Optimising turndown

Published:  25 February, 2014

Aligning plant operation to demand is clearly sensible but it needs to take account of fundamental engineering principles. Bernard Dawson* explains how this principle applies to burner and boiler turndown.

The term ‘demand control’ is one that is being used increasingly in relation to reducing energy consumption and carbon emissions. It is a well-established, and highly sensible, principle and one that is becoming increasingly important as control technologies become more sophisticated and the environmental performance of buildings improves.

An obvious example of this is reducing the output of heating plant (turndown) when demands for heat are low and ramping up output when demand increases. Where the heating is provided by boilers, this is achieved by using a modulating (or high/low) burner to vary the heat input to the boiler. In implementing such measures, though, it is important not to lose sight of the underlying engineering principles of the plant in question as these may impose practical constraints on the level of turndown that can be achieved. All boilers will have a minimum working capacity, determined by the water flow rate and the difference between the flow and return water temperatures. The temperature differential between flow and return temperatures influences whether condensation from the exhaust flue gases will occur in the heat exchangers and flue system.

Condensing boilers are clearly designed to allow for and encourage condensation, as this enables extra latent heat to be recovered from the flue gases and improves energy efficiency. A high burner turndown is therefore beneficial for condensing boilers. However, the heating plant in many industrial premises comprises low temperature hot water (LTHW) boilers, where the situation is very different. Many LTHW boilers are limited to a turndown of just 2:1 or 3:1 to ensure that the boiler exhaust gas temperatures are maintained above the 130-140°C required to avoid condensation. So this is something that needs to be borne in mind when specifying burners for use with LTHW boilers, but our experience suggests this principle isn’t always properly understood.

Clearly, when specifying burners it is tempting to opt for the largest possible turndown ratio, as this would seem to offer the optimum energy performance. As a result we have seen turndown ratios of 8:1 or even 10:1 specified for a burner that is to be used with a low temperature hot water (LTHW) boiler that is limited to a turndown of 2:1 or 3:1. If the burner was actually set to the specified turndown, then at lower firing rates the boiler exhaust gas temperature would not be high enough to avoid condensation, potentially resulting in serious damage to the heat exchangers and flue system. This illustrates the importance of seeking appropriate advice on the matching of burners and boilers to ensure that such damage is avoided.

With commercial and industrial high temperature hot water (HTHW) and steam boilers the situation is different again. Compared to LTHW boilers, they have lower minimum operating outputsso that higher burner turndown is possible. However, there are very few cases where a burner turndown of 8:1 or 10:1 is justified or beneficial. Indeed, it may even result in damage that increases maintenance costs and reduces the life of the plant. When specifying modulating burners it is also important to ensure that modulation is consistent and responds smoothly to changes throughout the life of the burner. Modulating control uses a servomotor to control the volume of air and gas required for correct combustion, with either an electro-mechanical cam or an electronic cam control system.

A potential problem with electro-mechanical cams is that mechanical wear can result in ‘slippage’ that results in lack of precision and reduced efficiency and performance. In contrast, mechanical wear and tear is virtually eliminated with electronic cam control systems, so that burner efficiency remains consistent. Electronic cam burner control can also be combined with a variable speed drive (VSD) and oxygen trim to achieve further efficiencies.

There are, therefore, two key points to be aware of. The first is that the relationship between a burner and boiler is more complex than many people realise. The second is that burners can vary in how they achieve modulation and this can have a significant impact on efficiency and performance. So it makes a lot of sense to engage with companies that can offer practical advice and guide you to the best solution.

For further information please visit: www.rielloburners.co.uk

* Bernard Dawson, technical director of Riello

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