System for sequencing chillers in a loop cooling plant and other systems that employ all variable-speed units

• US 6,185,946 B1
• Filed: 05/08/2000
• Issued: 02/13/2001
• Est. Priority Date: 05/07/1999
• Status: Expired due to Term

• Current Assignee: Armstrong Pump Incorporated, Optimum Energy LLC
• Original Assignee: Thomas B. Hartman
• Inventors: Hartman, Thomas B.
• Primary Examiner(s): Doerrler, William
• Assistant Examiner(s): NORMAN, MARC E
• Application Number:
  US09/566,681
• Time in Patent Office: 281 Days
• Field of Search: 621/75, 622/281, 622/283, 622/284, 622/285, 622/01, 625/10, 621/965, 621/963, 236/1.EA
• US Class Current: 62/175
• CPC Class Codes: F04D 15/029 for pumps operating in parallel
• CPC Class Codes: F25B 1/053 of turbine type
• CPC Class Codes: F25B 2400/06 Several compression cycles arranged in parallel
• CPC Class Codes: F25B 2600/13 Pump speed control
• CPC Class Codes: F25B 49/02 for compression type machines, plants or systems
• CPC Class Codes: F25B 49/025 Motor control arrangements
• CPC Class Codes: Y02B 30/70 Efficient control or regulation technologies, e.g. for control of refrigerant flow, motor or heating

Abstract

Methods of sequencing parallel units in HVAC systems are described to improve overall system operating efficiency. The improved method is applicable for sequencing parallel centrifugal pumps in a variable flow hydronic system, parallel fans in a variable air-flow system, and centrifugal chillers in an HVAC system having a plurality of variable speed drive centrifugal chillers arrange in parallel. The method calls for determining an operating point of the on-line units, comparing the current operating point to a natural curve of maximum efficiency, and adding or shedding units as required to move closest to an operating curve of optimal efficiency, without direct regard to actual loading. Natural operating curves relate head pressure and operating speed rather than load. Control logic that implements such operating characteristics often will result in operating more parallel units on-line that would be required by prior art to meet the load. However, in such cases, all of the on-line units are operated at lower speed resulting in lower overall system energy consumption.

Claims

1. A system comprising at least two variable speed chillers arranged in parallel, each chiller including a refrigerating cycle that includes a variable speed compressor, an evaporator, a condenser, and a refrigerant expansion device, and wherein the speed of all operating chillers is coordinated by a common controller to meet a cooling load, comprising:

means for transmitting a first control signal from each chiller to the common controller to communicate speed of the corresponding chiller;

means for transmitting a second signal from each chiller to the common controller to communicate a current head of the corresponding compressor;

the controller including first means responsive to the first control signals and the second control signals from all of the operating chillers for determining a current operating point of the system without regard to load;

the controller further including second means for selecting a specific number of chillers such that the selected number of chillers in operation at the current operating point of the system would be operating a closely as possible to their predetermined natural curve of operating efficiency; and

the controller further including third means for adjusting the number of chillers on-line such that it equals the selected number of chillers, thereby improving overall operating efficiency of the system, while still enabling the system to accommodate the load by adjusting speed of the adjusted number of on-line chillers.

2. A method according to claim 1 wherein the means for adjusting the number of chillers on-line comprises a predetermined procedure for adding a chiller on-line and a predetermined procedure for shedding a chiller.

3. A system according to claim 1 wherein the means of determining the compressor head pressure for each chiller comprises a pressure sensor arranged for measuring a difference between the chiller condenser and the chiller evaporator pressures.

4. A system according to claim 1 wherein the means of determining the compressor head pressure for each chiller includes:

means for measuring the entering and leaving condenser water temperatures;

means for measuring the entering and leaving chilled water temperatures;

means for determining a temperature differential between the log mean of condenser and chilled water temperatures and the refrigerant temperature;

means for calculating the refrigerant pressure at each condition by employing a look-up table that translates the refrigerant temperature to the saturated vapor pressure for the refrigerant employed in the chiller; and

means for calculating the head pressure by determining a difference between the condenser pressure and the evaporator pressure.

5. A system according to claim 1 wherein the means of determining the compressor speed includes means for measuring a power draw of the motor driving the compressor and means for calculating the motor speed based on the measured driving motor power draw.

6. A variable flow hydronic system comprising:

at least two variable speed pumps, a first signal from each pump is employed to communicate speed of that pump, and a second signal from each pump is employed to read the head or differential pressure across the pump;

a controller that operates in response to the first and second signals such that when the average pump head is a selected increment greater than an ideal pump head for the communicated speed, a pump is cycled off and the remaining pumps are operated at higher speeds as required to meet the load, and anytime the average head is a selected increment less than the ideal pump head for the current speed, an additional pump is cycled on, and all of the on-line pumps are operated at lower speeds sufficient to meet the load.

7. A system according to claim 6 for which the means of determining the pump head pressure for all pumps is a pressure sensor that measures the difference between the inlet and outlet headers for the pumping system.

8. A method of sequencing centrifugal chillers in a loop HVAC system having a plurality of variable speed drive centrifugal chillers arrange in parallel and coupled to a common distribution header comprising the steps of:

commencing operation of the HVAC system with at least one of the chillers on-line;

monitoring a current compressor operating speed and actual head pressure of the on-line chillers;

determining a head pressure shedding threshold based on the current compressor operating speed;

comparing the actual head pressure of the on-line chillers to the head pressure shedding threshold value for the current operating speed;

if the actual head pressure of the on-line chillers is greater than the head pressure shedding threshold value for the current operating speed, shedding one of the on-line chillers;

and then increasing the operating speed of the chillers remaining on-line, thereby improving overall operating efficiency.

9. A method according to claim 8 wherein said determining a chiller head pressure shedding threshold comprises determining a natural curve of efficient operation of the compressors.

10. A method according to claim 8 wherein the actual head pressure of the on-line chillers is determined as a ratio of an average current compressor head pressures compared to a predetermined design maximum for the chillers.

11. A method according to claim 8 further comprising monitoring refrigerant temperatures and wherein said monitoring the actual head pressure comprises estimating the actual head pressure based on the refrigerant temperatures.

12. A method according to claim 8 and further comprising monitoring condensing and chilled water temperatures, and estimating the actual head pressure based on the condensing and chilled water temperatures.

13. A method according to claim 8 wherein determining a head pressure shedding threshold value comprises calculating the shedding threshold value as a predetermined quadratic function of the operating speed.

14. A method according to claim 8 wherein the shedding threshold value is calculated according to the following formula:

Head pressure shedding threshold HPFS=((N/(n−

1))×

SP)²×

((n−

1)/n)^C2 where

N is the total number of centrifugal chillers arranged in parallel in the system;

n is the number of chillers currently on-line;

SP is a factor based on the current compressor operating speed; and

the exponent

c2 is a real number in a range between 0 and 2.

15. A method according to claim 14 wherein the current compressor operating speed is a nominal speed indicated by the system controller.

16. A method according to claim 14 further comprising determining the current compressor operating speed factor SP as a ratio of a sum of the speeds of all of the individual on-line compressors, divided by the total number N of centrifugal chillers available for sequencing.

17. A method according to claim 16 wherein N is in a range of 2 to 12.

18. A method of sequencing centrifugal chillers in a loop HVAC system having a plurality of variable speed drive centrifugal chillers arrange in parallel and coupled to a common distribution header comprising the steps of:

commencing operation of the HVAC system with at least one of the chillers on-line;

monitoring compressor operating speed and actual head pressure of the on-line chillers;

selecting a head pressure adding threshold value based on the current operating speed;

comparing the actual head pressure of the on-line chillers to the head pressure adding threshold for the current operating speed; and

if the actual head pressure of the on-line chillers is greater than the selected head pressure adding threshold value for the current operating speed, adding another one of the chillers on-line;

and then reducing the operating speed of all of the on-line chillers, thereby improving overall operating efficiency of the system.

19. A method according to claim 18 wherein said determining a chiller head pressure adding threshold comprises determining a natural curve of efficient operation of the compressors.

20. A method according to claim 18 wherein the actual head pressure of the on-line chillers is determined as a ratio of the average current compressor head pressures compared to a predetermined design maximum for the chillers.

21. A method according to claim 18 further comprising monitoring refrigerant temperatures and wherein said monitoring the actual head pressure comprises estimating the actual head pressure based on the refrigerant temperatures.

22. A method according to claim 18 and further comprising monitoring condensing and chilled water temperatures, and estimating the actual head pressure based on the condensing and chilled water temperatures.

23. A method according to claim 18 wherein selecting a head pressure adding threshold value HPFA comprises calculating the HPFA value as a predetermined quadratic function of the current operating speed.

24. A method according to claim 18 wherein the HPFA is calculated according to the following formula:

HPFA=((N/n)×

SP)²×

(n/(n+1))^C2 where

N is the total number of centrifugal chillers arranged in parallel in the system;

n is the number of chillers currently on-line;

SP is the current compressor operating speed; and

the exponent

c2 is a real number in a range between 0 and 2.

25. A method according to claim 24 wherein the current compressor operating speed is a nominal speed indicated by the system controller.

26. A method according to claim 24 further comprising determining the current compressor operating speed SP as a ratio of a sum of the speeds of all of the individual on-line compressors, divided by N—

the total number of centrifugal chillers available for sequencing.

27. A method according to claim 24 wherein c2 has a value greater than 1.

28. A method of sequencing centrifugal pumps in a variable flow hydronic system having a plurality of variable speed drive centrifugal pumps arranged in parallel and coupled to a common distribution header to circulate chilled or heating water, the method comprising the steps of:

commencing operation of the hydronic system with at least one of the pumps on-line;

monitoring operating speed and actual head pressure of the on-line pumps;

selecting a desired head pressure threshold HPT value based on the current operating speed;

comparing the actual head pressure of the on-line pumps to the head pressure threshold HPT for the current operating speed; and

if the actual head pressure of the on-line pumps is greater than the head pressure threshold HPT for the current operating speed, shedding one of the on-line pumps, and then increasing the operating speed of the pumps remaining on-line, thereby improving overall operating efficiency of the system.

29. A method according to claim 28 further comprising:

if the actual head pressure of the on-line pumps is less than a head pressure threshold HPFA for the current operating speed, adding one more pump on-line, and then decreasing the operating speed of the operating pumps, thereby improving overall operating efficiency of the system.