HEAT RECOVERY (MVHR)
A common utilization of this principle is in systems which have an exhaust stream or waste stream which is transferred from the system to its surroundings. Some of the energy in that flow of material (often gaseous or liquid) may be transferred to the make-up or input material flow. This input mass flow often comes from the system’s surroundings, which, being at ambient conditions, are at a lower temperature than the waste stream. This temperature differential allows heat transfer and thus energy transfer, or in this case, recovery. Thermal energy is often recovered from liquid or gaseous waste streams to fresh make-up air and water intakes in buildings, such as for the HVAC systems, or process systems.
The diagram shows how heat flowing from one side to the other can heat up and cool down depending on the airflow surrounding the ducts.
Neatafan has been manufacturing air handling units with air to air heat exchangers since 1984. The ability to recover what would otherwise be wasted heat in the exhaust route only required combining supply & exhaust fans in the same unit. The exception being polluted, corrosive or greasy exhaust air.
The recovery efficiencies achieved in practise are between 45 & 75% unless you have warm humid exhaust air, low velocities and narrow fin spacing. Also bear in mind that useful heat energy is only recovered when the exhaust air is approx. 5 degrees c warmer than the supply input air.
The system can also be used to cool the building during a summer evening, if the outside ambient falls sufficiently. The Neatafan DRC2 control system combined with a by- pass damper will optimise these opportunities.
The latest units can now benefit from EC/DC motors with built in speed control. Not only can the air volume be reduced for low occupancy, but it can also be reduced in very low ambient temperatures if the ventilation demand can be met. The power requirements of these motors falls away more rapidly than the equivalent inverter controlled AC motors.