Method for controlling an economizer circuit

1. A method for modulating economizer capacity in a vapor compression system to prevent a condition that may exceed system design limits, the method comprising:

providing an economizer circuit having a vessel, an inlet line to the vessel from a condenser, an outlet line connecting the vessel and a compressor, and a valve positioned in one of the inlet line or the outlet line;

measuring a system condition;

comparing the measured system condition to a predetermined value associated with an operating limit for the measured system condition;

incrementally closing the valve in response to the measured system condition exceeding the predetermined value;

reducing compressor capacity by throttling flow through the economizer circuit to avoid undesirable system conditions;

opening the valve in response to a compressor speed of the compressor reaching a predetermined compressor speed; and

closing the valve and reducing the compressor speed in response to a leaving fluid temperature dropping below a setpoint.

2. The method of claim 1 , wherein the measured system condition includes at least one of a compressor motor temperature, a compressor motor current, a discharge temperature of the compressor, or a discharge pressure of the compressor.

3. The method of claim 2 , wherein the vessel comprises a flash tank, the predetermined value is a predetermined limit, and the measured system condition includes a flash tank liquid level.

4. The method of claim 3 , further comprising stopping closing of the valve in response to the flash tank liquid level dropping below a predetermined level.

5. The method of claim 3 , further comprising periodically opening the valve until the flash tank liquid level is greater than the predetermined limit.

6. The method of claim 3 , further comprising determining a position of the valve at a predetermined interval.

7. The method of claim 3 , further comprising preventing opening of the valve in response to the flash tank liquid level being greater than a predetermined level and the valve being in a closed position.

8. The method of claim 7 , further comprising opening the valve at a predetermined rate in response to the flash tank liquid level being less than the predetermined level, a compressor frequency being greater than a predetermined frequency, and the valve being in the closed position.

9. The method of claim 3 , further comprising closing the valve at a predetermined rate in response to the flash tank liquid level being greater than a predetermined level and the valve being in an at least partially open position.

10. The method of claim 9 further comprising:

preventing closing of the valve in response to the flash tank liquid level being less than the predetermined level and the valve being in the at least partially open position; and

starting a timer for a predetermined time period in response to the flash tank liquid level being less than the predetermined level and the valve being in the at least partially open position.

11. The method of claim 3 , further comprising opening and closing a port valve in the economizer circuit in response to an additional operating parameter associated with the compressor and the flash tank liquid level to engage and disengage the economizer circuit.

12. The method of claim 1 , wherein a closure amount for the valve is a variable amount based on, or proportional to, a difference between the measured system condition and the predetermined value.

13. A method for controlling an economizer circuit having a flash tank, an inlet line to the flash tank from a condenser, and an outlet line from the flash tank connected to a compressor, the method comprising:

measuring a liquid level in the flash tank;

comparing the measured liquid level to a predetermined level;

comparing an outdoor ambient temperature with a predetermined temperature;

comparing a compressor operating time of the compressor to a predetermined time period;

opening a valve positioned in the outlet line from the flash tank in response to the outdoor ambient temperature being less than the predetermined temperature, the compressor operating time being less than the predetermined time period, a compressor speed of the compressor reaching a predetermined compressor speed, and the measured liquid level being less than the predetermined level, wherein opening the valve permits flow of refrigerant from the flash tank to the compressor; and

closing the valve and reducing the compressor speed in response to a leaving fluid temperature dropping below a setpoint.

14. The method of claim 13 , further comprising determining if the valve positioned in the outlet line is in an open position.

15. The method of claim 14 further comprising:

determining an output frequency provided to a compressor motor by a variable speed drive;

comparing the determined output frequency to a predetermined frequency;

comparing a current of the compressor motor to a predetermined motor current;

comparing a temperature of the compressor motor to a predetermined motor temperature;

determining if a first timer has finished; and

opening the valve positioned in the outlet line in response to the valve positioned in the outlet line being in a closed position, the measured liquid level being less than the predetermined level, the determined output frequency being greater than the predetermined frequency, the current of the compressor motor being less than the predetermined motor current, the temperature of the compressor motor being less than the predetermined motor temperature and the first timer being finished.

16. The method of claim 14 further comprising:

determining an output frequency provided to a compressor motor by a variable speed drive;

comparing the determined output frequency to a predetermined frequency;

comparing a temperature of the compressor motor to a predetermined motor temperature;

determining if a first timer has finished; and

closing the valve positioned in the outlet line in response to the valve positioned in the outlet line being in the open position, the determined output frequency being less than the predetermined frequency, the temperature of the compressor motor being greater than the predetermined motor temperature and the first timer being finished.

17. The method of claim 14 further comprising:

determining an output frequency provided to a motor of the compressor by a variable speed drive;

comparing the determined output frequency to a predetermined frequency; and

closing the valve positioned in the outlet line in response to the valve positioned in the outlet line being in the open position and the determined output frequency being less than the predetermined frequency.

18. The method of claim 13 further comprising:

determining whether the compressor is operating; and

closing the valve positioned in the outlet line in response to a determination that the compressor is not operating.

19. A method for modulating economizer capacity in a vapor compression system to prevent a condition that may exceed system design limits, the method comprising:

providing an economizer circuit having a flash tank, an inlet line to the flash tank from a condenser, an outlet line connecting the flash tank and a compressor, and a valve positioned in one of the inlet line or the outlet line;

measuring a system condition, wherein the measured system condition includes a flash tank liquid level;

comparing the flash tank liquid level to a predetermined level associated with an operating limit for the flash tank liquid level;

incrementally closing the valve in response to the flash tank liquid level exceeding the predetermined level;

closing the valve at a predetermined rate in response to the flash tank liquid level being greater than the predetermined level and the valve being in an at least partially open position;

preventing closing of the valve in response to the flash tank liquid level being less than the predetermined level and the valve being in the at least partially open position;

starting a timer for a predetermined time period in response to the flash tank liquid level being less than the predetermined level and the valve being in the at least partially open position; and

reducing compressor capacity by throttling flow through the economizer circuit to avoid undesirable system conditions.