Refrigeration system with efficient expansion device control, liquid refrigerant return, oil return, and evaporator defrost

1. A refrigeration system comprising:

an evaporator configured to receive a flow of refrigerant and transfer heat into the refrigerant within the evaporator to provide cooling for a temperature-controlled space;

an expansion valve operable to modulate the flow of refrigerant into the evaporator;

an oil level sensor configured to measure a level of oil accumulated within the component of the refrigeration system

a liquid level sensor configured to measure a level of liquid accumulated within a component of the refrigeration system, wherein the level of liquid is a total level of oil and liquid refrigerant accumulated within the component of the refrigeration system; and

a controller configured to operate the expansion valve to increase the flow of refrigerant into the evaporator or decrease the flow of refrigerant into the evaporator based on the level of liquid measured by the liquid level sensor;

wherein the controller is configured to:

determine a level of liquid refrigerant accumulated within the component of the refrigeration system by subtracting the level of oil from the total level of oil and liquid refrigerant; and

operate the expansion valve to increase the flow of refrigerant into the evaporator or decrease the flow of refrigerant into the evaporator based on the level of liquid refrigerant.

2. The refrigeration system of claim 1 , further comprising a liquid collection header coupled to the evaporator;

wherein the liquid collection header is the component of the refrigeration system within which the liquid is accumulated and the level of liquid is measured.

3. The refrigeration system of claim 1 , further comprising a liquid/vapor separator fluidly coupled to a refrigerant outlet of the evaporator and configured to receive a flow of the refrigerant exiting the evaporator;

wherein the liquid/vapor separator is the component of the refrigeration system within which the liquid is accumulated and the level of liquid is measured.

4. The refrigeration system of claim 1 , wherein the controller is configured to:

compare the level of liquid accumulated within the component of the refrigeration system to a threshold value;

modulate the expansion valve toward a fully open position to increase the flow of refrigerant into the evaporator in response to the level of liquid accumulated within the component of the refrigeration system being less than the threshold value; and

modulate the expansion valve toward a fully closed position to decrease the flow of refrigerant into the evaporator in response to the level of liquid accumulated within the component of the refrigeration system being greater than the threshold value.

5. The refrigeration system of claim 1 , further comprising one or more sensors configured to measure at least one of a temperature or a pressure of the refrigerant at a location within the refrigeration system;

wherein the controller is configured to:

determine an amount of superheat of the refrigerant at the location within the refrigeration system using one or more measurements from the one or more sensors; and

operate the expansion valve to increase the flow of refrigerant into the evaporator or decrease the flow of refrigerant into the evaporator based on both the level of liquid measured by the liquid level sensor and the amount of superheat of the refrigerant at the location within the refrigeration system.

6. The refrigeration system of claim 5 , wherein the controller is configured to:

compare the amount of superheat to a threshold value;

modulate the expansion valve toward a fully open position to increase the flow of refrigerant into the evaporator in response to the amount of superheat being greater than the threshold value; and

modulate the expansion valve toward a fully closed position to decrease the flow of refrigerant into the evaporator in response to the amount of superheat being less than the threshold value.