The requirements of regulations and the demands of building operators are changing the way we look at air handling units, as Dean Ward of Walter Meier (Climate UK) explains.
AS WITH all building services, the design of ahu systems is now subject to many more influences and pressures than was the case just a few years ago.

Consequently, there is a greater onus on system designers and the suppliers of the products they specify to address every detail of the system, with particular regard to energy performance and other factors contributing to the cost of ownership.

At the same time, end-user clients are more aware of their responsibilities to maintain high standards of indoor air quality and the associated maintenance and hygiene procedures required to achieve that. And, of course, in the current economic climate, customers are even keener to achieve best value.

The Energy Performance of Buildings Directive places emphasis on both energy efficient design and the efficiency of ongoing performance. This inevitably sets higher standards for air handling appliances, so not only do the units need to offer more efficient performance they also need to be easier to maintain, so that life cycle costs are minimised.

Similarly, the requirements of increasingly stringent hygiene standards also have an impact and need to be considered in the design of ahus.

Back to the drawing board

All of this means that ahu manufacturers are having to rethink their designs and come up with innovative new ideas. For example, we should look at the basic dimensions of the ahu, a fundamental consideration as it impinges on just about every other aspect of the unit's design.

Analysis of a wide range of projects has shown that the internationally accepted structural dimension of 610mm x 610mm for a standard bag filter is an ideal basis for the construction of a series of ahus. So, as a bag filter consists of six bags, it is logical to take this division (namely 610/6 = 101.67mm) as the basis for all lengths, widths and heights - so the cross-section of the unit can be fully utilised with no dead space. This is in contrast to many traditional designs where it is common to use a frame cover to hide parts that did not have the same dimensions as the filter.

The result of this approach is it then becomes easy to determine the position of fixing points and axes for components and fittings. Plus, because the same bottom, top and side panels, and the frame, can be used for different sizes or functional boxes there is more flexibility right up to the end of the production process. Even last-minute, on-site, design changes become easier to accommodate.

Multi-functional design

An additional benefit is the employment of a multi-functional design, rather than sticking to the traditional space-wasting approach of housing each function (filter, heater, fan, etc) in separate boxes. Instead, several functions can be housed together. Typical examples include combinations of filter/heater, heater/ventilation fan or filter/ heater/ventilation fan all in the same box.

Furthermore, increased flexibility makes it easier to incorporate the most appropriate heat recovery technologies, such as plate heat exchangers, run around coils and thermal heat wheels.
At the same time, the adaptability of the multi-functional design ensures that specifiers can easily choose between standard or high performance components to tailor energy performance to their requirements and budgets.

As well as enabling the units to be produced at lower cost, this reduces the overall dimensions of the ahu, important in refurbishments where higher capacity plant is required to fit into the space that was occupied by the previous plant. It also facilitates delivery to site and assembly.

In fact, with more organisations choosing to refurbish their existing buildings rather than move to new premises, on-site assembly is a critical factor for many projects. Here, features such as push-fit frames and durable plastic sealing strips instead of silicon can introduce time savings on installation.

With so many factors influencing the design of the built environment it's clear that there is no place for complacency in the design of building services products.

A fresh approach to the design of ahus shows what can be achieved across the board through a willingness to ditch tradition and re-apply proven engineering principles in a different way.

Walter Meier (Climate UK)
0871 663 0664