Pump Switching and Control

The switching configuration and control strategy of network pumps determines supply reliability and energy efficiency of a thermal network. Parallel configurations are suited for networks with high flow rates and flat characteristic curves, while series configurations are advantageous for high delivery heads with low flow rates. Network-based differential pressure control at the worst point — rather than at the pump itself — minimises pump electricity consumption and ensures that the required minimum differential pressure of typically 0.4—1.0 bar is available at every transfer station.

Pump Configurations

Parallel Configuration

Parallel pump configuration is advantageous in thermal networks with high volume flow rates and relatively low delivery head (flat network characteristic curve). Features:

• The pumps arranged in parallel should be of the same pump type
• Load-dependent control: at low load, a single pump operates alone; at higher load, both pumps are operated at synchronous speed
• One of the pumps serves as a standby pump (redundancy)

The delivery head remains constant in parallel configuration, while the volume flow rates add up.

Series Configuration

Series configuration is advantageous in thermal networks with high delivery head and relatively low volume flow rate (steep network characteristic curve):

• A bypass with automatic shut-off damper is arranged in parallel to each pump
• At low load, one pump operates alone; at higher load, both pumps operate together
• The volume flow rates remain the same, while the delivery heads add up

A booster pump in the periphery of the network is also operated in series with the main pump.

Redundancy and Expansion

Since the pumps are of central importance for the operation of a thermal network, redundancy of network pumps is recommended. Common arrangements:

• 2 x 100%: Simplest solution, but cost-intensive for large networks
• 3 x 50%: Good compromise between availability and cost
• 4 x 33%: For large networks, as investment costs of smaller pumps decrease disproportionately

During the initial operating years of a network still under development, only a fraction of the planned pumping capacity is typically required. It is advisable to initially install smaller pumps and replace them with larger ones as needed.

Low-Load and Summer Pump

The minimum speed of circulation pumps is approximately 25% of the maximum speed. For thermal networks with a large difference between maximum and minimum volume flow rate, it can be beneficial to provide a separate summer pump:

• Designed for low-load operation (< 20% of winter demand)
• Higher efficiency in the low load range
• Pump characteristic curve must overlap with that of the main pumps

Pump Control

Constant Pressure Control

With constant pressure control, the differential pressure across the pump is kept constant at varying volume flow rates:

• The operating point follows the constant pump control curve horizontally in the part-load range
• Simple control, but less efficient during part-load operation
• Suitable for smaller networks and systems with constant hydraulic resistance

Proportional Pressure Control

With proportional pressure control, the differential pressure decreases with decreasing volume flow rate:

• The operating point follows the proportional pump control curve in a declining manner
• More efficient than constant pressure, as less energy is consumed during part load
• Suitable for most thermal networks

Differential Pressure Control in the Network

For optimal control, the differential pressure is measured not at the pump itself, but at one or more reference measurement points in the network. These are typically located at the network critical point — the location with the lowest differential pressure.

The pumps are controlled such that the minimum required differential pressure (typically 0.4 — 1.0 bar) between supply and return is available at every transfer station.

Advantages of network-based differential pressure control:

• Optimal supply of all customers
• Minimum pump energy demand
• Prevention of over- or under-supply

Control with Multiple Feed-In Points

In thermal networks with multiple heat substations at different locations, the pump groups must be coordinated:

• One feed-in point as master: Takes over pressure control
• Further feed-in points as slave: Deliver a defined heat output or follow a characteristic map specification
• The algorithm must ensure that no flow stagnation occurs between the feed-in points

Pump Energy Demand

The annual energy demand of the pumps is determined from the mean values of the resulting flow rates and delivery pressures derived from the annual duration curve of the network. As a rough approximation:

where $P_P$ is the nominal pump power and $t_{\text{FLH}}$ is the full-load hours of the customers.

From experience, the annual energy demand of the pumps with optimal design is between 0.5% and 1.0% of the distributed heat energy. Values significantly above this indicate oversizing or suboptimal control.

Conclusion

The right combination of pump configuration and control strategy is crucial for the economic viability and reliability of a thermal network. Modern variable-speed pumps with network-based differential pressure control offer the greatest savings potential. The choice between parallel and series configuration depends on the network characteristic curve, while the redundancy strategy ensures supply reliability.

Further reading: Pump Sizing describes the dimensioning of volume flow rate and delivery head as the basis for pump selection, Network Control covers the overarching control strategies in interaction with pump control, and Network Operating Modes explains how sliding and constant operating modes affect the requirements for pump control.

References and Standards

• DIN EN 16297 — Pumps — Variable speed pumps — Specific requirements and tests
• VDI 2073 Part 1 — Hydraulics of water-based systems — Fundamentals
• AGFW FW 515 — Control and regulation in district heating networks