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Staying in control of turndown

Demand control is a key concept for optimising the performance of building services plant but it’s essential not to lose sight of the fundamental engineering principles. Bernard Dawson illustrates this point in relation to burner and boiler turndown

It is now a well-established principle that specifying and installing energy-efficient plant is only part of the equation when it comes to reducing energy consumption and carbon emissions. Controlling that plant efficiently is also critical so, in turn, the plant needs to have the inherent controllability to make that possible. In this way the operation of the plant can be aligned to the requirements of the building.

This principle is clearly illustrated with reference to heating systems, where there may be a requirement for full heat output at the beginning of the day to warm the building, or top up hot water, but that heat demand may then reduce as the day progresses. As the thermal insulation of buildings has improved in recent years there are now long periods where the space heating requires considerably less than full output from the boilers, so the ability to turn down the boiler and the burner has become increasingly important.

Resisting temptation

Consequently, it is often tempting to opt for the largest possible turndown ratio as this would seem to offer the optimum energy performance. However, it needs to be borne in mind that there are limits to the level of turndown that is possible. This is the result of fundamental engineering requirements that cannot, or should not, be ignored.

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. In many commercial and industrial buildings, though, the heating plant 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.

For example, we have seen turndown ratios of 8:1 or even 10:1 specified for a burner that is to be used with an LTHW boiler that is limited to a turndown of 2:1 or 3:1. If the burner was actually set to the specified turndown this could be catastrophic. 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. Consequently, it is essential that the burners and boilers are properly matched for each project.

With commercial and industrial high temperature hot water (HTHW) and steam boilers the situation is different again. These have lower minimum operating outputs than LTHW boilers, so 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.

Smooth modulation

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. Such systems may use an electro-mechanical cam with a single servomotor controlling the air and fuel flow rates via a mechanism of cams and linkages. Or they may use an electronic cam control system with separate servomotors for air and fuel control.

A potential problem with electro-mechanical cams is mechanical wear can lead to ‘slippage’ that results in lack of precision, as well as reduced efficiency and performance. In contrast, mechanical wear and tear is virtually eliminated with electronic cam control systems, so that burner efficiency remains consistent.

For further efficiency improvements,electronic cam burner control can be combined with a variable speed drive (VSD) and oxygen trim. Rather than adjusting an air damper to reduce air flow, VSD controls the fan motor speed in relation to the burner operation, potentially resulting in significant electrical energy savings and reduction in noise emission as the fan motor speed is reduced.

Oxygen trim control requires an oxygen sensor to be placed in the flue system to monitor excess air levels. This detects any changes in the parameters that were set at the time of commissioning, automatically reconfiguring the settings of the electronic servomotors to compensate, thereby ensuring optimum combustion and emissions at all times.

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.

// The author is the technical director of Riello //

1 July 2014

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