Rethinking part-load response

Published:  14 July, 2007

britta
A system approach to saving energy — Britta Frank.

Simple pumping less water round a heating system at times of reduced demand can save significant amounts of energy. Britta Frank explains how.

With energy costs continuing to rise, operations, maintenance and energy managers are constantly seeking ways to achieve further savings. Accessing the energy-saving potential of using hydraulics will meet the objectives of increasing efficiency and operating the heating circuit to at least factor four.

Electricity bills can be dramatically cut by utilising the pump and control valve as a component of a hydraulic system and working together effectively. This system approach makes possible savings that would otherwise be unachievable by using conventional methods.

EXTRA PICTURE

Rather than using a 3-way mixing valve to control flow temperature without changing the flow rate, KSB’s approach is to control both temperature and flow so as to enable the energy-saving benefits of variable-speed pumps to be enjoyed.

Mixing ratio

The system approach involves re-defining the mixing ratios of partial volume flows and taking into account the hydraulic correlation of volume flow rate and discharge head. Along with increased performance and overall improved operation for the user, reduction in electricity costs of up to 70% for circulator pumps is possible — irrespective of the type of pump application. Further cost reductions can be achieved through careful planning and commissioning techniques.

Tests have shown that heating systems run under part load conditions for anything up to 95% of operating time.

The system design is set against the lowest possible outside temperature, for example –12°C. The required thermal power is then calculated, and the pump design must meet these specifications (nominal volumetric flow and the respective pump head). With higher outside temperatures, less thermal power is required.

Delivering less thermal power is the main reason for having the control valve, as it sets up the mixing ratio. In conjunction with mass-constant hydraulics, the valve forces the pump to circulate cooler return water at nominal discharge head at the same time. The pump must run under nominal operating conditions to provide the required thermal output.

Variable-speed drives

This explanation then leads to how savings are achieved using a pump with variable-speed drive (VSD).

As thermostatic valves on radiators close, the pump reacts to reduce its rotational speed in accordance with the set pump curve. There is still too much discharge head created due to hydraulic resistance of the heating function. In single-pipe heating systems or those without regulating devices such as thermostatic valves, the VSD cannot reduce the speed of the pump, so no saving is possible.

It is at this stage that heat-control systems can be much more effective by matching the operation of the circulating pump and the control valve in line with each other. Such systems include KSB’s BOA-Systronic, which includes two straight-way control valves, one flow-measurement valve and a control box (see diagram). This system does not require constant-flow circuits fitted with a circulator pump and control valve and replaces conventional arrangements comprising a 3-port valve and 2-way straight-way valve arrangements.

Mixing ratios of the partial volume flows are specifically controlled, which in turn results in total volumetric flow which is adjusted according to the outside temperature, and is therefore determined by the controller actuating signal. At the same time, the required pump head value is communicated to the pump in line with the actual load conditions.

Such a system adjusts the appropriate volumetric flow in the main line automatically and continuously. The conventional balancing valve is therefore not needed and is replaced by the measurement and shut-off valve.

Such systems can be used in many different kinds of applications, whether for new or refurbishment projects, and can be integrated in any control system.

In summary, by re-defining the mixing ratio of partial volumetric flows and taking into account hydraulic correlation of volume flow rate and discharge head, it is possible to adjust the flow rate of a pump and discharge head according to outside temperatures. Significant savings of up to 70% on electricity costs can be made compared with constant-flow systems.

The pump is controlled to provide only the required level of discharge head, thereby avoiding high flow noise. Commissioning costs are reduced as static branch balancing on the circuit’s main feed manifold is an automatic function of the pump. Planning costs are also kept down by removing the need to calculate control valves, while at the same time ensuring a system’s hydraulic characteristics are automatically adjusted which will lead to increased reliability.

By reducing the amount of water that is circulated in a system, significant savings can be made on the electricity costs — and, with less energy usage, the environment will also be a winner.

Britta Frank is market sector manager for KSB Ltd.



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