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December 2005
Purposes of Measurement of Real Mass Air Flow
 

The function of sonic nozzles systems is to measure real mass airflow. This system is particularly useful in the measurement of the mass flow rate delivered by air compressors because:

• The system demands to expel to the atmosphere the gas that is being measured. From this point of view, air has two advantages: it is non-contaminant and cheap.

• The possibility of expelling to the atmosphere the gas and using the instrumentation of the compressor(manometer and thermometer) allows that the system be extremely simple and accurate. In addition, it doesn't have moving parts and the calculations are based directly on physical formulae and not on empirical approaches.

• The system of sonic nozzles allows the building of the real compressor's operation curve, this is, a curve that relates the mass flow delivered versus the pressure in the tank. If this real curve is known, and the load curve of the system to be driven by the compressor is known too (for example, a DTH hammer), it is possible to predict very accurately the point of equilibrium that will be obtained in actual operation.

Although the manufacturer delivers the characteristics of the compressor at the moment of its purchase(maximum pressure -maximum mass flow rate), these values are only referential because they are measured in standard conditions, that aren't necessarily the real conditions in which the compressor will operate. Actual conditions can reduce(or in exceptional cases increase) its mass flow rate for different reasons like:

• Altitude above sea level in which the compressor is operating: Density of atmospheric air in the suction is extremely important. A less dense air will reduce the delivered mass flow in a percentage that can be important with regards to the nominal flow.
• Mechanical conditions of the equipment: a deficient maintenance of the engine or of the compressor or the wear of its parts can notably reduce the mass flow delivered by it.
• Leaks in the feed system in the case of air networks.
• Pressure losses due to changes in the flow path and flow area.

The effects of the local atmospheric density are easily accounted for, due to the fact that any notorious discrepancy between this estimate and measurements obtained using the system of sonic nozzles can only be explained by the four factors in the previous list.
The measuring in different points of the air circuit can help detect the anomalies in the system.

 
System Operation
The basic system includes the following parts:

1. Set of sonic nozzles.
2. Straight Adaptor for the nozzles.
3. High pressure manometer.
4. Pre-Adaptor, that is used before the adaptor to make the connection with the particular thread of the system.

The nozzles (N°1 in the figure) are interchangeable and generally , between five and seven of the total available sizes are required to determine the compressor's characteristic curve.

 
The following figure shows the system setup:

 

The selection of the set to use is determined by three factors:

• Compressor's capacity (nominal pressure and nominal mass flow rate)
• Altitude above sea level
• Temperature at the moment of carrying out the test (this isn't essential).

The effect of altitude over compressor's mass flow rate is shown in the following diagram:

 

Mass Flow Losses due to Altitude Above Sea Level

To use this diagram it is required to know the altitude at which the compressor is operating and the atmospheric temperature. With these two values it is possible to determine a point within the diagram, after which, the correction line nearest to it is selected. This line will determine the correction factor that multiplies the nominal mass flow rate in order to obtain an approximated value of the real one.

If the ambient temperature isn't known, the same diagram can be used, where the straight line AE is a good way of estimating the atmospheric temperature only knowing the altitude of the place. Therefore, by entering with altitude and intersecting this straight line a point is obtained, after which, seeking the nearest correction straight line will result in an alternate correction coefficient.

Once the correction factor is determined, the nominal mass flow rate of the equipment is multiplied by it. This allows the range of nozzles for the compressor to be determined in a more precise way. In the following diagram the nozzles' selection chart is shown:

 

Nozzles Range

 

EXAMPLE:
if we have a screw compressor with nominal mass flow rate of 900 SCFM, the flow corrected for an altitude of 700 meters and an atmospheric temperature of 24°C (correction factor =0.9 ==>900*0.9 =810), is 810 SCFM. If the compressor was built to generate a maximum pressure of 350psig and a minimum of 120psig, the nozzles that we will have to use will be in the range of 10.5 mm to 16mm.
 
Once the range of nozzles to use is determined, they are tested individually. For that, they are simply connected to the adaptor firmly and the compressor is allowed to operate freely (There must not be significant obstructions in the path between the accumulator and the nozzle), giving time for readout in the pressure gauge of the adaptor or for the tank to stabilize (These two pressures differ in a value that increases with the distance between the tank and the nozzle. If this distance is higher than 5m, the pressure in the tank must not be used). Once the pressure is stabilized, it is entered into the particular nozzle chart and the flow is determined. In the following figure the diagram for a nozzle of 11.0 mm of diameter in the discharge is shown.
     
   
The different straight lines represent different temperatures in the accumulator (If the temperature value in the accumulator is not available, a value about of 70°C can be assumed without introducing an important error)
 
Example:
After the selected nozzles have been tested, all the measured points (P,Q) must be drafted and linked with lines in a blank grid. The next chart illustrates the results of a test carried out in the Hammer Test Bench's compressor at Drillco Tools' facilities (Compressor of 350 psig/900SCFM with less than 100 hours of use at the moment of being tested)
 
Q(SCFM) v/s Ptank
 
The blue curve corresponds to the real flow delivered by the compressor, determined in a precise way based on several operation parameters and its mechanical and geometrical characteristics.

The red curve corresponds to the solution of fluid mechanics equations in an exact manner using an iterative technique based on the pressure in the tank, the temperature in the tank and the diameter of the nozzle as boundary conditions. And the green curve represents an estimation of the mass flow rate based on the nozzles' graphs, which in turn base themselves on the solution of a simplified set of fluid mechanics equations.