Method and apparatus for variable refrigerant chiller operation

1. A refrigeration system comprising:

a compressor, a condenser, an expansion device, an evaporator, and an additional refrigerant vessel connected in a closed refrigerant loop;

the additional refrigerant vessel connected to the condenser at a high pressure side by a first valve and to the evaporator at a low pressure side by a second valve;

a controller to control operation of the first valve and the second valve, wherein only one of the first valve and the second valve is open at a time to allow additional refrigerant to be added to the closed refrigerant loop in response to the controller receiving a low refrigerant level indication of a refrigerant level in the evaporator, or to remove refrigerant from the closed refrigerant loop in response to the controller receiving a high refrigerant level indication of a refrigerant level in the evaporator, a chamber in fluid communication with the evaporator; and a fluid level sensor disposed within the chamber to provide a direct measurement of the refrigerant level to the controller;

the controller configured to:

count the number of surges occurring within a predetermined interval;

increment a surge count when a surge is indicated;

compare the surge count with a surge count threshold; and

in response to the surge count exceeding the surge count threshold:

observe a liquid refrigerant level in the evaporator with respect to a target refrigerant level; and

in response to observing no change in the liquid refrigerant level over the predetermined interval, return to the step of monitoring a parameter associated with the compressor and monitor a motor current for surge indication.

2. The refrigeration system of claim 1 , wherein only one of the first valve and the second valve may be open between the additional refrigerant vessel and the condenser or the evaporator.

3. The refrigeration system of claim 1 , wherein the additional refrigerant vessel is connected to the expansion valve line and the expansion valve line is connected to the condenser by a first line and to the evaporator by a second line.

4. The refrigeration system of claim 1 , wherein the controller determines an amount of additional refrigerant to be used in the additional refrigerant vessel based on a total refrigerant charge comprising the sum of a condenser refrigerant charge, an evaporator refrigerant charge, and system piping refrigerant charge.

5. The refrigeration system of claim 4 , wherein the controller computes the evaporator refrigerant charge by the equation:

Charge evap =LF evap ×ρ liq.evap ×Volume evap

Where:

(Charge evap )=the refrigerant charge in the evaporator

(LF evap )=the loading factor of the evaporator

(ρ liq.evap )=by the fluid density of the evaporator

(Volume evap )=by the volume of the evaporator.

6. The refrigeration system of claim 4 , wherein the controller computes the condenser refrigerant charge by the equation:

Charge cond =ρ liq.cond ×Vol sub +ρ cond.equiv ×Vol cond

where:

(Charge cond )=the refrigerant charge in the condenser

(ρ liq.cond )=the fluid density of liquid in the condenser

(Vol sub )=the volume of the liquid in the subcooler

(ρ cond.equiv )=equivalent fluid density in the condenser and

(Vol cond )=the volume of fluid the condenser.

7. The refrigeration system of claim 4 , wherein the controller computes the system piping refrigerant charge by the equation:

Charge piping =F piping ×(Charge evap +Charge cond )

where:

(Charge piping )=the refrigerant charge in the system piping

(F piping )=factor (%) of refrigerant charge in the system piping

(Charge evap )=the charge in the evaporator and

(Charge cond )=the refrigerant charge in the condenser.

8. The refrigeration system of claim 4 , wherein the controller computes a total system refrigerant charge by the equation:

Charge total =Charge evap +Charge cond +Charge piping

where:

Charge total =the total system refrigerant

(Charge evap )=the refrigerant charge in the evaporator

(Charge cond )=the refrigerant charge in the condenser

(Charge piping )=the refrigerant charge in the system piping.

9. The refrigeration system of claim 7 , wherein the factor (%) of refrigerant charge in the system piping is approximately 13% of the sum of the evaporator refrigerant charge and the condenser refrigerant charge.

10. The refrigeration system of claim 7 , wherein the total system refrigerant charge is used to determine an amount of refrigerant to add to the refrigerant vessel.

11. A refrigeration system comprising:

a compressor, a condenser, an expansion device, an evaporator, and an additional refrigerant vessel connected in a closed refrigerant loop;

the additional refrigerant vessel connected at an inlet to an expansion valve inlet line from the condenser, and at an outlet to an expansion valve outlet line from the evaporator; and

a controller configured to control operation of a first valve and a second valve, wherein only one of the first valve and the second valve may open at any time to allow refrigerant from the additional refrigerant vessel to be added to the closed refrigerant loop in response to the controller receiving a low refrigerant level indication of a refrigerant level in the evaporator, or to remove refrigerant from the closed refrigerant loop in response to the controller receiving a high refrigerant level indication of a refrigerant level in the evaporator, a chamber in fluid communication with the evaporator; and a fluid level sensor disposed within the chamber to provide a direct measurement of the refrigerant level to the controller;

the controller configured to:

count the number of surges occurring within a predetermined interval;

increment a surge count when a surge is indicated;

compare the surge count with a surge count threshold; and

in response to the surge count exceeding the surge count threshold:

observe a liquid refrigerant level in the evaporator with respect to a target refrigerant level; and

in response to observing no change in the liquid refrigerant level over the predetermined interval, return to the step of monitoring a parameter associated with the compressor and monitor a motor current for surge indication.

12. The refrigeration system of claim 11 , wherein the controller maintains a plurality of selected parameters of the closed refrigerant loop within preselected ranges.

13. The refrigeration system of claim 12 , wherein the plurality of selected parameters comprises: an input valve position, an output valve position, a fluid level in the refrigerant vessel, a fluid level in condenser, or a fluid level in evaporator.

14. The refrigeration system of claim 13 , wherein the controller employs continuous feedback from a plurality of sensors monitoring the respective selected parameters to continuously monitor and change the amount of refrigerant in the refrigeration system in response to changes in system cooling loads.

15. The refrigeration system of claim 14 , wherein the refrigerant level in the closed refrigerant loop can be varied by opening or closing the first valve or the second valve, and a capacity control device is correspondingly updated or revised in response to changes in cooling capacity capabilities resulting from the modified refrigerant level.

16. A method for controlling cooling capacity of a chiller system having a compressor, a condenser and an evaporator connected in a closed refrigerant loop, the method comprising:

providing a refrigerant vessel for a chiller system;

connecting the refrigerant vessel and an outlet line from the refrigerant vessel to the evaporator;

connecting an inlet line to the refrigerant vessel to the condenser, the inlet line including a first valve to control flow of refrigerant in the inlet line and the outlet line including a second valve to control flow of refrigerant in the outlet line;

monitoring a parameter associated with the compressor for indication of a surge condition in the chiller system;

in response to receiving an indication of an impending surge condition, directly observing a refrigerant liquid level in the evaporator with respect to surge indication frequency, and

adjusting the capacity of the chiller system in response to a change in the refrigerant liquid level in the evaporator, a chamber in fluid communication with the evaporator; and a fluid level sensor disposed within the chamber to provide a direct measurement of the refrigerant level to the controller;

counting the number of surges occurring within a predetermined interval;

incrementing a surge count when a surge is indicated;

comparing the surge count with a surge count threshold; and

in response to the surge count exceeding the surge count threshold:

observing a liquid refrigerant level in the evaporator with respect to a target refrigerant level; and

in response to observing no change in the liquid refrigerant level over the predetermined interval, returning to the step of monitoring a parameter associated with the compressor and monitoring a motor current for surge indication.

17. The method of claim 16 , wherein the parameter is a motor current flowing in a compressor motor.

18. The method of claim 16 , wherein the step of adjusting the capacity comprises:

in response to an increase in the refrigerant liquid level in the evaporator with respect to instances of surge indications over a predetermined period, opening the first valve to allow refrigerant to flow from the condenser into the refrigerant vessel to decrease the cooling capacity of the chiller system.

19. The method of claim 18 , wherein the step of adjusting the capacity further comprises:

in response to a decrease in the refrigerant liquid level in the evaporator with respect to instances of surge indications over a predetermined period, opening the second valve to allow refrigerant to flow from the refrigerant vessel into the evaporator to increase the cooling capacity of the chiller system.