Method for dehumidifying a refrigeration system

Abstract

A method for dehumidifying a refrigeration system especially to dehumidify a refrigerated transporting container is disclosed. The refrigeration system includes a refrigeration circuit including an evaporator, a compressor, an expansion valve and a condenser. The refrigeration system also includes a control unit and a cooling space, the evaporator is placed in the cooling space and air blows over the evaporator to be cooled down. The dehumidification method is stepwise, and the method includes a dehumidification mode and a re-establish mode. During the dehumidifying process the system shifts between the dehumidification mode and a re-establish mode stepwise dehumidifying the air in the container in such a way that the measured parameters especially the compartment temperature stays within acceptable limits.

Claims

1. A dehumidification method for a refrigeration system, the refrigeration system comprises a refrigeration circuit, a control unit, a cooling compartment, a target air temperature, a target air moisture percentage; wherein the refrigeration circuit comprises: a compressor, an expansion valve, a condenser, an evaporator; wherein the cooling compartment comprises a cooling space for cooling air, which is separated from the rest of the cooling compartment and which defines an air flow channel for providing cooled air into the cooling compartment, the cooling space comprising: means to blow air through the cooling space, the evaporator, a temperature sensor placed adjacent to the surface of the evaporator, a moisture sensor arranged upstream of the evaporator, heating elements arranged downstream of the evaporator; wherein the control unit determines a first shift condition to shift from a dehumidification mode to a re-establish mode, and a second shift condition to shift from the re-establish mode to the dehumidification mode; wherein the first shift condition to shift from dehumidification mode to re-establish mode is when an air temperature in the cooling compartment is more than a first preselected non-zero number of degrees Celsius different from the target air temperature; and the dehumidification method comprising the steps of: the control unit entering the refrigeration system into the dehumidification mode, the control unit shifting the refrigeration system to the re-establish mode when the first shift condition is reached, the control unit shifting the refrigeration system to the dehumidification mode when the second shift condition is reached, repeating the steps of shifting to the re-establish mode and shifting to the dehumidification mode until the target air moisture percentage is reached; wherein the dehumidification mode comprises the steps of: blowing air over the evaporator, measuring with the moisture sensor an air moisture percentage and air temperature of air blown through the cooling space before the air reaches the evaporator, determining by the control unit a dew point temperature based on the measured air moisture percentage and air temperature of the air blown through the cooling space, determining by the control unit a target evaporator surface temperature that is lower than the dew point temperature, regulating by the control unit the surface temperature of the evaporator by controlling the refrigeration circuit so the surface temperature of the evaporator corresponds to the determined target evaporator surface temperature, and warming up the air with the heating elements after the air passes over the evaporator; and wherein, during repeating the steps of shifting to the re-establish mode and shifting to the dehumidification mode until the target air moisture percentage is reached, each target evaporator surface temperature determined by the control unit is lower than the preceding determined target evaporator surface temperature wherein the second shift condition to shift from re-establish mode to dehumidification mode is that an air temperature in the cooling compartment is less than a second preselected number of degrees Celsius different from the target air temperature, the second preselected number degrees being less than the first preselected numbers of degrees; and wherein during the re-establish mode the refrigeration system is operated so as to cause the air temperature in the cooling compartment to increase up toward, or decrease down toward the target air temperature.

2. The dehumidification method according to claim 1, wherein the dehumidification mode further comprises the steps of reducing the amount of refrigerant in the evaporator so the evaporation takes place in the first part of the evaporator.

3. The dehumidification method according to claim 1, wherein the chosen target evaporator surface temperature is less than 10° C. lower than the dew point temperature.

4. The dehumidification method according to claim 1, wherein dehumidification starts when the air moisture percentage is higher than a predefined value.

5. The dehumidification method according to claim 1, wherein dehumidification starts when it is manually activated.

6. The dehumidification method according to claim 1, wherein defrosting is performed to remove ice from the evaporator, the method comprising the steps of: turning on the heating elements, and turning off the means to blow air over the evaporator, resuming previous operation when the ice is removed from the evaporator and the evaporator temperature T.sub.evap is above 20° C.

7. The dehumidification method according to claim 1, wherein the target air moisture percentage is reached via the control of superheat in the refrigeration system during the dehumidification mode.

8. A dehumidification method for a refrigeration system of a transport container, the refrigeration system comprising a refrigeration circuit, a control unit, a cooling compartment, a target air temperature, and a target air moisture percentage; wherein the refrigeration circuit comprises a compressor, an expansion valve, a condenser, and an evaporator; wherein the cooling compartment comprises a cooling space for cooling air, which is separated from the rest of the cooling compartment and which defines an air flow channel for providing cooled air into the cooling compartment, the cooling space comprising means to blow air through the cooling space, the evaporator, a temperature sensor placed adjacent to the surface of the evaporator, a moisture sensor arranged upstream of the evaporator, and heating elements arranged downstream of the evaporator; wherein the control unit determines a first shift condition to shift from a dehumidification mode to a re-establish mode, and a second shift condition to shift from the re-establish mode to the dehumidification mode; wherein the first shift condition to shift from dehumidification mode to re-establish mode is when an air temperature in the cooling compartment is more than a first preselected non-zero number of degrees Celsius different from the target air temperature; and the dehumidification method comprising the steps of: the control unit entering the refrigeration system into the dehumidification mode, the control unit shifting the refrigeration system to the re-establish mode when the first shift condition is reached, the control unit shifting the refrigeration system to the dehumidification mode when the second shift condition is reached, repeating the steps of shifting to the re-establish mode and shifting to the dehumidification mode until the target air moisture percentage is reached; wherein the dehumidification mode comprises the steps of: blowing air over the evaporator, measuring with the moisture sensor an air moisture percentage and air temperature of air blown through the cooling space before the air reaches the evaporator, determining by the control unit a dew point temperature based on the measured air moisture percentage and air temperature of the air blown through the cooling space, determining by the control unit a target evaporator surface temperature that is lower than the dew point temperature, regulating by the control unit the surface temperature of the evaporator by controlling the refrigeration circuit so the surface temperature of the evaporator corresponds to the determined target evaporator surface temperature, and warming up the air with the heating elements after the air passes over the evaporator; and wherein, during repeating the steps of shifting to the re-establish mode and shifting to the dehumidification mode until the target air moisture percentage is reached, each target evaporator surface temperature determined by the control unit is lower than the preceding determined target evaporator surface temperature wherein the second shift condition to shift from re-establish mode to dehumidification mode is that an air temperature in the cooling compartment is less than a second preselected number of degrees Celsius different from the target air temperature, the second preselected number degrees being less than the first preselected numbers of degrees; and wherein during the re-establish mode the refrigeration system is operated so as to cause the air temperature in the cooling compartment to increase up toward, or decrease down toward the target air temperature.

9. The dehumidification method according to claim 8, wherein the target air moisture percentage is reached via the control of superheat in the refrigeration system during the dehumidification mode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an embodiment of this invention, it is a transport container with the dehumidification system.

(2) FIG. 2 shows an I,x-diagram displaying an example of how the dehumidifying method runs.

DETAILED DESCRIPTION OF THE DRAWINGS

(3) FIG. 1 shows a transport container 1 with is the preferred embodiment of this invention. The container 1 comprises a cooling compartment 2 and a cooling space 3. The cooling space 3 is separated from the rest of the cooling compartment by a plate 4. In the cooling space 3 is placed an evaporator 5. The rest of the refrigeration circuit is placed outside the container, in FIG. 1 is shown the compressor 6, the expansion valve 7 and the condenser 13. The cooling space 3 comprises means to blow air through the cooling space 3, the evaporator 5, a temperature sensor 14 placed close to the surface of the evaporator 5, a moisture sensor 8 arranged upstream of the evaporator 5 and heating elements 9 arranged downstream of the evaporator 5. In the cooling space 3 beside the evaporator 5 is placed the moisture sensor 8 and heating elements 9. In the ends of the cooling space 3 is an air inlet 10 and in the other end is an air outlet 11. And outside the container is a control unit 12.

(4) In this case the cooling space 3 is an air channel build into the cooling compartment 2. The cooling space 3 can be a part of the container 1 or it can in an alternative embodiment be a separate unit mounted into the cooling compartment 2.

(5) At the air inlet 10 there are means to blow air into the cooling space 3; this could for instance be a fan. The moisture sensor 8 is placed upstream of the evaporator 5, so the air passes the moisture sensor 8 before it reaches the evaporator 5. The moisture sensor 8 measures the moisture percentage and the air temperature. The heating elements 9 are placed downstream from the evaporator 5, so the air reaches the heating elements 9 just after the air passed the evaporator 5.

(6) FIG. 2 is an I,x-diagram for moist air at 1013 mBar. The diagram can also be called a h,x-diagram or a Mollier chart. On the left side is air temperature T.sub.air, the horizontal lines follows the air temperature. On the right side the relative moisture RH percentage is following the curved lines. The temperatures following the slanting lines are not relevant for this invention. The x-axis of the diagram shows the moisture content in the unit [kg water/kg air]. The y-axis shows the Enthalpy, the Enthalpy is represented by the air temperature T.sub.air.

(7) The dehumidification process is initiated at the point A and the dehumidification mode is started. The air temperature in the container is 30° Celsius and the humidity is 90%. Going from point A vertically down to the 100% moisture line, the dew point temperature T.sub.dew is found to be 28° C. Now the target surface temperature T0 of the evaporator surface is by the control unit 12 chosen to be T0_1, which is 20° C. It is important that the T0 is lower than the found dew point temperature T.sub.dew, so a moisture percentage of 100% is reached for the air passing the surface of the evaporator. The air passing close to the evaporator then cannot hold all the moisture in the air, and therefore moisture condenses on the surface of the evaporator.

(8) After a while the air temperature has dropped to 25° C. Then a first shift condition is reached, and the system shifts to re-establish mode. The refrigeration system is now operated in such a way that the air temperature increases to the target temperature of 30° C. reaching point B in FIG. 2. Now the moisture percentage in the container has dropped to 72%. Reaching point B triggers a second shift condition and the system shifts back to dehumidification mode. A new target temperature for the evaporator surface is chosen. In this embodiment the algorithm used by the control unit 12 chooses the new target temperature T0_2 simply to be 5° C. less than the previous target temperature.

(9) So the surface temperature of the evaporator is now lowered to T0_2, which is 15° C. The air temperature is now slowly dropping and when it has dropped 5° C., again the system shifts to re-establish mode and the temperature is increase to the target temperature of 30° C. reaching point C, where the moisture percentages is now dropped to 60%.

(10) The procedure continues through two more steps eventually reaching point E, where the moisture percentages is dropped to lower than 50%, the target percentage is reached and the dehumidification process stops.

(11) While the present invention has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this invention may be made without departing from the spirit and scope of the present invention.