Table of Contents
- Conventional grain, fruit and herb drying often uses direct fossil-fired hot air above 80-120 C. That high temperature damages colour, aroma and nutrients and wastes energy. Heat-pump drying supplies warm air at 30-60 C, which protects product quality while still removing moisture. The catch is that the exhaust leaving the drying chamber is still warm and very humid - it carries away both sensible heat and a large amount of latent heat that would otherwise be lost.
- An air-to-air heat exchanger is placed between the exhaust and the incoming fresh (make-up) air. As the humid exhaust passes over one side of the core and the cool incoming air passes over the other, heat is transferred through the plates without the streams mixing: Sensible plate core - recovers heat only. The exhaust pre-heats the incoming fresh air, so the heat pump must supply less energy to reach the set drying temperature. Savings of 30-50% on heating energy are typical. Enthalpy (rotary or plate with desiccant) core - also transfers moisture. It pre-conditions the incoming air by moving both heat and water vapour, cutting the dehumidification and reheat load on the cycle.
- In a heat-pump dryer the refrigerant loop condenses moisture out of the process air and rejects that heat. Feeding pre-heated, partially dehumidified make-up air into the loop raises the coefficient of performance (COP): the compressor does less work per kilogram of water removed. Closed or semi-closed loops with recovery can lift COP from around 2-3 (no recovery) toward 4-5, sharply lowering electricity use.
- Frost and condensation - at low make-up temperatures the exhaust side can condense or frost; counter-flow plates with a bypass or a defrost cycle handle this. Contamination - exhaust from agricultural product can carry dust and organic load, so a cleanable plate or a wheel with a purge sector is preferred. Metric - specify the required sensible and latent effectiveness at the design airflow and drying temperature, not just peak rating. Used this way, the air-to-air exchanger turns a once-through dryer into a recovering loop, which is exactly the waste-heat-recovery idea covered across this Encyclopedia.
Conventional grain, fruit and herb drying often uses direct fossil-fired hot air above 80-120 C. That high temperature damages colour, aroma and nutrients and wastes energy. Heat-pump drying supplies warm air at 30-60 C, which protects product quality while still removing moisture. The catch is that the exhaust leaving the drying chamber is still warm and very humid - it carries away both sensible heat and a large amount of latent heat that would otherwise be lost.
An air-to-air heat exchanger is placed between the exhaust and the incoming fresh (make-up) air. As the humid exhaust passes over one side of the core and the cool incoming air passes over the other, heat is transferred through the plates without the streams mixing:
- Sensible plate core - recovers heat only. The exhaust pre-heats the incoming fresh air, so the heat pump must supply less energy to reach the set drying temperature. Savings of 30-50% on heating energy are typical.
- Enthalpy (rotary or plate with desiccant) core - also transfers moisture. It pre-conditions the incoming air by moving both heat and water vapour, cutting the dehumidification and reheat load on the cycle.
In a heat-pump dryer the refrigerant loop condenses moisture out of the process air and rejects that heat. Feeding pre-heated, partially dehumidified make-up air into the loop raises the coefficient of performance (COP): the compressor does less work per kilogram of water removed. Closed or semi-closed loops with recovery can lift COP from around 2-3 (no recovery) toward 4-5, sharply lowering electricity use.
- Frost and condensation - at low make-up temperatures the exhaust side can condense or frost; counter-flow plates with a bypass or a defrost cycle handle this.
- Contamination - exhaust from agricultural product can carry dust and organic load, so a cleanable plate or a wheel with a purge sector is preferred.
- Metric - specify the required sensible and latent effectiveness at the design airflow and drying temperature, not just peak rating.
Used this way, the air-to-air exchanger turns a once-through dryer into a recovering loop, which is exactly the waste-heat-recovery idea covered across this Encyclopedia.