Handling air sustainably

The days when building services engineers could simply concern themselves with meeting the necessary criteria to maintain comfort conditions are long gone. Quite rightly, other issues such as energy efficiency and sustainability are now considered just as important.

Energy efficiency, of course, has the highest profile but increasingly more emphasis is being put on the environmental impact of buildings and their services throughout the whole life cycle. In this respect, it makes a lot of sense to consider both the commercial and environmental impact of buying new plant when existing plant can be refurbished. This is something that applies to all items of plant, but air handling units (AHUs) illustrate the principle very well.

For example, even a 20-30 year old air handling unit can be refurbished very effectively – not just restoring it to its former glory but usually improving its performance, as is discussed in more detail below. Straight away this helps to minimise environmental impact by reducing consumption of materials and their associated embodied carbon. Thus, refurbishing AHUs can make a significant contribution to reducing the carbon footprint of a refurbishment project; whether the project involves just the AHUs is more broadranging in its scope.

Cost savings

Furthermore, the cost savings can be substantial and, depending on the extent of works required, we have found that refurbishment can cost between 20% and 50% less than the replacement option.

To use a common example, we can compare the costs related to a 2m x 2m x 6m AHU with an airflow of 7m3/s. It incorporates a volume control damper, bare tube frost coil (LPHW), panel filters to F4 efficiency, bag filters to G6, CHW cooling coil, LPHW reheat coil, fan and motors. Supply and installation of such a unit, based on good access, would cost in the order of £12,500. In contrast, to refurbish a similar unit, replacing all internal components, treating any corrosion, replacing damaged external panels, recommissioning and providing a 12 month warranty would cost £9,125 – a saving of 27%.

As mentioned earlier, the overall life cycle costs are also an important consideration and in this respect there are additional benefits. For instance, the newer components within a recommissioned system will reduce maintenance costs while also increasing energy efficiency, so the cost of ownership is lowered and the payback is faster. The operation and maintenance manuals will also be updated or, as with many older units, supplied for the first time in the life of the plant.

Choosing the best option

Recent initiatives, most notably the Carbon Reduction Commitment Energy Efficiency Scheme, Energy Performance Certificates and Display Energy Certificates, have certainly focused the minds of the end users on the performance of their plant. Building operators are coming under increasing pressure to improve their buildings’ energy performance, with a view to reducing the costs of energy, including carbon allowances in many cases, as well as possibly improving positioning in carbon emissions league tables.

Consequently, it’s hardly surprising that individual items of energy-consuming plant are coming under closer scrutiny and building services engineers are being asked to identify areas for improvement and propose suitable solutions.

In the case of AHUs, general wear and tear through the life of the plant will lead to deterioration of components and casework. Very often, though, the units continue to perform rather than breaking down completely, so the gradual fall-off in performance can go unnoticed for many years. This is where expert input can be of great value to the end client.

In such cases, the inherent flexibility of the refurbishment approach is a real advantage because it focuses just on the aspects of performance that need to improve, making it a more elegant and precise solution. For example, age deterioration will affect some components more than others and moving parts in fans and motors will clearly experience more wear and tear than static components. Cooling coils may also have suffered from the effects of corrosive oxidation, caused by condensate mixing with airborne pollutants. So there may also be geographic variation in line with air pollution levels.

As a result, the extent of the refurbishment can be tailored very precisely to the condition of individual items, so an initial inspection of the plant to identify opportunities for refurbishment is the first stage in any such project. Here, it often makes sense to bring in specialists in this field to carry out the inspection, produce a method statement, risk assessment and detailed costings.

This is also an ideal opportunity to identify ways of improving both energy and comfort performance, as it’s almost certain that the building’s usage will have changed since the plant was specified. We often encounter situations where heat gains have increased through higher occupancy densities and/or increased use of electrical equipment, so the plant may already have been struggling to cope with the higher cooling demands.

Using newer designs of components, that comply with modern standards and specifications, can help to boost performance – and do so more efficiently. For example, the original fans and AC motors may be replaced with backward-curved fans and DC motors to achieve higher efficiencies through reduced fan power consumption, while newer direct expansion refrigeration equipment will offer higher efficiency than older designs. Similarly, heating and cooling coils now deliver more efficient heat transfer than used to be the case, filters are more efficient and AHU panels with much improved thermal and acoustic insulation can be supplied.

In some instances, replacing the components may not be sufficient to achieve the required performance but a refurbishment can also include the replacement of whole sections, or addition of new sections to supplement what’s already there.

In all of these cases the refurbishment approach can also avoid the access problems that are often encountered in existing, occupied buildings and often make plant replacement a major challenge. Even when new sections are required these can be assembled on site, so that access is not an issue.

Minimise disruption

Working in occupied buildings also necessitates minimising disruption to the occupants and a well-planned AHU refurbishment can easily overcome this problem as the work can be carried out in phases outside normal working hours. And where the AHUs are serving critical areas these can be isolated so the client’s business-critical services are unaffected.

This isn’t to say that refurbishment of AHUs is always the best option but it is certainly one that ought to be considered more often, and probably will be as sustainability becomes a more important element of the project planning process. In this respect, the building services engineer can play a key role in making the end client aware of all of the options that are available. Given the potential financial and environmental benefits it also makes sense to team up with companies that have experience of AHU refurbishment and can provide a comprehensive service from feasibility studies and design through to commissioning.

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