The advancing designs of fuel cells and fuel processors require reliable, compact, and accurate means of precisely regulating hydrogen and other gas flow rates The brute-force solution of "valve+flowmeter+PLC" is neither simple, compact, nor reliable. Sonic chokes enable hydrogen flow rates to be regulated to ±1/4% solely by controlling upstream pressure.  Downstream pressure changes in the stack or processor (up to 85% of inlet pressure) have no effect on gas flow rates.  This solution, considered commonplace for forty years in the aerospace community, appears forgotten by the fuel cell industry.

Adjustable area sonic chokes can be used when hydrogen or gas flow rates must be varied precisely over a ten-or-fifteen-to-one range with flow rates still independent of ∆P.  When test stands require flow ranges of 200:1 or more, a set of fixed sonic chokes can be employed with on/off valves to elegantly and accurately accommodate this requirement.

1.     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. See Fig. 1.

2.     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.

3.     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.  See Fig. 3.

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.  Typical range for adjustable chokes is about 10:1 for small adjustable chokes (below a 1/2" line size) and 15:1 to 20:1 for larger valves.

4. 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.

4.     Regulating Gas Flows over a 250:1 Range: Digital Valves

In a lab setting, it is often desirable to have a very accurate system for regulating H2 or other gas flow rates over a very broad range.  This is a tall order for most conventional approaches, but fortunately, the aerospace industry once again solved this dilemma forty years ago.  The same solution is now perfect for use by the today's fuel cell designers.

The answer is called a digital valve and is very simple: a set of carefully sized sonic chokes are installed in parallel, all fed by one common inlet line and all discharging into a common exit manifold.  At the inlet to each valve is a solenoid on/off valve.  The chokes are sized in a binary pattern flowing, at a given inlet pressure, x, 2x, 4x, 8x, 16x, 32 x, 64x, 128x.  In this example, any flow rate between 1x and 255x can be selected by opening a combination of solenoid valves.  Unlike an adjustable area sonic choke, which permits almost infinite resolution, a digital valves can only select flows defines by discreet jumps of flow rate 'x', defined as the smallest choke in the set. (If one assumes a minimum machined orifice in a stainless sonic choke to be about 0.012 inches = 0.30 mm, then x at: a)  inlet pressure = 50 psia is about 0,2 slpm or 0.12 lbs/hr or b) inlet pressure = 25 psia is about 0,1 slpm or 0.06 lbs/hr ) The geometry of a digital valve can range from the simple and inexpensive for use in a lab (eight cheap solenoid valves coupled to eight sonic chokes with clunky tubing and fittings connecting them together)  to a tightly bundled, precision engineered package with the chokes and valves connected by passageways machined into a common housing.

Lawrence S. Fox

wee

Fox Valve Development Corp.

Hamilton Business Park,  Dover, NJ 07801

 Tel: 973 .328.1011   Fax: 973.328.3651  Email - larry@foxvalve.com

Curriculum Vitae:

1978 - Present 

            President, Fox Valve Development Corp, Dover, NJ

Fox Valve designs and manufactures venturi flow controls and venturi ejectors for high performance aerospace, industrial, and research applications.  Current projects include: supply of venturi flow controls for airborne laser; venturi ejectors for hydrogen recirculation for various fuel cell and reformer manufacturers, and steam ejectors for larch rocket engine test stands.

1973 - 75, '76 - '77  Princeton University, Princeton NJ USA

            . Received BSE in Aerospace & Mechanical Engineering, l977

            . Independent research with Dr. Thomas S Kuhn,  on History of Thermodynamics

1975 - 76  University of Southampton, Southampton, England 

         Visiting Student, College of Aeronautics & Astronautics