Introduction

Design and test engineers for fuel cells, reformers, and fuel processors all need to be able assert accurate flow control of hydrogen and other gasses into their systems. In addition to requiring that these flow rates are stable, accurate, and repeatable, the commercial realities of the evolving fuel cell marketplace demand that these systems, furthermore, are light, compact and highly reliable. These requirements are essentially identical to the needs spacecraft designers faced in the 1960’s when engineering small rocket engines and thrusters. The sometimes very low flow rates of propellants had to be controlled with accurate, robust, and highly reliable flow regulating equipment - and that solution was very often chokes. Even now, forty years later, critical flow venturies, also known as sonic chokes or Laval nozzles, are still the primary device for regulating a multitude of gas flow rates into the chemical laser at the core of the Airborne Laser being built by Boeing at Edwards AFB. This is an application where reliability, accuracy, and compactness are crucial - and chokes are the chosen solution. In contrast, most fuel cell test labs, fuel processing systems, and even fuel cells themselves have adopted a brute-force, expensive solution to flow regulation - the triple-headed combo of control valve+flowmeter+PLC, all tweaked thirty times per second to maintain fixed flow rates over the tiniest of changes in ∆P. Sonic chokes – an elegant solution considered commonplace amongst aerospace designers for forty years – offer substantial advantages.

A Sonic Choke: Flow Rate Independent of Differential Pressure (∆P)

There is nothing new about Sonic Chokes. Bernoulli understood them, their operational characteristics are described in detail in every fluid mechanics textbook, and they have been commercially available for over fifty years. What do they do? What sonic chokes do is very simple: When provided with a fixed inlet pressure, they maintain stable, constant flow rates that are unaffected by downstream pressure or changes in inlet-to-outlet differential pressure. (This is true as long as the outlet pressure is below about 88 - 90% of the inlet pressure - a value referred to as ‘recovery.’) In simpler terms - this means that if you set the inlet pressure to a sonic choke flowing hydrogen at 100 psia, then the discharge pressure can change from 15 psia to 50 psia to 75 psia to 85 psia with absolutely no change in flow rate. See Fig. 2. The only moving part in the entire system is perhaps the diaphragm in the upstream pressure regulator. The flow control elements of the this system - the sonic choke - has no moving parts at all. The flow rate, which can be calibrated to ±1/4%, is now solely a linear function of inlet pressure. Fuel cells or fuel processing systems already have a pressure regulating system. Coupled with a sonic choke, the existing pressure regulating system suddenly becomes transformed into a flow regulating system - and a very compact one at that. Therefore, with just a few extra psig/kPa on the inlet side to ensure that the minimum recovery level of 85-90% is achieved - flow rate into a fuel cell or reformer is fixed, stable, repeatable and unaffected by pressure changes in the stack or fuel processor.

Sonic chokes, which can be machined from any metal, are in use today with gasses with temperatures ranging from -450° F to +1500° F and with pressures ranging from 5 psia to 10,000 psia.
In the conventional, valve+flowmeter+PLC approach, every wisp of pressure fluctuation in the fuel cell or reformer causes a resultant change in ∆P across the valve, resulting in a change in flow rate, which is sensed by the flowmeter, which sends a signal to the valve, which adjusts the flow, which causes a new ∆P, which must be again compensated for, and so on. In a sonic choke, a shock wave at the venturi throat establishes a barrier that prohibits propagation of any downstream perturbations upstream beyond the throat. The inlet flow pattern into the throat - and hence flow rate - is undisturbed and unaffected by ∆P across the choke.

Adjustable Area Sonic Chokes

So far, we have discussed fixed area sonic chokes, where flow rate through a single venturi throat establishes a single curve - a straight line - relating inlet pressure to flow rate. What if we wish to be able to vary the H2 flow rate into a fuel cell, yet still take advantage of the features of a sonic choke where, once we establish the desired flow, it is unaffected by any changes in ∆P or backpressure.

This requirement is met by adjustable area sonic chokes,which have been used to vary flow rates into rocket engines and high energy lasers since the 1970’s. Precision-machined needles are inserted into a venturi throat, and can be accurately repositioned by manual, electrical, or pneumatic means. A calibration then determines the precise flow area corresponding to every valve position along its stroke. This “effective area” (CdA) can then be used to predict flow rate for any gas, at any pressure, at any temperature.

Elimination of Flowmeters: Regulating Flow Rates that Don’t Need to be Measured

It is important to remember that once sonic chokes are being used in a system, flowmeters should be eliminated from the process. This has sometimes been a difficult concept to understand. Sonic chokes - whether fixed or adjustable - can be calibrated traceable to the NIST to ±1/4% or better. Although you may wish to use them as a flow regulating device in your fuel cell, you can also remove them and use them as a calibration reference standard with which you can calibrate the other flowmeters (turbine, laminar flow, hot wire, etc.) in your facility. Do you have ISO-9000? If so - you don’t need to send your flowmeters out for recalibration if you have a calibrated sonic choke in your building: they can be calibrated against the choke. And certainly, you do not need to install a flowmeter downstream of a sonic choke in a reformer to verify performance, since the choke will probably be regulating gas flow rates with a higher precision than the flowmeter can measure.