Mixed model compressor

1. A chiller system, comprising:

a compressor section having a first refrigerant compressor having a first nominal cooling capacity and a second refrigerant compressor having a second nominal cooling capacity different from the first nominal cooling capacity, wherein each of the compressors in the compressor section is configured to operate independently of one another;

a controller configured to selectively operate the first and second refrigerant compressors; and

wherein the chiller system has a total maximum cooling capacity less than or equal to 700 tons, and a full-load efficiency of the chiller system is between about 0.5 and about 0.7 kW per ton.

2. The chiller system of claim 1 , comprising an evaporator upstream of the compressor section and a condenser downstream of the compressor section.

3. The chiller system of claim 2 , comprising an economizer configured to deliver fluid from downstream of the condenser to the compressor section.

4. The chiller system of claim 3 , wherein the controller is configured to selectively operate the economizer.

5. The chiller system of claim 1 , wherein the first and second refrigerant compressors are centrifugal compressors.

6. The chiller system of claim 1 , comprising a third refrigerant compressor.

7. The chiller system of claim 6 , wherein the third refrigerant compressor has a third nominal cooling capacity that is the same as one of the first nominal cooling capacity and the second nominal cooling capacity.

8. The chiller system of claim 6 , wherein the third refrigerant compressor has a third nominal cooling capacity that is different from the first and second nominal cooling capacities.

9. The chiller system of claim 8 , wherein the first nominal capacity is 300 kW, the second nominal capacity is 700 kW, and the third nominal capacity is 1200 kW.

10. The chiller system of claim 1 , wherein the compressor section has less than or equal to six refrigerant compressors.

11. The chiller system of claim 1 , wherein one of the first and second nominal cooling capacities is 700 kW.

12. The chiller system of claim 1 , wherein one of the first and second nominal cooling capacities is 300 kW.

13. The chiller system of claim 1 , wherein the first nominal capacity is 400 kW and the second nominal capacity is 1200 kW.

14. The chiller system of claim 1 , wherein the total maximum cooling capacity is greater than 400 tons.

15. A method of operating a chiller system, comprising:

providing a compressor section having a first refrigerant compressor having a first nominal cooling capacity and a second refrigerant compressor having a second nominal cooling capacity higher than the first nominal cooling capacity, wherein the chiller system has a total maximum cooling capacity less than or equal to 700 tons, and a full-load efficiency of the chiller system is between about 0.5 and about 0.7 kW per ton;

selectively operating the first and second refrigerant compressors with a controller; and

selectively operating an economizer with the controller.

16. The method of claim 15 , comprising operating the first refrigerant compressor at a different operating load than the second refrigerant compressor while the first and second compressors are both on.

17. The method of claim 15 , comprising operating only the first refrigerant compressor when the chiller system is operating at a low capacity.

18. The method of claim 17 , comprising operating only the second refrigerant compressor when the chiller system is operating at a medium capacity.

19. The method of claim 18 , comprising operating both the first and second refrigerant compressors when the chiller system is operating at a high capacity.

20. The method of claim 19 , wherein the low capacity is less than about 120 tons, the medium capacity is between about 120 and about 360 tons, and the high capacity is greater than about 360 tons.