Heat pump drying is a method of drying a product with warm, dry air that circulates in a closed loop. The air picks up moisture from the product, cools down on the heat pump's cold surface (the evaporator) and leaves that moisture behind as water, warms up again on its hot surface (the condenser) and returns to the product. Drying usually takes place between 35 and 60 °C, and because the moist air is not exhausted, most of the heat spent on evaporating the water is recovered. The result is a product whose colour, aroma and texture are better preserved, and a lower energy demand.
We do not make drying machines under our own brand; we design and build the heat pump module inside the machine for companies that manufacture dryers. This article covers what both a dryer manufacturer and a producer setting up a drying plant need to know about the heat pump side.
How does closed-loop drying work?
- Moisture pick-up: Warm, dry air passes between the products on the trays, takes up the water evaporating from their surface and cools a little in the process.
- Condensation: As the moist air passes through the evaporator it cools below its dew point; the moisture it carries condenses into droplets and is drained away. In this step, the heat the water absorbed while evaporating passes to the refrigerant.
- Reheating: The dried air passes through the condenser and is heated again by the heat collected in the evaporator plus the energy the compressor uses.
- Return: Fans send the warm, dry air back to the product. The loop continues until the product reaches the target moisture content.
In a conventional hot-air dryer, the moist air leaves through the stack and takes its heat with it. In a heat pump dryer that heat is captured in the evaporator and used again. We explain the basic heat pump cycle in our article what is a heat pump. When comparing dryers, the figure to look at is the SMER (specific moisture extraction rate): it shows how many kilograms of water are removed from the product for each kWh of electricity.
Why does a low temperature protect product quality?
When fruit is dried at a high temperature, its surface dries quickly and hardens. The moisture inside cannot get out through this crust, so the product ends up dry outside and moist inside. This is called case hardening. As the temperature rises, browning speeds up, volatile aroma compounds are lost and heat-sensitive nutrients such as vitamin C decline.
In a heat pump dryer, temperature and humidity are controlled together, so the drying rate can be matched to the rate at which water inside the product can reach the surface. In a closed loop, the dust, insects and humidity of the outdoor air never enter the drying room. Unlike sun drying, the result does not depend on the weather; production carries on through a rainy week.
Which products are dried?
- Fruit: apricots, figs, grapes, apple and pear slices, strawberries and other soft fruit. Soft fruit breaks down and darkens quickly at high temperatures, so it gains the most from low-temperature drying.
- Vegetables: tomatoes, peppers, aubergines, onions, carrots. With dried tomatoes and peppers, colour is the first sign of quality.
- Nuts: hazelnuts, walnuts, pistachios and almonds are brought down to a safe moisture level after harvest. This step prevents mould and loss of quality in storage.
- Medicinal and aromatic herbs: thyme, mint, sage, linden, bay leaf. Because essential oils are sensitive to heat, these products are usually dried at lower temperatures of around 30–40 °C.
Every product needs its own recipe. At the start of drying there is a lot of free water on the surface and the air velocity is kept high. As the product dries, water arrives more slowly from the inside, and temperature, humidity and air velocity are staged accordingly. A good dryer's control system lets these stages be programmed product by product.
Capacity: the water to remove, not the room volume
The heat pump in a dryer is sized not by the size of the room but by how many kilograms of water per hour have to be removed from the product. The calculation starts from the product's initial and target moisture content.
Example: let us assume we dry 1,000 kg of fresh apricots in one batch, and that the product contains 80% moisture at the start and 20% at the end. The dry matter is 200 kg and does not change; when drying is complete the product weighs 200 / 0.8 = 250 kg. So 750 kg of water has to be removed. If the batch is completed in 30 hours, the average moisture removal is 25 kg per hour; in the first hours of drying this figure rises well above the average. The latent heat of evaporation of water is about 2,400 kJ/kg, so 750 kg of water requires about 500 kWh of heat. In a closed loop, most of this heat is recovered in the evaporator and used again in the condenser.
The evaporator is chosen large enough to handle the peak moisture load of the first hours, while the compressor has to run efficiently at part load as well. This is why an inverter compressor is especially valuable in a dryer: towards the end of drying, as less moisture comes off the product, capacity is turned down too.
Closed, open and mixed layouts
Heat pump dryers are built in three layouts, depending on the air path:
- Closed loop: All of the air circulates inside and moisture is removed only by condensing it on the evaporator. It is independent of outdoor conditions and is the cleanest layout in terms of hygiene; it is preferred for aromatic herbs and delicate fruit.
- Open loop: Outdoor air is heated in the condenser and passed over the product, and the moist air is exhausted. It is a simple solution in regions and seasons where the outdoor air is dry and mild, but the heat of evaporation is not recovered.
- Mixed layout: Part of the air stays in the loop and part is renewed. The heat of the exhausted moist air can be passed to the incoming fresh air through an air-to-air heat exchanger. At the start of drying, when the moisture load is very high, the fresh air share is increased; towards the end the loop is closed.
The choice of materials depends on the product too. Fruit acids, and the sulphur dioxide released by products that are sulphured before drying (apricots, for example), corrode copper and aluminium surfaces in a humid environment. For these products, evaporator and condenser surfaces are specified with a corrosion-resistant coating, and the trays and racks in contact with the product are made of stainless steel. The amount of water drained from the evaporator is also a handy indicator for following the progress of drying.
The heat pump module inside the dryer
A dryer heat pump works under different conditions from a residential heat pump. The air reaching the evaporator is warm and humid, and the temperature of the air leaving the condenser has to be held precisely to the recipe. The excess heat the compressor adds to the loop also has to be rejected; an auxiliary condenser or a controlled fresh air inlet is used for this. That is why we work out the module together with the machine's air flow, tray layout and recipes.
Under our OEM solutions, capacity and refrigerant selection, the placement of the evaporator and condenser in the air duct, the control board and software, and the labelling and documentation are prepared for your machine. Because our R&D team develops the control board and its software, a board that works together with the machine's recipe controller can also be assessed at the technical proposal stage. One example of building a custom heat pump for a specific machine is the heat pump we made for SarMakina, for a machine that purifies acidic water.
Hot water for an existing drying room
If you have a drying room with a hot water coil, an air-source heat pump can prepare the water for that coil instead of a boiler. W series outdoor units produce water up to 55 °C and the Ayaz hybrid package up to 75 °C. No fuel is burnt in this arrangement, but the moist air is still exhausted and the recovery of evaporation heat that a closed loop provides is lost. In a new plant a closed-loop dryer, and when converting an existing room a hot water solution, may be the more practical choice.
For your drying project
If you manufacture dryers, share your machine's air flow, target temperature range and batch capacity; if you are setting up a drying plant, share the product, the daily quantity and the initial and target moisture content, through our contact form. We will work out the heat pump side with you from these figures.
