Energy Savings with the Correct Duty Point


Written by:
Jens-Uwe Vogel, VSX-Vogel Software GmbH
 

With continuously increasing energy costs, pump manufacturers must provide energy efficient solutions for fluid transfer. Besides hydraulically optimizing current technology, manufacturers are launching a number of new, highly efficient electrical drives and powerful control systems.

With the focus on optimizing current and new technology, the savings that can be generated with dimensioning a pipeline and specifying the right duty point for a pump in the planning phase are often underestimated.

Pump Power Requirements

A sample calculation will help illustrate the savings potential. A pump's duty point is usually defined by the required flow and the related pressure. The latter is often indicated as head. For turbulent flows of Newtonian fluids, the correlation between pressure loss and head follows a quadratic relationship. Formula 1 applies to a closed pipe system with a static pressure of zero:

 H ~ Q2

H = Head

F = Flow

Formula 1. Dependency of head and flow in closed pipe system.

The power requirement of a centrifugal pump is defined in Formula 2.

P1 = Q • H • p • g 

ηges

P1 =Power input

p = Density of the medium

g = Gravitational acceleration

ηges = Total efficiency of the unit

Formula 2. Power requirement of a centrifugal pump

Formula 2 implies a flow increase increases the power requirement by a factor of three. In other words, over-sizing the flow by 5 percent results in an increase of the energy demand by more than 15 percent (at constant efficiency). An increased flow of 10 percent raises the energy consumption to 30 percent.

Conversely, the influence of flow on the completion of the flow task depends in a high degree on the application. A heating system, for example, will reach more than 80 percent of its heating power if only half of the flow is provided.  In contrast, an undersized pump in the sewage or process technology can cause fatal consequences.

 

Figure 1. Increase of power requirement follows increase of flow in closed systems.

Planning Tools 

Pump manufacturers must support users in the planning phase to guarantee effective pump use. Support can be provided via consultation during the selection process. Various pump manufacturers also offer planning tools in the form of software, particularly web based applications. While complex pipe systems require extensive calculations, applications for unbranched pipes are also available. Numerous pump manufacturers are offering free access to calculation software in combination with a pump selection program.

Web-based software in particular offers a centralized updating process directly on the server and avoids the need for installations on local PCs. In web-based software, the required flow rate has to be determined to size the pipeline and calculate the pressure loss in the second step.

Flow Rate Determination

Each centrifugal pump application has a different calculation for required flow rate. For heating pumps, for example, the required flow results from the heat requirement calculation, but the flow rate in industrial applications depends on the particular transport task and the process parameters. In many applications, international engineering standards are available (which are typically integrated in pipe calculation software programs).

Domestic Wastewater 

According to EN 12056, the flow rate for domestic wastewater resulting from drainage can be calculated by adding the run-off coefficients of several sanitary fittings, depending on the building type (see Formula 3).

Qww = K · √ΣDU

Qww = Wastewater drainage

K = Drainage figure according to type of building

DU = Outflow value of sanitary fittings

Formula 3. Calculation of wastewater drainage according to EN 12056

Pipe calculation software programs integrate this formula and the necessary coefficients (see Figure 2).

Calculation of flow rate for domestic drainage water according to DIN EN 12056

Figure 2. Calculation of flow rate for domestic drainage water according to DIN EN 12056

Alternatively, the flow rate can be determined by the number of inhabitants of a settlement area and the specific peak flow.

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See also:

Upstream Pumping Solutions

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