A redesigned air-operated double diaphragm pump allows end users to reduce their energy costs.
by Paul McGarry, All-Flo Pump Co.
May 29, 2014

The shape of the diaphragm and the outer chamber were also optimized to reduce what is known as dead space. A pump has zero liquid dead space if the diaphragm, when fully extended, conforms 100-percent to the shape of the liquid chamber.

The optimized dead space on the liquid side of the diaphragm resulted in higher suction lift and higher displacement per stroke and yielded improved suction lift and a higher flow rate, respectively. Suction lift on the new design was increased from 13 feet-water to 24 feet-water, and the flow rate was improved from 155 gpm to 190 gpm.

Optimizing dead space on the air side of the diaphragm resulted in the diaphragm moving earlier in the overall cycle, which improved efficiency. As an analogy, think of a balloon connected to a large box and a balloon connected to a smaller box. The balloon connected to the smaller box will inflate sooner once air is applied. In the case of an AODDP, of course, the box cannot be smaller than the shape of the diaphragm. Some limits in the overall design exist. However, optimizing this space contributed to the efficiency improvements, though not as much as modifying the valve-shift point.

The Improved pump curve

The redesign resulted in a pump with dramatically improved energy characteristics (see Figure 3). In Figure 3, the new design is compared to the average AODDP from Figure 2.

Figure 3. Improved efficiency curve

Starting at the same point (the intersection of the 80 psi inlet and 20 psi discharge), the curve shows the operating point of the redesigned pump—125 gpm at a cost of 65 SCFM. The cost of operating the 2-inch redesigned pump during a standard 8-hour day can be:

65 SCFM x 1 hp/4 SCFM = 16.25 hp
16.25 hp x 0.746 kW/hp = 12.12 kW
12.12 kW x $0.07/kWh = $0.85 per hour

The redesigned pump costs its owner $0.85 per hour of operation; $6.78 per day; $3.94 per five-day work week; and $1,765 per year. When compared to the earlier calculations seen in Figure 2, $3,203 annually, the potential exists for dramatically improving the cost of doing business for the end user, especially an end user who operates up to 100 pumps. In this hypothetical scenario, the end user would save about $140,000 annually.

Redesign Benefits

The energy cost of several AODDPs will be analyzed from a different perspective. Instead of deciding to operate the pump at a certain air pressure, consider an application in which the pump must transfer 8,000 gallons of liquid every hour, 8 hours per day for an entire year. In this application, a basic transfer application with a liquid discharge pressure of 20 psi will be assumed.
In this example, 8,000 gallons per hour is equivalent to a flow rate of 133 gpm (8,000 gallons/hour/60 minutes). The pump operates at 133 gpm against a discharge pressure of 20 psi.

The air consumption required to operate this specific pump can be determined using Figure 4. Find 133 gpm on the horizontal axis and move up until it intersects the 20 psi line. The resulting point is the required operating condition to meet the 133 gpm requirement. From this point, the required air operating pressure in psi and the required air consumption in SCFM can be estimated.

Figure 4. Operation at 133 gpm with a liquid discharge pressure of 20 psi.

The point is midway between the two blue lines representing 80 psi and 100 psi, so the required operating pressure is estimated to be 90 psi. Similarly, the point also lies midway between the two red lines, 60 SCFM and 80 SCFM, so the required air consumption is estimated to be 70 SCFM.

Therefore, the operating cost can be estimated as follows:

70 SCFM x 1 hp/4 SCFM = 17.5 hp
17.5 hp x 0.746 kW/hp = 13.06 kW
13.06 kW x $0.07/kWh = $0.91 per hour

In this example, 8,000 gallons per hour can be transferred at a cost of $0.91 per hour. The total annual cost is approximately $1,900.

To understand how these numbers can vary dramatically from one AODDP to another, Table 1 details the variations between the pump manufacturer’s redesigned pump, the original pump and several other AODDPs.

Table 1. The operating requirements of the redesigned AODDP and other pumps

As shown in Table 1, both the operating pressure and required SCFM can vary greatly across these pumps. AODDP users no longer have to accept that the high cost of energy is part of the cost of doing business, opting to either raise prices or accept a lower profit.