Fins of the ratio, the fin efficiency and fin parameter selection
This section describes the finned tube two important concepts: fin ratio and the fin efficiency, and that should be considered in the choice of the fin parameter problem.
However, we must first finned tubes and fins on the structural parameters Tagging itself made the following suggestions:
(1) finned tube and fin structure annotation methods
First, the CPG represents finned tubes (CHIPIAN GUAN) abbreviation of the structural characteristics of finned tubes, material and processing method used the following series of numbers or symbols:
CPG (φDb × δ / Df / P / T-X / Y-A)
Where: CPG: finned tubes; φDb × δ: base tube diameter and thickness; Df: fin diameter, mm; P: fin pitch, mm;
T: fin thickness, mm; X: the base tube material; Y: fin material; wherein: Fe: iron; Al: aluminum; Cu: Copper
A: Processing method: I: frequency welding (not marked which is the default); others to be determined. See below the mark.
For example: CPG (φ32 × 3.5 / 64/8/1-Fe / Fe) Description of the finned tube outer diameter of the base tube 32mm, thickness of 3.5mm, an outer diameter of the fin 64mm, (i.e. the height of the fin 16mm),
Fin pitch of 8mm, fin thickness of 1mm, the base pipe and the fins are carbon, high-frequency welded pipe.
Further, there is need for a separate structural parameters of the fin itself, tagging, labeling methods are as follows:
CP (Db / Df / P / T-Y)
The meaning of each symbol with finned tubes that the same method. Examples are as follows:
For example: CP (32/62/8/1-Fe) Description The outer diameter of the fin base tube 32mm, outer diameter of the fin 62mm (fin height of 15mm),
Fin pitch of 8mm, fin thickness of 1mm, made of carbon steel.
(2) wing of the ratio
Is the ratio of the fin surface of the light pipe (base tube surface) after the installation of the fin surface multiples expansion, can be "β" to indicate that the
β = (original light pipe surface area) / (total surface area finned tube)
Calculation example:
There is a finned tube, CPG (φ25 × 2.5 / 50/4/1 -Fe/Fe), try to calculate the ratio of fin
1 meters the number of fin tube length n = 1000/4 = 250
1 meters long finned tube area
Af = 250 × [π / 4 {(Df2-Db 2) × 2 + π × Df × Y} = 0.775 m2
1 m tube length on the bare tube area, that area between the fins of the light pipe
Ao = π × Db × 1 × (PT) / P = 3.1416 × 0.025 × 1 × 3/4 = 0.0589 m2
1 m tube length of the light pipe area
Ab = 3.1416 × 0.025 = 0.0785 m2
Wings of the ratio
β = (Af + AO) / Ab = (0.775 +0.0589) / 0.0785 = 10.62
After that add fin heat transfer area of the original area of 10.62 times the light pipe.
Of several common types of finned tubes, the fin of the ratios calculated results are shown in Table 1, the order for reference:
(3) Fin Efficiency
When the fin to be "rooted" on the surface of the light pipe after pipe within a pipe by the rumor that the case of heat, the heat from the fin root pass outward along the fin height, while continuing to pass by way of convection to the surrounding fluid, the result is that the fin temperature is gradually decreased in the height direction. As shown below.
Fin temperature gradually decreased in the height direction, indicating the fin temperature difference between the temperature of the surrounding fluid is gradually reduced, the amount of heat exchange per unit area gradually reduced. Thus, the fin heat exchanger surface area to enhance the effectiveness of the decline. Fin higher, the increase of the area of the heat exchanger of the "contribution" is smaller. Therefore, it is necessary to introduce a new concept ---- fin efficiency.
Fin efficiency η = (fin surface of the actual amount of heat) / (assuming the fin surface temperature equal to the temperature of the heat dissipation wing root volume)
Because the fin efficiency is less than 1, indicating an increase of 1 times the fin cooling area, and can not increase one times the amount of heat, place a "discount", the "discount" is the fin efficiency.
The value depends on the efficiency of the fin fin shape, height, thickness, material, and more importantly also depend on the heat transfer coefficient of tube. Calculation is more complicated and time-consuming. The following commonly used for engineering finned tubes, given a set of computed values for selection. Table -1. Calculations show that the efficiency of fin height to fin affected, the higher the fins, the lower fin efficiency; secondly, the thermal conductivity of the fin material also has some effect, higher thermal conductivity of aluminum, carbon steel, other conditions are the same, aluminum fins than steel fin efficiency is higher. In addition, the fins and also the efficiency of the heat transfer coefficient of the tube, the fin efficiency value in the table is the heat transfer coefficient in a certain h = 50 W/m2. ℃ calculated under the conditions.
Table 1 fin structural characteristics
Fin specifications
Fin fin efficiency η of validity than β (β × η) Discussion
* - Recommended Level
CP (25/50/6/1-Fe) 7.4 0.82 6.07 *
CP (25/55/6/1-Fe) 9.2 0.78 7.18 **
CP (25/55/6/1-Al) 9.2 o.92 8.46 **
CP (32/62/8/1-Fe) 6.62 0.78 5.16 *
CP (32/70/8/1-Fe) 8.71 0.71 6.18 **
CP (32/62/6/1-Fe) 8.49 0.78 6.62 **
CP (38/68/8/1-Fe) 6.32 0.79 4.99 *
CP (38/76/8/1-Fe) 8.25 0.72 5.94 **
CP (38/68/6/1-Fe) 8.10 0.79 6.40 **
CP (51/81/8/1-Fe) 5.92 0.81 4.80 *
CP (51/89/8/1-Fe) 7.60 0.73 5.55 **
(4) The effectiveness of the fins
Fin validity means after the installation of the fin to the base tube (tubes) on the basis of the outer surface of the heat transfer coefficient increased many times in the end. The derivation of the following relationship:
ho = h × [(Ao + Af × η) / Ab]
Here, ho ---- In light pipe external surface area as the base of the convective heat transfer coefficient, which represents the total effect after the installation of the fin;
h ---- finned outer surface convective heat transfer coefficient;
AO, Af, Ab fin gap at the bare tube area, fin area, and the original light pipe area.
Because Ao << Af, so the above equation reduces to:
ho = h × η × [(AO + Af) / Ab] = h × η × β
Thus, the fin and the fin efficiency of the product ratio (η × β) as the effectiveness of the final index fin. Listed in Table 1 for the set of finned tubes, the validity of (η × β) values are also included in the table. For example, the table in the CP (38/68/8/1--Fe), it is assumed heat transfer coefficient of the outer surface of the fin h = 50 W / (m2. ℃), the effectiveness of the fins of 5.94 Finally, the outer surface of light tube convective heat transfer coefficient based on ho = 50 × 5.94 = 297 W / (m2. ℃).
(5) consideration of the fin parameters
5-1. Fin height choice: From the above table, the calculation results can be seen, for works on common high-frequency welding finned tube, when the fin height is 15mm, the fin efficiency is about 0.8,
When the fin height is 20mm, then reduced to about 0.7 fin efficiency. This shows that the choice of 15mm fin height is appropriate, if the selected fin height 20mm above, will have to be particularly careful fin due to their low efficiency, generally not be used. On the air with aluminum fins, the thermal conductivity of aluminum is much higher than carbon steel, high efficiency of their fins, the fin height increased to 22-25mm
Is also acceptable.
5-2. Fin pitch selection: Select a small pitch, which effectively increases the ratio of the wings. But in choosing the pitch, they should also be especially careful. Factors to be considered are:
* Gas flow around the nature and the possibility of fouling. Can be divided into three cases: first, fouling particularly serious situations, such as: steel plant furnace, converter, and certain industrial furnace exhaust, a large amount of ash, if the finned tube heat exchanger, Be sure to use a large fin pitch. Such a pitch of 10mm or more, but also complemented by rational design and selection of ash sootblowers.
The second situation is not very serious fouling, but it should also give attention to the occasion, such as: power plants and industrial boilers exhaust, fin pitch about using 8mm more appropriate, but to be complemented with a swank gray design capacity. The third situation is no fouling or fouling slight occasions, such as the combustion exhaust gas equipment, or air cooler, its 4-6mm fin pitch selection is possible. For aluminum cooler, its fin pitch often about 3mm.
* Fin processing technology and processing costs but also in the choice of factors to consider when fin pitch.
5-3. Fin thickness choice: the gas flow around the main consideration corrosive and wear. Corrosion and wear for serious occasions, the choice of thicker fins.
Key Words: Finned Tube, Fin Tubes, Extruded Fin Tubes, Embedded Fin Tube, Tubos Aletados