What Is The Formula For Mass Using Density And Volume Mixing of Air Streams at Different Pressures – A Graphical Approach

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Mixing of Air Streams at Different Pressures – A Graphical Approach

Introduction

Air flow mixing is a common scenario in many engineering applications. For the purposes of this discussion, let us have two incoming streams, A and B, and a resulting (output) stream, C. Normally, we know the conditions of A and B, and the requirement would be to determine the conditions of C. , sometimes the requirement would be to determine B knowing A and C.

Air properties are important when analyzing such situations:

Temperature – measurable

Pressure – measurable

Density (or specific volume) – not measurable

Moisture content (specific humidity, relative humidity, saturation percentage, vapor pressure are also shown) – relative humidity can be measured.

Background of the problem

One basic assumption is that energy losses due to flow behavior can be neglected, leading to the assumption that the energy (specific enthalpy * mass) of the output stream is equal to the sum of the energies of the input streams. It is also assumed that no mass loss occurs during mixing.

Direct measurement of enthalpies is not feasible. Therefore, they must be calculated using psychrometric relationships. Mass flow rates can be derived from volume flow rates and density. Density, which is not directly measurable, must be calculated from pressure, temperature and relative humidity. Thus, the parameters to be measured are pressure, temperature, and relative humidity, and the parameters to be determined are enthalpy and mass flow rate. It is obvious that psychrometric ratios must be used to solve such problems.

Variations on the problem

1. A, B and C are under the same pressure (the temperatures of A and B may be the same or different)

2. A and B are under the same pressure, but C is under different pressure (same or different temperatures)

3. A, B and C are all under different pressures. (same or different temperatures)

General solution method

Because Type 1 problems, knowing the temperature and relative humidity of the incoming streams (A & B), it is possible to mark the points (say P & Q) that meet the conditions of A & B. Draw a line between P & Q and find a point X such that the distances between X and P and X and Q correspond to the mass flow rates of B & A. The psychrometric values ​​at X would then indicate the condition of the output stream. This type of situation is easily represented on a standard psychrometric chart, since the pressures of all three streams are the same.

Because Type 2 for problems where the input streams are at the same pressure, the procedure is identical to that of type 1 up to the point X finding stage. However, since the output stream is at a different pressure, the psychrometric characteristics at X, as shown in this psychrometric diagram, would not be correct for the pressure of stream C. In this case, the solution would be to place the C pressure diagram on top of the previous diagram and read the values ​​from the new diagram.

Because Type 3 problems, we would need 3 charts, one for each of the three pressures. The same approach could be used because enthalpy is independent of pressure.

Conclusion

A graphical solution to flow mixing problems would be much easier if the psychrometric ratios at all three pressures were available from the same chart. Although superimposing complete psychrometric charts on top of other charts is so confusing as to make the scheme impractical, it is possible to draw only the relevant lines for each of the two or three pressures on the same chart and use the dynamic reading mechanism to display it. the set of values ​​of the psychrometric properties of the three relevant streams.

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