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Heat pump technology Dry sludge equipment
**1. Introduction**
Heat pump technology, as a heat source for drying and dehydration, is a relatively new method that emerged in the 1980s. Over time, it has found widespread application in various industries such as wood processing, chemical products, food preservation, vegetable dehydration, and sludge treatment, achieving excellent results. For sludge and fecal matter drying, heat pump drying stands out due to its low operating costs and no environmental pollution. Japan was among the first to successfully implement this technology in the early 1980s.
**2. Principle of Heat Pump Drying**
The heat pump drying system consists of a thermodynamic cycle and a hot-air circulation system. The heat pump provides heat to the drying chamber while simultaneously reducing the humidity of the air. In the hot-air drying system, the heated air comes into direct contact with the material, transferring heat to evaporate moisture and carrying it away.
A "heat pump" is an energy-efficient device that consumes mechanical work to absorb low-grade heat from the environment or waste heat and convert it into usable high-grade heat. It functions similarly to a water pump, which uses mechanical energy to lift water from a lower level to a higher one.
The heat pump system comprises a compressor, condenser, throttling device, and evaporator. The working fluid absorbs heat from the drying chamber, evaporates into low-pressure steam in the evaporator, and is then compressed into high-temperature, high-pressure steam. In the condenser, this high-temperature fluid releases heat to the drying air, condensing back into a high-pressure liquid. Through the throttling device, the fluid cools down and re-enters the evaporator to repeat the cycle.
During the process, the exhaust gas from the drying chamber, which is hot and humid (point A), passes through the evaporator. The temperature on the evaporator surface is below the dew point, causing condensation of moisture and removal of water vapor as liquid. The gas exiting the evaporator becomes cold and wet (point B), with a relative humidity of about 95–97%. After passing through the condenser, the gas is heated and dehumidified, becoming dry and hot (point C), ready to enter the drying chamber again. This closed-loop system ensures efficient moisture removal and energy recovery.
**3. Overview of Heat Pump Sludge Drying Equipment**
Since the 1980s, Chinese researchers have developed and refined heat pump drying technology. High-temperature heat pumps using refrigerant R142b and a unique hot-air circulation system (patent number: ZL.4) have been widely applied. These units operate at temperatures between 50°C and 85°C, with over 500–600 units currently in operation, especially in wood drying. For example, more than half of Shanghai's high-quality wood products are dried using this equipment, and many export brands rely on it.
There are two types of heat pump drying systems: continuous and batch. Materials like wood, agricultural products, and herbs are typically processed in batches. However, for sludge, which has a short drying cycle and uniform particle size, a continuous feed and discharge system is more suitable.
Our institute has developed a prototype for continuous sludge drying, as shown in Figure 2. It features a closed drying chamber equipped with adjustable-speed stainless steel conveyor belts and two sets of molding machines. After fermentation, sludge with 60–65% moisture content enters the drying chamber, where it is shaped, dried, and eventually reduced to 20% moisture, becoming granular fertilizer.
**4. Characteristics of Heat Pump Drying Technology for Sludge**
Heat pump drying offers several advantages:
**(1) Low Energy Costs**
Heat pump systems recover both sensible and latent heat during operation, making them highly energy-efficient. In Shanghai, the cost of heat pump drying is about half that of gas or fuel-based drying methods.
**(2) No Environmental Pollution**
All processes occur in a closed system, preventing odor and harmful gas emissions. The final product is odorless granular fertilizer, making it ideal for urban areas.
**(3) Stable Drying Quality**
The closed-loop system ensures consistent drying conditions, regardless of external weather. The temperature and humidity can be precisely controlled, preventing surface crusting and ensuring even moisture distribution.
**(4) Suitable for Modern Industrial Applications**
The compact design allows for easy scaling. A unit for 1-ton sludge per day occupies only 1.5m x 2.3m x 3.5m, and larger systems can be built with minimal space requirements. The high degree of automation makes it ideal for modern city environments.
**5. Energy Economy of Heat Pump Drying**
Electricity is the primary energy source for heat pumps. When compared to coal, gas, and fuel, the cost-effectiveness of heat pump drying is notable. While coal may seem cheaper, additional costs for boilers, maintenance, and environmental protection make it less favorable. Electricity, despite its cost, offers a cleaner and more controllable option.
**6. Conclusion**
Heat pump drying offers significant benefits for sludge treatment, including energy efficiency, environmental friendliness, and high-quality output. As environmental regulations become stricter, especially in densely populated urban areas, heat pump technology is increasingly recognized as a sustainable and effective solution for sludge management.
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