TECHNICAL FIELD

The present disclosure is directed to chiller plants, and more particularly, to methods of avoiding centrifugal compressor surge to enable more efficient chiller plant operation.

BACKGROUND

A chiller is an air conditioning system which provides a temperature conditioned fluid, usually water, for use in conditioning the air of a load such as a building. Chillers are typically used in large air conditioning systems which centralize the air conditioning requirements for a large building or complex of buildings by using water or a similar fluid as a safe and inexpensive thermal transport medium.

In its operation, the chiller provides conditioned water of a particular temperature for use in cooling air in a building by means of a first water loop referred to as the evaporator loop or chilled water loop. Heat is extracted from the building air, transferred to the water in the chilled water loop, and is returned via the chilled water loop to an evaporator, which again refrigerates the water to the desired temperature by transferring the heat of the water to the chiller's refrigerant. After the refrigerant is compressed by a compressor, the heat in the refrigerant is transported to a condenser and heat is transferred to a second water loop referred to as the condenser water loop. Centrifugal compressors are often employed for this purpose because of their high efficiency. The condenser loop transports heat from the condenser of the chiller system to a cooling tower or towers which then transfers the heat from the condenser loop to outdoor ambient air by direct contact between the ambient air and the water of the condenser loop. The cooled water returns to the condenser to again absorb heat transferred from the refrigerant.

Centrifugal compressors used in chiller systems are rated in terms of load and lift. Load refers to the compressor's air conditioning load capacity, and is typically expressed in tons or kilowatts. Lift refers to the maximum increase in pressure the compressor can achieve from inlet to outlet and may be expressed as a pressure (e.g., psi, kPa, or bar). Alternatively, because pressure and temperature are inextricably related, lift may also be expressed as a temperature (e.g., 50° F. of lift). If the difference in refrigerant pressure between inlet and outlet exceeds the lift capability of the compressor, refrigerant is forced backwards into the discharge port compressor, causing a sudden collapse of refrigerant flow known as compressor surge. Compressor surge has several undesirable consequences: energy that went into pressurizing the refrigerant is lost; extreme stress is placed on the compressor components such as impeller blades, bearings, housing and seals; and large swings occur in the electrical power drawn by the compressor motor. Continued occurrence of surge over time can cause compressor failure.

Known chiller plant control techniques utilize optimizing controllers that provide setpoint recommendations and attempt to optimize a quantity such as to minimize energy consumption, cost, other variables, or combinations thereof. Such optimizing controllers may employ, for example, artificial intelligence-based (AI) or Model Predictive Control (MPC) techniques. Such controllers may have drawbacks. For example, when optimizing controllers explore to find local minima with regards to their reward function, they can push the chiller system into operating conditions that cause compressor surge. A method to operate a chiller plant while reducing the risk of compressor surge would be a welcome advance.

SUMMARY

The following presents a summary to provide a basic understanding of one or more embodiments of the disclosure. This summary is not intended to identify key or critical elements or delineate any scope of the particular embodiments or any scope of the claims. Its sole purpose is to present concepts in a simplified form as a prelude to the more detailed description that is presented later.

According to an embodiment of the present disclosure a method of operating a chiller system includes determining a predicted surge lift temperature difference of a centrifugal compressor of the chiller system, calculating anti-surge condenser water minimum and maximum temperature limits, calculating anti-surge condenser water minimum and maximum flow rate limits, establishing an anti-surge condenser water temperature setpoint as a provisional condenser water temperature setpoint constrained between the anti-surge condenser water minimum and maximum temperature limits, establishing an anti-surge condenser water flow rate setpoint as the provisional condenser water flow rate setpoint constrained between the anti-surge condenser water minimum and maximum flow rate limits, and operating the chiller system in accordance with the anti-surge condenser water temperature setpoint and the anti-surge condenser water flow rate setpoint.

In embodiments, the method includes receiving a provisional entering condenser water temperature setpoint and a provisional condenser water flow rate setpoint from an optimization controller.

In embodiments, the method includes calculating a minimum and a maximum limit on the number of cooling tower cells, establishing a number of cooling tower cells setpoint as a provisional number of cooling tower cells setpoint constrained between the minimum and maximum limits on the number of cooling tower cells, and operating the chiller system in accordance with the number of cooling tower cells setpoint.

In embodiments, the method includes receiving the provisional number of cooling tower cells setpoint from an optimizing controller.

In embodiments of the method, determining a predicted surge lift temperature difference comprises determining an active chiller load percentage and determining the predicted surge lift temperature difference as a function of active chiller load percentage.

In embodiments, the method includes determining a minimum aggregate condenser water flow rate based at least in part upon a number of running chillers, the design minimum condenser water flow of each running chiller, and the design minimum water flow of each available cooling tower cell.

In embodiments, the method includes receiving design data characterizing non-alterable properties of the chiller system, wherein design data is selected from the group consisting of chiller design tons, chiller design rejection tons, minimum condenser water flow, maximum condenser water flow, minimum cooling tower cell flow, and maximum cooling tower cell flow.

In embodiments, the method includes receiving active data characterizing alterable properties of the chiller system, wherein active data is selected from the group consisting of active leaving chiller water temperature, active entering condenser water temperature, and active chiller tons.

According to another embodiment of the present disclosure, a device can comprise a processor and a memory that stores executable instructions that, when executed by the processor, facilitate the performance of a method as described above. According to another embodiment of the present disclosure, the described device can be included in a chiller system.

