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A cooling system has a tandem compressor, a condenser, an electronic expansion valve and an evaporator arranged in a direct expansion cooling circuit. The tandem compressor includes a variable capacity compressor and a fixed capacity compressor. A controller controls the electronic expansion valve a

A cooling system has a tandem compressor, a condenser, an electronic expansion valve and an evaporator arranged in a direct expansion cooling circuit. The tandem compressor includes a variable capacity compressor and a fixed capacity compressor. A controller controls the electronic expansion valve and selects which of a plurality of superheat control modes to use in operating the electronic expansion valve based on control parameters and a current operating status of each of the variable capacity compressor and fixed capacity compressor of the tandem compressor.

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1. A cooling system, comprising: a cabinet having an air inlet and an air outlet;an air moving unit disposed in the cabinet;a tandem compressor, a condenser, an electronic expansion valve and an evaporator arranged in a direct expansion cooling circuit;the tandem compressor including a variable capa

1. A cooling system, comprising: a cabinet having an air inlet and an air outlet;an air moving unit disposed in the cabinet;a tandem compressor, a condenser, an electronic expansion valve and an evaporator arranged in a direct expansion cooling circuit;the tandem compressor including a variable capacity digital scroll compressor and a fixed capacity scroll compressor;a controller coupled to the electronic expansion valve that controls the electronic expansion valve, the controller selecting which of a plurality of superheat control modes to use in operating the electronic expansion valve based on control parameters and a current operating status of each of the variable capacity digital scroll compressor and fixed capacity scroll compressor of the tandem compressor;wherein the plurality of superheat control modes include a gated mode, a system mapping mode and a constant mode, wherein: in the gated mode, each time the variable capacity digital scroll compressor unloads, the controller drives the electronic expansion valve from its current position down to a minimum open position until the variable capacity digital compressor stops unloading and once the variable capacity digital scroll compressor stops unloading, the controller adjusts the position of the electronic expansion valve to achieve a desired superheat;in the system mapping mode, each time the variable capacity digital scroll compressor unloads, the controller retaining the electronic expansion valve in its current position until the variable capacity digital scroll compressor stops unloading and once the variable capacity digital compressor stops unloading, the controller adjusts the position of the electronic expansion valve to achieve a desired superheat; andin the constant mode, the controller adjusts the position of the electronic expansion valve to achieve a desired superheat regardless of whether the variable capacity digital scroll compressor is unloading or loading. 2. The cooling system of claim 1 wherein the control parameters include a superheat mode threshold parameter A, a digital plus fixed superheat threshold parameter B and a digital plus superheat control parameter C, the controller: when only the fixed capacity scroll compressor of the tandem compressor is on, operating the electronic expansion valve in the constant mode; when only the variable capacity digital scroll compressor of the tandem compressor is on, operating the electronic expansion valve in the gated mode when a pulse width modulation loading percentage of the variable capacity digital scroll compressor is less than the superheat mode threshold parameter and when the pulse width modulation loading percentage of the variable capacity digital scroll compressor is greater than or equal to parameter A; and when both the variable capacity digital scroll compressor and fixed-capacity scroll compressor of the tandem compressor are on, operating the expansion valve in the constant mode when parameter C is set to constant, operating the electronic expansion valve in the gated mode when parameter C is set to a system mapping and the pulse width modulation loading percentage of the variable capacity digital scroll compressor is less than parameter B, and operating the electronic expansion valve in the system mapping mode when parameter C is set to the system mapping and the pulse width modulation percentage of the digital scroll compressor is greater than or equal to parameter B. 3. The cooling system of claim 2 wherein cooling system includes a second direct expansion circuit that also has a tandem compressor, a condenser, an electronic expansion valve and an evaporator, the tandem compressor of the second direct expansion circuit also including a variable capacity digital scroll compressor and a fixed-capacity scroll compressor; the cooling system having a plurality of separate cooling stages including an upstream cooling stage and a downstream cooling stage with each cooling stage having one of the direct expansion circuits, the evaporators of the cooling circuits arranged in the cabinet so that air to be cooled passes over the evaporators in serial fashion, first over the evaporator of the direct expansion cooling circuit of the upstream cooling stage and then over the evaporator of the direct expansion cooling circuit of the downstream cooling stage; the controller also coupled to the electronic expansion valve of the second direct expansion cooling circuit and controlling the electronic expansion valve of the second direct expansion cooling circuit based on current operating status of each of the variable capacity digital scroll compressor and fixed-capacity scroll compressor of the tandem compressor of the second direct cooling circuit and otherwise in the same manner as it controls the electronic expansion valve of the other direct expansion circuit. 4. The cooling system of claim 3 wherein the controller has a separate set of control parameters for each of the direct expansion cooling circuits. 5. The cooling system of claim 1 wherein cooling system includes a second direct expansion circuit that also has a tandem compressor, a condenser, an electronic expansion valve and an evaporator, the tandem compressor of the second direct expansion circuit also including a variable capacity digital scroll compressor and a fixed-capacity scroll compressor; the cooling system having a plurality of separate cooling stages including an upstream cooling stage and a downstream cooling stage with each cooling stage having one of the direct expansion circuits, the evaporators of the cooling circuits arranged in the cabinet so that air to be cooled passes over the evaporators in serial fashion, first over the evaporator of the direct expansion cooling circuit of the upstream cooling stage and then over the evaporator of the direct expansion cooling circuit of the downstream cooling stage; the controller also coupled to the electronic expansion valve of the second direct expansion cooling circuit and controlling the electronic expansion valve of the second direct expansion cooling circuit based on current operating status of each of the variable capacity digital scroll compressor and fixed-capacity scroll compressor of the tandem compressor of the second direct cooling circuit and otherwise in the same manner as it controls the electronic expansion valve of the other direct expansion circuit. 6. The cooling system of claim 5 wherein the controller has a separate set of control parameters for each of the direct expansion cooling circuits.