The heat pump as a heat source for drying, drying and dehydration and other operations, is the 1980s developed a new type of drying technology. At present, the technology has been applied in various fields, such as wood , chemical products, food drying, vegetable dehydration and sludge stool drying, etc., have achieved good results. For sludge and excrement drying problems, heat pump drying technology with low operating costs and environmental pollution without any two major advantages. As early as the early eighties of last century Japan has been successfully used.
2, heat pump drying principle
Heat drying apparatus by the heat pump system thermodynamic cycle and a hot-air circulating drying system. Heat pump circulation system for hot air drying system to provide heat and reduce hot air humidity. Hot air drying system, direct contact with the material through the hot air circulation, to provide heat evaporation moisture, take away the moisture in the material.
The so-called "heat pump" is a kind of energy-saving device which consumes a certain amount of mechanical work and absorbs the low-grade heat energy in the environment or wastes to increase it into usable heat energy. Just like a "water pump", it takes a certain amount of mechanical work to raise the water from a low water level to a desired water level.
The heat pump system consists of compressor , condenser , throttling device and evaporator. The working medium in the system absorbs the heat from the drying chamber to discharge the hot and humid air in the evaporator and evaporates into low pressure steam from the low pressure liquid refrigerant and becomes the high temperature and high pressure steam after being pressurized by the compressor; in the condenser, the high temperature and pressure The working fluid heat releases heat to enter the air in the drying chamber, while the working fluid itself condenses from the gas to a high pressure liquid. Through the throttling device, the liquid working fluid produces a blocking effect, reducing the pressure and temperature, becoming a low pressure and cryogenic liquid and re-entering the evaporation Absorption of hot and humid heat in the gas, so repeatedly circulating the low temperature heat transfer to the high temperature medium to form a heat pump cycle.
Drying chamber exhaust gas is a high moisture content of hot and humid gas, the state shown in Figure hd point a, its relative humidity ψ = 70 ~ 80% or so, through the heat pump evaporator, the evaporator surface The temperature below the dew point of the air not only reduces the temperature of the air but also condenses the water vapor on the evaporator surface and exits the system as liquid water. The gas leaves the evaporator to become cold and wet, as shown in Figure 1. The relative humidity is generally ψ = 95-97%. After desorption of gas in the heat pump condenser to get wet heating, increased gas temperature, but also reduces the relative humidity, as shown in a point C into a hot dry gas into the drying chamber. In the drying chamber, the dry hot gas is in direct contact with the material being dried, providing the heat of the material to dry, while also removing moisture. Air in the drying chamber isenthalpic humidification cooling process, when leaving the drying chamber and returned to a point a. As can be seen from Figure 1 per kilogram of gas through a cycle of water can be removed △ d = da-dc.
Figure 1: heat pump hot air circulation diagram on hd
From the above principle of heat pump drying point of view, the difference with the general drying process is no hot and humid gas emissions, through two closed loop system, the water in the material eventually to liquid water to exclude. There are two advantages of using this principle to dry sludge:
(1) energy saving: in heat pump drying. Do not discharge hot and humid gas and its sensible heat and latent heat all recovered, only spent a certain amount of mechanical work. Commonly used to measure the heat pump heat recovery capacity of the heat pump coefficient of performance to characterize, that is
COP = Condenser heat / compressor power consumption = QC / W0
In the sludge drying equipment COP = 4.0 ~ 5.0, both the heat pump provides heat is 4 to 5 times the power consumption.
Another important indicator of the energy consumption of drying equipment heat pump than the rate of dehydration WPE
WPE = dehydration / heat pump energy consumption = kg (water) / kw.H (electricity)
In the current equipment heat pump WPE = 5.0 ~ 6.0. That can be dehydrated per kilogram of 5 to 6 kg.
(2) do not pollute the environment: As the heat pump dry sludge in a closed environment, all the stink and harmful gases generated in the drying process can be done without leakage, the surrounding environment can be reduced to a minimum of pollution, favorable Drying is done near the settlements.
3, heat pump dry sludge device profile
Since the 1980s, our scientific and technical personnel have systematically researched and developed the heat pump drying technology. A series of high temperature heat pump dehumidifiers have been developed using a high temperature thermal performance refrigerant R142b and a unique hot air circulation system (patent number: ZL.4). The unit's drying temperature in the 50 ~ 85 ℃ range can work, so far have been 500 to 600 sets (sets) of equipment put into operation, especially in the wood drying has achieved excellent results; such as Shanghai's high-quality wood products more than half, I use my equipment for drying. Export brand-name wood products, almost all of my equipment for drying process. In 2000 won the "national key new product" title, since 1997 I was named "high-tech enterprises in Shanghai."
Heat pump dryer group with the matched continuous drying of the drying chamber and drying a batch of two ways. The general material is relatively coarse, the drying cycle is longer or the material does not form batch batch drying, that is, the material is loaded and unloaded intermittently. Such as wood, agricultural and sideline products, herbs , handicrafts and more in this way.
Figure 2: sludge drying device
For the sludge drying cycle is relatively short, bulk and larger, forming the material is small and the particle size is more uniform and therefore should adopt continuous feed and discharge continuous drying. A prototype of a sludge drying continuous heat pump drying device developed by our institute is shown in Figure 2. Its appearance is shown in the photo 1. Heat pump dryer is still using my fruitful, mature unit. Drying room equipped with adjustable speed stainless steel belt conveyor belt group. In order to dry the process all in a closed environment, in the drying chamber equipped with two sets of molding machines.
Sludge drying continuous heat pump drying device
After fermentation, the powdery sludge with a water content of 60-65% passes through the closed conveying cylinder and enters the drying chamber. In order to be easy to be dried, the sludge is prevented from flying up after being dried. After preliminary shaping, the sludge is poured on the conveyer belt. Speed with the material transport, transport back and forth by the number of layers of the conveyor belt so that the sludge drying to a moisture content of about 40% for the second time into the second granulation fertilizer, and then transported back and forth by several conveyor dry, and finally reached 20% moisture content of finished products Of the granular fertilizer (see photo 2) out of the drying chamber, and then sent to the packaging plant closed by the conveyor, Sheng bag out.
Hot air from the heat pump dryer sends the air vertically to the conveyer through the circulation fan and then passes through several layers of stainless steel mesh belt to absorb the moisture of the material to return to the heat pump for dehumidification. dry. In this way, it has also successfully dried to a moisture content of 60% for a drainage sludge with an unfermented moisture content of 80%. Of course, it is more difficult to form in the drying process, but this also creates the conditions for further expanding the scope of the sludge treatment.
4, heat pump technology characteristics of dry sludge
After heat pump drying device research and test and analysis, heat pump dry sludge has the following advantages:
(1) low energy costs
The heat pump dry sludge device can recover the sensible heat and latent heat of the hot and humid air during operation, and the energy is fully and rationally utilized, which is a recognized efficient energy-saving device. According to the current energy prices in Shanghai, the energy cost of heat pump operation is about half that of heating and drying with gas and fuel (see Economic Analysis).
(2) Do not pollute the environment
As the whole process of heat pump dry sludge is carried out in a closed system, without the need to dehumidify the surrounding environment, but also emit harmful, odor gas. If all the incoming and outgoing system in the pipeline closed, will make the entire operation to maintain a clean environment. Dried sludge has become odorless granular fertilizer. Open the drying chamber after drying sludge cleaning, no obvious odor overflow. This is a unique advantage over other drying methods.
(3) sludge drying quality
The drying medium in the heat pump drying device circulates in a closed space. It is stable under the same conditions regardless of the external climatic conditions. Therefore, the drying quality is stable. Sludge is a jelly-like solid micelles precipitated from urban drainage through flocculants. The composition of the sludge is complex and has strong hydrophilicity. The temperature of heat pump drying can not only be adjusted at 50-85 ℃, but also can control the humidity of the drying chamber to meet the ideal conditions of drying organic sludge, which will not cause the outer wall of sludge to crust and cause the difficult dehydration. Test proved that the moisture content of dried products uniform, but also to maintain the organic fertilizer granules. In addition, since the cycle is carried out in a closed space and is not affected by the external weather conditions, it operates under the same conditions all year round and the quality of the drying is stable.
(4) Easy to form a modern industry
At present, the prototype machine for daily processing of one ton of sludge heat pump dryers has been equipped with heat source, drying chamber, air blower and control equipment in a confined space of 1.5m wide, 2.3m high and 3.5m long. It is very compact . If expanded to handle five tons of sludge equipment, its volume will not exceed B × H × L = 2 × 3 × 5 m. Its area is very limited. In addition, the unit can be combined operation. If the daily processing capacity of 20 tons of sludge, you can make four units in close parallel operation, the total area is small, of course, in the daily volume fluctuations can also adjust the number of operations. Due to the heat pump drying operation for a single electric energy, high degree of automation , very suitable for supporting modern city applications.
5, heat pump drying energy economy
The heat pump is driven by means of electricity force, high power costs generally mistaken, easily accepted. Now on the use of coal, oil, gas and heat pump as the heating device for drying, economic comparison, the calorific value of a variety of heat sources and prices Set as follows:
(1) Coal heat value: 6700 kcal / kg, price: 0.35 yuan / kg.
(2) gas calorific value: 3500 kcal / M3, price: 0.8 yuan / M3.
(3) fuel heat value: 10800 kcal / kg, price: 2.4 yuan / kg.
(4) Electricity value: 860 kcal / kWh, average price: 0.45 yuan / kWh.
(0.6 yuan / kWh during the day and 0.3 yuan / kWh at night)
If the treatment of a ton of 60% moisture content of sludge drying into 20% moisture content of granular fertilizer, for example, to provide equal heat of drying, the energy cost calculation results are shown in the following table:
Can be seen from the above table, a variety of heat source drying device, except for coal, the use of heat pump drying, fuel oil, gas costs are low. However, the actual costs of using coal are higher than those listed in the table above, because in addition to drying equipment, there is a need to invest in large boiler rooms, boiler equipment and to pay for complex management costs such as adding environmental protection facilities, managing operators, Field, transportation of coal and slag, etc.
6, Summary
The sludge drying into granular fertilizer, the use of heat pump desiccant drying device and other heating means of the drying device, the advantages and disadvantages of the following table:
Therefore, as environmental requirements increase, especially in cities with high population densities, the use of heat pumps to dry sludge has unique advantages.
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