Get your ducts in a row

Readers of CONTROL respond with comments, suggestions and a solution to the problem: What can we do to get six identical air flows from our air plennum splitting system?

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PROBLEM:

we have an air plenum splitting system that consists of six identically sized round pipe ducts with the same valve in each pipe. We simply cannot control the airflow so that the same flowrate moves through each splitter. What should we be doing, with either hardware or software, to get six identical air flows?

-- from August 2004 CONTROL

SOLUTIONS:

Match the Master
This is a relatively simple problem. Even if all the ducts are not the same, the motor/inverter drive/fan equipment would be the same, except that each duct would have a flowmeter sensor on the pipe output for feedback to the drive of that pipe. An external master controller would set the required flow for the first pipe. Each drive would be connected to use the flowmeter sensor on the master pipe output as a setpoint to all the drives. The flow level of the master pipe would then be matched by the other five pipe systems.

John Malinowski, Baldor Electric Co.

Overcontrol Syndrome
Sounds like the ever-present overcontrol syndrome. Assuming these valves are control valves, I can understand why the flows are not equal and the valves are probably constantly oscillating. A different view might be to simplify the system by putting a variable speed control on the blower and installing dampers instead of the control valves.

Paul Sparks, Dow Reichhold

A Trick Question
This is a trick question. If it is constant volume, get a balance contractor! If variable, they could get fancy and place flow measuring stations in each duct, go to a controller, and modulate the dampers.

Jeff Miller, ABB Drives

Bill Brown, Control Engineer

Algorithm Required
This sounds like a good application for a most-open-valve flow splitting algorithm. This approach requires a master valve position controller and an airflow controller for each of the valves.

Will Lloyd, Lloyd Engineering, PLC

Verify Capacity First
First you must verify that there is enough capacity for the plenum, both in pressure and volume. Control will require some pressure drop. If there is extra pressure available, simple orifices could balance the flow. If more sophisticated control is required, flowmeters can be installed. For simple, even distribution with only minor downstream pressure changes, orifices with significant pressure drop could provide good distribution. If this is not available, increasing the source (for instance, faster fan speed) is often more practical than adding flow controls.

Bob Hershey, NGK Ceramics USA

In addition, a very well though-out solution to this problem appears as a
WEB ONLY BONUS!

Submitted by Rick Rys of r2controls: www.r2controls.com

APPROACH TO PROBLEM:

1) Process Understanding:
The process is not described in detail so let's make some assumptions:

Q: Is the source of the air provided from a blower?
A: Assume yes.

Q: The Flow into the plenum is variable?
A: Assume the flow varies with a typical centrifugal blower performance curve.
(i.e. max pressure at zero flow and the blower is fairly close to the plenum).

Q: The plenum is pressure controlled?
A: Assume not, plenum pressure floats on the blower output pressure.

Q: The ducts are independent?
A: Let's assume each duct is essentially a pipe resistance to atmosphere.

Q: What are the fluid dynamics?
A: Lets' assume that the clean dry air is near atmospheric pressure (and temperature), say 1 PSIG in the plenum if all 6 valves are shut, dropping to 0.2 PSIG if all 6 valves are wide open. With the valves full open the pressure loss in the valves is nil and the 0.2 PSIG is due to duct pressure drop.

Q: Are there any upsets? Let’s assume that we can get some upsets in the downstream pressure on each of the 6 ductsâ€¦
A: maybe a spike of 0.1 PSIG from time to time.

Q: What is the process control objective?
A: The process objective is to allow equal flow in all 6 ducts up to the blower capacity, with the possibility to regulate the flow in a roughly 5:1 turndown.

Q: What kind of valves?
A: Simple low cost butterfly valves, with pneumatic actuator, but no positioner.

Summary: This is a process with interaction, in that increasing the flow in 1 duct will reduce the plenum pressure and steal flow from the remaining ducts. If the existing control system is 6 single loop flow controllers, the interaction could easily result in all 6 controllers oscillating. All 6 loops have the same dynamics and will have the same natural period so an oscillation in one loop will tend to excite the remaining 5 loops. So let’s focus on solving the interaction problem.

2) Determining Interaction:
In 1966 Edgar Bristol wrote a key paper on the measurement of interaction where he introduced "Relative Gain". Shinskey and others utilized this extensively to determine interaction and develop control approaches that deal with interaction. Multivariable controllers like: DMC+, DOT Products, PCL Connoisseur, and related technologies are particularly good at dealing with such interactions and use some related mathematics to achieve multivariable control of interacting processes.

We can compute the relative gain to understand and to quantify the interaction:
Î»i,j Is the relative gain (dimensionless). Where c is the "Controlled Variable" (Flow in a duct), m is the "Manipulated Variable" (Flow Valve) and Î» is the relative gain. i is the instance of the controlled variable and j is the instance of the manipulated variable. mother=constant means that all other manipulated variables are held constant during the evaluation of the derivative (numerator). cother=constant means that all other controlled variables are perfectly regulated at their setpoint (when evaluating the denominator). Here we consider regulating the Flow in Duct 1 with the valve in duct 1 and thus would compute the gain Î»1,1.
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