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Heat exchange element

The core component of rotary air preheater is heat storage element, commonly known as heat exchange element. Heat exchange element and element box are important parts of rotary preheater, their weight sometimes accounts for about 50% of the preheater. The performance of the preheater is closely related to the heat exchange elements.

key word:Heat exchange element

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Product description

1. Brief

The core component of rotary air preheater is heat storage element, commonly known as heat exchange element. Heat exchange element and element box are important parts of rotary preheater, their weight sometimes accounts for about 50% of the preheater. The performance of the preheater is closely related to the heat exchange elements. The selection of heat exchange elements is very important for the thermal characteristics, comprehensive economy and operation and maintenance of the preheater.

The heat exchange element of rotary air preheater is usually composed of corrugated plate and locating plate rolled by thin steel plate. The corrugated plate and locating plate are overlapped with each other to form flue gas and air passage in the middle. When the air flow is in the middle, it is strongly disturbed, which strengthens the convective heat transfer process between the air flow and the heat exchange element.

The material selection of heat exchange elements is determined according to the fuel characteristics, working temperature, combustion conditions, whether the system is equipped with SCR denitration system and the temperature field distribution of heat exchange elements. For coal-fired units, common carbon steel is used in hot end and middle temperature section in most cases. In order to resist low-temperature corrosion, low-alloy corrosion-resistant steel is mostly used in cold end heat exchanger, and Corten steel or equivalent material is commonly used. For the air preheater of fuel oil and inferior coal, the cold end heat exchange elements are sometimes enamel plated.

 

2. Design principle of pattern selection of heat exchange element

The design of plate type of heat transfer element depends on its application

2.1. Good heat transfer performance, i.e. high air velocity, good disturbance and strong mixing.

2.2. The heating surface per unit volume is large.

2.3. Good resistance performance, small resistance under the same thermal characteristics.

2.4. It is not easy to accumulate ash (especially at the cold end).

2.5. It is easy to manufacture in production line and the cost is relatively low. Many requirements are contradictory, so the overall economic benefits, including maintenance, should be considered comprehensively.

 

3. Brief introduction of heat exchange element pattern

 

Nowadays, there are many types of boards in the world. Different types are used in different situations, while only a few types are commonly used, but the specific sizes are different. Each type of plate has to be tested for many times, and then formally used as a standard and applied for a patent. j and f curves are given. j is related to heat transfer and f is related to resistance. j and f are functions of Reynolds number Re. j and f are the key data in the performance calculation of preheater. In the performance test comparison of various plate types, a Puming power rating data is often used.

At present, DU, DNF, NF and FNC are the most commonly used plate types. However, the specific sizes of different countries are different, and the test data are confidential. The same is DU board type, the performance may vary greatly, so attention should be paid to the design.

According to the characteristics of the flow channel, the waveforms can be divided into two types, one is the open waveform, the other is the closed waveform. The open waveform has strong disturbance in the process of flue gas flow, so the heat transfer is better, but the resistance is poor. The closed waveform is on the contrary. Therefore, from the principle of economic design, the open waveform is generally used at the hot end to improve the heat transfer efficiency, The cold end usually uses closed waveform to achieve the purpose of anti blocking.

 

4. Performance characteristics of different heat exchange element pattern

Pattern Wave type Geometric characteristics Heating exchange Resistance characteristics application
NF closed It consists of a plate without oblique wave and a plate Less heat exchange area per unit volume, very poor Straight channel, low resistance coefficient cold
NU/UNU open cross wave Excellent normal hot
DN/DN3/DUN closed cross wave good excellent cold
DL/DL3 open loose slant straight wave good normal hot
UNF semi-closed wave plate good good both
DU2.5/ open slant cross wave good normal hot
DU2.78/
DU2.8
/DU3
/HS7
/HS8
/HS8E
/HE
/HE1
/K2/K6
DNF/TC1/KE-1/DU3E/HE2 closed closed slant wave normal good cold
FNC open completely cross Excellent not good hot
CU/HC open big/small slant cross wave normal normal hot

 

5. Comparison of the performance of heat exchange elements

 

6. Suggestions on type selection in current situation

The heat transfer effect, flow resistance and the possibility of ash plugging should be considered in the selection of element plate type. The use of heat transfer elements with good heat transfer effect can reduce the manufacturing cost, but it is not necessarily that the flow resistance is small or the ash blocking resistance is resistant, resulting in the increase of operating cost. For example, a company uses HS7, HS8 and other DU series wave elements in the cold section of the preheater. Although the weight of the preheater rotor becomes lighter, when the fuel ash content becomes more, or when it is used in SCR preheater, it is easy to cause ash blockage and increase the resistance. Heat transfer effect and flow resistance often constitute a pair of contradictions, because to improve the heat transfer effect is to strengthen the local turbulence of the airflow channel, that is, to increase the ripple density or inclination angle of the heat transfer surface, but this way also increases the flow resistance coefficient.

Therefore, the selection of heat transfer elements is based on the characteristics of the fuel to select the appropriate plate type of heat transfer elements. Generally, DU, DN, DL and other wave type heat transfer elements are selected in the hot section and medium temperature section of the preheater to match the anthracite to lignite fuel. Compared with the Du series waveform (such as HS7 and HS8 of a company) used for any kind of coal, it can give consideration to the heat transfer performance and anti blocking ability. Considering the characteristics of Chinese coal, for high ash coal and lignite, in the cold section of preheater, the general preheater adopts nf6 wave heat transfer element which has small resistance and is not easy to block ash; In SCR preheater, DNF / KE-1 type heat transfer element with good heat transfer effect and closed flow channel is usually used in cold section. The good heat exchange effect can ensure that the exhaust gas temperature will not decrease when the height of the cold section element increases, and the good soot blowing effect of the closed channel can ensure that the soot is not easy to be blocked.

It is also necessary to calculate the temperature field of metal and gas in the preheater rotor during type selection, and determine the height of heat transfer elements in each layer according to the temperature field and dew point temperature of sulfuric acid and ammonia hydrogen sulfate, with a certain margin to adapt to the change of coal type. On the one hand, this method ensures that the deposition area of ammonia hydrogen sulfate in SCR preheater is completely within the scope of heat transfer elements in the cold section, and when high sulfur coal is used, sulfuric acid is also completely deposited in the cold section layer, so as to avoid inter layer ash plugging, improve the flushing effect, and ensure the economy and safety of the equipment.

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