Humidity control in a refrigerated transport container with an intermittently operated compressor

Abstract

There is a growing need for dehumidification control in refrigerated transport containers with an intermittently operated compressor. The disclosed system and method of dehumidification control, controlling the speed of one or more evaporator fans and a heating unit, meets that need using one or more dehumidification need indicators and one or more condensation indicators. More specifically a system for and method is disclosed, comprising: controlling the operation of the one or more evaporator fans ( 10 ) for dehumidification purposes according to: if a need for dehumidification is determined using at least one dehumidification need indicator and if at least one condensation indicator indicates that the evaporator ( 16 ) is sufficiently cold respectively too warm for condensation or deposition of water vapor on its external surface, then setting or maintaining the speed of the one or more evaporator fans ( 10 ) to a reduced speed (LOW) in order to increase a condensation or deposition rate respectively to an increased speed (HIGH) in order to increase heat load to the evaporator ( 16 ), respectively. In other embodiments the system and method controls the heating unit as well.

1 . A method of dehumidification control in a refrigerated transport container ( 1 ), the refrigerated transport container ( 1 ) comprising at least one heating unit ( 20 ), a transport volume ( 45 ), a cooling unit ( 40 ), and a control unit, wherein the cooling unit ( 40 ) comprises at least a compressor ( 6 ) and an evaporator ( 16 ) comprising one or more evaporator fans ( 10 ), the heating unit ( 20 ) is situated downstream of the evaporator ( 16 ), where the compressor ( 6 ) is intermittently operated between a first active state and a second less active state, wherein the method comprises:
 controlling the operation of the one or more evaporator fans ( 10 ) for dehumidification purposes according to
 if a need for dehumidification is determined using at least one dehumidification need indicator and if at least one condensation indicator indicates that the evaporator ( 16 ) is sufficiently cold for condensation or deposition of water vapor on its external surface, then setting or maintaining the speed of the one or more evaporator fans ( 10 ) to a reduced speed (LOW) in order to increase a condensation or deposition rate, and/or 
 if a need for dehumidification is determined using at least one dehumidification need indicator and if at least one condensation indicator indicates that the evaporator ( 16 ) is too warm for condensation or deposition of water vapor on its external surface, then setting or maintaining the speed of the one or more evaporator fans ( 10 ) to an increased speed (HIGH) in order to increase heat load to the evaporator ( 16 ). 
   
     
     
         2 . The method according to  claim 1 , wherein the at least one dehumidification need indicator is one or more selected from the group of:
 a difference between current or recent relative humidity (RH) and relative humidity setpoint (RHset), or a function thereof,   a time elapsed since a most recent start of operating in dehumidification mode, and   that the evaporator fans ( 10 ) were operating at a reduced speed (LOW) the last time the compressor ( 6 ) was in the first active state.   
     
     
         3 . The method according to  claim 1 , wherein the at least one condensation indicator is one or more selected from the group of:
 that the compressor is in the first active state,   that a supply air temperature is colder than a return air temperature,   that the supply air temperature is colder than a setpoint temperature,   a measured temperature of at least a part of an external surface of the evaporator ( 16 ) is below dewpoint, and   a function of a boiling temperature of the refrigerant inside the evaporator is below dewpoint.   
     
     
         4 . The method according to  claim 1 , wherein the method comprises the step of
 controlling the operation of the one or more evaporator fans ( 10 ) for dehumidification purposes by setting or maintaining the speed of the one or more evaporator fans ( 10 ) to be a reduced speed (LOW) thereby increasing condensation or deposition if:
 the compressor ( 6 ) is in the second less active state, the evaporator fans ( 10 ) were operating at a reduced speed (LOW) the last time the compressor ( 6 ) was in the first active state, and a supply air temperature is colder than a return air temperature, and/or, 
 the compressor ( 6 ) is in the first active state and the difference between current relative humidity (RH) and relative humidity setpoint (RHset) is determined to be above a second predetermined dehumidification need threshold (RHlim2), and/or 
 the compressor ( 6 ) is in the first active state, a time elapsed since a most recent start of operating in dehumidification mode is less than a predetermined period of time. 
   
     
     
         5 . The method according to  claim 1 , wherein the method comprises the step of
 controlling the operation of the one or more evaporator fans ( 10 ) for dehumidification purposes by setting or maintaining the speed of the one or more evaporator fans ( 10 ) to be an increased speed (HIGH) thereby increasing heat load to the evaporator if:
 the compressor ( 6 ) is in the second less active state and if the difference between current relative humidity (RH) and relative humidity setpoint (RHset) is above a first predetermined dehumidification need threshold (RHlim1) and if at least one condensation indicator indicates that the evaporator ( 16 ) is too warm for condensation or deposition of water vapor on its external surface. 
   
     
     
         6 . The method according to  claim 1 , wherein the method comprises controlling the operation of the heating unit ( 20 ) for dehumidification purposes by activating the heating unit ( 20 ) thereby increasing heat load to the evaporator ( 16 ) when a need for dehumidification is determined using at least one dehumidification need indicator. 
     
     
         7 . The method according to  claim 6 , wherein the heating unit ( 20 ) is activated if:
 the compressor ( 6 ) is in the first active state and the difference between current relative humidity (RH) and relative humidity setpoint (RHset) is determined to be above the second predetermined dehumidification need threshold (RHlim2), and/or   the compressor ( 6 ) is in the first active state, and a time elapsed since a most recent start of operating in dehumidification mode is less than a predetermined period of time, and/or   the compressor ( 6 ) is in the second less active state, the heating unit was active at the end of a last period the compressor ( 6 ) was in the first active state and the difference between current relative humidity (RH) and relative humidity setpoint (RHset) is determined to be above the second predetermined dehumidification need threshold (RHlim2), and/or   the compressor ( 6 ) is in the second less active state, and the difference between current relative humidity (RH) and relative humidity setpoint (RHset) is determined to be above a third predetermined dehumidification need threshold (RHlim3).   
     
     
         8 . The method according to  claim 1 , wherein the first predetermined dehumidification need threshold (RHlim1) is lower than or equal to the second predetermined dehumidification need threshold (RHlim2). 
     
     
         9 . The method according to  claim 7 , wherein the second predetermined dehumidification need threshold (RHlim2) is lower than or equal to the third predetermined dehumidification need threshold (RHlim3). 
     
     
         10 . The method according to  claim 1 , wherein the state of heating unit, compressor and evaporator fans may only change if one or more minimum duration times for being in a given state for the heating unit ( 20 ) and/or the compressor ( 6 ) and/or the evaporator fans ( 10 ) have elapsed. 
     
     
         11 . The method according to  claim 1 , wherein the method is only performed if the temperature is within specification by a predetermined margin. 
     
     
         12 . A method according to  claim 1 , wherein the refrigerated transport container is not a transport container but another type of refrigerated space in connection with a refrigeration unit. 
     
     
         13 . A system for dehumidification in a refrigerated transport container ( 1 ), the refrigerated transport container ( 1 ) comprising at least one heating unit ( 20 ), a transport volume ( 45 ), a cooling unit ( 40 ), and a control unit, wherein the cooling unit ( 40 ) comprises at least a compressor ( 6 ) and an evaporator ( 16 ) comprising one or more evaporator fans ( 10 ), the heating unit ( 20 ) is situated downstream of the evaporator ( 16 ), where the compressor ( 6 ) is intermittently operated between a first active state and a second less active state, wherein the system comprises a processing unit adapted to:
 control the operation of the one or more evaporator fans ( 10 ) for dehumidification purposes according to
 if a need for dehumidification is determined using at least one dehumidification need indicator and if at least one condensation indicator indicates that the evaporator ( 16 ) is sufficiently cold for condensation or deposition of water vapor on its external surface, then setting or maintaining the speed of the one or more evaporator fans ( 10 ) to a reduced speed (LOW) in order to increase a condensation or deposition rate, and/or 
 if a need for dehumidification is determined using at least one dehumidification need indicator and if at least one condensation indicator indicates that the evaporator ( 16 ) is too warm for condensation or deposition of water vapor on its external surface, then setting or maintaining the speed of the one or more evaporator fans ( 10 ) to an increased speed (HIGH) in order to increase heat load to the evaporator ( 16 ). 
   
     
     
         14 . The system according to  claim 13 , wherein the at least one dehumidification need indicator is one or more selected from the group of:
 a difference between current or recent relative humidity (RH) and relative humidity setpoint (RHset), or a function thereof,   a time elapsed since a most recent start of operating in dehumidification mode, and   that the evaporator fans ( 10 ) were operating at a reduced speed (LOW) the last time the compressor ( 6 ) was in the first active state.   
     
     
         15 . The system according to  claim 13 , wherein the at least one condensation indicator is one or more selected from the group of:
 that the compressor is in the first active state,   that a supply air temperature is colder than a return air temperature,   that the supply air temperature is colder than a setpoint temperature,   a measured temperature of at least a part of an external surface of the evaporator ( 16 ) is below dewpoint, and   a function of a boiling temperature of the refrigerant inside the evaporator is below dewpoint.   
     
     
         16 . The system according to  claim 13 , wherein the processing unit is adapted to
 control the operation of the one or more evaporator fans ( 10 ) for dehumidification purposes by setting or maintaining the speed of the one or more evaporator fans ( 10 ) to be a reduced speed (LOW) thereby increasing condensation or deposition if:
 the compressor ( 6 ) is in the second less active state, the evaporator fans ( 10 ) were operating at a reduced speed (LOW) the last time the compressor ( 6 ) was in the first active state, and a supply air temperature is colder than a return air temperature, and/or, 
 the compressor ( 6 ) is in the first active state and the difference between current relative humidity (RH) and relative humidity setpoint (RHset) is determined to be above a second predetermined dehumidification need threshold (RHlim2), and/or 
 the compressor ( 6 ) is in the first active state, a time elapsed since a most recent start of operating in dehumidification mode is less than a predetermined period of time. 
   
     
     
         17 . The system according to  claim 13 , wherein the processing unit is adapted to
 control the operation of the one or more evaporator fans ( 10 ) for dehumidification purposes by setting or maintaining the speed of the one or more evaporator fans ( 10 ) to be an increased speed (HIGH) thereby increasing heat load to the evaporator if:
 the compressor ( 6 ) is in the second less active state and if the difference between current relative humidity (RH) and relative humidity setpoint (RHset) is above a first predetermined dehumidification need threshold (RHlim1) and if at least one condensation indicator indicates that the evaporator ( 16 ) is too warm for condensation or deposition of water vapor on its external surface. 
   
     
     
         18 . The system according to  claim 13 , wherein the processing unit is adapted to control the operation of the heating unit ( 20 ) for dehumidification purposes by activating the heating unit ( 20 ) thereby increasing heat load to the evaporator ( 16 ) when a need for dehumidification is determined using at least one dehumidification need indicator. 
     
     
         19 . The system according to  claim 18 , wherein the heating unit ( 20 ) is activated if:
 the compressor ( 6 ) is in the first active state and the difference between current relative humidity (RH) and relative humidity setpoint (RHset) is determined to be above the second predetermined dehumidification need threshold (RHlim2), and/or   the compressor ( 6 ) is in the first active state, and a time elapsed since a most recent start of operating at increased condensation or deposition rate is less than a predetermined period of time, and/or   the compressor ( 6 ) is in the second less active state, the heating unit was active at the end of a last period the compressor ( 6 ) was in the first active state and the difference between current relative humidity (RH) and relative humidity setpoint (RHset) is determined to be above the second predetermined dehumidification need threshold (RHlim2), and/or   the compressor ( 6 ) is in the second less active state, and the difference between current relative humidity (RH) and relative humidity setpoint (RHset) is determined to be above a third predetermined dehumidification need threshold (RHlim3).   
     
     
         20 . The system according to  claim 13 , wherein
 the first predetermined dehumidification need threshold (RHlim1) is lower than or equal to the second predetermined dehumidification need threshold (RHlim2).   
     
     
         21 . The system according to  claim 19 , wherein the second predetermined dehumidification need threshold (RHlim2) is lower than or equal to the third predetermined dehumidification need threshold (RHlim3). 
     
     
         22 . The system according to  claim 13 , wherein the state of heating unit, compressor and evaporator fans may only change if one or more minimum duration times for being in a given state for the heating unit ( 20 ) and/or the compressor ( 6 ) and/or the evaporator fans ( 10 ) have elapsed. 
     
     
         23 . The system according to  claim 13 , wherein the processing unit is adapted to only perform dehumidification if the temperature is within specification by a predetermined margin. 
     
     
         24 . A system according to  claim 13 , wherein the refrigerated transport container is not a transport container but another type of refrigerated space in connection with a refrigeration unit.