Tony Millard, sales director of Carrier Air Handling Units, reviews the latest Eurovent publication and discusses the energy efficiency of air-handling-unit-based air conditioning systems
THE publication of Recommendations for Calculations of Energy Consumption for Air Handling Units by Eurovent has so far received scant attention in the UK. But the increasing interest in improving the energy-efficiency of air conditioning systems make the availability of such a document more interesting. It is the first time Eurovent has turned its attention to air handling in this way and it is proving a valuable tool to engineers concerned with getting the most out of air-handling-unit-based air conditioning systems.

Although seemingly one of the older air conditioning technologies, there is every reason to consider ahus when designing today's energy conscious, energy efficient heating, cooling and ventilation solutions. Air handling systems can operate without circulating refrigerant in the conditioned space and can introduce outside air, enabling ventilation needs to be met. Energy recovery technology can make this cost-effective.

One of the aspects of energy efficiency within air handling systems which has similarly received limited attention in the UK, is the technology of rotary heat exchangers. These can be incorporated as a heat recovery wheel in the system.

Commonly, the rotary heat exchanger is a wide, coiled aluminium sheet wheel of large diameter mounted in a stable frame, fixed within an ahu, set against the air flow. In the heat wheel, sensible and latent heat (the latter strongly dependent on the applied rotor type - see below) is transferred by means of a matrix structure with micro channels created in the aluminium construction of the wheel. This structure rotates between the supply air and the exhaust air channels.

Heat exchangers are regenerative and operate according to a counter-flow in the air stream giving them a high recovery ratio. The matrix contains a large heat and mass transfer surface and provides a low pressure drop as the air passes through it. Sealing strips separate supply air and exhaust air channels. The wheel is rotated by means of a belt-drive and motor. Changing the speed of rotation controls the recovery ratio of the wheel or rotor.

To prevent 'polluted' extract air being carried over to the supply air side, most rotary heat recovery sections are equipped with a purging sector. The sector opens up an air passage between outdoor air and extract air and, by means of a pressure differential, the rotor's passageways can be emptied of extract air before they pass into the supply air duct. The angle of the sector is adjusted according to the rotor speed and the pressure differential between outdoor air and extract air.

Importantly from an energy efficiency view, latent as well as sensible heat can be recovered. Latent heat is transferred when a synthesized metallic substrate within the wheel removes moisture from the air stream that has the higher humidity ratio through condensation and/or adsorption and releases the moisture through evaporation and/or desorption into the air stream that has the lower humidity ratio.

There are three different types of heat exchange rotors - condensation, hygroscopic and sorption. As the name suggests, the condensation rotor only transfers water vapour from the rejected warm air when condensation is formed, during low temperature outside air conditions, in the wheel. The hygroscopic wheel has the collection matrix chemically treated to improve this effect by sorption.

However, the sorption rotor's treatment and design makes it the most effect at recovering latent as well as sensible heat. It can recover moisture virtually independently of condensation. It has a sorption material on the heat exchange surface which absorbs moisture from the extract channel. This version can achieve moisture recovery rates of more than 70%. From observation of these principles it is apparent latent seasonal efficiencies of (particularly) condensation rotors and hygroscopic rotors will be substantially lower than the rated efficiency at design (ie minimum) outdoor temperature. However, the sorption rotor, with its almost constant latent effectiveness, independent of the outdoor temperature, can also be relied upon to transfer moisture during summer operation.

The objective of any type of energy recovery in an air handling system is to lower operating costs and reduce emissions.

The possible operating cost savings depend on the following factors:

1. System air volume;

2. Mass flow ratio between outside air and exhaust air;

3. System design (heating, cooling, humidifying);

4. Energy recovery efficiency at design conditions;

5. Resistance over the recovery system;

6. Temperature and humidity
pattern;

7. Geographical system location;

8. Seasonal energy recovery
efficiency;

9. Operating times;

10. Seasonal thermal energy
generation efficiency;

11. Price of electrical and thermal energy.

The first five factors are determined by the system design. A temperature and humidity pattern determines the relation between the outside air condition (independent variable) and the current entering and return air condition (dependent variables). For any operating cost calculation we must first determine the pattern, as there would otherwise be large differences between the calculated results. The diagrams show an example of the temperature and humidity pattern of an air handling system that can heat, humidify and cool.

A manufacturer such as Carrier can provide independently verified computer calculations which show the energy efficiency of the equipment under any specified conditions and what savings can be expected. Any consideration of pay-back against the higher capital costs of this equipment must take into account the reduced operating costs and the lower investment required in heating, humidification and cooling.