Higher humidity –> More latent heat, Higher temperature –> More sensible heat -Overall heat recovery rate
Heat transferred in the heat recovery unit can be divided into the two separate terms
- Sensible heat recovered due to the cooling of hot fluid flow
- Latent heat recovered due to the released latent heat from condensation of the condensable gases
Effect of the humidity level is presented in our article How much increased water content in dryer section exhaust air will improve the heat recovery efficiency? In this article is accomplished the view explaining how temperature and humidity affect to heat recovery rate.
As a case example we have heat exchanger dimensioned for the following process conditions:
- mass flow: 20 kg/s dry air
- water content: 140 g H2O/kg dry air
- temperature in: +70 °C
- mass flow: 15 kg/s
- temperature in: +30 °C
Dimensioning diagram for the base case
Effect of the parameters
By doing an off-design study for this heat exchanger the following variation for the output values can be estimated:
As expected: either increased humidity or increased temperature improves the heat recovery performance. However, if these figures are compared to energy consumption which needed to create this additional heat recovery, the following results are found:
- Water content 140 g/kg and temperature increased from 70 –> 90°C increases heat recovery rate ca. 9%. However, heating up process air from 70 °C –> 90 °C will increase energy consumption ca. 20%!
- Water content 160 g/kg and temperature increased from 70 –> 90°C increases heat recovery rate ca. 8%. However, heating up process air from 70 °C –> 90 °C increases energy consumption ca. 19%!
The presented example shows that for efficient heat recovery system operation, it is preferred to focus on higher humidity level instead of higher temperature difference. Higher humidity level gives also savings on ventilation system operation. Also, it is important to notice that higher the temperature level is raised higher are the system losses which we have not counted on this analysis. However, higher humidity level sets much harder demand for the overall process ventilation system like hood and ductwork.