Method and apparatus for the defrosting of an evaporator in connection with an air handling unit

1. A method of defrosting of an evaporator ( 1 ) in a heat pump ( 2 ), which the heat pump ( 2 ) comprises a refrigerant system ( 3 ) with a refrigerant ( 4 ) and in a flow direction of the refrigerant are arranged at least one compressor ( 5 ), at least one condenser ( 6 ), at least one first expansion valve ( 7 ) and at least one evaporator ( 1 ), and further the heat pump ( 2 ) comprises at least one four-way valve ( 8 ) which is arranged in the flow direction of the refrigerant after the compressor ( 5 ) and before the condenser ( 6 ), and further the four-way valve ( 8 ) is arranged to shift the flow direction of the refrigerant ( 4 ) in the refrigerant system ( 3 ) for reversible operation such that, in the reversible operation, the refrigerant ( 4 ) flows from the four-way valve to the evaporator ( 1 ) instead of the condenser ( 6 ) without passing through the expansion valve ( 7 ), and further the heat pump ( 2 ) is arranged in connection to an air handling unit ( 9 ), which air handling unit ( 9 ) comprises a controllable heat recovery device ( 10 ) arranged to recover energy from a first air stream ( 11 ) and transfer the recovered energy to a second air stream ( 12 ), and the evaporator ( 1 ) is arranged in the first air stream ( 11 ), in a flow direction of the first air stream after the heat recovery device ( 10 ), and the condenser ( 6 ) is arranged in the second air stream ( 12 ), in a flow direction of the second air stream after the heat recovery device ( 10 ), the method comprising the steps of:

when frost formation occurs in the evaporator ( 1 ), increasing the temperature of the refrigerant ( 4 ) through the evaporator ( 1 ) simultaneously as the heat recovery device is regulated down and the heat recovery is decreased, and

accomplishing a simultaneously heating of the evaporator ( 1 ), both from inside by the increased temperature of the refrigerant ( 4 ), and from the outside by the passing first air stream ( 11 ), wherein the first air stream temperature increases when the heat recovery is decreased, and thereby the evaporator ( 1 ) is defrosted.

2. The method according to claim 1 , wherein the temperature raise of the refrigerant ( 4 ) at defrosting is accomplished by that the heat pump ( 2 ) runs reversible by that the four-way valve ( 8 ) shifts the flow direction of the refrigerant ( 4 ) in the refrigerant system ( 3 ) so that the heated refrigerant ( 4 ) is sent to the evaporator ( 1 ) instead of to the condenser ( 6 ).

3. The method according to claim 1 , wherein the temperature raise of the refrigerant ( 4 ) at defrosting is accomplished by that the heat pump ( 2 ) comprises a hot gas valve ( 13 ) which is arranged in the flow direction of the refrigerant after compressor ( 5 ), and where the hot gas valve ( 13 ) partially or fully opens during operation and thereby directs a portion of heated refrigerant ( 4 ) directly to the evaporator ( 1 ) without the refrigerant ( 4 ) passing the first expansion valve ( 7 ), simultaneously as a remaining portion of the refrigerant ( 4 ) is sent to the condenser ( 6 ).

4. The method according to claim 3 , further comprising the step of simultaneously increasing a speed of the compressor ( 5 ) as the hot gas valve ( 13 ) partially or fully opens.

5. The method according to claim 1 , wherein the temperature raise of the refrigerant ( 4 ) at defrosting is accomplished by that the cooling power of the evaporator ( 1 ) is decreased by reducing the compressor ( 5 ) speed.

6. The method according to claim 1 , further comprising the step of measuring the temperature (T1), between the heat recovery device ( 10 ) and evaporator ( 1 ), and when frost formation occurs in the evaporator ( 1 ), defrosting the evaporator based on the temperature (T1).

7. The method according to claim 1 , wherein the heat pump ( 2 ) further comprises a reservoir ( 14 ) and a second expansion valve ( 15 ), which are arranged in the flow direction of the refrigerant between the first expansion valve ( 7 ) and the condenser ( 6 ), and the first expansion valve ( 7 ) controls a refrigerant ( 4 ) flow over the evaporator ( 1 ) and the second expansion valve ( 15 ) controls the refrigerant ( 4 ) level in the reservoir ( 14 ) in interaction with the first expansion valve ( 7 ), to obtain a controlled amount of refrigerant ( 4 ) and balanced counter pressure in the condenser ( 6 ) at a partial load of the compressor ( 5 ).

8. The method according to claim 7 , further comprising the step of achieving the defrosting by raising a super heat above a normal operation set point by reducing a speed of the compressor ( 5 ) and simultaneously throttling the first expansion valve ( 7 ), and decreasing the cooling power of the evaporator ( 1 ).

9. A device for defrosting of an evaporator ( 1 ) in a heat pump ( 2 ), which heat pump ( 2 ) comprises a refrigerant system ( 3 ) with a refrigerant ( 4 ) and in a flow direction of the refrigerant are arranged at least one compressor ( 5 ), at least one condenser ( 6 ), at least one first expansion valve ( 7 ) and at least one evaporator ( 1 ), and further the heat pump ( 2 ) comprises at least one four-way valve ( 8 ) which is arranged in the flow direction of the refrigerant after the compressor ( 5 ) and before the condenser ( 6 ), and further the four-way valve ( 8 ) is arranged to shift the flow direction of the refrigerant ( 4 ) in the refrigerant system ( 3 ) for reversible operation, whereby, in reversible operation, the refrigerant ( 4 ) flows from the four-way valve to the evaporator ( 1 ) instead of the condenser ( 6 ) without passing the expansion valve ( 7 ), and further the heat pump ( 2 ) is arranged in connection to an air handling unit ( 9 ), which air handling unit ( 9 ) comprises a controllable heat recovery device ( 10 ), arranged to recover energy from a first air stream ( 11 ) and transfer the recovered energy to a second air stream ( 12 ), and the evaporator ( 1 ) is arranged in the first air stream ( 11 ), in a flow direction of the first air stream after the heat recovery device ( 10 ), and the condenser ( 6 ) is arranged in the second air stream ( 12 ), in a flow direction of the second air stream after the heat recovery device ( 10 ), and the heat pump ( 2 ) comprises a hot gas valve ( 13 ), which is arranged after the compressor ( 5 ), in the flow direction of the refrigerant ( 4 ), and the hot gas valve ( 13 ) is arranged to partially or fully open during operation and thereby direct a portion of heated refrigerant ( 4 ) directly to the evaporator ( 1 ) without the refrigerant ( 4 ) passing the first expansion valve ( 7 ), simultaneously as a remaining portion of the refrigerant ( 4 ) is sent to the condenser ( 6 ),

wherein the heat pump ( 2 ) comprises a reservoir ( 14 ) and a second expansion valve ( 15 ), which are arranged between the first expansion valve ( 7 ) and the condenser ( 6 ), and the first expansion valve ( 7 ) is arranged to control a refrigerant ( 4 ) flow over the evaporator ( 1 ) and the second expansion valve ( 15 ) is arranged to control refrigerant ( 4 ) level in the reservoir ( 14 ) in interaction with the first expansion valve ( 7 ), to obtain a controlled amount of the refrigerant ( 4 ) and balanced counter pressure in the condenser ( 6 ) at a partial load of the compressor ( 5 ).