Influence of fin structure on dust accumulation and pressure drop in fin-tube heat exchanger

June 21, 2022
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The types of fins that are widely used at present include straight fins, corrugated fins and window fins. The dust accumulation on the surface of the heat exchanger of these fin types will seriously affect the heat exchange efficiency of the heat exchanger. Therefore, in order to clarify the long-term performance changes of finned-tube heat exchangers with different fin structures, it is necessary to understand the influence of different fin structures on the surface ash characteristics of finned-tube heat exchangers.
In this paper, we will discuss the characteristics of the ash on the surface of the lower fin heat exchanger. The influence of the fin structure on the dust deposition and pressure drop of the fin-tube heat exchanger was analyzed, and the influence of different fin structure parameters on the dust deposition on the surface of the heat exchanger and the pressure drop on the air side were analyzed.

1. Experimental principle and test samples

The test bench consists of three parts:
1) An air duct system that provides and guides dry air at a specific wind speed to the test sample;
2) Dust generation system, which can adjust the mass flow of dust to provide dust-laden airflow with specific dust concentration;
3) Visualize the test section, which is used to photograph the dust morphology on the surface of the test sample and measure the amount of dust deposition and air-side pressure drop.

The visual test section includes transparent plexiglass air ducts, test samples, analytical balances, differential pressure sensors, vertical lifts, trays and sponges. The sample is embedded in the 2 mm deep groove of the tray and fixed, while the 15 mm deep groove is carved around the tray and filled with sponge, and the transparent air duct squeezes the sponge in the tray groove to seal the test section. The tray is placed on the analytical balance, and the elevator is used to adjust the lifting height of the tray, so as to measure the weight of the sample and observe the appearance of the dust during the process of dust accumulation. Differential pressure sensor is used to measure the pressure drop data on the air side of the sample during the fouling process.

 

1.2 Experimental conditions and test samples

The experimental parameters include fin type and fin spacing. The fin types are selected as window fins, corrugated fins and straight fins, and the fin spacing is selected as 1. 5 mm and 1. 8 mm, covering common types and sizes of outdoor heat exchangers for air conditioners. Test sample real test sample physical object and structure

According to the regulations of GB 13270-91, the test dust used in the experiment contains 72% kaolin and 28% carbon black, the dust density is 2.2 × 103 kg/m3, and the median diameter is 10 μm. Due to the low dust concentration in the actual outdoor environment, in order to speed up the experimental process of fouling, at the same time, according to the normal upstream wind speed of the fin-tube heat exchanger in the outdoor unit of the air conditioner, the dust concentration of 10.8 g/m3 and the wind speed of 1 are selected. . 5 m/s for ash deposition experiments. The total length of dusting time was 255 min to ensure that the dust deposition amount was stable. The wind speed is adjusted by air compressor, flow meter and flow valve, and the powder concentration is controlled by screw feeder, control cabinet and mixing box.

 

2. Data processing method and error analysis

The pressure drop and wind speed can be read by the differential pressure sensor and flow meter respectively, and the dust deposition amount and the dust concentration are determined by a specific relationship.

 

2.2 Error analysis

The parameters include direct measurement parameters and indirect measurement parameters. The direct measurement parameter error can be obtained through the accuracy of the experimental instrument. The direct measurement parameters include air side pressure drop, air volume flow and sample weight. The indirect measurement parameter error can be obtained by the R.J.Moffat method.

 

3. Experimental results and analysis

The dust deposition distribution characteristics of samples with different fin types when the dusting concentration is 10.8 g/m3, the wind speed is 1.5 m/s, and the dusting time is 255 min. It can be seen from Figure 3 that the surface of the flat finned tube heat exchanger deposits the least dust, and it is mainly deposited on the heat exchange tubes; the heat exchange tubes and corrugated fins of the corrugated finned tube heat exchanger have a certain amount of dust deposited on the surface, and the area is The dust level is more serious than that of the straight fins; the dust level on the surface of the windowed finned tube heat exchanger is the most serious, and the finned windows are almost completely blocked by dust, and the surface of the heat exchange tube is also prone to form dirt blocks.

Analysis of the deposition characteristics of dust on the surface of these three types of heat exchangers shows that: the intermittent gaps protruding on the surface of the window fins face the dust-laden airflow, and the dust particles are more likely to directly hit and deposit on the intermittent gaps, making the surface of the window fins more likely to be deposited. Easily clogged with dust. At the same time, due to the small distance between the gaps on the surface of the window fins, the dust is likely to form a relatively tight dirt group between the gaps, resulting in a serious degree of dust accumulation.

In addition, since the surface structure of the straight fins and the corrugated fins is simpler than that of the windowed fins, under the same cross-sectional area of the heat exchanger, the contact area between the two and the dust-laden air flow is smaller, so that the dust particles can be reduced. The probability of collision deposits with the heat exchanger surface is small. At the same time, since the spacing between the straight fins and the corrugated fins is larger than the spacing between the gaps of the window fins, when the accumulated dust grows to a certain thickness, it is easy to fall off the surface of the fins under the action of gravity.

 

4.Effect of dust deposition on pressure drop

For windowed finned tube heat exchangers, the smaller the fin spacing is, the more significant the effect of fouling on the pressure drop is. It can be seen from the analysis in Fig. 5(a) that the small fin spacing can rapidly increase the pressure drop per unit time. The amount of ash deposition increases the pressure drop significantly. At the same time, the critical point of fouling with small fin spacing is higher, because the smaller the fin spacing is, the less likely the fouling layer blocked on the fins and heat exchange tubes will fall off.