Heating and Ventilating

It is now standard practice to include heat recovery in ventilation systems and the choice of heat exchanger can have a major impact on other aspects of the system. Alasdair Howie explains
When selecting heat recovery for an air handling system there are essentially three options - cross-flow heat exchangers, counter-flow heat exchangers and thermal wheels. As a manufacturer of all of these we don't have a particular
axe to grind but our experience shows that each has a knock-on effect on other aspects of the system and its overall performance. For instance, there is little to be gained from using high efficiency motors in a unit that has a low efficiency heat exchanger.

Consequently it's important to identify the most appropriate technology at an early stage in the design process.

Cross-flow heat exchangers are suitable for a wide range of applications, up to airflows of around 12,000 cu m/h, and have a low pressure drop so they have a relatively low impact on the performance of the system. However, they will only deliver 50-65 per cent efficiency and while this is sufficient to meet the current requirements of Part L of the Building Regulations it may not satisfy the end client's energy efficiency aspirations.

Counter-flow heat exchangers have longer air paths and a longer dwell time, enabling them to achieve efficiencies of up to 90 per cent (typically 65-85 per cent). However, they also have a higher pressure drop, which takes the fan up to a higher point on the pressure curve. This has a small impact on efficiency but is more than compensated for by the improved heat exchange efficiency. Counter-flow heat exchangers are best suited to projects with low airflow requirements (up to 500 m3/h).

For large applications with very high air flows thermal wheels are the obvious - and often only - choice. They do have some downsides, though, which need to be taken into account. They are bulky and heavy, which may influence where they can be sited, and also have higher maintenance requirements (compared to static plates) because of the belt driven wheel. The fact that the wheel rotates also means it's more difficult to achieve a seal so a small amount of leakage should be anticipated.

Inevitably, one aspect of the design process will be to establish a balance between energy efficiency and project budgets. For example, the lowest cost option is a cross-flow heat exchanger but if the project requires efficiency of, say, 80 per cent then an alternative will need to be considered.

Similarly, Part L requires a maximum Specific Fan Power (SFP) of 2.2 W/l/s so if this can't be achieved with a balanced ventilation unit fitted with crossflow heat exchanger it will be necessary to consider an alternative, perhaps fitted instead with a thermal wheel.

An additional influence on system performance is the controls package that is included in the project - something that can have a significant impact on overall project costs. Here, the important thing is to align the level of control functionality to the requirements of the building and its individual spaces. There is no point in the end client making a major investment in a full 'bells and whistles' system if they are only going to use 5 per cent of its functionality.

So it's important to consider whether the system will need to control airflow in a single zone or in a number of different zones, as this will influence the choice of controls package.

If the system serves a single zone, such as a classroom, it is generally best to use a variable air volume (VAV) configuration to deliver the lowest energy use whilst providing demand controlled ventilation in relation to occupancy. Typically, the system will be linked to a room sensor that monitors an indicator of occupancy levels, such as carbon dioxide and increases the ventilation rate only when necessary by increasing the fan speeds. For single zones, local control with some timer functionality will be perfectly adequate in most cases.

Some systems serving single thermal zones may use a constant air volume CAV configuration, especially in small buildings and residences, using dampers to vary the air pressure. CAV systems tend to be a little less efficient than VAV because the system pressure tends to be higher and so doesn't offer the same savings on fan energy consumption, but they are also more complex with higher initial costs. The control considerations remain similar to a VAV system.

Where multiple zones are being ventilated the best solution is to use a constant operating pressure (COP) configuration where each room has a supply and exhaust circuit and sensor, and airflows are controlled by local dampers and variable speed fans. This situation is clearly more complex that VAV or CAV and will generally require a building management system with full Modbus connectivity.

It's also worth bearing in mind that variable airflows will alter the dwell time of the air passing through the heat exchanger which, in turn, will influence the efficiency of heat recovery.

There are number of other factors to consider when designing air handling systems. For example, when working with high airflows it is also necessary to include some pre-heating using either low pressure hot water or an electric heater. This will prevent freezing in the heat exchanger, which can obstruct the air flow and compromise ventilation. Externally sited units will also require higher levels of insulation to prevent excessive recovered heat losses.

At the other extreme of ambient temperatures, it is usually necessary to include a summer bypass valve to prevent pre-heating of supply air with recovered heat from the extract air. In such cases, the bypass should be fitted to the extract circuit to ensure the supply air continues to be filtered.

For all of these reasons the choice of heat recovery mechanism and the control strategy to be used need to considered early on in the design process. In many cases it will make sense to seek specialist advice to ensure that all of these interacting parameters have been taken into account.

// The author is marketing manager with S&P UK Ventilation Systems //