximately 0.05. 
     In one embodiment, the method includes the step of operating the fan at an increased airflow determined by the formula: Current Airflow=Initial airflow×(Current airflow multiplier+airflow offset). In one embodiment, the method includes the step of operating the fan at a decreased airflow determined by the formula: Current Airflow=Initial airflow×(Current airflow multiplier−airflow offset). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments and other features, advantages and disclosures contained herein, and the manner of attaining them, will become apparent and the present disclosure will be better understood by reference to the following description of various exemplary embodiments of the present disclosure taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a component diagram of an HVAC system according to the present disclosure; 
         FIG. 2  is a schematic flow diagram of a method for heat exchanger freeze protection for an HVAC system; and 
         FIG. 3  is a continuation of the schematic flow diagram of the method for heat exchanger freeze protection for an HVAC system. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of this disclosure is thereby intended. 
       FIG. 1  illustrates a HVAC system, generally referenced at  10 . The HVAC system  10  includes an indoor unit assembly  12 . The HVAC system  10  further includes an outdoor unit assembly  14  operably coupled to the indoor unit assembly  12  via a suction line  18  and a liquid line  16 . The HVAC system  10  further includes a controller  20  operably coupled to the indoor unit assembly  12  and the outdoor unit assembly  14  for the control thereof. It will be appreciated the controller  20  may be disposed within either the indoor unit assembly  12  or the outdoor unit assembly  14 . It will also be appreciated the controller  20  may be a thermostat. 
     In one embodiment, the indoor unit assembly  12  includes a heat exchanger  22  including a suction port  24  and a liquid port  26 . In one embodiment, the suction port  24  is coupled to the suction line  18  and the liquid port  26  is coupled to liquid line  16 . The indoor unit assembly  12  further includes a fan  28  configured to circulate air across the heat exchanger  22  into an interior space  30 . The indoor unit assembly  12  further includes a temperature sensor  32  operably coupled to the heat exchanger  22 . In one embodiment, the temperature sensor  32  is affixed to the suction port  24  of the heat exchanger  22 . For example, the temperature sensor  32  may be affixed to the suction port  24  of the heat exchanger  22  for ease of access by installation and service personnel. It will be appreciated that the temperature sensor  32  may be affixed to the liquid port  26  or on the heat exchanger  22 . In one embodiment, the indoor unit assembly  12  may be an air handler. In another embodiment, the indoor unit assembly  12  may be a furnace in combination with an evaporator coil. In one embodiment, the outdoor unit assembly  14  may be an air conditioner. In another embodiment, the outdoor unit assembly may be a heat pump. 
       FIG. 2  illustrates a schematic flow diagram of an exemplary method  100  of heat exchanger freeze protection for an HVAC system  10 . The method  100  includes the step  102  of operating the indoor unit assembly  12  and the outdoor unit assembly  14  in a cooling mode, and operating the fan  28  at an initial airflow. For example, the outdoor unit assembly  14  is configured to circulate a refrigerant from the outdoor unit assembly  14 , through the liquid line  16 . The refrigerant enters the heat exchanger  22  through the liquid port  26  and exits through the suction port  24  where it enters the suction line  16  and returns to the outdoor unit assembly  14 . As the low pressure, low temperature refrigerant flows through the heat exchanger  22 , the fan  28  operates at an initial airflow to distribute chilled air within the interior space  30 . It will be appreciated that any medium used to reduce a temperature of the heat exchanger  22  may be used, such as chilled water to name one non-limiting example. 
     In one embodiment, the method  100  includes step  104  of operating the temperature sensor  32  to measure a temperature value of the heat exchanger  22 , at the expiration of a first predetermined time period. In one embodiment, the first predetermined time period is adjustable. In one embodiment, the first predetermined time period is less than or equal to approximately five minutes. For example, at every five minute interval, the temperature sensor  32  measures the temperature on the suction port  24  as the low pressure, low temperature refrigerant enters the heat exchanger  22 ; then, the controller  20  reads the temperature value from the temperature sensor  32 . The temperature sensor  32  will continue to measure the temperature on the suction port  24  and the controller  20  will continue to read the temperature value at every five minute interval. 
     In one embodiment, the method  100  includes step  106  of determining whether the temperature value is greater than or equal to a first temperature preset value and less than or equal to a second temperature preset value. In one embodiment, the first temperature preset value and the second temperature preset are adjustable. In one embodiment, the first temperature preset value is less than or equal to approximately 35 degrees F. In one embodiment, the second temperature preset value is greater than or equal to approximately 37 degrees F. If the temperature value is greater than or equal to the first temperature preset value and less than or equal to the second temperature preset value, the method  100  proceeds to step  108  of operating the fan  28  at the current airflow rate. For example, during the beginning of a cooling mode operation, the fan  28  may operate at an initial airflow of approximately 1000 CFM. If the temperature value at the suction port  24  measures approximately 36 degrees F., the fan  28  will continue to operate at the current airflow rate of approximately 1000 CFM. 
     In one embodiment, to reduce the likelihood of the heat exchanger  22  freezing if the temperature value is less than the first temperature preset, the method  100  proceeds to step  110  of determining whether a current airflow multiplier is equal to a maximum airflow multiplier limit. The current airflow multiplier is a factor in which the initial airflow may be increased or decreased to circulate more or less air across the heat exchanger  22 . In one embodiment, the maximum airflow multiplier limit is adjustable. In one embodiment, the maximum airflow multiplier limit is greater than or equal to approximately 1.50. For example, during the beginning of a cooling mode operation, the current airflow multiplier may be equal to 1.00, which designates that the fan  28  operates at the initial airflow. It will be appreciated that the current airflow multiplier is reset to 1.00 at the beginning of each cooling cycle. If the temperature sensor  32  measures and the controller  20  reads a temperature value of 34 degrees F., the controller  20  determines whether the current airflow multiplier (e.g. 1.00) is equal to the maximum airflow multiplier limit (e.g. 1.50). If the current airflow multiplier is equal to the maximum airflow multiplier, the maximum amount of air that may be circulated across the heat exchanger  22  has been achieved, and the method  100  proceeds to step  112  of operating the controller  20  to start a freeze delay timer. Then, method  100  proceeds to step  114  of operating the controller to determine whether the freeze delay timer is equal to a second predetermined time. In one embodiment, the second predetermined time is adjustable. In one embodiment, the second predetermined time is approximately sixty minutes. For example, if the maximum airflow multiplier has been reached, a freezing condition has occurred. The controller  20  starts a freeze delay timer to allow for conditions of the heat exchanger  22  to improve to return to normal operation. 
     If the freeze delay timer is equal to the second predetermined time, the method  100  proceeds to step  116 , wherein the controller  20  commands the outdoor unit assembly  14  to stop operating in the cooling mode and commands the fan  28  to operate at the current airflow. Commanding the outdoor unit assembly  14  to stop operating in the cooling mode stops the refrigerant from flowing through the heat exchanger  22 . Continuing operation of the fan  28  allows warmer air to flow across the heat exchanger  22 ; thus, raising the temperature of the heat exchanger  22 . It will be appreciated that a signal may be shown on the controller  20  to alert a user that the outdoor unit assembly  14  has stopped operating in a cooling mode. 
     If the freeze delay timer is not equal to the second predetermined time, the method  100  proceeds to step  118  of determining whether the temperature value is greater than or equal to the second temperature preset. If the temperature value is less than the second temperature preset, the method  100  proceeds to step  116 , wherein the controller  20  commands the outdoor unit assembly  14  to stop operating in the cooling mode and commands the fan  28  to operate at the current airflow. If the temperature value is greater than or equal to the second temperature preset, the method  100  returns to step  102  of operating the indoor unit assembly  12  and the outdoor unit assembly  14  in a cooling mode, and operating the fan  28  at an initial airflow. For example, if the continuing operation of the fan  28  increases the temperature such that equals or surpasses 37 degrees F., the condition of the heat exchanger  22  is such that a cooling operation may resume. 
     If the current airflow multiplier is not equal to the maximum air flow multiplier, the method  100  proceeds to step  120  of increasing the current airflow multiplier by an airflow offset. The airflow offset is a factor in which the current airflow multiplier may be increased or decreased. In one embodiment, the airflow offset is adjustable. In one embodiment, the airflow offset is approximately 0.05. For example, after the temperature sensor  32  measured and the controller  20  read a temperature value of 34 degrees F., and the controller  20  determined the current airflow multiplier was not equal to the maximum airflow multiplier limit, the current airflow multiplier may be increased by the airflow offset (e.g. 1.00+0.05=1.05) to move more air across the heat exchanger  22 . 
     After the current airflow multiplier has been increased by the airflow offset, the method  100  proceeds to step  122  of operating the fan  28  at an increased airflow determined by the formula:
 
Current Airflow=Initial airflow×(Current airflow multiplier+airflow offset)
 
For example, after temperature sensor  32  measured and the controller  20  read a temperature value of 34 degrees F., and the controller  20  determined the current airflow was not equal to the maximum airflow multiplier limit, and the current airflow multiplier was increased by the offset airflow factor (e.g. 1.00+0.05=1.05), the controller  20  commands the fan  28  to operate at an increased airflow of 1000 CFM×1.05, or 1050 CFM to circulate more air across the heat exchanger  22 ; thus, increasing the temperature of the heat exchanger  22  to reduce the likelihood of freezing the heat exchanger  22 .
 
     After the current airflow is increased, the method returns to step  104  where the temperature sensor  32  measures the temperature value of the heat exchanger at the expiration of the predetermined time period. For example, after the fan  28  increases the current airflow to 1050 CFM, the temperature sensor  32  will again measure the temperature of the suction port  24  at the five minute interval. It will be appreciated that the aforementioned steps will be repeated until the temperature value is greater than or equal to the first temperature preset value or the current airflow multiplier equals the maximum airflow multiplier limit after the expiration of the predetermined time period. 
     In one embodiment, if the temperature value is greater than the second temperature preset, the method  100  proceeds to step  124  of determining whether the current airflow multiplier is equal to a minimum airflow multiplier limit. In one embodiment, the minimum airflow multiplier limit is adjustable. In one embodiment, the minimum airflow multiplier limit is less than or equal to approximately 1.00. Continuing from the prior example where the current airflow multiplier is equal to 1.05 causing more air to circulate over the heat exchanger  22 ; thus, increasing the temperature of the heat exchanger  22 , if the temperature sensor  32  measures and the controller  20  reads a temperature value of 38 degrees F., the controller  20  determines whether the current airflow multiplier (e.g. 1.05) is equal to the minimum airflow multiplier limit (e.g. 1.00). If the current airflow multiplier is equal to the minimum airflow multiplier, the minimum amount of air that may be circulated across the heat exchanger  22  to provide efficient operation of the HVAC system  10  has been achieved, and the method  100  proceeds to step  126  wherein the controller  20  commands the fan  28  to operate at the current airflow. The fan  28  will continue to operate at the current airflow until the temperature value may drop again below the second temperature preset. 
     If the current airflow multiplier is not equal to the minimum air flow multiplier, the method  100  proceeds to step  128  of decreasing the current airflow multiplier by the airflow offset. For example, after temperature sensor  32  measured and the controller  20  read a temperature value of 38 degrees F., and the controller  20  determined the current airflow multiplier was not equal to the minimum airflow multiplier limit, the current airflow multiplier may be decreased by the airflow offset (e.g. 1.05−0.05=1.00) to move less air across the heat exchanger  22 . 
     After the current airflow multiplier has been decreased by the offset airflow factor, the method  100  proceeds to step  130  of operating the fan  28  at a decreased airflow determined by the formula:
 
Current Airflow=Initial airflow×(Current airflow multiplier−airflow offset)
 
For example, after temperature sensor  32  measured and the controller  20  read a temperature value of 38 degrees F., and the controller  20  determined the current airflow multiplier was not equal to the minimum airflow multiplier limit, and the current airflow multiplier was decreased by the airflow offset (e.g. 1.05−0.05=1.00), the controller  20  commands the fan  28  to operate at a decreased airflow of 1000 CFM×1.00, or 1000 CFM to circulate less air across the heat exchanger  22 ; thus, decreasing the temperature of the heat exchanger  22  to provide more efficient operation of the HVAC system  10 .
 
     After the current airflow is decreased, or left unchanged, the method returns to step  104  where the temperature sensor  32  measures the temperature value of the heat exchanger  22  at the expiration of the predetermined time period. For example, after the fan  28  decreases the current airflow to 1000 CFM, the temperature sensor  32  will again measure the temperature of the suction port  24  at the five minute interval. It will be appreciated that the aforementioned steps will be repeated until the temperature value is less than or equal to the second temperature preset value or the current airflow multiplier equals the minimum airflow multiplier limit after the expiration of the predetermined time period. 
     It will therefore be appreciated that the controller  20  may command the fan  28  to circulate more or less air across the heat exchanger  22  based upon the temperature value of the suction port  24  of the heat exchanger  22  during a cooling mode. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only certain embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.