Control system for a dual evaporator refrigeration system

1. A method of operating a refrigerator appliance, the refrigerator appliance comprising a fresh food storage compartment evaporator, a freezer storage compartment evaporator, and a compressor, the steps of the method comprising:

receiving T ff-sp , a set point temperature for the fresh food compartment;

receiving T ft-sp , a set point temperature for the freezer storage compartment;

measuring T amb , an ambient temperature;

measuring T ff a temperature of the fresh food storage compartment;

measuring T fz , a temperature of the frozen food storage compartment;

measuring T ff-evap , a temperature of the fresh food storage compartment evaporator;

measuring T fz-evap , a temperature of the frozen food storage compartment evaporator;

calculating {tilde over (V)} ff-ss , an air flow rate for the fresh food storage compartment needed to maintain its temperature at T ff-SP during steady state conditions;

calculating {tilde over (V)} fz-ss , an air flow rate for the freezer storage compartment needed to maintain its temperature at T ft-SP during steady state conditions;

calculating {tilde over (V)} ff-sp , an air flow rate needed to reduce the fresh food storage compartment to T ff-sp over a predetermined time period t s - red - ff ;

calculating {tilde over (V)} fz-sp , an air flow rate needed to reduce the freezer food storage compartment to T fr-SP over a predetermined time period t s - red - fz ;

determining {tilde over (V)} ff-total , the total air flow rate required for the fresh food storage compartment, from {tilde over (V)} ff-ss and {tilde over (V)} ff-sp ;

determining {tilde over (V)} fz,-total , the total air flow rate required for the freezer storage compartment, from {tilde over (V)} fz-ss and {tilde over (V)} fz-sp ;

determine S ff - fan , a fan speed that provides {tilde over (V)} ff-total , including using a predetermined relationship between speed of the first evaporator fan and a resulting air flow of the first evaporator fan;

determine S fz-fan , a fan speed that provides {tilde over (V)} fz-total , including using a predetermined relationship between speed of the second evaporator fan and a resulting air flow of the second evaporator fan;

operating a first evaporator fan in the fresh food storage compartment at S ff-fan , and

operating a second evaporator fan in the freezer storage compartment at S fz-fan .

2. A method of operating a refrigerator appliance as in claim 1 , further comprising the steps of:

providing S ff-fan-min , a minimum fan speed for the first evaporator fan; and

determining if the compressor is operating and

maintaining the first evaporator fan at a fan speed of at least S ff-fan-min if the compressor is operating, or

deactivating the first evaporator fan of the compressor is not operating.

3. A method of operating a refrigerator appliance as in claim 1 , further comprising the steps of:

providing S fz-fan-min , a minimum fan speed for the second evaporator fan; and

determining if the compressor is operating and

maintaining the second evaporator fan at a fan speed of at least S fz-fan-min if the compressor is operating, or

deactivating the second evaporator fan of the compressor is not operating.

4. A method of operating a refrigerator appliance as in claim 1 , further comprising the step of:

providing S ff-fan-min , a minimum fan speed for the first evaporator fan;

providing S fz-fan-min , a minimum fan speed for the second evaporator fan;

ascertaining the speed of the first evaporator fan;

ascertaining the speed of the second evaporator fan; and

deactivating the compressor if the first evaporator fan speed is at S ff-fan-min and the second evaporator fan speed is at S fz-fan-min .

5. A method of operating a refrigerator appliance as in claim 4 , further comprising the step of

deactivating the compressor if T ff is more than a predetermined value below T ff -SP .

6. A method of operating a refrigerator appliance as in claim 1 , further comprising the step of pressurizing a zeotropic refrigerant with the compressor.

7. A method of operating a refrigerator appliance as in claim 1 , further comprising the steps of

calculating an excess enthalpy H ff of the fresh food compartment using T ff-sp and T ff ,

calculating an excess enthalpy H fz of the frozen food compartment using T fz-sp and T fz ;

determining an instantaneous cooling capacity required Cap to reduce the excess enthalpy H ff and the excess enthalpy H fz substantially to zero within a selected time period T s ;

calculating S c , a compressor speed that delivers at least the instantaneous cooling capacity required Cap while minimizing power consumption of the compressor; and

operating the compressor at compressor speed of at least S c , as determined from said step of calculating.

8. A refrigerator appliance, comprising:

a fresh food compartment;

a frozen food compartment;

a refrigeration loop comprising

a fresh food compartment evaporator for providing cooling to the fresh food compartment;

a frozen food compartment evaporator for providing cooling to the frozen food compartment;

a condenser;

a compressor; and

an expansion device;

a first evaporator fan configured to provide an air flow over the fresh food compartment evaporator;

a second evaporator fan configured to provide an air flow over the frozen food compartment evaporator;

one or more controllers configured to

receive T ff-sp , a set point temperature for the fresh food compartment;

receive T fz-sp , a set point temperature for the freezer storage compartment;

receive T amb , a measurement of ambient temperature;

receive T ff ; a measurement of a temperature of the fresh food storage compartment;

receive T fz , a measurement of a temperature of the frozen food storage compartment;

receive T ff-evap , a measure of a temperature of the fresh food storage compartment evaporator;

receive T fz-evap , a measure of a temperature of the frozen food storage compartment evaporator;

calculate {tilde over (V)} ff , an air flow rate for the fresh food storage compartment needed to maintain the fresh food storage compartment at the temperature T ff-SP during steady state conditions;

calculate {tilde over (V)} fz-ss , an air flow rate for the freezer storage compartment needed to maintain the freezer storage compartment at the temperature T fz-SP during steady state conditions;

calculate {tilde over (V)} ff-sp , an air flow rate needed to reduce the fresh food storage compartment temperature to T ff-sp over a predetermined desired time period t s-red-ff ;

calculate {tilde over (V)} fz-sp , an air flow rate needed to reduce the freezer food storage compartment temperature to T fz-sp over a predetermined desired time period t s-red-fz ;

determine {tilde over (V)} ff-total , the total air flow rate required for the fresh food storage compartment, the {tilde over (V)} ff-total including {tilde over (V)} ff-ss and {tilde over (V)} ff-sp ;

determine {tilde over (V)} ft-total , the total air flow rate required for the freezer storage compartment, the {tilde over (V)} fz-total including {tilde over (V)} fz-ss and {tilde over (V)} fz-sp ;

determine S ff - fan , a fan speed that provides {tilde over (V)} ff-total , including using a predetermined relationship between speed of the first evaporator fan and a resulting air flow of the first evaporator fan;

determine S fz-fan , a fan speed that provides {tilde over (V)} fz-total using a predetermined relationship between speed of the second evaporator fan and a resulting air flow of the second evaporator fan;

operate the first evaporator fan at S ff-fan ; and

operate the second evaporator fan at S fz-fan .

9. A refrigerator appliance as in claim 8 , wherein the one or more controllers are further configured to:

provide S ff-fan-min , a minimum fan speed for the first evaporator fan;

determine if the compressor is operating; and

maintain the first evaporator fan at a fan speed of at least S ff-fan-min if the compressor is operating, or

deactivate the first evaporator fan if the compressor is not operating.

10. A refrigerator appliance as in claim 8 , wherein the one or more controllers are further configured to:

provide S fz-fan-min , a minimum fan speed for the second evaporator fan;

determine if the compressor is operating; and

maintain the second evaporator fan at a fan speed of at least S fz-fan-min if the compressor is operating, or

deactivate the second evaporator fan if the compressor is not operating.

11. A refrigerator appliance as in claim 8 , wherein the one or more controllers are further configured to:

provide S ff-fan-min , a minimum fan speed for the first evaporator fan;

provide S fz-fan-min , a minimum fan speed for the second evaporator fan;

ascertain the speed of the first evaporator fan;

ascertain the speed of the second evaporator fan; and

deactivate the compressor if the first evaporator fan speed is at S ff-fan-m and the second evaporator fan speed is at S fz-fan-min .

12. A refrigerator appliance as in claim 11 , wherein the one or more controllers are further configured to:

deactivate the compressor if T ff is more than a predetermined value below T ff-SP .

13. A refrigerator appliance as in claim 11 , wherein the refrigeration loop further comprises a zeotropic refrigerant.

14. A refrigerator appliance as in claim 13 , wherein the refrigeration loop further comprises a phase separating component configured to substantially separate different phases of the zeotropic refrigerant.