Simple Steps to Understand the Air Consumption Curve of Pneumatic Diaphragm Pumps

"Given a flow rate of 100 LPM, what would be the approximate air consumption?"

"After knowing the air consumption, how do I choose the appropriate horsepower (HP) for the air compressor?"

Are these questions something that often puzzles you?

Let’s dive in and understand what air consumption is all about!

A pneumatic diaphragm pump operates by using compressed air as a power source, with a diaphragm that moves back and forth to pump fluids. In the performance curve of an air-operated double diaphragm (AODD) pump, besides flow rate and vertical head, air consumption is also a crucial performance parameter. The ability of the air compressor to meet the pump’s air consumption needs directly affects the pump’s performance stability. If the air compressor cannot supply more air than the pump’s consumption, the pump’s efficiency will drop. Below, we’ll guide you through how to read the air consumption from the performance curve.

Understanding the Structure of the Performance Curve:

The performance curve is displayed as an XY graph, where the horizontal axis (X-axis) represents the flow rate, measured in liters per minute (LPM) or gallons per minute (GPM).

The vertical axis (Y-axis) indicates either the vertical head or working pressure (negative pressure) at the pipeline’s end. The vertical head is measured in meters or feet, while the working pressure is in bars or psi.

In the chart, solid lines represent different air pressure driving forces, measured in bars, while the dashed lines show the air consumption under specific conditions, measured in liters per minute (LPM).

Setting Known Conditions:

By knowing two out of the three parameters—vertical head (working pressure), input air pressure, and flow rate—you can estimate the air consumption range for a specific pump model.

Finding the Corresponding Air Consumption Range Based on Set Conditions:

a.Known Vertical Head (Working Pressure) and Input Air Pressure: Find the vertical head (working pressure) position on the Y-axis, then extend horizontally to intersect with the chosen air pressure curve. The intersection point will give you the corresponding air consumption range.

b.Known Vertical Head (Working Pressure) and Flow Rate: Locate the vertical head (working pressure) on the Y-axis and the flow rate on the X-axis. Extend both points towards each other until they intersect, and from that point, you can determine the air consumption range.

c.Known Input Air Pressure and Flow Rate: Identify the input air pressure curve, then locate the flow rate on the X-axis. Extend the flow rate point vertically along the Y-axis until it intersects with the input air pressure curve. The intersection will reveal the air consumption range.

Cross-Referencing:

The performance curve also provides corresponding flow rate values, allowing you to cross-reference and find the most efficient operating conditions.

Determining the Appropriate Pump Size:

To ensure the pneumatic diaphragm pump operates efficiently and has a longer lifespan, it’s generally recommended to use a stable operating pressure of 1.5 to 3.5 bars. Lower pressure reduces friction between the seal ring and its counterpart, prolonging pump life and durability.

For example: If the required flow rate is 40 LPM, and the maximum flow rate for a 1/2-inch pump is 48 LPM while the maximum flow rate for a 3/4-inch pump is 85 LPM, you should choose the 3/4-inch diaphragm pump. Although the 1/2-inch pump can meet the required flow rate, it would require higher operating pressure, thus affecting the pump’s efficiency and lifespan. Therefore, selecting a larger diaphragm pump is recommended.

Note: The performance curve is measured under conditions involving water, low negative pressure, and minimal pipeline resistance. Real-world scenarios require consideration of factors such as fluid density, viscosity, pipeline resistance, diameter, valves, distance from the air compressor, and air compressor specifications, all of which will affect the actual flow rate and pressure values at the outlet.

Example 1: What is the air consumption at a working pressure of 4 bars and a flow rate of 56 LPM?

Locate the curve for 4 bar air pressure.
On the X-axis, find the point corresponding to a flow rate of 56 LPM.
Move vertically from the 56 LPM point along the Y-axis until it intersects the 4 bar curve.
At the intersection point, read the air consumption range on the dashed line. The air consumption is between 500 and 750 LPM.

*By following these steps, you can accurately read the air consumption for specific operating conditions from the performance curve. Understanding this data helps in optimizing pump operation and calculating operating costs and energy consumption.

Example 2: How do I choose the correct horsepower (HP) for the air compressor? (Using the data from Example 1)

Using the method from Example 1, the maximum air consumption value is found (750 LPM).
Apply the data to the air compressor horsepower (HP) calculation formula: HP = (PxQ) / (1714xη).

P = test rated pressure in psi (1 bar ≈ 14.504 psi).

Q = air consumption (Scfm) (100 LPM ≈ 3.531 Scfm).

η = air compressor efficiency constant, usually between 0.7 and 0.9 (use 0.8 as the average).

The test rated pressure for air consumption is 8 bars. Plugging the values into the formula:

HP = (116.032 x 26.4825) / (1714 x 0.8) = 3072.81744 / 1371.2 ≈ 2.241 HP.

Considering potential overheating during continuous operation, the value is multiplied by 1.3, resulting in ≈ 2.9133 HP. Therefore, it’s recommended to choose an air compressor with at least 3 HP.

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