Active stall prevention in centrifugal fans
A system, method, and computer program to adjust the effective width of centrifugal fan wheel(s) in a fan system during operation. The reason for the adjustment is to prevent stall when running the fan over a wider range of system operating points. This is of particular use in high turndown variable air volume systems commonly encountered in HVAC systems. An additional option function is to isolate a single fan, in a fan system, if that fan should fail, to prevent reverse flow through the failed fan.
1. A fan system comprising:
at least one variable speed fan comprising an adjustable valve for controlling a volume of air for delivery and for varying an effective width of a fan wheel;
a means for monitoring a flow rate of the volume of air;
a sensor configured to measure a static pressure differential across the fan;
a microprocessor configured to calculate a stall static pressure based on the volume of air from the means for monitoring; and
a first controller configured to adjust the adjustable valve based on the measured static pressure differential and the calculated stall static pressure.
2. The fan system of claim 1 wherein the means for monitoring comprises at least one sensor.
3. The fan system of claim 1 wherein the means for monitoring a flow rate comprises a calculated flow rate.
4. The fan system of claim 1 wherein the microprocessor configured to calculate a stall static pressure comprises using the formula
SP
s
=
SP
b
*
(
Q
c
Q
b
)
2
.
5. The fan system of claim 1 wherein the first controller is configured to vary a speed of the fan.
6. The fan system of claim 1 further comprising a second controller configured to vary a speed of the fan.
7. The fan system of claim 1 wherein the adjustable valve comprises a linear operator.
8. The fan system of claim 1 wherein the at least one variable speed fan comprises a centrifugal fan.
9. The fan system of claim 1 wherein the adjustable valve further comprises a fan fault closing valve.
10. A method for adjusting characteristics of a centrifugal fan, the method comprising the steps of:
varying flow characteristics of the fan with an adjustable valve;
monitoring a flow rate of the volume of air;
measuring a static pressure differential across the fan
calculating a stall static pressure from the monitored flow rate; and
adjusting the adjustable valve based on the measured static pressure differential and the calculated stall static pressure.
11. The method of claim 7 wherein the step of monitoring comprises using a sensor.
12. The method of claim 7 wherein the step of monitoring comprises calculating a flow rate.
13. The method of claim 7 wherein the step of calculating a stall static pressure comprises using the formula
SP
s
=
SP
b
*
(
Q
c
Q
b
)
2
.
14. The method of claim 7 further comprising the step of varying a speed of the fan.
15. The method of claim 7 wherein the step of varying the volume of air comprises moving a conical valve linearly.
16. The method of claim 7 further comprising the steps of:
detecting a fan fault condition; and
closing the adjustable valve until the fault condition has ceased.
17. A non-transitory computer-executable storage medium comprising program instructions which are computer-executable to implement a variable volume of air to prevent a stall condition in a fan assembly comprising:
program instructions configured to cause a measurement of a flow rate of a volume of air;
program instructions configured to cause a static pressure differential measurement across the fan;
program instructions configured to cause a calculation of stall static pressure of the volume of air from the measured flow rate; and
program instructions configured to cause a controller to vary the flow characteristics of the fan by adjusting a valve based on the program instructions configured to cause a static pressure differential measurement and the program instructions configured to cause a measurement of a flow rate.
18. The non-transitory computer-executable storage medium of claim 17 wherein the program instructions configured to cause a measurement of the flowrate comprise a sensor.
19. The non-transitory computer-executable storage medium of claim 17 wherein the program instructions configured to cause a measurement of the flowrate further comprise program instructions configured to cause a calculated flow rate.
20. The non-transitory computer-executable storage medium of claim 17 wherein the program instructions configured to cause a calculation of a stall static pressure differential comprises using the formula
SP
s
=
SP
b
*
(
Q
c
Q
b
)
2
.
21. The non-transitory computer-executable storage medium of claim 17 further comprises program instructions configured to cause a variation of a speed of the fan.
22. The non-transitory computer-executable storage medium of claim 17 further comprises:
program instructions configured to cause a fan fault condition be detected; and
program instructions configured to cause the adjustable valve to remain closed until the fault condition has ceased.