Heat recovery

Heat recovery

Given the current and future energy prices, the use of heat recovery systems in supply air systems is an effective measure to reduce energy costs for companies in old systems or to achieve best values for new systems from the outset.

The following example of a supply air system with an exhaust air and supply air volume flow of 10,000 m³/h illustrates this effect:

Technical data:

• Exhaust air, supply air volume flow: 10.000 m³/h

• Density of the air: 1,2 kg/m³

• Specific heat of the air: 0,24 kcal/kg x grd

• Desired supply air temperature: 20 °C

• Lowest outisde temperature: - 15 °C

• Heat recovery coefficient: 65 %

• 860: Conversion factor in kcal/h x kW

Required heating capacity without heat recovery
Q = 10.000 x 1,2 x 0,24 x (20 - (-15)) /860 = 117,2 kW

Required heating capacity with heat recovery
Q = 10.000 x 1,2 x 0,24 x (20 - (- 15)) x (1 - 0,65) /860 = 41 kW

This means that the heat recovery reduces the connected load of the heating controller in the air supply unit to approx. 41 kW, since at the design point a heat output of approx. 76 kW is recovered from the exhaust air.

The annual savings in thermal energy depend on the customer-specific data (location, operating mode, supply air temperature parameter, etc.), so that a reliable statement can only be made in a specific individual case.

Systems

Plate heat exchanger 

The plate heat exchanger is the most frequently used system in the medium flow rate range, as it has proven to be a very economical solution in practice with the following additional advantages:
- simple construction 
- no mixing of the air streams 
- simple bypass solution
- heat recovery rate up to approx. 65 %

Closed-loop system 

The closed-loop system is used when the applications require separate supply and exhaust air units.

Heat exchangers are installed in the supply air and exhaust air units, which are connected to each other on the water side.

With closed-loop composite systems, heat recovery rates of up to approx. 50 % can be achieved.

Rotary heat exchanger

This system is a solution variant for relatively large air volumes, especially when moisture transfer is also desired for process-related reasons.

Another advantage is that heat recovery rates of up to approx. 75 % can be achieved.