David Applegate, managing director of Thermal Technology, discussses how the ahu has evolved and how today's units are more complication but also more efficient than previously.
Air Handling Units have changed considerably during the last 20 years. Rather than a simple box containing a filter, heater battery and fan, they are now much more complicated with energy recovery devices, built-in control packages, inverter-controlled high efficiency fans, thermal break pentaposts and more optional extras.

Production of the sheet metal components has also moved forward. Most of the larger manufacturers have invested in state-of-the-art, computer-controlled, sheet metal handling plant and all the cutting, punching, notching and folding is automatic. This ensures a high quality end product.

The selection process has been simplified with computer software permitting the input of limiting variables determined by consultant's specifications, such as coil face velocities, fan discharge velocities, fan efficiencies and heat recovery efficiencies. Autocad-based software produces dimensional drawings which can be forwarded to consultants and contractors.

The inclusion of energy recovery devices, to meet the Building Regulations, is commonplace. Forty years ago many thermal wheels were supplied, but most of them were ducted up to ahus, rather than as an integral component. Heat recovery devices help to achieve higher efficiencies, with lower air side pressure drops.

Thermal wheels are typically specified with a required efficiency of between 70 per cent to 80 per cent, which means pressure drops in the order of 100 to 200Pa.

Efficiency

Single plate recuperators can achieve 60 per cent to 70 per cent, with moderate pressure drops. For applications with large supply and exhaust air temperature differences, such as swimming pools, duplex recuperators (two recuperators in series) can give operating efficiencies of 80 per cent to 85 per cent with combined air pressure drops of no more than 200 to 250 Pa.

Run around coils, always a good heat recovery solution where space is at a premium, or where the supply and extract air ducts are separated, can achieve 50 per cent to 60 per cent. Coils with six or eight tows are about the optimum.

Control

Thermal wheels, plate recuperators and run around coils all feature different forms of controlling efficiency. The thermal wheel is perhaps the simplest as the efficiency varies with the speed of rotation, maximum efficiency being achieved at around 10 to 12 rpm. Plate recuperators are usually supplied with face and bypass dampers. These may be fitted with modulating damper actuators to vary the amount of air passing through the recuperator and the bypass duct. Run around coils may be controlled by turning the pump on and off, or by fitting a 3 port mixing or diverting valve.

With the requirement to meet specific fan powers (SFP) now in the Building Regs, fan selections have become more important too. The higher efficiencies are achieved with backward curved centrifugal fans with aerofoil impellers or plug-type fans. Efficiencies of 70 per cent to 85 per cent are achievable. Plug fans are direct drive so there are no transmission losses. They should be fitted with motor inverters, which accept various input signals to achieve constant air pressure.

Another trend in ahus is for manufacturers to offer units wired and fitted with a controls package. These are generally standardised and are either BMS enabled or contain a control outstation for greater interrogation.

Standardised packages are all good, provided they have all of the features and controls. Bespoke options are more likely, available from the middle/smaller sized manufacturers which operate on a more flexible production policy. Factory fitted controls are better quality, and tested before they leave the factory.

The recession has meant fewer new build projects, but there are opportunities in the refurbishment market. Certain ahus can be supplied in flat-pack component form for on-site assembly.

Ahus, using construction techniques taken from the double glazing window market, are generally easier and quicker to assemble than more traditional units.

Ahus have changed considerably and will continue to evolve. To help this process, Thermal Technology has adopted the optimised component selection principle which ensures that each item within each ahu has been selected to give its optimum level of performance and efficiency.