Temperature and humidity – importance for dryer’s heat recovery efficiency

It is a common rule that increased humidity level in dryer’s exhaust air flow improves heat recovery rate. Besides this there is a fact that increased temperature difference between hot and cold fluid flows increases heat transfer as Newton’s law of cooling defines. However, the scale of influence of these two factors is not so obvious for all designers. In this article is presented with an example the importance of these parameters.

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.

Case study

As a case example we have heat exchanger dimensioned for the following process conditions:

Exhaust air:

mass flow: 20 kg/s dry air
water content: 140 g H2O/kg dry air
temperature in: +70 °C

Circulation water:

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:

Analysis

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%!

Summary

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.