Load estimator for control of vapor compression cooling system with pumped refrigerant economization

1. A cooling system, comprising:

a cabinet having an air inlet and an air outlet;

a first cooling circuit that includes an evaporator disposed in the cabinet, a condenser, a compressor, an expansion device and a liquid pump;

the cooling system having a direct expansion mode wherein the compressor is on and compresses a refrigerant in a vapor phase to raise a pressure of the refrigerant and thus a condensing temperature of the refrigerant and the refrigerant is circulated around the first cooling circuit by the compressor and a pumped refrigerant economizer mode wherein the compressor is off and the liquid pump is on and pumps the refrigerant in a liquid phase and the refrigerant is circulated around the first cooling circuit by the liquid pump and without compressing the refrigerant in vapor phase;

a controller coupled to the liquid pump and the compressor that turns the compressor off and the liquid pump on to operate the first cooling circuit in the pumped refrigerant economizer mode and turns the compressor on and the liquid pump off to operate the first cooling circuit in the direct expansion mode; and

the controller including a load estimator that estimates real-time indoor heat load on the cooling system and uses the estimated real-time indoor heat load to determine an outdoor temperature control parameter and operates the cooling system in the pumped refrigerant economizer mode when a temperature of outdoor air is less than or equal to the outdoor temperature control parameter and operates the cooling system in the direct expansion mode when the temperature of outdoor air is greater than the outdoor temperature control parameter.

2. The cooling system of claim 1 wherein the load estimator calculates the real-time indoor heat load based on a temperature of indoor return air, a temperature of supply air and a volume of air flow across the evaporator.

3. The cooling system of claim 2 wherein the load estimator uses compressor loading information and the temperature of at least one of the supply air or the return air to estimate the real time indoor heat load.

4. The cooling system of claim 1 including a receiver/surge tank coupled between the condenser and the liquid pump.

5. The cooling system of claim 1 further including a second cooling circuit having an evaporator disposed in the cabinet, a condenser, a compressor, an expansion device and a liquid pump, the cooling system having one of the first or second cooling circuit included in an upstream cooling stage and the other one of the first or second cooling circuit included in a downstream cooling stage wherein the evaporator of the first or second cooling circuit included in the upstream cooling stage is an upstream evaporator and the evaporator of the first or second cooling circuit included in the downstream cooling stage is a downstream evaporator, wherein the upstream evaporator and the downstream evaporator are arranged in the cabinet so that air to be cooled passes over the upstream evaporator and the downstream evaporator in serial fashion, first over the upstream evaporator and then over the downstream evaporator, the first and second cooling circuits are operable in the direct expansion mode and the pumped refrigerant economizer mode wherein when the first cooling circuit is operated in the pumped refrigerant economizer mode the compressor of the first cooling circuit is off and the liquid pump of the first cooling circuit is on and the refrigerant is circulated around the first cooling circuit by the liquid pump of the first cooling circuit, and when the second cooling circuit is operated in the pumped refrigerant economizer mode, the compressor of the second cooling circuit is off and the liquid pump of the second cooling circuit is on and the refrigerant is circulated around the second cooling circuit by the liquid pump of the second cooling circuit in the liquid phase.

6. The cooling system of claim 5 wherein when the upstream cooling stage is in the pumped refrigerant economizer mode, the downstream cooling stage must be in the direct expansion mode, the controller operates the upstream cooling stage in the pumped refrigerant economizer mode by turning the liquid pump of the upstream cooling stage on and the compressor of the upstream cooling stage off, and operates the downstream cooling stage in the direct expansion mode by turning the compressor of the downstream cooling stage on and the liquid pump of the downstream cooling stage off.

7. The cooling system of claim 5 wherein for each of the first and second cooling circuits the controller further determines the outdoor temperature control parameter using the estimated real-time indoor heat load and operates each of the first and second cooling circuit in the pumped refrigerant economizer mode when the outdoor air temperature is at or below the outdoor air temperature control parameter for a respective one of the first or second cooling circuit.