Description

A virtual water flowmeter was developed that uses thechilled-water control valve on an air-handling unit (AHU) asa measurement device. The flow rate of water through the valve was calculated using the differential pressure across the valve and its associated coil, the valve command, and an empirically determined, steady state valve characteristic curve. To compensate for the loss in accuracy using the valve command to indicate the valve open position, in this paper, valve dynamic behavior was empirically described using valve stiction (S)and stiction plus dead band (slip jump: J) to convert valvecommands to actual valve positions. Stiction is defined as static friction resistance to valve movement, and dead bandrefers to hysteresis involved in the reversal of valve movement directions.

To calculate the S and J parameters, extra measurements of the system, especially valve reaction time, needed to be taken. To determine which values to use for S and J, very small incremental changes were made manually via valve command overrides within the control software and the actual valve positionwas closely monitored for any changes. The stiction valuesmay not have remained constant throughout the period of valvemovement. Therefore, three valve command ranges were alsoexamined. In this study, low, medium, and high ranges of valvecommands were considered. Accuracy improvement of thevirtual flowmeter using converted valve commands was also demonstrated by a case study. Uncertainty analyses were conducted, revealing significant improvements in virtual flowmeter accuracy. Using the converted valve commands, the mean relative error of the virtual flow calculation improvedfrom 13.73% when the valve command was used in the virtual flow rate calculation to 7.26%, when the valve dynamic behaviorr is reflected for low flow rate conditions.

Citation: 2018 Winter Conference, Chicago, IL, Technical Papers

Product Details

Published:

2018

Number of Pages:

11

Units of Measure:

Dual

File Size:

1 file , 1.5 MB

Product Code(s):

D-CH-18-007