Craig McFadyen, system products manager at Fl kt Woods, looks at the issues surrounding the different types of heat recovery units
MOST designers and contractors are now aware of the different types of energy recovery devices in air handling units. They are also aware of the benefits they bring in terms of lowering energy usage and, hence, reducing the end users running costs.

One thing which comes out of this knowledge is the understanding that it is imperative to opt for the most efficient method of energy recovery possible.

Realistically, where double deck units can be installed, a thermal wheel is usually the most energy efficient method.

Thermal wheels are the most efficient method of energy recovery. These will recover up to 90% (typically 75-85%) of the energy in the extract air.

The wheels are typically made of aluminium (although other materials are used) which rotate slowly perpendicular to the supply and extract airflows. As the extract air passes through the wheel the heat or 'coolth' transfers into the thermal mass of the wheel. As the wheel turns, it moves into the supply side where the supply air picks up the energy.

Obviously the major benefits are the highest levels of energy recovery and that they are only 500mm in length but they also have a relatively low airside pressure drop. Because of the high levels of recovery, it is advisable to run these at any time of year where there is a temperature differential between ambient and extract air.

The only downside of thermal wheels is that some of the extract air can be transferred into the supply side as the wheel rotates. And even though this can be minimised by a damper on the extract side, there is always a risk. Therefore, thermal wheels cannot be used in critical areas such as operating theatres in hospitals, or even in pharmaceutical production areas where contamination cannot be allowed to occur.

In these cases, or where you have to split the supply and exhaust unit, you are better off using a system such as Econet.

Econet is a run-around system which has energy recovery efficiencies of 65-75%. In other words, it is more efficient than a plate heat exchanger. On top of this, it constantly optimises itself maintaining the efficiency in all conditions.

Econet utilises large coils in both the supply and extract airstreams. This helps it maximise the amount of energy we can transfer.

Now the fact we have such large coils enables us to use them for heating and cooling as and when required. We do this by supplementing the Econet system either by injecting LPHW or CHW directly (if no glycol in the system) or through water heat exchangers (if there is glycol in the system). This means that no subsequent coils are required unless, of course, you are dehumidifying and then reheating where you would need a very small reheater.

The other fact is that because of the size of the coils, low grade cold or hot water can be used, so it is ideal for use with waste water from chillers for heating etc. or bore hole water for cooling. It also means that if you do run with the normal temperatures (60C and 820C), then the water flow rates required are lower (we allow for higher deltaT on waterside) reducing pipe sizes and valves etc.

Systems to reduce overall energy use

We can also look at using components in a smarter way to reduce the energy consumption.

· Twin wheel system

For systems where dehumidification and reheat are necessary, the twin wheel system is designed, not only to eliminate any heating requirement but also to reduce the chiller load by around 50%.
The system works with two thermal wheels, one hygroscopic and one non-hygroscopic, as well as a cooling coil.

Knowing what temperature is required off the cooling coil to dehumidify, we can calculate the conditions of the reheating (non-hygroscopic) wheel. As it is reheating, the cooling is transferred into the extract side which then goes on to the pre-cooling wheel (hygroscopic). This cooling is then transferred back to the supply side by the pre-cooling wheel. Because of this pre-cooling, the amount of cooling the coil is required to do is approximately half of what it would be if it had to cool down from typical summer ambient conditions. And because of the reheating wheel, we no longer require a reheater.

This system is ideal for chilled beam systems to avoid any moisture problems.

· Individual evaporative cooling

By the addition of an evaporative humidifier in the extract airstream before the energy recovery device, we can reduce the cooling load required from a chiller by up to 50%.

Because of efficiencies, only non-hygroscopic thermal wheels and Econet are used for these systems but basically they work by cooling down the exhaust air from the space by putting it through an evaporative humidifier. This cooler air then goes on to the energy recovery device and is transferred over to the supply side where it pre-cools the air before the cooling coil.

As we said at the beginning, it is imperative we use the most energy efficient systems on air handling plant. This reduces the life cycle cost of these systems and benefits the client for 25 years.

But, by being smarter, we can also reduce the energy consumption of the other parts of the air conditioning systems like chillers and condensing units.


Flakt Woods T: 0121 717 4680