Real Time Dryer Energy Efficiency Monitoring

In today’s carbon conscious environment, it is important to optimize dryer efficiency. The first step in improving dryer efficiency is to monitor in real time. In many cases, the efficiency can be improved by simply adjusting air flow to an ideal value. Other applications will need to look at modifications like feed improvements and better end-product moisture control.

Real-time dryer efficiency measurement is also very useful in identifying upset conditions, indicating when maintenance is needed and recognizing difficult products to dry.

For monitoring dryer efficiency (air side only), measure the inlet dryer temperature, the inlet /ambient air humidity ratio, dryer exhaust humidity ratio (using H2O sensor) and airflow.  Below are the steps to monitoring air-side dryer efficiency using these four real-time measurements:

• Calculate enthalpy (kJ/kg) of ambient air, ha. ha = CpT + (ri*((hfg+(Cpv*T)))) where T is temperature, ri is humidity ratio (kg h2O/ kg dry air) and hfg is enthalpy of vaporization at given T. Cp is specific heat of dry air. Cpv is specific heat of water vapor.
• Calculate the enthalpy of inlet gas to dryer (after being heated) to a temperature Ti, hi. Follow same steps as step 1 and humidity ratio will stay the same as step 1 or you might have to add the moisture from combustion on direct fired dryers which is easy to do.
• Calculate the delta enthalpy, hd:  hi – ha
• Calculate mass flow rate (kg/hr), mf: Multiply gas density (kg/m3) x gas flow rate (m3/hr).
• Multiply delta enthalpy by mass flow rate (kg/hr) to determine energy required to heat air per hour (kJ/hr), Ei:  Ei = hd x mf
• Calculate evaporation rate (kg/hr) across dryer, Er. Er = (ro – ri) * mf, where ro is exhaust humidity ratio, ri is inlet humidity ratio and mf is mass flow rate.
• Calculate minimum energy for evaporation rate (MJ), Em. 2.3 MJ/kg (constant) times evaporation rate: Em = Er x 2.3MJ/kg
• Calculate dryer efficiency, n: n = Em/ Ei * 100
• For detailed analysis, see our enthalpy calculator worksheet which has all equations and constants.